Spectroscopy

[theodog]

After talking with my Physics class about spectra and stars I thought I would see if there were any simple spectroscopes to be made from the web. Found this:

http://www.uwm.edu/~awschwab/specweb.htm

I am aware that many of these projects may not work so I made one myself (1st image) and took the second image with a cheap digital camera of an energy saver light globe.
Truely amazing.:thumbsup:
Something to do on those overcaste nights with the kids.
:D

[Merlin66]

Just got back from a few days in Italy!! A very welcome break from the clouds here in the UK.
Terry,
The S Shed slit is very good for what it is; I use it on both my spectroscopes and my Foucault tester. Unfortunately I believe it's currently out of stock.
To make it more usable with transmission gratings ( similar to Robin's idea with the eyepiece) I'm measuring up a couple of "adaptor" sleeve's which would allow it to be used in a 1.25" focuser, and a 1.25" female, so we could add an extension tube with a lens and a SA/ Baader grating etc etc.
BTW the limited trials with the 50 and 9 micron optical fibre went well; just need to rig up a plate to hold the outer end on the slit. This would allow me to use a optical "head" on the scope ( much lighter!) and leave the spectroscope on the table nearby......

[sheeny]

Never mind the kids... I'll have a go myself!:D

BTW My copy of Kaler arrived today. More reading.:thumbsup:

Al.

[theodog]

Hi All,
Back imaging again, new computer so no previous images.
I started with a familiar one earlier this evening, eta carina, details on image.
:D

[Merlin66]

Good to see you're back up and running!
Can you post the .pic file for that one... need something to "play" with; it's still p**** down here, and looks set in for the next week....again!

[theodog]

Ok here it is, the fits image from the ST7.

Could do with some of that rain during daylight here, getting a bit dry.
PS have you found the drawings of the spectroscope you mentioned previously. I have managed to make an adjustable slit using a razor blade -I have been decifering the pictures from your projects article.
:)

[Merlin66]

Thanks for that!
I'll do a sketch of the BAA transmission type; you can use that as a template.
Need a couple of old binocular lenses, 2-3 front surface mirrors ( old photocopier mirrors are pretty good to cut up!) some 6mm customwood, or equivalent......
The other one was the TRAGOS,
http://www.burwitz-astro.de/spectrographs/tragos/index.html
but omit the last lens and use the camera lens....

[Merlin66]

In each design you can use a 3-4degree prism ( Edmund have them) infront of the grating to divert the first order back onto the optical axis. (GRISM)
The BAA could be turned into an L shape by omitting M1 and turning the collimator by 90degs; but I think the original design looked nice!!
If a flip mirror is used in M3 then an eyepiece can be placed at the end of the box for visual work without moving the camera... neat!
If the wall section supporting the grating is made "slide in" then you can mount different gratings in the slot to suit - quick changeover!
The adjusting Knob can be replaced with a cheap micrometer for more accurate readings...
The surplus shed slit could be incorporated into either design. Old camera lenses will work just as well.

[Merlin66]

Looking at Jeff's latest EtaCar spectra I keep seeing a definate emission around 6035 which doesn't (??) appear on any of the earlier ones going back to Jan.
I can find the "Pa" match.....
Any other ideas????

[theodog]

I'll try for another tonight, to confirm the emission, if skies allow.;)
:)

[theodog]

OK Tonight (30/3/09) these are my results.
I have posted
etacar -from VSpec -red hydrogen, green helium, purple spec of eta
etacar-20sec-specfitraw.zip -raw spec image
etacar-20secfit-speciris.zip -image through iris y=7
etacar-20sec-speciris.zip -.spc file from VSpec
I hope, for you to fiddle.
**I think my previous spec may have been contaminated by a fluoro light**
Given the shape of the Ha area.
Have fun, I did, I spent the afternoon chaseing a 1m black snake:eyepop: away from the observatory, I still don't know where it went . Double checked every inch of the obs floor before entering tonight.
:D

[Merlin66]

Still caught up trying to process some Ha solar images; but a quick look shows that the emission has disappeared....
This one now looks more like the early spectra... how did you figure the "contamination"

[theodog]

Last night while imaging I noticed the trees around light up very slightly. I asked my partner if she had turned a light on and yes she had. I restarted the run. She told me she did the same during the nights observing and I didn't complain, - I was inside using MaximDL sequence so I didn't notice the light.:mad2:
I don't know if it did effect the spectra, but it has to be noted.
:)
Yes we are still talking.:D

[sheeny]

Woohoo! Daylight saving is gone... clear sky and in the obs tonight!

This is the first spectrum (half) processed with my SA in my new filter wheel.

Shot this spectrum of Eta Carinae so I can measure the dispersion of the new setup - 8.98 A/pixel with the camera directly behind the filter wheel.:thumbsup: It means maybe a bit more processing with the DMK21 since it's nearly always 2 frames to join to get the full spectrum, but that's OK at least for now. Maybe a DMK41 might appear on the scene:P.

Seeing was pretty ordinary.

I shot some more spectra of Sirius, betelgeuse, Hadar, Alpha Cen, Gamma Crucis, etc yet to be processed.

Al.

[Terry B]

Very good.
I've has cloud for over a week and the 100 hrs stuff over the last few nights (also with mostly cloud)
I will try again soon.

[sheeny]

OK here's the corrected Eta Carinae and the corrected Sirius Spectrum I used to get a better camera response curve. ;)

That'll be about as much processing for me tonight though:whistle:. Time for bed!

Al.

[theodog]

Nice spectra Al, I do like the Sirius spread:P. The hydrogen lines seem to line up a cracker.
I couldn't greet the end of daylight saving with starlight as I have my gear striped down for RYSTARS and a school observing night. As soon as I have my gear together I will have a go at Sirius myself.
:)

[sheeny]

OK guys. Something to think about... I think.:P

Attached is a spectrum of Betelgeuse captured last night. It has been corrected for camera response based on the response curve generated from my Sirius Spectrum of the same night. Now if the camera response curve is accurate, shouldn't the corrected spectrum agree with the library spectrum?:shrug:

The blue spectrum is my Betelgeuse corrected with the camera response from my Sirius spectrum. The green is the library spectrum for M2i (which is the nearest to Betelgeuse in the VSpec library).

Notice the library spectrum is low at low wavelengths, and high at high wavelengths. This suggests to me that the camera response curve is not correct.

I've been thinking for a while that to get a true camera response curve would require averaging the response curves from the full range of star classes (O to M) so that the low signal to noise portions of the spectrum can be handled properly.

Thoughts?

Al.

[Merlin66]

Who's been a busy boy then...
I've been using the few minutes of sunshine to image with the SM60 ( and get practise in processing Ha images) The nights have still been too cloudy to be able to do ANY imaging!!!
Al, I think the fall off of the camera responce below 4000 and above 7000 is so rapid/ steep that the smallest error in compensation with throw things out dramatically. I know I have that problem with the Canon 350 ( the 300D has the Baader UV-IR filter fitted, and that gives a very clean cut-off at 3950 and 7000)
Certainly try different reference stars, you may be able to find a compromise.
What if you calibrate on the Betelgeuse and then "correct" the Sirius spectra????

[sheeny]

G"Day Ken.

I've been playing about with similar arcane ideas :P:lol:.

Another thing that could influence this is the black point on the image. This lot of spectra that I took has the black point at about 11000 counts on the intensity axis. Probably the result of either not having the brightness control set up zero, or maybe from using so much gain in order to keep exposure time down (due to seeing). 11000 divided by next to nothing gives a fantastic lie for camera response...:P

That zero error or black point noise I think could be contributing to the spurious camera response curves. What do you think?

I started having a play with subtracting a constant from the spectrum and I think that's part of the way to go. I'm about the call it quits for the night though.

So, I'm starting to think I need to put a step into my work flow, that I subtract the black point offset from the raw spectrum before doing any other operations.

I think that if that doesn't fix the problem, then that combined with averaging the response curves (across the star classes) should.;):whistle:

Ideas?

Al.

[theodog]

Hi Al,
Do you do all the processing frames; flats, dark, bias?

I have the same issues that my instrument responce seems untransferable but I figure it's because I only do darks.

I had the same thoughts about instrument responce for all classes.
Perhaps we could use others final spectra as library and compare?

It will be the weekend before I'm up and running again.
:D

[sheeny]

G'Day Jeff

I haven't been doing darks or flats at all.:whistle: So far most of my spectra have been captured as AVIs and stacked in registax, so that smooths out the noise. So far it's only Eta Carinae that I've captured as a still (as it's not very bright). But you are right, subtracting a dark should remove a lot of the black point noise if it is due to brightness/gain settings on the camera.

Subtracting the black point offset in VSpec should have a similar effect though.

Al.

[sheeny]

I see lines! I see lines!:rolleyes::screwy::lol:

Here's the fruits of tonight's foray to the fringes of sanity.:P

I reworked Sunday night's Sirius and subtracted the Black Point offset before doing the division to generate the camera response. The results of the reworked spectrum are in the first image. I found the spectrum was crap below about 3400A.

I then used the new camera response curve to process Betelgeuse captured on the same night. The result was rubbish. It slightly shifted the hump in the raw spectrum but did not get close to the library spectrum above about 7000A.

So I generated another camera response curve from Betelgeuse. Again I subtracted the black point offset first, taking care not to make any values negative. It was a bit of suck-it-and-see to get the subtraction right, if it isn't right the division curve will look like rubbish where it approaches zero:P, but it's a simple matter to correct the offset adjustment and try again.

The final camera response I got from Betelgeuse is very different to what I got from Sirius. What I like about it, or what I find encouraging at this stage, is that it is similar in shape to the chip manufacturer's published response curve:thumbsup:.

I have put all 3 camera response curves in the last image for comparison.

