Starship Asterisk*

Nitpicker wrote:A noob question which may have been asked before ...

Assuming I wanted to create a 10-minute exposure, would there be any technical difference between stacking, say, 20x30-second sub-frames and 5x120-second sub-frames? Obviously, it is more convenient to stack less sub-frames, but easier to track a scope accurately with shorter sub-frames. Would there be any difference in the signal-to-noise ratio, or anything else, if the total exposure time was the same?

geckzilla wrote:I was curious about the zero readout noise sensor and how much one costs but apparently you can't just buy one. It's a thing you have to build yourself, I presume?

Nitpicker wrote:Is there an obvious/practical/easy way to determine the point at which the sky background noise dominates the readout noise? I've never considered readout noise before.

Chris Peterson wrote:

Nitpicker wrote:Is there an obvious/practical/easy way to determine the point at which the sky background noise dominates the readout noise? I've never considered readout noise before.

There are a number of exposure calculators out there, like this one. Google for more. Most describe the math involved, as will any good imaging reference.

Nitpicker wrote:So, the answer is no. :ssmile:

Nitpicker wrote:Thanks Chris. Just dipping my toes in for now. Mainly wanting a sanity check at this stage, to ensure I'm not just wasting my time entirely. Thus far, the best tip I've gleaned from this topic is to take more subexposures.

I still don't really know how to look at a series of test shots and say "the readout noise is dominating the sky glow noise at this point", or if that is even feasible. I'm still struggling to understand what readout noise is exactly, and how one can tell it apart from some of the other noise sources. That was more where my question was directed. Whenever I google information on deep sky photography, I am baffled by jargon and perplexed at the apparent contradictions spread by so many. A classic case of too many cooks. If I'm to dive in to this seriously at some point in the future and understand this subject, I shall need to avoid google and software wizards, and start from first principles. Thanks for the book references.

Chris Peterson wrote:Noise is noise. It's an uncertainty on the signal, and there's no way to separate noise sources in a single image. There are all sorts of tests you can do with your camera to isolate and measure individual noise components, though. They are explained in the above references. For instance, if you take an image with the telescope covered, there will be no sky background at all. The only signal will be from the dark current, and the only noise will be the dark current noise and the readout noise.

Nitpicker wrote:So, by subtracting a dark frame, I'm subtracting the dark current noise and the readout noise. But I assume the readout noise is random from one sub to the next, whereas the dark current noise should have a more constant pattern if generated under the same conditions as the "light" subs. If so, then subtracting a dark frame is not quite as clean an operation as I had previously imagined, especially if my readout noise is dominating my sky glow, as you suspect.

Furthermore, it occurs to me now that by ramping up my ISO settings, as I described above, I could, in fact, be increasing my readout noise faster than my sky glow. I had previously assumed it was all sky glow. [Sound of penny dropping and forehead slapping, fade to noisy black.]

Chris Peterson wrote:You can't subtract noise. In fact, when you subtract a dark frame, you actually increase the noise, since both the light and dark frames contain noise, and that noise adds quadratically. We typically subtract dark frames because removing offsets and fixed pattern artifacts usually improves the image, even at the expense of a little more true noise.

Chris Peterson wrote:Yes. The choice of ISO setting with a DSLR isn't obvious. Astronomical cameras don't typically have adjustable gain (which is what your ISO settings represent). You can frequently get better S/N using a lower ISO setting. Keep in mind that with a properly exposed image, the primary noise component is just the statistical noise on the signal itself, which is the square root of the number of photons you collect. Changing the gain of the amplifier at the output of the sensor obviously has no impact on the number of electrons you collect. Basically, it cranks up the value for both the signal and the noise the same amount.

Ultimately, you improve S/N by collecting more light. That means a larger aperture or a longer exposure.

stephen63 wrote:In response to your original question, adding(stacking) your exposures does not equal the sum of the exposures. It just makes for a cleaner short exposure. What taking a lot of shorter exposures DOES do is increase your SNR. Here is a link explaining just that.
http://www.astropix.com/HTML/J_DIGIT/COMPEXP1.HTM
This website has a lot of good information on DSLR astrophotography.

http://www.astropix.com/HTML/J_DIGIT/COMPEXP1.HTM
The formula for calculating the signal to noise increase when images are composited by averaging (adding the pixels in each image and then dividing by 2) is: n/SQR n... (n divided by the square root of n), where n is the number of negatives to be added.

Averaging two negatives together will increase the signal to noise ratio by 1.414x (2/SQR 2 = 2/1.414 = 1.414). Averaging four negatives together will result in a 2x increase in the signal to noise ratio.

stephen63 wrote:

http://www.astropix.com/HTML/J_DIGIT/COMPEXP1.HTM
The formula for calculating the signal to noise increase when images are composited by averaging (adding the pixels in each image and then dividing by 2) is: n/SQR n... (n divided by the square root of n), where n is the number of negatives to be added.

Averaging two negatives together will increase the signal to noise ratio by 1.414x (2/SQR 2 = 2/1.414 = 1.414). Averaging four negatives together will result in a 2x increase in the signal to noise ratio.

Chris Peterson wrote:In terms of S/N, it is always better to take a single exposure than to break that exposure into shorter subexposures and sum them. There are, of course, good reasons to build an image up from shorter exposures- advantages that often outweigh the reduced S/N that you inevitably get.

stephen63 wrote:

Chris Peterson wrote:In terms of S/N, it is always better to take a single exposure than to break that exposure into shorter subexposures and sum them. There are, of course, good reasons to build an image up from shorter exposures- advantages that often outweigh the reduced S/N that you inevitably get.

I think you have it backwards, Chris. First of all, you don't sum them, you average them. Be it median or mean combining the exposures. Do you normally sum them? That's the whole point of taking multiple exposures! The random noise is reduced by lowering the standard deviation in the final image. The flux of the target in question remains relatively constant so the average remains constant as well. As far as the read noise and dark current, the first step in processing is to calibrate the exposures with the master dark and bias frames. Why are you focused on noise that can be effectively removed with simple procedures? If want to image something that requires a 30 minute exposure to detect, no amount of 30 second exposures are going to capture it!

geckzilla wrote:Coincidentally, I am messing around with four pretty bad exposures from the HLA with low S/N ratio and a crapload of cosmic rays. I'm getting some interesting results by taking four different possible combinations of pairs of them and using minimum which I presume is some kind of floor function and then creating a median stack out of those four combinations. I thought it might be a good way to recover some of the signal loss from using only a minimum stack.

Chris Peterson wrote: In terms of S/N, there is no difference between summing and averaging. It doesn't matter which you use. The average is just the sum multiplied by a constant, which affects the signal and noise equally. Median averaging results in poorer S/N compared with the sum or mean, but has the advantage that you can eliminate certain artifacts, so the S/N hit may be worthwhile.

The reason for taking multiple exposures has nothing to do with S/N. The reason is to get around poor tracking, to provide a mechanism to eliminate cosmic ray hits, airplanes, satellites, and other transient phenomena, to increase dynamic range in cases where a single long exposure would result in saturation. It does so at the cost of reduced S/N.

Calibration with darks and flats removes absolutely no noise, it only adds noise.

In the absence of readout noise, something that requires 30 minutes to "detect" will be equally captured using 60 30-second exposures summed together. But the readout noise that nearly all cameras have means that the actual number of 30-second subs will be much larger- perhaps impractically large.

stephen63 wrote:So, that when summing the images, the signal increases linearly with number of exposures, while the noise increases more slowly, sqrt(n).
Correct?