Optimal Sub length calcs

Discussion of using SharpCap for Deep Sky Imaging
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balimester
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Joined: Sat Mar 20, 2021 11:34 pm

Optimal Sub length calcs

Post by balimester »

Hi All,

I've just watched Dr Glover's presentation on Deep Sky Astrophotography with CMOS cameras, it was just amazing and inspiring, and I loved the scientific approach despite the fact I was not the best-in-class physics student, so there might be something I missed in the plethora of new information:

When the optimal sub length is detemined, doesn't the brightness of the object should come into calculations? I mean, the Orion nebula is like much, much brighter than the Dolphin's head for example. Or like the photons hitting the sensors is like kind of a constant number, unrelated to the brightness of the DSO? Quite confused here.
For now I only use a Sony MILC, did a few pics, nothing fancy. But I couldn't seem to get the same amount of details out of let's say the Triangulum galaxy and the Andromeda with the same exposure length (okay my post processing skils definitely need improvement, but still).
Or is there something I completely misunderstood?

Thanks for the help in advance, stay safe everyone!
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admin
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Re: Optimal Sub length calcs

Post by admin »

Hi,

the reason we ignore the brightness of the object is that usually you want to see the faint bits of the target - the bits on the edge that are the thinnest wispiest parts of the nebulosity or galaxy. The brightness there is only fractionally above the sky background brightness, so it makes sense to optimize for the sky background level.

cheers,

Robin
balimester
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Joined: Sat Mar 20, 2021 11:34 pm

Re: Optimal Sub length calcs

Post by balimester »

Thanks a lot, that makes sense I think! :)
So basically what I understand, even if it is a very faint object, with an f/5 scope that I have on a mono CMOS, b5 skies around 17-20 secs should be enough (and 3x of that for my RGB filters, etc) rather than what I see from lots of astrophotographers, going for 300-600 secs even for L or RGB subs, because that would only do minor improvement on the SNR.

Thanks Robin, really appreciate it!
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Re: Optimal Sub length calcs

Post by admin »

Hi,

it's one part of the information that helps you make your decision. What the calculations tell you is that you will not see any more detail in the faint stuff by going beyond that calculated exposure value.

You might want to use longer exposures though (if your mount tracking is good enough) for other reasons - for instance

* Less frames to process at the end of the evening
* Make the stars brighter if you are having trouble with aligning the frames

cheers,

Robin
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