The next step will be to test the Betelgeuse response curve, and see what it does to the Sirius Spectrum...

Al.

[sheeny]

Here's the test results.

This is Sirius corrected with the camera response curve made from the Betelgeuse spectrum. I'm not sure whether it's encouraging or disappointing:shrug:.

I think there's more to this camera response business than I've got a handle on yet...:whistle:

Al.

[Merlin66]

The Imaging Source curve for the DMK21 chip (ICX098BL) shows about 50% at 4000 and 7000.... this seems to match the Pink curve??????
Very interesting stuff!
A dark and sky correction may make some difference , but looking good...

[Terry B]

Al
I think the camera response curves are going to be dodgy for a few reasons. I'm not sure how we can avoid this. The A stars (like sirius) arn't very bright in the IR so the levels are low compared to the green etc. When you try to divide another spectrum by this curve it enhances the IR more than it should. I tried this with a red star and got a vastly enhanced IR end to the spectrum that wasn't real.
The same problem will occur if you use a red M star as the response curve and try to calibrate a hotter star but with the opposite effect of over enhancing the blue end.
I suppose a more realistic result would be achieved by making a curve from a "B G and M' star and averaging them. The difficulty here would be how to normalise them first so that the weighting for each spectrum was even. Maybe normalise all of them at a specific green frequency and go from there.:shrug:

In the end I don't think it matters terribly as we arn't measuring the flux at a particular frequency although this is possible with reference stars etc.

[sheeny]

I'll have to examine the curve again:whistle: ... from my memory of it - based on shape only, I thought the green curve was better.:rolleyes::whistle:



I agree. In theory, if we had a uniform light source across all frequencies, we could very simply get a perfect camera response.

Because we are using non-uniform light sources, any area where the signal is low becomes very inaccurate.


I think I can see 2 possible ways to establish a wide spectrum camera response curve:
Average the normalised camera response curves from B, G and M stars as you suggested (if necessary expand that to include the other classes too, but I don't think that would be necessary);
Join the low wavelength section from the B spectrum, to the middle from the G, to the long wavelength section of the M class. Might be a bit fiddly scaling the connections, but this should address the magnification of errors/noise that a low signal causes.:)

Al.

[theodog]

I don't want to sound defeatist but, are we asking for an accuracy that is unattainable? Is it too hard to map the CCDs?

Would we need to ensure that the spec falls onto the same pixels each time? Maybe each pixel has a different responce to each wavelength. So if the spec image is moved between two exposures it would give a different response.

A single dimention pixel array should be easier to calibrate.
One way to check this maybe to allow the spec to drift up across the camera array and measure the levels along the extended Ha line for example. Do they change along the Ha line? (Illustrated by Terry's image)

ie I think we need to get the image to land on the same pixels each time.

I hope this makes sense.
:)

[sheeny]

I see what you're getting at, Jeff. The only problem with drifting the spectrum that I see is when it comes to fainter stars - more background stars to affect the spectrum. After thinking about it a bit, I don't think the pixel to pixel variation will matter too much, unless you're using a slit or can focus the zero order image into 1 pixel (or near enough to). Even a well focussed spectrum is several pixels wide so any pixel variation is just a small part of the noise in the image.

After some more playing about I'm just about at the point of shelving it for a while. Basically, for my camera anyway most of the response curves I make are pretty close in the range from 4000 - 7700 A which is visible light (OK it's a bit short on the violet end:P).

Outside of that range is really where the trouble starts for me. At the UV end the response is useless IMO, but there's a tantalising signal amongst the noise in the IR, and this is what I've been trying to sort out.

My main reason for pushing all this is for stars like Eta Carinae for which we can't access a library spectrum like the others. Unless the camera response spectrum we use is repeatable and transferable, then what do we use? The camera response from the hottest star we have access to?

So I'm just about at the point of giving the camera response thing a rest for a while, concentrate on 3900-7700 A and take it from there.

BTW I played about making a hybrid camera response curve. I averaged the black point adjusted Sirius and betelgeuse curves, and then cropped it at 7700A and joined on the IR end of the Betelgeuse curve. Like all things, it was a compromise.:lol:

Al.

[theodog]

Hi All,
I haven't seen this site here before.
Has some interesting info on spectroscopes.

http://www.astroman.fsnet.co.uk/optics.htm

Are others aware of it?
:D

[Merlin66]

It was raised way back...
Maurice has been active with spectroscopes for over 20 years!
He lives about 10 miles from me, in SW London!!
Been over to visit a couple of times......

[theodog]

:doh::doh::doh:Sorry, the thread is so big I missed it.
Nice info 'though.
:)

[Merlin66]

Not much to report from this end!
The weather seems to be improving and there should be some clear weather later this week.
Re camera calibration - I'd just use the "best" we can find from the standard library curves... I don't think the calibration varies over the CCD frame unless there's some optical effects ( vignetting??)
You could try some exposures with various filters and see if there are any differences?
I'll set up the Littrow for Solar and check out the resolution....

[theodog]

Been quiet here too observation wise.

I have however, spent some time constructing my BAA style spectroscope. All seems to be going well so far, mirrors arrived today. Lenses are from a cheap 10x25mm binocculars.

Modifications from the BAA plans I have seen include a flip mirror for alligning and a movable camera mount to assist focusing the spectra.
Just a trial and I've had the time. :thumbsup:

If it works I'll look for larger Binoc lenses and build a bigger one.;)

[Merlin66]

Looking good!!
Will you use a razor blade slit or go for a Surplus Shed adjustable??

[Terry B]

Great start.
Too much cloud here for anything.:mad2:

[sheeny]

Yeah... 2 weeks holidays (3/4 over) and all I've had is cloud and drizzle...:rolleyes:

Al.

[theodog]

I have started with a fixed slit (ground by myself) -not razors.
The mirrors and lenses have been fitted and an image of the slit can be formed ( and moved by the movable mirror -right top) on a piece of paper held about 30mm above the hole at the top.:thumbsup:

Off fishing for the weekend:cool::cool: so no further progress until next week.

[sheeny]

Well, didn't the blue sky surprise me this arvo!:D

I got about an hour of play tonight... fog brought the innings to an end. Everything is so damp and cool you have to expect that:rolleyes:.:whistle:

But I managed to do a little experiment.:thumbsup:

I'm aware of the theory that a shorter focal length will improve the purity of the spectra from the SA, but in the past my attempts with the focal reducer have been less than encouraging. So tonight, I set down to do the experiment. I captured spectra of both gamcru and etacar at f/10. Then I put the f/6.3 focal reducer in and took them again.

The results of the Eta Carinae spectra are attached. I haven't processed the Gamma Crucis sectra yet, as VSpec has been testing me out with funny errors and not saving SPC files correctly:rolleyes:. But from this test, the focal reducer is a clear winner:thumbsup:.

Al.

[Merlin66]

If you look at the shape of the zero order image you see the improved (!!??) image size profile.
Smaller star images, with a no slit configuration will give you better resolution; what you can do to improve things even further is to move the grating away from the chip to give a higher dispersion; then you get the best of both worlds... smaller stars and larger spectrum spread!!!???

[sheeny]

Yep.

Here's a corrected spectrum.

Al.

[theodog]

Nice one Al.
Good to see some observing being done.
I must admit I've been a bit lazy.

[sheeny]

Here's part 2 of my focal length experiment. The subject was Gamma Crucis this time. I found the camera response curve was a bit screwy below 4000A with the focal reducer, so that's cropped out. Not sure what's going on there, the response curve took a jump up:shrug:. Makes no sense at all... I could understand it blocking frequencies...

The depth of the absorption lines is improved with the FR and you can see some detail starting to emerge. Definite :thumbsup::thumbsup:.

I found out the problem I was having last night with VSpec. I was saving the SPC files with the header applied. When I go to reload the spectra afterwards I get an "Input after end of file" error.:shrug: Any ideas on how to fix/work around that?

Al.

[sheeny]

Oh yeah... after thinking about the wrestling I was doing in my last session with the black point in the spectra, this time I made a point of making sure the brightness setting for the DMK was always on 0... it greatly simplifies the processing of the spectrum:whistle: cause there's no offset to remove before doing the divisions for camera response.

:)

Al.

[Terry B]

Well done Al.
I spent the clear night :thumbsup: doing time series of the nova in the LMC so no spectra. Interesting result with the focal reducer.

[Merlin66]

Some good results there Al!!!
The reducer certainly pulls out a bit more detail; what do you think you'd get with a bit greater distance to the chip??
With the small DMK chip can you still get the whole spectrum or will you have to go to "sections"???

[sheeny]

At the moment I have to do 2 sections to get the zero order image and the spectrum. When I first put my SA in the filter wheel, I could find the star image but not the spectrum. Eventually I found it... well over one frame width away... I had the diagonal between the camera and the filter wheel. I could try to stretch it a bit more with a short barlow body, but ATM I only have the 1/3 of the spectrum on the zero order image frame, so going much more will make it difficult to stitch the frames together accurately. Wouldn't a DMK 41 be nice? (I'd still be doing spectra in sections no doubt!) Al.

[Merlin66]

Al,
There's a couple of options:
Now that you have a calibrated spectra you don't need the zero order..just use the Ha and Hb lines...
Alternatively add a 4 degree prism ( Edmunds) and turn it into a Grism; this will then bring the first order spectra back close to the optical axis.
I made one up for the P-H gratings I have and it worked as advertised.

[bojan]

Just in case some of us did not know about this (I didn't), here is this link:
http://www.astrosurf.com/vdesnoux/
Christian Buil is a legend :-)

[sheeny]

I may progress to that as I gain experience. Not all stars have Ha or Hb lines though, so I still have some learning to do.:)


That would make the SA a "Grism-ism" wouldn't it?:P

The 4° prism would move the spectra back towards the axis of the scope, but wouldn't it also move the zero order image?



