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should flats be pure white?

Astrophotography
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#1 belliott4488

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Posted 03 March 2021 - 08:02 PM

I attempted my first-ever set of flats this morning, following the advice of Trevor from AstroBackyard and using the morning sky to light my double layer of tee-shirt stretched over my 8" Newt. I was surprised (although I'm not sure why, in hindsight) that my flats all came out pale blue. I guess the sensor was "seeing" the blue sky through the tee-shirt.

 

Is this okay? Disastrous? Does stacking software consider the color content of the flats or only the monochrome value?



#2 dhaval

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Posted 03 March 2021 - 08:05 PM

You should look at the histogram and figure out if it is within the linear range (typically between 1/3 to 2/3 of the maximum well depth for a 16-bit camera). The "color" makes no difference, unless what you are seeing is complete saturation of all the pixels (in which case the histogram is way beyond the 2/3 well depth).

 

CS!


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#3 belliott4488

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Posted 03 March 2021 - 08:52 PM

You should look at the histogram and figure out if it is within the linear range (typically between 1/3 to 2/3 of the maximum well depth for a 16-bit camera). The "color" makes no difference, unless what you are seeing is complete saturation of all the pixels (in which case the histogram is way beyond the 2/3 well depth).

 

CS!

Yes, thanks - I did check the histogram when I was choosing my exposure time (shutter speed), and it was between 25% and 50%. I'd expected a sharp peak but instead got more of a bell shape on the left with a steep cutoff on the right. Not sure if that's optimal ...



#4 dhaval

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Posted 04 March 2021 - 11:52 AM

Yes, thanks - I did check the histogram when I was choosing my exposure time (shutter speed), and it was between 25% and 50%. I'd expected a sharp peak but instead got more of a bell shape on the left with a steep cutoff on the right. Not sure if that's optimal ...

I would think that should be fine. Have you tried calibrating lights with the flats? Try and see what happens. If it over or under corrects, you will know you have a problem. 

 

CS!



#5 Borodog

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Posted 04 March 2021 - 02:18 PM

I think the master flat is typically converted to monochrome before being used for correction, so as long as all the channels are well exposed I think you should be fine.


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