Separating colours

[Guest]

In the AAVSO guide to variable star photometry it says that you should process the colour channels separately but I'm not sure how debayering affects how I do that. Does that screw up the procedure?

At the moment I preprocess the CR2 files from my canon camera using deep sky stacker which outputs the resulting calibrated and registered images as FITS files. Those fits files are then taken into AIJ for plate solving and photometry.

One thing I found is that I can imports the FITS files into AIJ. At that point the 3 colour channels are separate. When I plate solve using AIJ it adds the found coordinates to the FITS header and then saves that as another FITS file. But that file only appears to have one channel so presumably it is now a composite image. I have been reading those files back into AIJ before doing the photometry. This is convenient because I can more easily find the comparison stars.  But that must be wrong because the colour channels are no longer separate.

 

[Xilman]

2 hours ago, woodblock said:

In the AAVSO guide to variable star photometry it says that you should process the colour channels separately but I'm not sure how debayering affects how I do that. Does that screw up the procedure?

At the moment I preprocess the CR2 files from my canon camera using deep sky stacker which outputs the resulting calibrated and registered images as FITS files. Those fits files are then taken into AIJ for plate solving and photometry.

One thing I found is that I can imports the FITS files into AIJ. At that point the 3 colour channels are separate. When I plate solve using AIJ it adds the found coordinates to the FITS header and then saves that as another FITS file. But that file only appears to have one channel so presumably it is now a composite image. I have been reading those files back into AIJ before doing the photometry. This is convenient because I can more easily find the comparison stars.  But that must be wrong because the colour channels are no longer separate.

 

First question: under what OS are you running your software? If it is Linux I can tell you what I have done. Anything else and you may have to investigate further, but my info might be helpful.

The basic utility is "rawtran" (which requires dcraw to do its donkey work) and is used as follows:

rawtran -c Ri -o red_file.fits file.cr2

rawtran -c Gi -o green_file.fits file.cr2

rawtran -c Bi -o blue_file.fits file.cr2

You now have three files, each of which contain a single channel. Process them separately.

Both dcraw and rawtran are in the standard Ubuntu repositories. My guess is that they are easily available for other distros. A search engine will find them for you if you have difficulty.

[Guest]

Thanks. I'm on windows 10. I had a quick search for a windows version of rawtran and dcraw but didn't find anything so I'll have to come back to that.

In the meantime I found if I use deep sky stacker to produce aligned FITS files these files contain all three colour channels. If I take those into AIJ and do the photometry it does actually produce three separate apparent magnitudes - one for each channel. SO it seems possible to keep the three colours separate all the way through without splitting the files. I tried the photometry on a single file and it gave me three magnitudes and it seems to take the three channels as three 'slices'.

[happy-kat]

When you open a fits file in fitsliberator you can save each pane as a split channel. From memory (please check) pane 1 is red, 2 is green and 3 is blue.