Thanks Bojan. Good site. There's a lot of people in the astronomy field that provide excellent information and software to amateurs for free! Good on 'em!

Al.

[sheeny]

Has anyone played with the SDSS data?

I stumbled on this:

http://www.sdss.org/dr5/algorithms/spectemplates/index.html

and thought it may help when trying to identify lines (for us L platers:P).

They also have spectra available apparently (I haven't tried a download). I think they are plots not images though:shrug:.

Here's another useful page of link from the CSIRO:

http://outreach.atnf.csiro.au/education/senior/astrophysics/spectroscopy_links.html

Al.

[theodog]

I have used the AT outreach for lessons before. It's a cracker of a site for learning about spectroscopy.

I use the Project CLEA stellar spec program to demonstrate to my students the procedure for taking and analysing spectra.

http://www3.gettysburg.edu/~marschal/clea/CLEAhome.html

Many useful simulations.

[theodog]

I have used the AT outreach for lessons before. It's a cracker of a site for learning about spectroscopy.

I use the Project CLEA stellar spec program to demonstrate to my students the procedure for taking and analysing spectra.

http://www3.gettysburg.edu/~marschal/clea/CLEAhome.html

Many useful simulations.
:D
Al, the top link seems very heavy going.:sadeyes: Might take some time to digest.:)

[theodog]

Sorry, don't know what happened with the last post:shrug:.
Just in case you suffer deja vu:doh:.

[sheeny]

Thanks Jeff, I'll have a look in my spare time.;)

I'm sure it has potential to be;). I liked the fact that the spectra templates on the page can be viewed in your browser as a GIF file - makes a very quick and easy reference to help identify lines on a similar class spectrum (I think).

I'm sure you can get as deep as you like in the real SDSS data!

Al.

[Heian]

Hi everyone,
I haven't caught any spectra for a while; I'm so slack..:shrug:

There's an interesting post on the staranalyser yahoo group from a guy who's used his SA to measure the redshift of the quasar 3C273...
You may have seen it, but I thought I'd mention it... It's quite impressive what an 1 1/4" filter can do :)

cheers
Mark

[Merlin66]

Al,
You're correct, the prism just brings the spectrum back towards the centre of the the field, but it does make things easier to image.
Getting calibration without the zero order is good practise for your next project! I'm sure some of us will migrate towards a slit spectroscope.
Pete has just finished a very nice build of a WPO design based on a 50mm camera lens.
The BAA transmission design also improves resolution and will open up new areas of investigation.
I'm reading Youngs "The Sun" published in 1882 and he has some interesting spectra of sunspots showing line reversals..... Hmmm I feel another project coming on!!

[sheeny]

Yep. I can practice that now:P... It's just a matter of becoming more familiar with what lines are what, and what to expect in each different spectral class. Ultimately, more resolution/purity will make the job easier as fainter lines should become detectible. ATM I have some spectra of the B class stars in Orion that I can't resolve a line in:(... no doubt there is some detail there if I had adequate resolution and purity.

I'm still half tempted to seek out a Meade f/3.3 reducer. Theoretically that should get the Raleigh Criteria for my scope down to about 1.6 pixels which will help again with line resolution.

Al.

[Merlin66]

Al,
I've just recently dumped my x.33 reducer!! It was useless for me! Couldn't get a good focus with the gear I have.....
The x0.66 with a x0.5 in the "nosepiece" of the camera/ filter holder I think would give better results ( haven't verified this, although I have the 1.25 x0.5 reducer!).... another thing to do/ check!!!

[sheeny]

This is the sort of valuable information I like to hear from you, Ken - at least, it's good to hear it before buying an f/3.3 reducer!;)

Hmmm... I'd like to hear other's experiences with FRs!:whistle:

Al.

[sheeny]

I've had another play with the camera response for the DMK.

This time, for something completely different, I created a camera response curve from the published chip response curve.:P

I copied the response curve from the chip document and printed it enlarged, then scaled off the curve every 100A. I put the data into Excel, and the curve looked like a good copy of the published curve. I then saved the spreadsheet as a .txt file, and then renamed it to .dat.

VSpec opens the .dat file OK.

To use it, open the spectrum you want to correct, then open the response file. Copy the response spectrum and paste it into the same window as the spectrum you want to correct - this will recalibrate the response curve to the same dispersion as the spectrum:thumbsup:.

From the Gamma Crucis test below, you will see the response is excellent for 4000A-7100A. Above 7100A the corrected curve is low, suggesting the response curve is high (more likely that the rest of the light train is having an impact on the raw signal).

Al.

[sheeny]

Another test. This time I reworked one of my Sirius spectra with the DMK theoretical response curve.

Notice now the response is low below 4300A.

So this tells me the theoretical response curve is probably fine for my scope between 4300A and 7100A. Less than 4300A and above 7100A the signal starts to be attenuated in my scope (probably corrector plate and coatings:shrug:).

The beauty of having the response curve in Excell is it's easy to make manual adjustments to it. It remains to be seen if a repeatable scope/camera response curve can be constructed though.:):whistle:

Al.

[Merlin66]

Al,
Very good piece of lateral thinking! Works out very well!!
I've just resigned myself to fitting a Baader UV-IR cut-off filter (4000-7000) in the light train and if I EVER get some night time observing (!!) I'll work within these limits. It will suit the modded 300D as well ( has the same filter)

[sheeny]

There's always that, Ken.;)

I'm intrigued to see if I can construct a response curve that can be used on any spectrum that's a bit wider than that though. I think the key is to use a red star to develop the red end, and a blue star to develop the blue end of the response (in the absence of having a uniform frequency light source to simply catch one;)). The key will be collecting good repeatable spectra (and ignoring the end of the spectrum where the S/N ratio is crap).

The Sirius spectrum I used in the last test wasn't a particularly good one. It was one I captured with the brightness control bumped up, so I had to subtract a constant before dividing by the response curve.:rolleyes:

All good fun. It keeps the grey matter juicy...:P

Al.

[theodog]

Nice work Al.
I've got to get my gear going again.

[Heian]

Al,
just on your comments about the use of a FR, I've got a 0.6x FR from Steve Mogg that I use all the time. It's placed in front of the SA.
My understanding is that by reducing the size of the star image on the SA grating, you reduce the number of lines it covers, and hence improves the definition of the spectra.
Provided you have enough focus travel, it also allows for the SA to be moved further from the CCD, reducing the ang/px.

cheers
Mark

[Terry B]

This is a very good result. I will have to try this with my kodak chip.

[sheeny]

Not quite, Mark, but you've got the right result.:)

The dispersion formula for the SA is

D = (10000 x p)/(100 x d)

where p = pixel size in µm and d = distance the SA is from the CCD (and the 100 is the l/mm for the SA).

Resolution is determined by Raleigh's criteria:

r = fL/D

where r = radius of the airey disc image of the star, f = scope focal length, D = diameter of scope (so f/D = FR of scope), and L = wavelength of the light.

For my scope (C8), the theoretical resolution (ignoring seeing and imperfections) is:

r = 9.4 µm for red light at f/10 (4 pixels diameter at 5.6 µm)
r = 5.9 µm for red light at f/6.3 (just over 2 pixels diameter)
r = 3.1 µm for red light at f/3.3 (just over 1 pixel diameter)

The smaller r is, the easier it is to detect fainter lines, so the "purity" of the spectra improves. Purity is a function of resolution and dispersion. I hadn't heard the term purity till I read Kaler.;)

Obviously, as the FR reduces, this limits how far away the SA can be placed from the CCD. For a f/3.3 scope, a full aperture light cone of say 30mm at the SA, will focus at just 99mm, which limits the dispersion to 5.66 A/pixel. Not quite sure what would happen at d>99mm in that case, I assume focus could still be achieved but the spectra would be dimmed as not all the light from the star would make it through the SA:shrug:.

:)

Al.

[Heian]

Al,
with my 120mm refractor + various spacers/FR's + DSI3 I can get down to about 5 ang/px.
Trying to go lower, results in focus becomes an issue as the "fishtails" at either end of the spectra are getting quite severe, almost to the point of having to focus at different areas and then "stitch" them together later.
I can get down to 2 ang/px by placing the SA directly in front of the camera lens on the 350D, working with a 30mm aperture and 270mm lens gives approx 2 ang/px. It's good for the brighter objects at least:)

Mark

[sheeny]

I was seeing the fishtails on my f/6.3 spectra and initially it put me off using the FR, but after doing the last FR tests I have to admit the FR gets a big :thumbsup: from me.:) Focussing is very critical though, and I can imagine that going to shorter FR could result in the need to focus on sub-bands of the spectra:sadeyes:.

Hopefully that is not the case if the FR is also a good field flattener:shrug:. Focussing will get more difficult/critical at lower FR though...

I have had a couple of plays with the SA in front of my E510 with the 300mm lens, and while I've achieved spectra that I can see visually in the photos, I haven't been successful at extracting anything with identifiable lines in VSpec.:P I found it quite fiddly and, well, I tend not to persevere with fiddly things that don't give me good results:lol:.

Al.

[sheeny]

Following on from my post #318, I started wondering if there was an "optimum" setup for my scope, in terms of a focal reducer and its effect on the spectra. So I ran a few calcs, and found some interesting results (I think;)).

I decided to calculate the minimum band width I could resolve in a spectrum with the SA, for each FR with my scope. I found that regardless of which FR I used, the smallest bandwidth I could resolve with my scope is 6.25 Å.

At f/10, the maximum dispersion is 1.86 Å/pixel, but the resolution is 3.36 pixels; at f/6.3, the maximum dispersion is 2.963 Å/pixel, but the resolution is 2.1 pixels; and at f/3.3, the maximum dispersion is 5.7 Å/pixel, but the resolution is 1.1 pixels. These all multiply out to about 6.25 Å for my scope and camera.

