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  #41  
Old 17-06-2014, 12:46 PM
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Octane (Humayun)
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Cheers, Ray!

Will do this tonight (provided it's clear!).

H
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  #42  
Old 19-06-2014, 09:52 AM
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Thanks Ray, thats really great, I always just wing it and until something is visible I'll give this a go next time it's clear.

Erik
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  #43  
Old 08-08-2014, 05:54 PM
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Thanks Guys.

Rick just pointed out that Stan Moore derived essentially the same equation in:
http://www.stanmooreastro.com/eXtreme.htm

Looks like I didn't invent anything after all . I suppose one consolation is that I seem to have got it right at least.

Apologies to Stan.

regards Ray
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  #44  
Old 08-08-2014, 05:58 PM
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multiweb (Marc)
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We'll call you Stan from now on.
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  #45  
Old 09-08-2014, 07:19 AM
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I've been mulling over this for a time, and maybe this is not correct.

Is read noise fixed or or random? I suspect the latter, and in fact this is exactly what "skipper CCD''s" utilise by being read-up hundreds of times to get read noise to sub electron levels.

Further, if read noise were fixed, you'd see a gradual pattern build up after summing multiple frames. To date, I've not seen that happen even after summing dozens if frames.

Assuming then read noise is indeed random, would not the noise reduce by the square root of the number of subs.
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  #46  
Old 09-08-2014, 07:40 AM
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Quote:
Originally Posted by Peter Ward View Post
I've been mulling over this for a time, and maybe this is not correct.

Is read noise fixed or or random? I suspect the latter, and in fact this is exactly what "skipper CCD''s" utilise by being read-up hundreds of times to get read noise to sub electron levels.

Further, if read noise were fixed, you'd see a gradual pattern build up after summing multiple frames. To date, I've not seen that happen even after summing dozens if frames.

Assuming then read noise is indeed random, would not the noise reduce by the square root of the number of subs.
I don't understand what bit you find to be incorrect Peter.

I agree with your statements - read noise is indeed random and the power increases with the square root of the number of reads (ie it reduces by the square root if you take an average). There is also no fixed pattern to read noise with a CCD. However, the equation is based on the noise from a single read event, so, as far as I can see, none of the above applies.

The method is designed to provide a single goal ADU value that you can use to set sub exposure length. If you use it, the read noise contribution in the final image will be less than than 5% of the total noise (ie you bury the read noise under the shot noise). The underlying maths is a recast of that used in the various on-line calculators, but it provides a very quick and universal method for checking exposures on the fly. When you have determined the goal ADU for your camera (once only), you can use it to set subs with any combination of scope, filter etc.

Last edited by Shiraz; 09-08-2014 at 08:25 AM.
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  #47  
Old 09-08-2014, 07:41 AM
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Quote:
Originally Posted by Shiraz View Post
Thanks Guys.

Rick just pointed out that Stan Moore derived essentially the same equation in:
http://www.stanmooreastro.com/eXtreme.htm

Looks like I didn't invent anything after all . I suppose one consolation is that I seem to have got it right at least.

Apologies to Stan.

regards Ray
I'd be happy to call it an independent discovery, so still well done, Ray!

Quote:
Originally Posted by Peter Ward View Post
I've been mulling over this for a time, and maybe this is not correct.

Is read noise fixed or or random? I suspect the latter, and in fact this is exactly what "skipper CCD''s" utilise by being read-up hundreds of times to get read noise to sub electron levels.

Further, if read noise were fixed, you'd see a gradual pattern build up after summing multiple frames. To date, I've not seen that happen even after summing dozens if frames.

Assuming then read noise is indeed random, would not the noise reduce by the square root of the number of subs.
Yes, read noise is random (if it wasn't then it would be unwanted signal, not noise) and its effect on SNR reduces by stacking. However, the whole point of the Stan-Ray rule of thumb is that you can make the effect of read noise inconsequential by making your subs long enough. Then you are only at the mercy of the noise sources you can't control like shot noise from the target and shot noise from the sky glow.

Cheers,
Rick.
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  #48  
Old 09-08-2014, 09:29 AM
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Quote:
Originally Posted by Shiraz View Post

..
The method is designed to provide a single goal ADU value that you can use to set sub exposure length. If you use it, the read noise contribution in the final image will be less than than 5% of the total noise (ie you bury the read noise under the shot noise). ..
Yes, I get that you are simply looking for a sky limited exposure time. I was simply pondering, given read noise is random, it's effects disappear with more and more read-ups, or more and more subs, with say, everything else being equal.

P.s. "Correct" was a bad choice of words, I was more concerned about how multiple read ups also improve S/N even though one might be exposing for less than a sky limited time.

