Re: Exposure comparisons with CCD cameras


Benoit Schillings
 

Median combined can be an issue for very slight background variation.
How confident are you of the software auto-scaling... I would do a
simple mean for the comparaison, even if you end up with a few hot
pixels

-- benoit

On Tue, Aug 18, 2020 at 6:07 PM W Hilmo <y.groups@hilmo.net> wrote:

“I'm not shooting RGB, I'm shooting NB where the 10 min object signal is at 700 or so out of 65,000.”



I realize that you weren’t shooting RGB, and I know that you can’t do a single 60 minute broad band exposure without saturating the sensor. I am saying that if you were shooting RGB (and if the sensor had sufficient well depth to deal with that), I would expect the two results to be closer to each other. The reason for that is that that the dominant noise would be shot noise from a much higher signal in the background sky. Since you are shooting narrow band, both the signal and shot noise are very small. That makes the read noise relatively more significant.



In terms of S/N, the difference between 1x3600 seconds and 6x600 seconds is the read noise of the camera. You may also be giving up something by using median combine instead of a combination of statistical rejection and mean combine – but the specifics of that are less clear to me.



From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@aol.com> via groups.io
Sent: Tuesday, August 18, 2020 2:53 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Exposure comparisons with CCD cameras





If you were shooting RGB, or if you were using a camera with very low read noise, I’m guessing that the final comparison between the two results would be much closer.

I'm not shooting RGB, I'm shooting NB where the 10 min object signal is at 700 or so out of 65,000. And the faintest detail in the thin wisps next to the brightest part are on the order of 160 in that exposure time. In the 1 hour exposure they are 6 times higher, which is still low compared to the max level before saturation. I could go 10 hours before reaching saturation of the brightest part of the nebula.



I was not trying to compare RGB, only Narrowband for extremely faint objects. I cannot do RGB here due to massive light pollution, so it's a moot point. I'm simply saying that for very faint stuff, long exposure NB has an advantage. It's not only me, but others have shown similar results. The noise in the CCD is the same for both exposures - that is, the noise produced by the camera is high for both images. The download noise may be the limiting factor, so therefore it behooves to increase the exposure time and thus minimize the number of downloads if you want to capture the faintest possible detail.



By the way, I have changed the image posted on AstroBin with some more info.

https://www.astrobin.com/916uf7/C/



Rolando





-----Original Message-----
From: W Hilmo <y.groups@hilmo.net>
To: main@ap-gto.groups.io
Sent: Tue, Aug 18, 2020 4:06 pm
Subject: Re: [ap-gto] Exposure comparisons with CCD cameras

I think that to understand what’s going on, you need to separate read noise from shot noise.



In the single 60 minute image, there is 60 minutes worth of signal, 60 minutes worth of shot noise and 1 instance of read noise. In the 6 x 10 image, there is 60 minutes worth of signal, 60 minutes worth of shot noise and 6 instances of read noise. In theory, I believe that the math suggests that with a zero read noise camera, there would be no difference in S/N between the two final images. If I remember correctly, you are using a camera with a KAF-8300 CCD, which has pretty high read noise. Also, you are imaging a narrow band object, so the signal level in the faint areas is very low.



If you were shooting RGB, or if you were using a camera with very low read noise, I’m guessing that the final comparison between the two results would be much closer.



From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@aol.com> via groups.io
Sent: Tuesday, August 18, 2020 1:56 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Exposure comparisons with CCD cameras



Aha, thank you for the explanation.



Rolando







-----Original Message-----
From: sbasprez via groups.io <beneckerus=aol.com@groups.io>
To: main@ap-gto.groups.io
Sent: Tue, Aug 18, 2020 3:52 pm
Subject: Re: [ap-gto] Exposure comparisons with CCD cameras

The lower background noise floor in the 60 minute image is easily explained mathematically. Noise in a sum of stacked of images is increased by the square root of the number of subs stacked. So summing the stack of 10 subs results in 3.16 time the noise of a single sub. The signal on the other hand adds linearly. So the signal in the summed stack is equal to the 60 minute exposure, but the noise floor in the stack is higher than the 60 minute single frame.

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