Note: the above calculations ignore the effect of seeing and imperfections, too. So to do better than 6.25 Å I believe I'd have to introduce a slit to the system...

So the choice of which FR to use has to be from other factors:


the length of extension tubes between the SA and the CCD; and
the number of frames required to capture a given bandwidth.

For f/10, the maximum dispersion is achived when the CCD is 300mm from the SA. That's a long extension tube! And to capture , say, the whole visible spectrum of 4000Å would take about 5 or 6 frames!

For f/6.3, the max distance from SA to camera is a more reasonable 189mm, and the visible spectrum could be captured in about 3 frames.

For f/3.3 (if I could find a suitable FR for the job:P) , the max distance would be 99mm, and the visible spectra could be captured in 2 frames of the DMK 21AU04.AS.

So I'm a long way from pushing the limits of the SA!:lol:

It would be interesting to see if my scope has enough focus travel to handle the extreme distance from SA to CCD.:shrug: I guess an experiment is the only way to tell.:P

For the time being (in the absence of an f/3.3 solution) the f/6.3 reducer still seems like the best setup for me ATM. Plenty of scope to explore yet!:thumbsup::whistle:

Al.

[Merlin66]

Al,
I like your analysis!
Basically I think you're correct, but I'd add in a couple of "real life" variables....
The image on the SA will always be controlled by the seeing conditions, over here its got to be very good to get much below 4 sec star images, and that's for the faint stars; the brighter ones are "bloated"
Also, the larger the out of focus star image on the SA grating, the more lines are illuminated and the better the resolution. Add these to the dispersion and you may get different results???

[sheeny]

Yeah, Ken... I did say I was ignoring seeing and imperfections. Seeing should be less of a problem at lower f/ ratio (the star image should be smaller) but the relative effect should be the same in terms of Å on the spectrum.

The defocussed star image at the SA is a full disc of 30mm diameter (that's my assumption for maximum dispersion) since if we go bigger than that then the SA becomes the field stop.

Al.

[Merlin66]

Point made!

[sheeny]

:lol:

I just did a little practical experiment...

I have the f/6.3 reducer on my scope, and I get about 9 A/pixel with the SA in the filter wheel.

So decided to try my short barlow body to increase dispersion a bit. I could not get a thing on the screen, and got lost driving the motorfocus bacdkwards and forwards:rolleyes: so I took it all out and put he diagonal in and my 10mm EP... can't achieve focus...

Oh well, thats a result worth knowing... but I got enough of an image to know I was at the right end of the focus adjustment, so I put the short barlow body and camera back in... adjust exposure to get an image and refocus...

No way! I still run out of focus. So the best I can do ATM is maybe 9 A/pixel with the f/6.3 FR. I might be able to stretch it a little bit by fitting the SA extension ring...:P

That being the case, an f/3.3 FR wouldn't reach focus at all...

I guess that's what you were saying before, Ken...:whistle:

:lol:

Al.

[sheeny]

A little puzzle, guys...

This is the first spectrum I captured tonight. I plugged NGC3372 into the scope and it slewed straight to this. I took one look at it and immediately thought I had Eta Carinae. But do I?

The continuum is a very different shape to the raw etas I've captured before - this one is much bluer / hotter... and there is a strong double line bluewards of Hb.:shrug:

Can anyone verify if Eta Carinae is changing, or have I picked up another star?

Al.

[Merlin66]

Woooh......
That certainly looks odd! Whatever you got is a Be type star, emission in Ha and Hb; don't know what the peaks around 4640/4675 are....
I've taken the liberty of posting on the Spectro-L forum ( for the LiHiresIII guys) see what they know...
Can you get another spectra tonight... and check if it is Eta???

[sheeny]

That's cool. I'll try for another spectrum tonight, and maybe have a hunt around the area.

A thought that struck me this morning before i got out of bed... the blue wouldn't be the effect of scattered moonlight would it? I estimated the limiting magnitude in Crux to be 4.7 (due to moonlight), and Eta is not real bright. Whatever the star is, it is about the same brightness as Eta. I had the same settings as previous Etas wrt to exposure, etc.

I started processing another spectrum of Arcturus as well last night (before VSpec spat me out) and it also looked a little strange due to a lot of blue:shrug:. I'm sure I had Arcturus:P:lol: this time, but maybe the full moon is a factor?:shrug:

Al.

[sheeny]

I have attached some odf the spectra I captured last night: Antares (x2), Arcturus, and the red star at the centre of the Jewel Casket. The spectrum of the red star at the centre of the Jewel Casket also had a strange blue response (like the eta car spectrum). I think that is the result of scattered blue light from the moon on a longer exposure. I have cancelled the blue out during corection for the camera response.

I have also attached a list of neighbours to eta carinae from CdC. There are a few possibilities with PEC spectra.:)

Al.

[Heian]

Al,
I'm guessing this is a situation where the use of an aperture stop in front of the SA would be an advantage...
What do you think would happen if you just put a mask over the front of the scope? :shrug:

cheers
Mark

[Merlin66]

Al,
I think you're getting the hang of this game!! Those spectra are great!
If and when I write "the" book I'll ask your permission to use them!!

[sheeny]

I dunno, Mark. A mask in front of the scope will reduce the aperture, so it will change the f/ ratio, reduce resolution and then the exposure time making it longer... a catch 22 I think. :shrug:

I think maybe it's just something to be aware of when capturing spectra of the dimmer stars. And, it's another reason why a standard camera response curve is a problem:P. A standard response curve wouldn't compensate for an increase in scattered blue light from moonshine:rolleyes:. I think I'm starting to realise why it's important to do a camera response curve for each session at least, and maybe each spectral class.:shrug: In this case, actually, it's the exposure that's important.

:lol: Thanks Ken. I feel like I'm getting head around it, slowly at least... though sometimes I think I'm doing everything the same but VSpec obviously thinks otherwise and spits me across the room:lol::whistle:.

I've come to the conclusion that the filter wheel has got to go! I can get the SA 30 to 40mm further up the light train by not using the filter wheel. That gives me 30 to 40mm more back focus. Hopefully that will allow me to increase dispersion with the f/6.3 reducer with my short barlow body or possibly the star diagonal:shrug:. I feel another experiment coming on;).

I think I'll leave my SA in the barlow body if that works out OK. I don't use my 2x barlow much anyway now I have a Powermate;). Will that make it a "SArlow"?:whistle:

Al.

[sheeny]

Not that I know what I'm doing :P but I've just had a browse through the finding list on NIST (http://physics.nist.gov/PhysRefData/Handbook/Tables/findinglist11.htm) and my guess at the origin of these two lines are Nd I, and Al II.:whistle:

I just browsed the list for about 10Å each side of the peak wavelength and picked the elements with the strongest lines. Al II has two strong lines at 4663 and 4666, and Neodymium has two strong lines at 4634 and 4631.

The Al II seems possible, but the Nd? Seems a pretty heavy element (atomic number 60) :shrug: so it doesn't seem so likely to me. But having a better guess requires a bit more knowledge than I have ATM.:P

Al.

[Glenpiper]

Hi Al, Ken,

While a lurker on IceInSpace I'm sorry to say I wasn't aware of this spectroscopy forum ... until alerted to the possibility by Ken's posting on Spectro-L. Melbourne's inner city clouds lifted long enough last night for me to grab a 100s low-res slitless grism spectra ... copy attached. While I'm no expert on this star it does look as 'normal' as can be said for anything regarding this far from normal star. While Eta car does have regular HeII (4686A) variability I don't believe it is anywhere near as strong as your spectra shows, and I don't see it in my spectra (recorded with Sony chip which has good blue response). So I would agree with you in questioning if you imaged Eta Car ... I strongly doubt it. But top marks for trying, and keep up the effort as it needs more monitoring :)

Bernard Heathcote

[sheeny]

Thanks Bernard. It confirms my suspicion that it was another star (and that I was sloppy at locating eta carinae:P:whistle:).

So that's another little project to work on... find that star again.:D

Al.

[Glenpiper]

Al,

I forgot to mention that the first thing that made me doubt you had imaged Eta Car was the poor signal/noise ratio of your spectrum ... much more like that of a mag 9-10 star than Eta's current mag 4.7

Bernard

[sheeny]

Yeah, its a bit furry now that you mention it. I did notice that at the time but dismissed it as extra background noise due to moonlight, but could just as easily be a dimmer star. I was using the exact same exposure (0.6s) that I normally use on Eta, but I didn't notice the spectrum as particularly dim.

Al.

[Glenpiper]

I think a 0.6sec exposure is way too short, unless you have a very large scope and a very bright target. I assumed you were using a C8+DSLR, is this correct and if so, is the DSLR a CMOS sensor? I'm using a C8 with a monochrome, binned, CCD and still need to stack 5 x20sec exposures to get a reasonable SNR.

Bernard

[Glenpiper]

Ooops, error ... my exposures were 5x 5sec (where the star was included) but I reduced it to 5x 4sec for just the spectrum.

As the attached raw image shows, the star and Ha are saturated but the areas of interest have a reasonable SNR. The stars are out of focus, which is normal when the spectrum is focused.

Bernard

[sheeny]

I used the C8, with an f/6.3 FR, the SA and my DMK 21. I find that with brightness =0, Gain = 1023, and exposure = 0.6s the eta spectrum uses most of the histogram without clipping and the homunculus is just starting to appear in the zero order image.

Very interesting - thanks for posting Bernard. I haven't been exposing my spectra anywhere near as much as that!

I've attached a jpg copy of the spectrum as captured from the eta impostor the other night, and two previous eta spectra including the zero order.

Part of the reason I think I ballsed up the eta the other night was I was practicing taking spectra without the zero order. Maybe I'd have noticed the lack of homunculus in the zero order if I'd done it the old way:shrug:.

