I'm working my way through all the information about transformation. I'm using a DSLR for my work. I've downloaded the V26. spreadsheet and associated instruction document by Mark Blackford.
I've also been reading the CCD Photometry guide and especially the section on transformation. I don't understand it all but I get the idea that you work out the transform coefficients using measurements from images taken of one of the standard star fields (M67 for example). I thought that those coefficients would be calculated once and could be used multiple times but should be recalculated at least every year or when anything changes in my setup.
However, working through the way the excel spreadsheet works it seems to calculate the transform coefficients on any set of images not just a set from a standard star field. Should I use that spreadsheet only for standard star fields or can I use it for any variable star along with the reference and check stars on the variable star's particular chart.
Have I got this wrong?
Cheers
Steve
Hi Steve,
that spreadsheet was designed for wide field DLSR images where differential extinction across the image is large enough to affect the magnitudes measured from stars in different parts of the image. The spreadsheet does simultaneous extinction correction and transformation, but relies on the comparison stars having accurately known V magnitudes and B-V color indices, and on knowing the B-V of the target variable.
I wrote the spreadsheet and DSLR manual quite a while ago and there has been some criticism of the methodology since then. Some people have recommended that transformation coefficients should be derived from images of standard clusters (like M67) in the normal way used in CCD photometry. And that is my recommendation too if you are imaging through a telescope where the field of view is small enough that differential extinction is negligible. If your equipment doesn't change then your transformation coefficients don't change much either. So you only need to determine them a few times a year.
However, if you are imaging with a camera lens you should also correct for differential extinction so extinction coefficients also need to be derived. They change all the time depending on air quality so need to be established each night. There are established ways to do that but they require extra images. And you will have to write a spreadsheet to apply the extinction and transformation corrections.
The DSLR manual does need to be updated but I won't be doing that until I find a way of easily (i.e. a simple Windows program) converting RAW DSLR images to FITS format so any photometry software can be used to calibrate and measure them. If anyone knows of such a program please let me know. Cheers,
Mark
I'm using a…
Thank you Mark,
I'm using a telescope and my field of view is about 1.2 degrees across. I assumed that differential extinction would not be a problem in that case. The problem I have is that many of the standard star fields are only 20 minutes across. I struggle to find enough reference stars. I've taken a series of M67 images and the central area is too crowded to do photometry on the stars there so I have to use the stars in the outer part. I lose some of them because they are too blue or too red or too dim and I ended up with only 7 useable stars. In the CCD photometry guide it actually says you should have 20 to 30 comparison stars but many of the standard star fields don't have any where near that many.
I'm finding the spreadsheet very useful because I'm working through it and can see how the maths works out alongside the various manuals from this website. I misunderstood it's purpose initially. I thought it was there only to work out the coefficients and I couldn't understand why it needed a target star and a check star.
I'm beginning to understand why photometry using a DSLR is rather limited. I'm not too bothered at the moment. I want to learn the ropes first and then might invest in a CCD or CMOS camera later on. At any rate it's always interesting.
Thanks
Steve
sorry for my slow…
Hi Steve,
sorry for my slow reply, I only check AAVSO forum posts every few days. With your field of view differential extinction should be negligible if your target is above about 40 degrees elevation. As you get closer to the horizon it becomes more significant.If the target, comp and check stars are all close together in the image then differential extinction is smaller than when those stars are spread across the whole image. Also, bluer stars suffer more extinction than red stars. So a good rule of thumb would be to image at high elevation, use comp and check stars close to the variable and with similar B-V color index. Sounds simple but sometimes you can't meet all those criteria.
I'm not familiar with northern hemisphere standard fields, perhaps someone else can help select a more suitable one for your field of view.
My plan was always to move from DSLR to CCD photometry once I retired and set up a permanent observatory. I've made that transition now. Starting with DSLR is a good thing, allows you to try photometry without too much expense. Have fun learning the process, no point doing it if you don't enjoy it. Cheers,
Mark
Greetings,
I have used DeepSkyStacker for converting and debayering DSLR RAW to FITS for several years.
