Fri, 08/05/2022 - 16:35
Hello,
yesterday I followed Lauren's advise to check Siril to stack frames and I was impressed. The only "problem" I have: It's a bit like in a supermarket, just too many offers how to stack. Which one is best for spectra? I tried several and they indeed led to some differences in the spectra, mainly smoothness.
Maybe using the average stacking, for normalization "additive" without scaling and for rejection median sigma clipping (but I think there will not be any rejections normally).
Anyone else any experience?
Matthias
Most stacking programs for astrophotography do not work well with images of spectra. I use the BASS Project software, created just for spectroscopy by the BAA, to stack and process images for spectroscopy. ISS by Christian Buil is powerful and excellent, but I found it more confusing to use than BASS. It might be more powerful, but for me as a beginner, I have not yet attempted more than the basic processing.
ISIS is probably the…
Hello.
ISIS is probably the most powerful, but i agree with you on its difficulty. It is very complicated to master. Sometimes i think it was designed difficult on purpose - just kidding-.
BASS stacks images using stars for alignment, but using a slit spectroscope there are no stars to align with. If using a Star Analyzer there will be some stars, but they will be doughnuts, not pinpoint stars.
Other option with BASS that sometimes works -not always- is to extract individual spectra from each image and afterwards align the spectra using an emission or absortion line.
I use Demetra for stacking. It works for Shelyack spectroscopes only. It just stacks the images without aligning them. On the other hand, if guiding was good, i find aligning the images is not necessary, unless the run was very long, with many images and temperature or dflection changed the spectrum lines positions.
Cheers.
David Cejudo.
I was under the impression that we were talking about slit spectra. In that case, the instructions state that when stacking spectra from slit spectragraphs, you should select "no alignment' within the stacking tool. The assumption is that each of the images will have the spectral "line" located at the same location in the image. It works quite nicely for me.
I also tried…
I confirm...
I also tried several processing software and I always go back to Bass which is very complete, lots of options and very user-friendly.
For the stacks, I also remove the alignment option and it works very well for me too.
JBD
Hello,
Everybody enourages everybody to use the BASS Project software, but there is no way to download it at the provided link in IO groups. Should you be accepted in the group first? If so, my join request has been sitting there for months and has not been approved yet. So, the BASS software may be great, but as far as I see is not so free for download.
On the other hand I use Siril successfully to stack spectra. Lauren has a video showing the process very clearly. This video can be found in AAVSO's YouTube channel. I use one star registration, and average stacking with no normalization. Do apply as well dark frames to calibrate the images first.
Hope this helps.
Here are my experiences combining spectrum images
Slitless
Align and stack is commonly done as it can be used to avoid tracking issues and the need to guide. Note it should only done on a single point (normally the zero order) as the spectrum always lies in the same direction and you don’t want field rotation corrections to rotate the spectrum or distort the dispersion
(I use Siril currently)
Slit
Spectrum images can be stacked unaligned but it is generally better to treat each image separately and combine the resulting spectra as this offers several advantages (ISIS for example works this way)
Easier to identify and heal cosmic ray hits and other anomalies (Individual spectra can be examined for variability between them and outliers investigated and rejected)
Optimise the SNR by weighting the contribution from each image. (also estimate the SNR accurately by dividing one sub spectrum by another)
Track any movement of the spectrum in the slit direction due to guiding errors, minimising the contribution from the sky background which would otherwise be the case in an unaligned stack of images.
Potential to track and correct any movement in the dispersion direction (eg by cross correlation) due to spectrograph thermal and mechanical instability which would otherwise smear the spectrum reducing resolution.
(I use ISIS which has these capabilities)
Cheers
Robin
www.threehillsobservatory.co.uk
https://britastro.org/observations/user.php?user=146
Celestron C11, EQ6, ATIK 314, 428
Star Analyser 100,200, ALPY 600,200 LHIRES III Spectrographs