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RGB from Bortle 7 skys

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#1 tomb18

tomb18

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Posted 09 April 2020 - 10:41 PM

Hi
Well galaxy season is here and I want to try my luck with some rgb imaging from the suburbs of Montreal.
The scope is a FSQ106ED, and the camera a STL11000 monochrome.
I am used to narrowband by necessity where 30 minute subs was the norm.
So, if anyone can give some hints as to what I should be doing it would appreciate it.
Thanks, Tom

#2 steinw

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Posted 10 April 2020 - 04:24 AM

Find the optimal exposure time and add lots of time. Really lots of time.

 

The factors deciding your singel exposure length is the read noise of the camera vs the overexposure clipping of stars and wanted signal.

Your single frames should be exposed until you swamp the read noise. This is not an exact term, but if the signal from read noise (RN) is very small compared to the total background signal (sky noise) you can be quite confident that stacking lots of frames will give you a signal/noise ratio that only depends on the total integration time. Now there are various opinions of how much "very small is", ranging from 5 * RN^2  to 20 * RN^2.

 

See this post for the basic equations: https://www.cloudyni...1600/?p=8200280

 

For you camera with a read noise of 13e and a gain of 0.8 e/DN, the numbers add up to a DN of around 4200 for 20 times RN squared. So if you can expose until your sky background reads out at 4200 ( + the general offset your camera may give) without blowing the signal I would go for that. However if too many of the stars are clipped, you should start trading off on the swamping factor.

 

Then just keep adding frames. Broadband from Bortle 7 is possible, you just need time and calibrate your expectations. This was taken from my Bortle 7 skies, close to 18 hrs of exposure:

 

M51 HaLRGB

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#3 tomb18

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Posted 10 April 2020 - 05:45 PM

Hi

Thanks.  I do have one picture from a few years ago, where I exposed for 600 seconds.  Many of the stars are clipped but the background is right around 4500.  I might go down a bit in exposure.

Going to give it a try if the sky and moon cooperate.

Thanks, Tom 




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