Al.

[Merlin66]

Al,
With the DMK set-up, are you just using a single frame capture at 0.6 sec? Somehow, I'd assumed you were taking a short 10 second or so video at 0.6 sec per frame and stacking the result?
Can you confirm your processing?
BTW I found with the Baader (207lpm) at f7.6 on the ED80 that the spectra of first mag stars ( Altair/ Vega) was well over exposed at 10 sec with the modded Canon 300D.

[sheeny]

For bright stars I usually capture a video, and stack it in registax. When I was capturing the zero order and and the first order in separate videos (with some overlap) I would then save them as BMPs and merge them in photoshop, open the resulting BMP in Registax again and save as a FITS.

If I capture only the spectrum without the zero order, I just save the stacked image as a FITS.

Now for fainter stars like eta, I was capturing say 5 or 6 still images of 0.6s and stacking those in Registax (only the sharpest ones), but I have to confess to getting slack and not doing that in recent ones.:whistle:

BTW after a lot of playing around I've been maxing out the gain in IC Capture to keep the exposure time low (to reduce the effect of seeing as much as possible), and keeping the brightness on zero to avoid the black point offset problem with the spectrum divisions (camera response).

Al.

[Glenpiper]

Al,

Is your DMK 21 the colour or monochrome version?

I've plotted one of your Eta jpg images (attached) and the limitations of an 8-bit camera are very evident, with the peak values limited to 256 ADU. This, and being uncooled, are going to be a severe limitation on your ability to get a decent SNR, In other words, an 8-bit videocam is going to limit the quality of your spectra ... but multiple image stacking will certainly help get the best result possible.

Bernard

[sheeny]

Its mono.

What software are you using, Bernard - just out of interest? I notice not VSpec...

Al.

[Glenpiper]

Al,

I normally use Maxim DL (Pro) for image acquisition, calibration, stacking, etc and also for the amplitude plots of the raw images I've posted. I certainly do use Vspec ... have done so for years, great program ... for spectral calibration and processing.

Bernard

[sheeny]

OK. I was just intrigued that your plots were a very different format to VSpec. VSpec is a great program even with it's little quirks (dare I say bugs?) especially at the price!:D

Al.

[Glenpiper]

Fully agree about the value of VSpec. While it is still buggy it isn't anywhere as bad as it was a few years back (vers 1 & 2), when I used to think of it as VSPECtacularCrash:)

Maxim DL is another brilliant program ... but not in the same price range as VSpec :mad2:

Bernard

[theodog]

I use the same but old MaximDL 3.
Works well.;)

Interesting investigations Al.:thumbsup:

[Merlin66]

A bit OT, but I find with the DMK, at least for solar Ha imaging, that I have to keep the gain below 500 ( Use around 380) to minimise stack noise in RegistaxV5. Increasing the exposure times, to me, is prefered to increasing the gain setting...
Just my 2c

[sheeny]

Had a nasty day here today (weather wise), so I was very surprised to see stars when I went outside a little after 21:00. So I tested my latest SA arrangement in between the returning clouds.

Here is eta carinae captured with the C8, f/6.3, SA100, DMK 21 again but now I've ditched the filter wheel, and using the 2x barlow body and SA extension between the SA and the camera. This gives me near enough to 6 A/pixel.:thumbsup:

Just to show that I do listen (sometimes:P) I captured 8 x 13.7s images and subtracted a master dark made from 4 DFs. Temp 5C, brightness = 0, Gain =380;).

Noise is much better I think you'll agree.:whistle:

Good H, He and Fe lines.

I may yet do a trial using the diagonal as the spacer between the SA and camera yet, but unless it provides a big advantage I think the barlow body is very user friendly:thumbsup:.

Al.

Whooopppsss! Take no notice of the time on the image... missed by 12 hours!:rolleyes: How embarrassment...:lol:

[Glenpiper]

Al,

That is an excellent result, and it also supports my earlier results. I'm glad you went for the longer exposures :)

While it isn't possible to do a correct profile comparison of our spectra, as mine wasn't corrected for instrumental response (was yours?), it is interesting to note that the H alpha / H beta amplitude ratio has decreased in the last few days ... which I interpret to mean that the source (envelope) has cooled a bit.

The clouds cleared here earlier on ... the first night in over a week, and the night i had to be out to dinner ... good old sod's law.

[Glenpiper]

Re the decreasing hydrogen amplitude ratios: I think it is real, and not a result of Ha saturating. I will be attending the RASNZ conference in Wellington next weekend and, fortuitously, Bill Allen will be presenting a paper on Eta Carinae. He has been observing this star for 30 years and knows a lot about its behavour ... in fact his paper puts forward the theory that it has a companion with a period of 5.5 years. I will discuss the H amplitude matter with him.

[Glenpiper]

Oops ... in my message 351 where I said "the source (envelope) has cooled a bit" I meant to say it has heated up a bit.

[Merlin66]

Bernard,
It's good to have you around...look forward to your report back after NZ trip.
Al,
Love it!
Why 13.7sec exposures ( why not!?)
Before anyone asks
"How do you get coloured spectra from a mono DMK camera"
No - you don't use RBG filters!!!!

[Glenpiper]

Ken,

If it isn't a trick question then I assume you are referring to the colour spectra at the top of Al's plots ... a pseudo spectra that can be generated with Vspec. Now that we always use the 2D plot approach I'm not sure what value this type of 1D plot is.

[sheeny]

No Bernard, this spectrum was not corrected for camera response (yet). I captured a spectrum of Antares as well last night so when I process that, I hope to get a response curve I can use on this. I had to do this one first though to determine my dispersion with the new spacer arrangement though.;)

Here comes a newbie question :P ... When you say the amplitude ratio, I assume you are simply comparing the height of the peaks between Ha and Hb and not comparing FWHM?


13.7s was about as long as I could go without saturating the Ha line I reckoned. The whole histogram in IC Capture was used with just a few zeros flashing in at the top end.

BTW at this exposure the image of eta carinae itself was ugly... the star image was an oval due to the homunculus. This is something I had been trying to avoid with the shorter exposures.

Al.

[sheeny]

They look pretty, Bernard!:P

I read somewhere (one of my books?) that some people "respond better" to the spectrum image (or synthesized image) than the graph, but there's no doubt the profile is more quantitative. Personally, I think in a forum like this where there may be people browsing who might have an interest but little knowledge, it helps them to see that "red is at this end and blues is at the other". I still use it to tell what colour a line is:P.:whistle: (Yes... L plate firmly displayed, officer...):)

Al.

[Glenpiper]

I won't argue about them looking pretty.

One of the aspects (probably the only) of Kaler's "Stars and their Spectra" I dislike is that most of his spectra are 1D plots, and b&w to make them even less appealing and informative.

Re your earlier question on the amplitude ratios: yes, only the ratio of the maximum amplitudes of the Balmer lines. Unless there are very close spectral lines from other elements involved (very likely) I would expect the FWHM values to be about the same for each of the hydrogen lines.

[Glenpiper]

Ooops, another 'senior moment' ...I reversed my 1D and 2D references! 2D being the 'raw' spectral image and 1D being the scaled amplitude vs wavelength plot. I nearly always have trouble with this as I find it counterintuitive, chronologically speaking ... as the process is to go from the 2D to the 1D plot.

[Merlin66]

Re: Coloured spectra - yes it was a bit of a trick question... No one mentioned ( or no one thought to ask...) how the colour was achieved!
Not cheating, but as Al mentioned "synthetic" rendering based on wavelength.
It does, not just for the L-plate guys, sometimes help to visualise in colour. When you see the comparison "graphs" for various colour filters ( LRGB, OIII, Ha, SII etc) most for them are presented "colourised"
I hit a site recently and the spectra illustrated threw me completely ( I know, I know..not a hard thing to do!) by having the Red end on the LHS! Like driving the car via the rear view miror.....
I'm no physicist, but in a "normal" Balmer Hydrogen series, I think the intensity of the Hb is 50% of the Ha...so any diffference in the ratio Hb/Ha shows something else is happening... correct Bernard???

[sheeny]

Don't rush in Bernard.:sadeyes:

Here's a little comparison of 2 spectra captured last night. The blue profile is the 13.7s exposure, the pink is a failed attempt to capture the long exposure as an avi file. The exposure in the avi file was not as specified in IC Capture but was something less than 1/3.75 of a second (3.75 frames per second frame rate).

While the exposure on the avi isn't ideal, the ratio of Ha/Hb is clearly different. This suggests to me that the longer exposure was overexposed and the Ha line was clipped.

Thoughts?

Al.

[Merlin66]

Al,
You could set the exposures of the DMK independent of the frame rate..
For some of my prom shots I need to go to 1/6 sec and capture AVI ( It says 30/sec on the screen.. but this is actually only the play back speed according to Stephen at IC)
You should, haven't tried it yet, be able to increase the individual frame exposure to 2 or 3 sec and get a better SN outcome. What do you think??

[sheeny]

I thought it did that, Ken, but I couldn't get it to do it last night. I ended up writing it off as remembering how K3ccdtools works, and took a series of stills instead.

I was quite sure it wasn't working, because live view behaves right, and updates after each exposure, but when I chose capture, the image dimmed, and the frame count was ticking up at the right rate for 3.75 frames per second.:shrug: Couldn't get it to do any different...

Al.

[Glenpiper]

Al,
I think you are correct, which is a relief as I was scratching my head trying to come up with a viable physical explanation. The fact that your H alpha/beta ratio is around four for your pink/AVI curve while mine is just over three made me wonder if my Ha was clipped, or in the non-linear zone. Turns out the latter is probably the case. I've not yet got around to determining the linear range of my new DSI Pro III ... but I think I exeeded it with my Ha!

