Al, the top link seems very heavy going. Might take some time to digest.
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!
Hi everyone,
I haven't caught any spectra for a while; I'm so slack..
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
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!!
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.
Yep. I can practice that now... 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,
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!!!
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!!!
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!
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.
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.
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).
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).
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.
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)
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.
All good fun. It keeps the grey matter juicy...
Al.
Last edited by sheeny; 30-04-2009 at 09:01 PM.
Reason: afterthought
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.
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).
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.
Al.
This is a very good result. I will have to try this with my kodak chip.
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.
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
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
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 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.
Hopefully that is not the case if the FR is also a good field flattener. 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. I found it quite fiddly and, well, I tend not to persevere with fiddly things that don't give me good results.
Al.
Last edited by sheeny; 01-05-2009 at 11:56 AM.
Reason: treatment for schizophenia...
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Might take some time to digest.
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... no doubt there is some detail there if I had adequate resolution and purity.
Camera Response again
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