Last edited by Peter Ward; 09-08-2014 at 09:52 AM.
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  #49  
Old 09-08-2014, 11:45 AM
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I just ran the numbers on some red data for NGC1097 taken under darkish skies. I was using 15 minute subs which were still nowhere near sky limited (I'd need to do about 1 hour subs to get all the way there...)

Comparing the SNR for a 15 min sub vs a stack of 3 x 5 mins the SNR is 4% better for the bright galaxy core, 20% better for a bright/non-core galaxy area and 34% better for a dimmer area on the outskirts of the galaxy.

To get the same SNR as a 15 minute exposure in the dim part of the galaxy requires stacking more than 5 x 5 minute subs. For the bright/non-core area you need to stack more than 4 x 5 minute subs to get the same SNR. For the galaxy core, a stack of 4 x 5 minute subs has better SNR than the 15 minute sub.

Just a random example, but I think it shows that read noise can have a very significant effect on SNR. It also demonstrates nicely that this effect is much greater for dim targets.

Cheers,
Rick.
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  #50  
Old 16-12-2014, 10:18 AM
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There's now a PI script that does the calculation that Ray described in this thread: http://pixinsight.com/forum/index.php?topic=7877.0

Cheers,
Rick.
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  #51  
Old 13-11-2016, 06:11 PM
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Quote:
Originally Posted by RickS View Post
I just ran the numbers on some red data for NGC1097 taken under darkish skies. I was using 15 minute subs which were still nowhere near sky limited (I'd need to do about 1 hour subs to get all the way there...)

Comparing the SNR for a 15 min sub vs a stack of 3 x 5 mins the SNR is 4% better for the bright galaxy core, 20% better for a bright/non-core galaxy area and 34% better for a dimmer area on the outskirts of the galaxy.

To get the same SNR as a 15 minute exposure in the dim part of the galaxy requires stacking more than 5 x 5 minute subs. For the bright/non-core area you need to stack more than 4 x 5 minute subs to get the same SNR. For the galaxy core, a stack of 4 x 5 minute subs has better SNR than the 15 minute sub.

Just a random example, but I think it shows that read noise can have a very significant effect on SNR. It also demonstrates nicely that this effect is much greater for dim targets.

Cheers,
Rick.
Good post. Thanks for that Rick. I probably should go for longer subexposure legnths. I use mostly 10 minutes but if tracking is behaving I sometimes go for 20minutes. I could go longer though as both my mounts usually work very well. Occassionally minor gremlins seem to interfere but overall they probably could go one hour.
Although a 1 hour exposure is likely to get fatter stars just from the average of PE in different directions. Perhaps mixing in some shorter exposures for stars or not worth the trouble?

Well depth may be the main limitation here.

Greg.
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  #52  
Old 13-11-2016, 08:05 PM
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It's very easy to check Greg.

Assuming slow readout, just look at the sky background in one of your subs - (lum, RGB, Ha, whatever and on any of your scopes) - and if it is much less than 600ADU + biasADU then you could use longer subs. If the sky is much greater than 600ADU + biasADU, then you could reduce the sub length without noticeable penalty.
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  #53  
Old 14-11-2016, 08:21 AM
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Merlin66 (Ken)
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Rick,
Just interested...
What were the actual SNR you were getting?
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  #54  
Old 14-11-2016, 08:46 AM
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Quote:
Originally Posted by Merlin66 View Post
Rick,
Just interested...
What were the actual SNR you were getting?
Ken, I'll have a rummage around when I get home and see if I can find my calculations, but it was three months ago...
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  #55  
Old 14-11-2016, 10:16 AM
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It might be worth noting that the factor of 10 in the equation isn't necessarily fixed - it represents a trade-off.

The idea is to choose a factor so that sky noise dominates the read noise.

For narrowband, I'd often use a factor less than 10, as little as 2-3 to keep sub lengths shorter.

I think that using factors less than 10 is still valid, it just means that the sky noise doesn't "overpower" the read noise as much.

Using a factor of 10 is good because the total noise is only increased by 5% by the read noise contribution.

The same calcs show that using a factor of 5 would give a 10% increase, x2.5 gives 20% and so on.

My thinking is that using smaller factors can be compensated for by adding extra subs (to achieve the same integrated SNR).

Hence, if you need an ideal 100 total minutes when using long exposure lengths, the following shows how many minutes are required using each factor:
x10 -> Needs 110 mins in total
x5 -> Needs 120 mins
x2.5 -> Needs 140 mins

I've found that sometimes it's better to aim for more shorter exposures that fewer longer exposures (more of which sometimes get rejected).
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  #56  
Old 17-11-2016, 08:06 AM
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graham.hobart (Graham stevens)
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sub exposure times

Very good post folks.
One basic query.
I have Maxim DL 5 and it measures sub exposure as a value of I:=n
Is this directly related to ADU?
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