[sheeny]

I think Eta Carinae poses a real problem for getting the exposure right because there is so little of the frame that is bright.:shrug:

I gotta say, I'm learning heaps and I like the more analytical side of spectroscopy - results should be repeatable and testable, not just look good:whistle:, and I really appreciate the help you guys have given me.:thumbsup: Thanks guys!

Al.

[Glenpiper]

It is good to be able to swap experiences and, as you say, the analytical side of spectroscopy is very rewarding ... as is also the case with photometry. I quickly found out that the science that amateurs can do iin astronomy is far more rewarding than taking 'pretty pictures'.

I tried to get an Eta Car BVRI and a spectra earlier this evening but as soon as I was ready to shoot the clouds arrived thick and fast :astron:

[Merlin66]

Al, and others-
We're all in this together, learning from each other..

" the pinnacle of amateur astronomy - spectroscopy" -Richard
" If optical spectroscopy had not been invented then fully 75% of ALL astronomical knowledge would be unknown today" -Kitchin

Let's continue to hold hands in the traffic and "boldly go where no amateurs have gone before" ( couldn't help myself!)

Al,
I only use the DMK21 for solar imaging, but I'll set it up and play with long exposure settings....keep you posted!!

[Glenpiper]

Ken,

I notice you list UBVRI filters among your equipment ... do you really have a U filter ... and use it?

(This question isn't really that far OT, given that photometry is just extreemly low res spectroscopy ;))

[Merlin66]

Bernard,
I was lucky enough last year to get a full set of Schuler UBVRI filters from the Maria Mitchell Observatory in the US ( they were "upgrading")
I'll be honest, although they are now mounted in an AtiK -USB electronic filter wheel, as yet I haven't done any serious work with them.
Why do you ask???

[Glenpiper]

I don't know of any photometrist that has been able to use the U filter successfully without having a large (at least 1M) scope and/or a back illuminated CCD. Many even have problems with the B filter.

If you, or anyone, is interested in variable star work you should join AVSON (Austral Variable Star Observer Network) http://tech.groups.yahoo.com/group/AVSON/. I set this forum up a few years back as a meeting point and target information dissemination centre for all those interested in southern hemisphere variable star work. While there are less than 100 members they include just about anyone doing amateur astro science in this hemisphere.

[sheeny]

:rofl:

I wish you could see the image in my mind!

Al.

[sheeny]

I don't know, Ken. I vaguely remember reading about this (in Kaler?). If I remember correctly the relative heights of the different lines did depend on temperature. When star temps get high enough to reveal the Balmer Series, Ha is dominant, but as temperature increases more of the H is energised high enough to bring Hb and Hc up. It made sense when I read it, but left me a little confused about the significance of the relative strengths of the lines.

I remember reading about a secondary method of temperature determination based on the relative strength of the lines, but it wasn't explained and gave the impression it wasn't a simple method.

I'll have a look in Kaler tonight... see if I can find it.

Al.

[Merlin66]

Al,
Re-reading it.....

Village People!!:rofl:
Not exactly what I had in mind when I wrote it!!!

[sheeny]

I've had a scratch through Kaler...

Refer Section 4.3 - 3rd paragraph. It's on page 91 of my paperback copy. He describes a method of comparing the relative strengths of absorption lines to determine temperature.

I'm not sure whether I've read the same w.r.t. emission lines, but I see no reason why there wouldn't be an equivalent temperature effect.:shrug:

Al.

[Merlin66]

In Robinson's Book "Spectroscopy -the key to the stars", p97 he talks about the "Theoretical Balmer decrement" which is based on the ratio of Hb to the bluer emissions of Hgam, Hdel etc etc
To quote " The Ha is often so strong that lines further up the series would have very small values indeed if the Ha was used as the standard"
The other thing I came across the the ratio of the FWHM of the Ha/ Hb ( see Robinson, p144) to determine the total amount of Hydrogen gas in the object, when compared with similar ratios for other elements we get ..."Cosmic Abundances"....

Kitchin ("Optical Astronomical Spectroscopy") ,p233 talking about Cataclysmic variables and Novae
"...During the outburst..the spectrum changes from that characteristic of a star to that characteristic of an interstellar nebula. The change arises from the enormous expansion in the outer layers of the star that are thrown off during its explosion... As the outburst progresses, the spectral type changes towards lower temperatures and emission lines start to appears. The emission lines become stronger and stronger,and the continuous component weekens, until emission lines dominate the spectrum."

[Merlin66]

http://www.stark-labs.com/blog/files/GainAndOffset.php

This may be of interest.

[Glenpiper]

Thanks Ken, that gain & offset tutorial is excellent.

[Merlin66]

Robin Leadbeater has shown that you can mate a slit to the SA100; reduces background light etc etc.
http://tech.groups.yahoo.com/group/staranalyser/files/User_Folders/robin_staranalyser/DSLR_tests/Slit_FR_SA_90mm_350D.jpg
I've been playing around with "standard" adaptors/ spacers and found what I think is a pretty good solution.
I'll upload in two parts

[Merlin66]

I've just trialed the set up's visually and they both work 100%
I'll get the webcam/ Canon/ DSI connected to get some images.
Oh, and finish filing the Surplus Shed slit assembly!!!

[Glenpiper]

I've never really understood the various attempts to convert the simple grating into a 'real' spectrograph (slit with collimated beams), other than for its technical challenge or budget reasons. For me the main, and a very big advantage, of the simple uncollimated grating is that it is quick and easy to use. No need for difficult slit guiding (a real plus) and it can easily be used in a filter wheel ... which also allows for automated 'quick look' spectra. I see the ideal amateur spectroscopist's equipment being the 'quicklook' grating (RO, SA or Baader) plus a slit spectrograph for detailed work (SBIG, Lhires, etc).

[Glenpiper]

The following probably needs it own thread.

It has always interested me to note over the years, from observing the contents of the many astronomy forums (forii?), and contact with individuals, that the amateur astronomy community is easily divided into two classes of participants, namely:

a) Those that see their equipment as aids to study the night sky ... the 'seekers'

b) Those that see the night sky as an aid to help them develop and perfect their equipment .. the 'tinkerers'

Where do you readers stand?

[sheeny]

I think that would make a good thread in its own right, with a poll.:thumbsup:

The impression I get from a lot of people in IIS is aften they can be split pretty easiy into the two categories, but there those who're somewhere in between. Personally, I think I'm a fence sitter. I do a mix of seeking and tinkering... at present I think I'm largely tinkering with Spectroscopy - trying to learn what I and my gear can do. The seeking will follow.

:)

Al.

[Terry B]

Very good point.
I started as a tinkerer but now am mostly interested in what I can find and measure.
The tinkering is very prominent in lots of forum members I think.:D

[Merlin66]

My 2c
I think there are probably two types of Tinkerers..
There's the one's on restricted Budgets, limited resources, but keen to be able to "do something". In the early days these were the guys who built their own telescopes ( grinding the mirrors from scratch), even building their own eyepieces (??!) as well the the mountings, drives etc etc. The lack of commercial equipment ( at affordable prices) continued to drive this process. Just look at what John Dobson did.....

Then there's the "cloudy night" guys... nothing else to do...( BTW these seem to make up the majority over here in the UK) who adapt, modify and "improve" their telescopes and equipment. Adding finders, changing focusers, collimating and re-collimating the scope, adding adaptors etc for CCD imaging etc etc. They always seem to be fiddling around with something!
The bottom line: we are a group of interested, dedicated amateurs and it's always been the role of amateurs (look at all the inventions and discoveries made by the Victorians during the Golden Age of astronomy) to dabble, fiddle and keep asking questions.....

[Merlin66]

I agree with the comments that having both a grating spectroscope and a higher resolution reflection grating spectroscope is a wonderful, and very useful combination to have. Each has a different role and purpose, as Bernard said.
I also acknowledge that the hight cost of entry to Hi-Res spectroscopy, means that many novices start with the SA100/ Rainbow Optics and gain satisfaction and experience as they climb the learning curve.
These grating spectroscopes are primarily star spectroscopes and rely on the image of the star to form a "virtual" slit. (Even used as an aid to visual observers they can help in identifying planetary nebulae etc - Hartung made great use of his "prism" in preparing his 'Astronomical Objects'), but they can't easily be used to obtain the spectra of nebulae or galaxies....this really needs a slit.
This was one of the drivers; to see if I could add a slit, easily, to the SA100 and further expand it's role for the amateur. Option 2 does this.
So, by using "off the shelf" components, we can now:
- obtain low resolution spectra of nebulae and galaxies
- gain good experience and practise using and guiding on spectroscope slits

When we move onwards and upwards to the hi-resolution spectroscope, we can look back on this as completion of "Astronomical Spectroscopes - 101".

[Glenpiper]

Hi, from windy (and cold) Wellington.
Another very great advantage of a slit spectrgraph is that it gives you the ability to have calibration lines from, for example, neon or thorium lamp spectra. You can also, to a degree, adjust the resolution by adjusting the slit width, e.g. narrower slit -> more resolution -> less flux -> longer exposure .... and visa versa.

The downside, as indicated above, is that the price of high resolution is the need for long exposures and, hence very careful guiding to keep the target on the narrow slit ... not easy as I can tell you from my own experience (I'm sure Ken will confirm this). In my own work on the Ha monitoring of Be stars it is usual to have to make one hour exposures in order to achieve an acceptable SNR of at least 50.

I LOVE taking spectra with a simple 'star spectroscope' :D

[Merlin66]

http://tech.groups.yahoo.com/group/astronomical_spectroscopy/?yguid=322612425

Just to add to the enjoyment and give us more opportunity to share with others.......

[Terry B]

I joined.:D

[Heian]

never let it be said I'm not a trend follower! :D

I joined as well...

[sheeny]

I joined too!:whistle: It's just like a little IIS home away from home...:P

Al.

