Creating the Image Processing File

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You have finished your image session and you have brought the valuable sub-exposure data indoors to be processed into a wondrous deep-sky image. There are a number of hurdles to be overcome before we end up with a pretty picture. The first thing to do is to convert your individual FITS sub-exposures into IEEE floating point format using Maxim DL's "Batch Save and Convert" routine. This step is not entirely necessary if you do not have the software to carry out the conversion. You now need to convert the files to colour RGB files. Assuming you have the SXV-H9C (or similar) one-shot colour camera, you run the "Convert RGB" function as a batch conversion using the "Command Sequence Window". The Command Sequence Window allows you to convert all your subs into RGB frames using the Convert RGB command. For the SXV-H9C camera you need to have both the X-offset and the Y-offset boxes ticked in the Convert RGB dialog, you will also use the "High Quality" deBayer mode. If you are using the SXVF-M25C camera you will only need the Y-offset box ticked, but you will still use the High Quality deBayer routine. Having created a set of RGB colour-converted FITS files from your sub exposures, you now put all these files into their own separate folder on your computer for further processing. Using your acquisition/processing software, open up each of the RGB converted subs in turn and check them out carefully for defects, you may want to screen stretch the data just to see if there are any problems lurking in the shadows. With Hyperstar/H9C data I delete all subs that have say a satellite trail, a plane trail, a download glitch, or a movement glitch (uncompensated PEC glitch). I am told that this is unnecessary as the Sigma combine process will remove the satellite and plane trails, and the download glitches, but once again, I have personally never found this to work in practice, so I take the easy option and I delete the offending files. I cannot afford to throw away sub-exposures so easily with the Sky 90 M25C combination, as there are far fewer of them in an imaging session due to the longer sub-exposure times I must use. For this data, I bite the bullet, and spend a lot more time during the image processing

stage removing all these annoying defects. You may also have some subs with poor, or no data, due to clouds. Again, these files should be deleted. If your subs are say 5 minutes each, then it can be a wrench deleting any of them as you feel you are throwing away something valuable. You feel that if there is 5 minutes worth of data there, even if it is not top quality, then surely it is worth adding in. It is not worth adding in! All sub-optimal data will actually detract from the quality of the final image, which is why I prefer to go to shorter length subs (so there is less chance of an unwanted glitch occurring during the sub-exposure), and you don't lose so much valuable imaging time in deleting shorter exposure subs. Finally of course the signal to noise ratio of your final image improves as the square root of the number of subs you take, so a larger number of shorter subs is in my opinion the best route to take (though many experienced astrophotographers will disagree with this approach). It is true that using a larger number of shorter sub-exposures will lead to a "shallower" image, but does it really matter that your image's limiting magnitude is 20 rather than 21? Usually it does not.

So you now have a file containing all your high-quality (i.e. sifted) RGB converted FITS files, the next step is to combine all these separate files into a single file. To do this you use the "Combine Files" function of your acquisition/processing package. I prefer to use the SD mask combine, although many experienced astrophotographers use the Sigma combine. Having created this single large combined file, my combined files tend to be around 16.5Mb in size using the SXV (F)-H9C camera; you are now ready to start processing the data.

To summarise, for the H9C camera:

1) Convert the raw 2.76Mb FITS data files (the sub-exposures) into IEEE floating point format files using "Batch Save and Convert"

2) Convert the 5.52Mb floating point files into RGB colour-converted files using "Convert RGB" and the "High Quality" deBayer routine. Remember to check both X-offset and Y-offset for the SXV (F)-H9C camera, other cameras may need different combinations of offset checked.

3) Check each colour-converted file to see if there are any defects in the image, if there are delete the image.

4) Combine the remaining 16.5Mb RGB files into a single 16.5Mb file using either the SD mask or the Sigma combine function.

For the M25C camera I take a slightly different approach.

1) Colour-convert the raw 11.6Mb FITS data files (the sub-exposures) into RGB files using "Convert RGB" and the "High Quality" deBayer routing. The Y-offset box should be ticked. Each RGB colour-converted file will now be 34.9Mb in size, these are getting rather large!

2) Check each colour-converted file and delete any that have defects that you feel you cannot fix in the processing software.

3) Combine the remaining individual 34.9Mb files into a single IEEE floating point file using either the SD mask or the Sigma combine function. The resulting single file will be 69.9Mb in size.

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