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Why take multiple flats?

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#1 NGC 2419

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Posted 25 June 2019 - 09:48 PM

This question was asked by another member in another thread but was not the main topic so it went unanswered.

(Paraphrasing the original question.) Why take multiple flats when the signal to noise ratio of a well exposed flat is so high?

Thanks for any insight.

Clear skies!

Edited by NGC 2419, 25 June 2019 - 09:52 PM.


#2 Brett Waller

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Posted 25 June 2019 - 10:12 PM

Any noise in your flat is going to be transferred to your calibrated images, so the idea is to minimize the amount of random noise by combining many frames.  Don't get confused, just because the SNR is high does not mean the noise is low, or that the noise can't be reduced.

 

Brett



#3 pfile

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Posted 25 June 2019 - 10:13 PM

i guess "so high" is relative to the paltry signal in the lights.

 

https://www.ccdware....cdap5/hs150.htm

 

middle paragraphs discusses the effect of the flat SNR on the final calibrated SNR.

 

rob


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#4 WadeH237

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Posted 26 June 2019 - 07:36 AM

I use sky flats.

 

With my current setup, a CMOS camera with electronic shutter, I can take very short flat exposures - short enough that I can take flats in broad daylight against a blue sky.  With this setup, I take 15 flats per channel and reuse them for all of the subs that I take while the scope is set up in the same configuration.

 

When I use my CCD with mechanical shutter, I have to take longer duration flats to avoid shutter effects.  I consider 3 seconds to be the minimum exposure.  With this setup, I have to use a twilight sky to get a usable level of signal with the longer exposures.  Further, I often use a camera rotator with this camera.  That means that I need flats for each channel and each rotation angle that I use.  And I can't reuse the flats, since each image has a different rotation angle.  This means that I need lots of flats, every dusk and every dawn, and it needs to happen with a sky that's rapidly changing brightness.  I would not be able to do this without automation that continuously adjusts exposure length to match the sky conditions, and the total window of time is very narrow.

 

So with the CCD/rotator configuration, I can generally get 6 flats per channel if I use both dusk and dawn and have a single target.  With two targets, I can get 3 flats per channel per target.  To prepare a master flat from only 3 raw flats, I use a mean combine while clipping the high value pixels.  This is pretty good at eliminating stars (which inevitably appear in twilight flats).  So most of the pixels in my master flat are the average of 3 values, and the ones where there were stars in the raws are the average of the two remaining values.

 

The thing is, that master flats made from just 3 raw flats work surprisingly well.  So well, that I cannot distinguish the difference during processing or in the final image.



#5 NGC 2419

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Posted 26 June 2019 - 10:20 PM

Thanks for the replies. Good info at that link.

Cheers!


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