Summing short exposures or taking long ones ?

[ollypenrice]

Beautiful work! Was this just one 4 minute exposure?

I think it was a minute each of LRGB at F5 with our Tak FSQ106/Atik 11000.

Olly

[lensman57]

Astrostuart's image is lovely but a comet in a starfield doesn't demand much exposure time and the example doesn't scale up to targets needing long exposure. This is my Comet Panstarrs which got a whopping four minutes:

This four minute experience might be misleading if one were trying to image the Integrated FLux Nebula!!

Olly

Perhaps you should have gone for 100 X 4s exposure. Just joking. First class as usual.

A.G

[glowingturnip]

Returning back to the original OP's question though...I don't believe that doing what he is asking is fruitful...

yep, I strongly suspect you're right, after all, if my idea worked, we'd all be doing it already !

I will have an experiment at some point though, find out just how bad the differences are.

What do people think about my sky glow question though, surely sky glow arrives at the same rate as the signal does, so so long as you don't saturate the signal, then there is no different in signal-to-skyglow ratio regardless of exposure time ?

[sharkmelley]

What do people think about my sky glow question though, surely sky glow arrives at the same rate as the signal does, so so long as you don't saturate the signal, then there is no different in signal-to-skyglow ratio regardless of exposure time ?

Yes, in principle that is absolutely true but with the following caveats:

1) As you are already aware, the longer the exposure, the more chance that brighter objects will saturate because their "head room" is reduced by the sky glow. 

2) As you continue to reduce exposure time there comes a point when the dominant source of noise in your images becomes read noise instead of sky glow + thermal noise.

Mark

[dph1nm]

As a DSLR user, is it possible to set an ISO to give a unity gain though ?

Not exactly, but really you just need the gain to be better than this (i.e. >1 ADU per photoelectron) For the Canon's, unity gain is somewhere between ISO400-1600, depends on camera model.

NigelM

[dph1nm]

In this case please conduct an experiment and take 1000 X 1s exposures and then take 10 X 100s exposures and compare the results, if you can get the same amount of detail out of 1000 X 1s exposures as in the 10 x 100s one then I stand to be corrected. I doubt very much that this would be the case.

Unfortunately, in astro work it is very difficult to get enough background noise in 1s to overcome read-noise, so almost inevitably you are going to be read-noise limited and hence less efficient with the 1s exposures.

However, if you have a 'zero' read noise ccd (see e.g. http://www.ast.cam.ac.uk/research/instrumentation.surveys.and.projects/lucky.imaging/lucky.imaging.methods ) then there really will be little or no difference between adopting either of the  two sub lengths.

NigelM