[Shiraz (Ray)]

Originally Posted by naskies

If you don't accept quantisation noise/errors, then may I ask what your explanation is for the limiting magnitude under a given set of conditions?

It clearly applies, otherwise we could all just take huge numbers of 1 min sky limited exposures in the heart of an urban centre, and get nicely detailed mag 30 galaxies...?

With a sum combine as you suggest, then yes - those 1 electron subs will register a signal. However, with sum combine noise increases linearly with the number of subs (SNR ∝ n) therefore stacking doesn't increase SNR. For mean combine it's SNR ∝ sqrt(n)), hence noise is effectively reduced. For our current cameras where read noise >> 0 e-, sum combine isn't practical beyond a few frames at most.

Anstey included empirical data in his article. I've also done a few experiments myself, though nothing rigorous enough to share publicly.

Hi again dave. As understand it, you reach limiting magnitude when a dim target has a 3x SNR. My system typically produces a few hundred electrons noise per sub from background sky, so an object at the limiting magnitude needs to produce maybe 1000 electrons. This is way above any possible quantisation noise, which really is limited to 1 electron either way - quantisation noise seems to be a non-event at my system limit. We cannot reach very dim targets simply because we have to image through the bright sky and we have a fixed noise background. It just takes too long to integrate much below the nominal sky levels.

Sum combine and mean combine have the same result if implemented in floating point - the only difference is a fixed scaling factor (the sub number). SNR increases with Sqrt(N) in both cases.

Anstey talked about empirical results, but I couldn't find any hard data in his article - might have been me not understanding what he was presenting though.

Originally Posted by SpaceNoob

I just checked my bias for the 8300 both binned 2x2 and unbinned 1x1.

The read noise value does increase in the 2x2 binned bias; however it is by a factor of ~ 2.1 or so. With there being 4 pixels in the single logical pixel, would I be correct in assuming there is an improvement of around half in this case?

Not sure how the sony sensor would look here or if I am heading down a rabbit hole.

From what I have read, seems that all sensors implement a compromise process in binning - you don't get the full gain expected from binning and there is even some argument that software binning after the event may be a better approach. I understand that manufacturers may change the internal gain to keep the output stages from severe overload, so test data may be hard to interpret. I haven't tested the performance of the 694 when binned, except to ensure that it worked OK.

Originally Posted by gregbradley

A lot of this theory is tempered by the reality of imaging.
Clouds, light pollution, poor tracking, bad autoguider performance, flexure, lack of clear nights, lack of time due to work.

So you tend to end up with some sort of subexposure length that optimises both performance of the camera and performance of your tracking in your setup.

Also 40 minute subs sound great if you have the tracking and weather for it. The occasional fast cloud would mean 40 minute subs are unwise.

Poor tracking would make it impractical anyway.

Greg.

Hi Greg - agree, the therory is only ever a starting point, but it is way better than waving one's finger in the wind and hoping - which is more or less what I used to do.

Originally Posted by Placidus

Assuming floating point (not integer) arithmetic, there is no difference in signal to noise ratio between a sum combine and a mean combine.


Suimmary: Dividing by a constant does not change the coefficient of variation. Changing from sum to mean is just a change of scale, like meters to centimeters. It does not change the accuracy of our photo. It does not change the signal to noise ratio.

agree

Originally Posted by Peter.M

This seems counter intuitive to me. If a one second sub is insufficient to overwhelm read noise but a 2 minute sub can, that means that the image signal is increasing at a faster rate than the read noise of the camera. Logically then a longer sub would increase the separation between the read noise and the signal. Obviously this is assuming that the one source of noise is the read noise which is not the case.

Hi Peter, the read noise is a fixed injection at the end of a sub. The signal from the sky rises as the sub length increases and eventually you get to a point where the signal is large enough that the associated shot noise completely swamps the single burst of read noise - there is no point in having longer subs, since read noise has been removed from consideration and the SNR is determined for all practical purposes by the shot noise and you cannot do anything about that. That is the basis for all of the sub length calculators. You could use fewer but longer subs if you wished, but you would not significantly change the SNR in the final combined image.