#41  
Old 28-09-2015, 11:37 AM
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Slawomir (Suavi)
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I had a look at that calculator about 6 months ago, I'd never seen the recommended number of exposures change from 27. I was just putting in random numbers however as I didn't have a CCD camera at that stage.
It does change, but you need to update the numbers in the SNR section.
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  #42  
Old 06-10-2015, 11:57 AM
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So I ran the aforementioned PI script on some new images. Turns out I need 135 mins of Ha, 38mins of B to be "sky limited". The curse of dark skies I guess...
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  #43  
Old 06-10-2015, 12:00 PM
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A blessing and a curse
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Old 10-10-2015, 05:48 PM
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Atmos (Colin)
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Apparently my imaging time for Luminance without a moon is ~1m... Not very long at all
I think I need to head up to my rural property and test it up there!
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Old 11-10-2015, 10:27 AM
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Hi Colin,

To my understanding, the darker the skies, the cleaner the camera and the brighter the object the shorter the subs to get a good SNR. And inversely, with light polluted skies and with a noisy camera and when imaging a dim object exposures need to be much longer to achieve the same SNR. Or am I missing something obvious?

So the optimum exposure times should vary for different DSOs even for the same gear.
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Old 11-10-2015, 04:52 PM
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Originally Posted by Slawomir View Post
To my understanding, the darker the skies, the cleaner the camera and the brighter the object the shorter the subs to get a good SNR. And inversely, with light polluted skies and with a noisy camera and when imaging a dim object exposures need to be much longer to achieve the same SNR. Or am I missing something obvious?
Suavi,

To restate what you said more correctly: the darker the skies, the cleaner the camera and the brighter the object the shorter the total exposure time needed to get good SNR.

The effect of dark skies on sub length is the tricky bit. Because they are dark there is little sky glow and hence little sky glow noise (which is the sqrt of the number of photons detected). That means that read noise makes a much bigger contribution to the total noise and long subs are needed to overcome this. This seems counter intuitive but it makes sense if you think about it.

Note that the SNR and quality of a sky limited sub taken under dark skies will be much, much better than a sky limited sub taken under bright skies. The latter sub will contain a lot of noise associated with the sky glow (as well as the unwanted sky glow signal itself.) Despite needing long subs under dark skies, the total exposure time required to get acceptable overall SNR will be much shorter.

Cheers,
Rick.
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Old 11-10-2015, 05:10 PM
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With my Sky Limited being 60-80s in Luminance, is it worth while pushing that out to 600s just to shift the histogram further to the right?

I personally am finding that 600s exposures are easier to work with than 200s, even though I am getting like 12,000 ADU backgrounds.
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  #48  
Old 11-10-2015, 05:33 PM
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Suavi,

To restate what you said more correctly: the darker the skies, the cleaner the camera and the brighter the object the shorter the total exposure time needed to get good SNR.

The effect of dark skies on sub length is the tricky bit. Because they are dark there is little sky glow and hence little sky glow noise (which is the sqrt of the number of photons detected). That means that read noise makes a much bigger contribution to the total noise and long subs are needed to overcome this. This seems counter intuitive but it makes sense if you think about it.

Note that the SNR and quality of a sky limited sub taken under dark skies will be much, much better than a sky limited sub taken under bright skies. The latter sub will contain a lot of noise associated with the sky glow (as well as the unwanted sky glow signal itself.) Despite needing long subs under dark skies, the total exposure time required to get acceptable overall SNR will be much shorter.

Cheers,
Rick.
Thank you Rick for your clarification and disposing of my confusion, it makes perfect sense now
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Old 11-10-2015, 09:02 PM
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With my Sky Limited being 60-80s in Luminance, is it worth while pushing that out to 600s just to shift the histogram further to the right?

I personally am finding that 600s exposures are easier to work with than 200s, even though I am getting like 12,000 ADU backgrounds.
You lose a bit of dynamic range and run a larger risk of losing a sub due to cloud or guiding error, but there's no major downside to going longer unless you start running out of well depth for details you want to keep.

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Thank you Rick for your clarification and disposing of my confusion, it makes perfect sense now
Glad I could help, Suavi. It took me a while to make sense of it too.
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