[Merlin66]

More like KISS!:lol:
Just to give us a bit more space so we can compare notes and ideas with others, hopefully worldwide, without boring everyone else.
You KNOW IIS will always be home!!:D

[jinxsta]

Yeah I joined its kinda like the ISS holiday house really.:lol:

[Merlin66]

I see from the ABC news you're getting your share of wild weather!!
(and the swine flu virus!!)
The good news is that the weather will clear and you will have excellent seeing conditions; I believe there will also be power outages in Sydney and Melbourne, to give +5mag naked eye skies from the city centres commencing when I arrive back on the 14th June!!!!!
Make the best of it ;)
Still haven't had the opportunity to do any observing over here!!
Al/ Jeff/ Bernard/ Mark/ Terry... any more observations of Eta Car????

[justafew]

I don't know about Terry but I would love to have a copy of your spectra exercise! I'm way down here in the shallow end of the spectroscopy pool.

Best regards,
Alice

feel free to email it directly to few_2001 AT yahoo DOT com
Thanks a million

[theodog]

OK I'm in.:D

[Merlin66]

Guys/ Gals,
I truly appreciate the support!
Please, Please, remember it only what we make it!!
If it helps the amateur develop his/her skills in spectroscopy, then it's all worthwhile...
Thanks,

[sheeny]

Permanently or just a visit?

Not yet... weather...

Al.

[Terry B]

No more obsrecently due to a computer failure in the observatory and lots of cloud.

[theodog]

No obs due to work and construction project.:sadeyes:
Thoughts of this weekend, -where did THAT cloud come from?:eyepop:
:D

[Heian]

Ken,
tried to do some Be stars in Musca + Centaurus before the cloud parked itself, but no joy :cloudy:
The scoundrels in my local club enticed me to the "other" side of imaging and I got all excited about doing some widefield instead of spectro's...:)

cheers
Mark

[Merlin66]

Al,
I wish!
No, we're only getting back for a couple of weeks; visit all the kids ( we have 7... Brady Bunch stuff!)
Taking the mother-in-law to Toowoomba ( Why!!!!) for a few days then hope to catch up with some ol' astro buddies in Melbourne before heading back to wet and cloudy UK.

[sheeny]

Finally got to see some stars tonight.:D

I spent some time doing other stuff (tweaking polar alignment, etc), but eventually managed to capture these spectra of Eta Carinae as an experiment.

Each spectra is a stack of 4 images. The blue profile is an exposure of 4.841s, the magenta is 8.158s exposure. I wanted to get more at about 2s, etc but the clouds noticed me... The aim of the experiement was to find how much exposure I can us before the DMK ceases to be linear. This shows 8s is too long. Next opportunity I'll do a 4s cw 2s just to confirm 4s will be OK... it looks like it might be:shrug:. Good agreement between the two spectra, except for Ha. Both profiles are corrected for camera response and normalised.

Al.

[Merlin66]

Al,
I looked at the graph and couldn't understand why the Ha responce in 8sec was LOWER than that at 4sec...these chips should be pretty linear and it's only the noise build-up that becomes an issue.... then I saw you NORMALISED the curves!!!
This will effectively "hide" the differences in the ADU figures...
What you should consider, I think, is what's the max signal you can get without saturation. The DMK is a 8 bit chip so you should aim to get 256adu max before saturation. ( maybe Bernard can confirm!!)
What was the max readout on the raw data???

[sheeny]

Yyyyyyeeeeeeessssssss..........

I'm not sure I'm on the same page as you, Ken... so there's probably something to learn here... :lol:.

I normalised the spectra to validate the comparison. Both spectra agree quite nicely except for the Ha line, which tells me the Ha line is saturated - at least in the 8s exposure. I would expect that the nonlinearity comes in as soon as some of the pixels in the image of the Ha line become saturated.

I don't follow how to compare the values in the raw spectra, because that's a product of the binning process, not just the ccd.:shrug:

I've attached a comparison plot of the raw spectra. Both are scaled to Ymax = 340000. The peak intensity for the 4s exposure is 251035, and that for the 8s exposure is 309404.

I find the histogram in IC capture doesn't give a good indication of the saturation of such a small section of the whole image, hence the experiment to try to find the limits of linearity so I can ultimately compare spectra and intensity changes over time.:)

I've also included the zipped spc files if anyone wants to play.

Let me know if you think I'm off the rails...:whistle:

Al.

[sheeny]

I've been doing more experimenting. :)

I captured a couple of spectra of Gamma Crucis the other night. One covering the visible spectrum, and one covering the red to IR section.

I stacked them in Registax 5, and saved them as 16 bit fits files. I then opened them in Iris and "Remove(d) the sky to a 2D spectra...".

Then I opened them in Visual Spec, calibrated and joined them to make a broad band spectrum. I then loaded both spectra for an M3III and an M4III and added them together to simulate an M3.5III. No idea if this kocher but it makes sense in my mind:P. I used the result to produce an instrument response curve for my scope and camera (the DMK).

The theoretical response for the DMK that I produced earlier overestimates the response in the blue and IR. The instrument response I produced (above) seems to be the best I've achieved so far:thumbsup:.

From the attached comparison with the theoretical response of the DMK, it matches well, but with slightly degraded response in the blue and the IR. It seems to look the part so far:thumbsup: even though the S/N ratio in the IR gets quite ordinary.

I think there's 3 things that have contributed to this being a better result for instrument response:


Using Iris to "Remove the sky" (this addresses the black point issue I was exploring earlier)
Creating the response continuum by suppressing lines rather than picking points
Simulating a more accurate reference spectrum (if an exact match can't be found).

Al.

[Merlin66]

Still not 100% recovered, but beginning to get there!
I got fed up waiting on Surplus Shed making the T2 adaptor for their adjustable slit (and the lathe work was taking for ages to organise!) so I just got out the Dremel and ground the inside diameter of the 15mm T2 spacer from 38 to 39mm...only took about 15 mins. Now the slit sits securely in the spacer!
I'm going to set it up tomorrow with the camera adaptor and 25mm eyepiece with the SA and see if it performs as good as or better than the prototype.
Seems an easy way to move up towards a slit configuration without building a Littrow etc.
Keep you posted.

[sheeny]

Finally got a clear night (till the fog rolled in). Here's a couple of tonight's spectra: Arcturus and Vega.

Al.

[Merlin66]

Al,
A couple of nice results there! Well done!!
The Spectral Atlas I have only concentrates in the K stars around 450nm
The "dip" at 430nm is probably the G band ( CH band) and a bit of Hgama.

[Merlin66]

Can't do much else at the moment, so I made up a spreadsheet for the dispersion, resolution and spectrum size for a transmission grating (no slit) in a converging beam i.e. infront of a CCD in a telescope!
It allows you to enter telescope aperture, focal ratio, CCD details and distance to CCD chip and gives you an output.
You can then check if the full zero and spectrum will fit across your CCD chip, and by varying the parameters see the effect of varying the focal ratio and/or the distance to the CCD.
Give it a go. I'd obviously appreciate feedback.
I'll also post on the Astronomical Spectroscopy group.
ps Also "redone" Buil's Simspec to accommodate slits etc.

[sheeny]

Thanks Ken. Yeah I think the Arcturus spectrum has some strong atmospheric lines. When I first binned it, I thought "hmm I don't know these lines" but I was able to pick the O2 line at 7600Å and do a rough calibration. The spectrum type tool provided the clues though - Fe.:thumbsup: It was easy then to load the library spectrum and recalibrate more accurately.:)

I've downloaded your Excel spreadsheet to have a look. I already have my own version, but it's always good to compare other people's work.:)

Al.

[sheeny]

I stumbled on this one... Just browsing Stellarium lookking for something interesting and found Regor - a WC8 + O9I.:D It was very low in the SW sky at the time.

I've also been playing with drift imaging to effectively increase the dynamic range of the DMK (if that's the way to describe it:P). I've attached the single drifting image I used to produce the spectrum of Gamma Crucis, as well as the comparison of that spectrum with a previous one made from multiple stacked tracked images. I think there's slightly more detail in the single drift image (and less noise in the red end).

I'm interested in seeing eventually if I can use the drift image method to capture an eta car spectrum without clipping the Ha line on the DMK:shrug::whistle:.

Al.

[Merlin66]

Al,
Very interesting results! IMO the stacked version is slighty better! ( coming from someone who hsn't seen the sky for the last few months!!!)
Re spectroscope objects...
I'm attaching a copy of the "draft" version of - "Best Celestial objects for Small Spectroscopes" - you'll see that REGOR, WR 11, had already made it to the list as it's one of the brightest ( if not THE brightest WR star visible)
The WC8 classification means high carbon emmissions
CIII/ CIV @ 4650
HeII @4686
CIV @ 4441
some HeII @4861
can't find a definate reference but I think the emissions in the 5600 area are C2 ( there's a band a 5635) [Edit: found a spectrum by Maurice Gavin for WR 120 ( WC7 +O) so very similar to W 11- there's CIII @ 5660 and CIV @ 5780 - interestingly he found the 5660 peak is only about 25% of the 5780 peak??? Does this mean anything?????]
Anyway, a very good catch!! Well done.

[sheeny]

Thanks Ken.

That document is a work in progress I assume? I couldn't find section 2 in it, so I figure it's not complete yet, rather than it lost some along the way...:P

Al.

[Merlin66]

Al,
Definately a "work in progress"!!
Coming up with the list was the easy part; finding examples of spectra from the mountains of reference material, forums and websites is most definately a "labour of love"!!
At least the list should provide a good starting point for suitable objects.

I'll keep it up to date as the seach continues.

I am intrigued by the differences between WR 120 and WR 11 ( WC7 and WC 8). Anyone have any ideas???