DeepSkyStacker may be configured to write calibrated (you don't want calibrated & registered) and debayered intermediate files as fits. It does the flatfielding. Set the light frame to average (if you are stacking images for photometry), dark and flat to median. The individual frames are flat-fielded then written as fits and you may save the stacked averaged image to fits at the end of the process if you want to do photometry on a stacked image. DSS also works fine for flat-fielding and/or stacking my Atik490EX & Astro Photography Tool produced CCD images.
Also I have always been able to extract and transform DSLR R,G,B to standard Rc,V,B using the standard CCD transformation method. Rc is always significantly noiser but if you have any signall at all to work with it is usable.
Jim (DEY)
Thanks Jim,
Sounds like DeepSkyStacker ticks all the boxes for conversion and calibration of RAW DSLR images. I'll check it out. Cheers,
Mark
Greetings,
If you have any questions about configuring DSS let me know.
Jim (DEY)
Could someone provide a link to said spreadsheet for transformation coefficient calcs? I am having no luck using the AAVSO TG app with python due to lack of experience but am pretty facile with excel. Have M67 with V and B filters to work with
thanks
https:…
Hi, here is the link
https://www.aavso.org/dslr-camera-photometry-guide
It's part way down the page under "Additional Material" along with instructions for use.
Hi Bryan,
that spreadsheet isn't meant to be used for calculation of normal CCD photometry transformation coefficients. I recommend making your own spreadsheet based on the method described in the AAVSO CCD Photometry manual (see page 51 onwards). Cheers,
Mark
Thanks for the recommendation
I have followed the examples and am working through my data
Why are the measurements for my b and v mags entered as negative numbers on the spreadsheet?
bryan
Hello Mark,
For the Canon camera, I believe that "Astronomy Photographic tools" (APT) does this directly in fit files when taking pictures. https://www.astrophotography.app/
Otherwise the free software "Iris" does it too and do the convertion : http://www.astrosurf.com/buil/iris-software.html
I would also be very interested in the latest version of this Excel spreadsheet.
JBD
Jean-Bruno,
Are you referring to conversion of DSLR RAW to FITS files?
I've used APT for image capture for years, with a DSLR camera and a dedicated astrophotography camera, and have never been aware of a file conversion facility. I can't see any reference to file conversion from a quick perusal of the link you posted.
Roy
You are absolutely right ...
I was based on my bank of flats files which were taken RAW from MaximDL and saved as Fits files. It's the right software that makes the conversions.
I love you too this super efficient little software (APT) which can run in conjunction with PalneWave's Platesolve 2 and make "platesolves" easily and quickly.
I had discussed with the developer and the conversion of RAW images to Fits files was still in his development plans.
Sorry for my mistake.
JBD
Hi JBD,
Perhaps future versions of APT will save DSLR images as FITS.
I used MaxIm DL to control a DSLR camera and save images as FITS files, and control a mount as well. It is a great piece of software but beyond the budget of most of the people doing the DSLR photometry courses in the past. In its most basic form DSLR photometry is done with a camera+lens on a fixed tripod using a remote shutter release. RAW images then need to be converted to FITS before most photometry software can read them. The DSLR manual and the C.H.O.I.C.E. DSLR photometry course need to cater for these people as well as those using more sophisticated equipment and software. Running the course was particularly hard as everyone had different software and computer skills. So I'm looking for a simple (preferably free) set of software that everyone doing future courses will use to learn the basics of DSLR photometry. Afterwards they can use whatever software they prefer. Cheers,
Mark
I use APT/EQASCOM on an old laptop to control my DSLR and scope outside. It is an excellent piece of software. But inside when I process the images I use Maxim DL on my desktop. I tried various free software like IRIS, AIP4WIN, AIJ, Muniwin and so on but I seemed to have problems with all of them one way or another - crashes, odd things happening, not opening or saving files properly. It may have been a problem that I'm now on WIndows 10 on my inside computer. So I decided to take the plunge and tried Maxim DL and Pixinsight. I couldn't get along with pixinsight so I settled on Maxim DL and the more I use it the more I like it. Since I use APT with EQASCOM to take the images I only needed to buy the IP version of Maxim DL which is considerably cheaper than the full version.
In fact I also use AIP4WIN on my old laptop to check images while I'm working outside. It's excellent for that but some things just don't work on it so I can't use it for photometry or image processing. Also I don't think it's supported any longer.
Cheers
Steve