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.
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. Good agreement between the two spectra, except for Ha. Both profiles are corrected for camera response and normalised.
Al.
Last edited by sheeny; 06-06-2009 at 01:01 AM.
Reason: forgot stuff
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???
I'm not sure I'm on the same page as you, Ken... so there's probably something to learn here... .
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.
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.
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. 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.
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 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.
Quote:
Originally Posted by 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.
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).
Al.
Last edited by sheeny; 07-06-2009 at 08:57 PM.
Reason: premature submission
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.
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.
TransSpec - calculations for Transmission gratings
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.
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.
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. 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.
I stumbled on this one... Just browsing Stellarium lookking for something interesting and found Regor - a WC8 + O9I. 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). 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.
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.
Last edited by Merlin66; 24-07-2009 at 11:42 PM.
Reason: corrected edit!
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...
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???
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.
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,
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????
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????
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) where the absorption lines are in the G2V spectrum, but I haven't done anything to dampen or remove them.
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...
The mystery line is very close to Fraunhofer B... which worries me. I assume Fraunhofer B is atmospheric O2 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... . I'm not aware that Ganymede has O2......
. Good agreement between the two spectra, except for Ha. Both profiles are corrected for camera response and normalised.
.
Camera response yet again!
. I used the result to produce an instrument response curve for my scope and camera (the DMK).
.