[Merlin66]

Found the following reference, which explains the different spectra between WR120 (WC7) and WR 11 (WC8)

Ciii 5696 emission is very sensitive to
mass-loss rate, so weak winds for LMC WC stars would produce negligible Ciii
5696 emission (WC4 subtypes) and strong winds interior to the Solar circle
would produce strong Ciii 5696 emission (WC8-9 subtypes)

See
http://arxiv.org/PS_cache/astro-ph/pdf/0610/0610356v2.pdf
for more detail!!

[sheeny]

I had another play tonight (it's glorious outside!). For a challenge I thought I'd try collecting spectra of the Galilean moons. I had some mixed success.:whistle:

Io and Europa were very close together at the time and the glare from Jupiter ruined that spectra anyway. The Callisto spectrum didn't show much in the way of structure at all, but I got a definite line at approx. 6888A in the Ganymede spectrum.

My processing was as follows:
I captured a Gamma Crucis spectrum first for reference and calibration.
The Ganymede spectrum was normalised, corrected for instrument response, and then I divided the corrected spectrum by the G2V library spectrum. There's a bit of noise in the spectrum but the line at 6888A is definite. I'm guessing it would be a molecular band rather than an element. Any clues?

Al.

[Merlin66]

Al,
Can you upload your spectra?
( I'm in transit, and will get back to a computer, telescope and hopefully a spectroscope early next week!)
BTW the kids got me a neon driver circuit board Ex-Bunnings inspection light... works a charm! I'll be able to mount it with a 3V battery on the Littrow when I get home.
From memory... I think there's some profiles in PAS for molecular bands????

[sheeny]

Here we go, Ken.

I've posted both spectra: one is corrected for camera response (...ccr) and the other os correct for camera response and solar corrected (..ccrsc) by dividing by a G2V spectrum. Both are also included in the zip file.

The raw spectrum is a bit noisy as it is only a stack of 2 images. It was just an experiment to see what I got, so I plan to put more work into the next attempt. Dividing by the G2V spectrum also adds some noise (false emission lines:P) where the absorption lines are in the G2V spectrum, but I haven't done anything to dampen or remove them:whistle:.

I'm in the process of flicking through PAS... I haven't found a lot yet, the odd reference for particular lines like CN... I'll have a proper browse later - I'm in the middle of a shutdown at work, doing long days and feeling shattered...

Al.

[sheeny]

The mystery line is very close to Fraunhofer B... which worries me. I assume Fraunhofer B is atmospheric O2 :shrug: as is Fraunhofer A. The H2O band is visible in the spectra too.

The puzzle to me is that this Ganymede spectrum was the only one to show this band/line... it doesn't appear noticeably in the spectra for the other Galilean moons nor on the GamCru spectrum I used to check calibration...:shrug: . I'm not aware that Ganymede has O2...:P...

Al.

[robin_astro]

Hi Al,

It does look suspiciously like a Telluric line. Compare for example Christian Buil's annotated spectra of Vega
http://astrosurf.com/buil/us/vatlas/vega3.gif
http://astrosurf.com/buil/us/vatlas/vega4.gif
from this page
http://astrosurf.com/buil/us/vatlas/vatlas.htm
Was the spectrum taken at a lower elevation compared with the others (higher air mass)?

Cheers
Robin

[sheeny]

G'Day Robin,

No. The other Galilean Moons were captured at essentially the same time. Separate images for each to optimise exposures, so Ganymede was the shortest exposure of the 3. Callisto was the longest exposure, but this line doesn't show there, and it doesn't show in the Io/Europa conjunction spectrum either:shrug:. That's the puzzle...

I'll just have to do it again:D:P:whistle:. It would be interesting to see what other's get if anyone wants to have a go too;). It won't hurt to do it at higher elevation anyway... I guess it was about 45° elevation when I captured these.

Al.

[Merlin66]

Again from memory, the profiles were under Nick Glumac's chapter in PAS...his 2 spectroscope designs, I think the Hi dispersion section....

[sheeny]

Yeah I found them, thanks Ken. There wasn't anything there of use. I'm part way through browsing Kaler again.

The other thing I found the other day was a Molecular Spectrum database... there heaps of data in the IR band, but I'll have to find the URL (I found it after giving up on the NIST Moleular database) and work through it methodically for the visible / near IR band. I found I could get simulated spectra out of it, but the data is bogged down with lots of extra stuff that we don't need to construct an artificial spectrum for Vspec. I need to spend more time working out the best way to extract what I need:P.

I suspect it is a telluric line, but why it appears in this spectrum and not the others defies me:shrug:. I need to repeat it for that reason.

Al.

[Terry B]

I haven't been using my Sa much recently but I did take some images of Nova Sgr2009 no3 on 13th August. At the time it was V mag 9.66 and R mag 8.61.
The spectrum is a composit stack of 5 frames of 20 sec exposures.
It is very low res at just under 32A/pixel.
The Ha dominence is very obvious.

[Merlin66]

Very nice!!!!
Just goes to show what can be done with a (good!) 8" telescope and a Star Analyser grating.
Here we have an almost 10 mag star spectrum recorded in less than a two minute total exposure.

[Merlin66]

Christian Buil has just published his results from the 10th through the 13th Aug. C11, 2.1A/pixel and exposures from 35 to 90min!!


http://astrosurf.com/buil/nova_sgr2009_n3/obs.htm

[Swanssm]

Hey Folks,

I am interested in doing a Blog Post on my blog about amateur spectroscopy, featuring a bit of the local "legenday" IceinSpace talent.

As I don't know much about it myself...I am curious, but also haven't seen much comment on this other than here in Iceinspace.

If a couple of folks are happy to answer a few questions (message me) I'd be keen to do a feature, link to some interesting examples/photos etc & highlight some of your talent.

Just reading though some of your work on this thread sounds like some amazing stuff is going on. If you interested message me with the answers to a few questions and may be provide a quote and or varified "discoveries/interesting work".

Q: How did you get stated?
Q: What equipment do you use? (Basic, Intermediate and advanced)
Q: Examples of "build your own" (websites/photos to link to etc)
Q: Whats your most exciting work so far?
Q: Any examples of where you have collaborated/confirmed professional astronomers etc
Q: What are the limits/boundries/best practices

:thanx:
Peter
SWANSSM
(or as I now like to say Page 79 of the compendium)
aartscope.blogspot.com

[Swanssm]

Hi all,

My article has been published on Universe Today. Enjoy....thanks to Ken and those who assisted.

http://www.universetoday.com/2009/11/30/amateur-spectroscopy/

Keep up the good work!!!!
Regards
peter

[sheeny]

Well done. Its a good read.

Al.

[Waxing_Gibbous]

Thanks for that. I think this is a fascinating branch of astronomy and I've decided to take it up. Your article was very helpful.
Regards,
PJH

[sheeny]

I've finally managed to get out and capture some spectra again.:thumbsup:

This is the first run with a new setup: ED80, SA 100 on the end of an Orion 3x barlow body and the DMK 41AU02.AS. On paper I thought I could get about 4 A/pixel dispersion with this setup if I can achieve focus.

Well I can achieve focus:thumbsup: but ATM the focus is not very good:lol:... With my C8 and Motofocus I could be sitting at the puter and tweaking the focus:thumbsup:. Unfortunately, I'm not that sophisticated with the ED80, and its obviously much harder to get a good sharp focus looking at a screen on the other side of the obs!:P I think I need an assistant!:lol:

Anyway, the dispersion is encouraging:thumbsup:. Actually I think the 3.4 A/pixel is better than I calculated (estimating distances etc), I just need to work on focus, and maybe guiding for dimmer objects. If I can nail focus, I reckon I should achieve a resolution of low 20's A/pixel.

ATM I could always go back to the C8 if I want to...

Unfortunately, I can't achieve focus at this sort of dispersion with either the C8 or C11 :rolleyes:. The extra aperture would be nice!:D

Al.

[Merlin66]

Al,
Good to see you've managed to some results.. the weather ( everywhere?) has been terrible!!
The ED80 is a nice instrument, you could fit one of those SW electronic focusers....
Increasing the distance can give some increase in plate scale, but the resolution is swamped by the aberrations. Adding a Grism would help!
Good result though!!
Keep at it!

[Terry B]

I'm been pretty slow at processing this but I took a simple spectrum of the nova in Eri early in Dec. It is designated KT Eri
It is taken with my SA grating that I had put in the filter wheel of my camera. It is too close to the CCD and the result is that the resolution is ~25A/pixel.
At least it gives the idea.

[Heian]

Nice one Terry!

It's been so long since I did any sort of spectra collection and analysis, I'm pretty sure I've forgotten exactly how to do it! :eyepop:

cheers
Mark

[Terry B]

Likewise. I had the analysis program crash about 10 times before I realised I was pushing an incorrect button.
I will try to get a better system going to improve the resolution.

[Merlin66]

Increasing the distance to the CCD chip, as Al has done, will give a marginal improvement. The next step would be to add a Grism, this will reduce the aberrations and improve resolution.
To go further, then we're moving into a collimated design either with the SA or a reflection grating.......

Sitting under 4" of snow at the moment and more to come.....

[sheeny]

I see SBIG have released a hires version of the SGS with an 1800 l/mm grating:

http://www.sbig.com/sbwhtmls/online.htm

It's in the price list but nowhere else on the web page that I can find.:shrug:

LHires III must be being felt...

Al.

[Glenpiper]

I think an SGS (not STS) 1800 l/mm grating has been available from SBIG, as a special request, for a long time. I recall reading a few years back that a US amateur (I think it was Dale Mais) had been able to talk SBIG into this.

Bernard

[sheeny]

Whoops! Fixed the typo now.:thanx:

I had read in several places that SBIG may release higher resolution grating for the SGS, but I hadn't realised that it had already happened.

Al.