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Multi night multi temp dark frame processing

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#1 Binsky3434

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Posted 10 July 2018 - 02:55 AM

Hello. I recently over over the last 2 weeks have had 4 good night to get sum solid data on the veil nebula with my canon 60Da but of course it is during the heat wave of the summer. My subs have quite the range in temp so I decided to try and make a dark library for the camera setting ( 780 seconds at ISO 800 ). After literally 4 days straight of round the clock dark frame exposures I believe I have a decent amount to start calibrating my light frames but I was unsure of the best way to do it. I have flat and bias that r different for each night so I will probably have to calibrate each night separately I’d assume. As far as what dark temps to use I came up with an idea. For each night, I find out the temp of each light frame and use a dark frame of the same temp when I start picking out what to use for dark frames. So I may have a variation in light temps but if it’s matched up with the same variation in darks I feel like it should subtract the noise correctly. I also would assume that if I could use 2 or more darks of the same temp for each temp that it would only help even more. Any comments and advice would be great cuz it’s a lot of data to trial and error it. Thanks and clear skys

#2 bulrichl

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Posted 10 July 2018 - 04:21 AM

After literally 4 days straight of round the clock dark frame exposures I believe I have a decent amount to start calibrating my light frames but I was unsure of the best way to do it. I have flat and bias that r different for each night so I will probably have to calibrate each night separately I’d assume. As far as what dark temps to use I came up with an idea. For each night, I find out the temp of each light frame and use a dark frame of the same temp when I start picking out what to use for dark frames. So I may have a variation in light temps but if it’s matched up with the same variation in darks I feel like it should subtract the noise correctly. I also would assume that if I could use 2 or more darks of the same temp for each temp that it would only help even more. Any comments and advice would be great cuz it’s a lot of data to trial and error it. Thanks and clear skys

 

The individual dark frames contain a lot of noise. When calibrating light frames with single dark frames, this noise is added to the calibrated frame. This is not what you want.

 

By stacking a large number of dark frames (building a MasterDark) the noise is greatly reduced, by about square root of the number. So a better idea is, to group the dark frames according to their corresponding temperature and build a MasterDark of each group of dark frames. (The temperature ranges of the MasterDarks should be chosen reasonably, dependent on the overall temperature range that you experienced during image acquisition. The number of dark frames should be about the same for each group.) Group your light frames according to the same temperature limits and calibrate each group of light frames with the MasterDark whose temperature range matches.

 

Example: your overall temperature range during image acquisition was 20 to 5 °C, group both the dark frames and the light frames in 3 groups (20 to 15, 15 to 10 and 10 to 5 °C), make 3 MasterDarks, calibrate each group of light frames with the MasterDark whose temperature range matches.

 

The appropriate approach will also depend on the software that you use for preprocessing. Some software packages support the "scaling" of a MasterDark. Scaling is usually applied only for different exposure times, but PixInsight is able to optimize the MasterDark for noise, quasi for different exposure times AND different temperatures. Take a look at the documentation of the software that you are using.

 

Bernd



#3 Binsky3434

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Posted 10 July 2018 - 07:30 AM

That seems like a better idea than my first one for sure. The only thing I’m still unsure of is the flat and bias frames. I use nebulously 4 for my pre processing and I usually subtract the bias from the flat so I then use darks and flats to calibrate the lights ( pretty much what the most popular YouTube video on nebulosity 4 demo instructs ). Since I have different flats and bias for each night my question would be if I should calibrate the lights with the flats and bias then split them up into temp ranges to calibrate the darks? Or is that an incorrect method?

#4 Binsky3434

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Posted 10 July 2018 - 01:27 PM

I think I may have explained my original idea incorectly. I think what I meant to say was something like this. So for example I have 3 different nights of data ( all the same except temp, bias, and flat frames since I took the camera off the scope ). I was thinking of just calibrating each nights worth of lights at a time so the master flat would work. As far as making a master dark I’ll use and example. If I had 4 images for a night ( closer to 30 ) and the temps were 33, 25, 16, and 21 then I was wondering if I should make 1 master dark but for the 4 images for the dark I would use a dark of 33, 25, 16, 21 and average them since the variation in the light frame temps would be the same as the darks. And to make it a better dark I could use 8 images ( 33,33,25,25,16,16,21,21). I guess that is what I was trying to say the first time. Ur method sounds better but I’m just worried if it’s ok to preprocess the lights with flats first then do darks according to temp

#5 terry59

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Posted 10 July 2018 - 03:35 PM

shrug.gif Perhaps part of the problem with lack of help is your software choice 


Edited by terry59, 10 July 2018 - 03:35 PM.


#6 Binsky3434

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Posted 10 July 2018 - 04:37 PM

Possibly. But regardless of the software the question still remains if it would be correct to calibrate light frames using a master flat and then after wards calibrate the “flat calibrated lights” with a master dark. I mean all that really happened is that u corrected for the flats and the dark noise still remains right? So then if u take ur flat corrected lights and sort them by temp to subtract temp specific mater darks shouldn’t that calibrate the lights?

#7 sharkmelley

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Posted 12 July 2018 - 03:27 PM

Possibly. But regardless of the software the question still remains if it would be correct to calibrate light frames using a master flat and then after wards calibrate the “flat calibrated lights” with a master dark. I mean all that really happened is that u corrected for the flats and the dark noise still remains right? So then if u take ur flat corrected lights and sort them by temp to subtract temp specific mater darks shouldn’t that calibrate the lights?

Regardless of the software, the master dark must be subtracted from the light frames before calibrating with the master flat.  The master dark should not be applied after the lights have been corrected by the master flat.

 

Mark




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