I thought I would put together a quick visual comparison of what different forms of noise look like in a single frame of a deep sky image. My hope here is to give beginner and intermediate imagers who have never looked at a "single-row" or "single-column" noise plot of an image an understanding of what it does look like, and what it means when we say things like "swamp the read noise". These plots were produced with data acquired with an ASI183MM Pro.
There are three plots here, sourced from data that has been calibrated (so no amp glow or hot pixels are represented here), sampled every pixel horizontally across the image (see image inlay for sample row):
DARK RED = Read Noise
DARK GRAY = Dark & Read Noise
BLUE = Photon Shot & Dark & Read Noise
The dark red plot represents the minimum noise of the camera, read noise. This is the baseline read noise, and represents the MINIMUM noise any part of the image can have. You cannot have less noise than read noise in any given sub-exposure no matter what you do. This is an interesting fact when it comes to doing narrow band imaging, where at a dark site, you may have some areas of the frame that have only read noise, even if you manage to expose for an hour or two per sub-exposure. That is actually just fine...what that means is, you are getting the maximum possible contrast! Read Noise == Minimum Possible Noise.
* Note that the vertical median location of the plots here is representative of the total "offset" in the signal. For read noise, that offset represents the bias offset.
The dark gray plot represents the read noise combined with the dark current shot noise. The two are added in quadrature, so mathematically this means SQRT(Ndark^2 + Nread^2). Even with exposures of 5-10 minutes, the total dark signal is very tiny, and represents only a small increase in total noise. In fact, read noise is still the dominant noise term here, indicating that dark current, which with this particular camera is only about 0.002e-/s, so even with a 10 minute exposure, total dark current shot noise is actually less than read noise even at unity gain.
* Note that the offset is only marginally higher than read noise (dark red) in the dark noise (dark gray) plot. That represents the bias offset + about 1 electron of dark signal offset.
The blue plot represents the read and dark noise combined with the photon shot noise of the image signal. All three terms are again added in quadrature, so mathematically this means SQRT(Ssky + Sobj + Ndark^2 + Nread^2). With this particular exposure, read noise was swamped by the photon shot noise by about 5-7x. That is fairly middle ground, and an amount by which I think many beginners and intermediate imagers could achieve. It can be done with 5 to 10 minute exposures, depending on the gain used. You should note a few things about the blue plot. First, it has a noticable increase in even the minimum offset compared to the read and dark noise offsets. That is the combination of bias offset plus skyfog offset (the minimum average offset at the edges of the blue plot) plus the object signal offset (the higher average offsets within the middle areas of the blue plot). I have plotted a 54-period moving average, which is 1/100th the total number of samples, which better represents the actual signal level (offset).
* Note how the blue plot has significantly higher variation (noise) than the other two plots. This larger variation includes both the photon shot noise, as well as the read and dark noise. The read and dark noise are there, but they overall represent a tiny fraction of the unexpected deviations from the mean compared to the photon shot noise. DESPITE the noise here, though, you can actually fairly easily follow the changing height of the plot, and correlate that height with the brightness of the row marked in the inlaid image (from which the plot was sampled). Where the image gets brighter, the blue plot gets higher, and where the image gets darker, the blue plot gets lower.
You can also pick out some of the stars...there are quite a few, and they are usually represented by the large outlier spikes in the blue plot. Some of the stars are barely larger than one standard deviation of the noise in the blue plot, and are harder to pick out from the noise...their SNR is low. Other stars are much brighter than the standard deviation o the noise in the blue plot, and are easy to pick out from the noise...their SNR is high. The average SNR of the entire plot is high enough that you can pick out the relative brightness differences...it is easy to differentiate a bright area from a dark area in the plot, and also easy enough to pick out moderately brighter areas from medium brightness areas. The SNR is good...not ideal, it is a single sub-exposure, but it is good, and the signal is strong enough to clearly represent the structures of the object in the image.
Edited by Jon Rista, 08 November 2018 - 07:01 PM.