Detecting Secondary Fogging in Subs

[glend (Glen)]

I have before me over one hundred subs from an old imaging run. I suspect the night i captured these subs that there might have been some fogging on the secondary of the newt. Rather than go through every sub to check, is there a simple way to identify and discard the affected subs? I can't be sure when the fogging started.
I was thinking that if i just go ahead and process them normally in DSS i will get a score for each sub. If i then watch for a deteriouration of the scores i should be able to determine when the fogging started. Does that sound reasonable? What would you do?

[billdan (Bill)]

Hi Glen,

That sounds like good strategy, the star count and score should drop off when it encounters blurred stars from fogging.

I use Astroart for stacking and you can set parameters for the subs, such as setting the FWHM to reject bad subs, in theory it should only stack the good subs.


Cheers
Bill

[Atmos (Colin)]

If you get a trial version of CCDInspector (if you don't have it already) it will look at all of your subs. If you put in order of capture you should begin to see a noticeable change in both the background and contrast when the secondary begins to fog.

[glend (Glen)]

Thank you Bill and Colin. I went ahead with DSS this morning and sure enough i could see degradation in the scores in the stack. I did the red channel first as i shot those first and half way through the scores started declining. When i looked at the final red channel stacked image i could clearly see a white fog in the middle of the image. Since the other colours were shot after that i have had to delete most of the subs. I think i finished up with about 25 red channel keepers. I will need to get the secondary heated before going back. Thanks