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Yet another FLAT question

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3 replies to this topic

#1 Lostone

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Posted 25 January 2020 - 06:42 AM

The simple question I have is this.  When creating your flat's I know the camera temp needs to be the same as your lights, but what about your Gain,  Does it need to be the same?  i.e. 90 Gain, 130 Gain ect.???



#2 jerahian

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Posted 25 January 2020 - 07:23 AM

Honestly, I don’t see why the temperature or gain of flats need to be the same as lights.  They should get calibrated with flat darks anyway.  I shoot mine at room temp and 0 gain, because I shoot them indoors the next day.  Of course, your flat darks then need to be the same temperature and gain as you chose for your flats.



#3 OhmEye

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Posted 25 January 2020 - 08:26 AM

Temperature doesn't matter for flats since their purpose is purely optical calibration. That said, you still need to calibrate out sensor noise from the flats themselves just as we do with lights, so temperature is important for that. So the bias or dark flats need to be at the same temperature as the flats, but that temperature doesn't have to be the same as the lights.

 

I only shoot new flats after I make a change to my optical train, which is usually just rotation. I pick a convenient temperature and shoot a full series of LRGBSHO flats and dark flats and have a directory where I keep them in dated folders. I can reprocess any of my data and use the appropriate flats based on the date, and I can use a single set of flats for quite a long time if I don't rotate my camera or make other changes.



#4 Madratter

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Posted 25 January 2020 - 09:11 AM

Flats are not just about optical calibration for things like vignetting and dust. Of course they do function in that way.

 

However, they also correct for uneven pixel responsiveness. And for that, things like gain (and perhaps temperature) do matter.

 

You could go your entire imaging career ignoring this or being unaware of it. The result will be subtle. You'll have slightly higher noise. Or you might have some banding. Nonetheless, it is a factor and personally, I would rather avoid avoidable noise.


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