Probably the most important factor in getting the best out of our camera is that of maximizing the "signal-to-noise" ratio, S/N. A whole book could be devoted to this topic! It is frequently thought, particularly by those new to CCDs, that because they are more sensitive than film, only short exposures are needed. However, while it is true that an exposure of just a few seconds will produce some results and reveal the target object, there is a world of difference between such an image and one that has maximized the S/N. The former will be heavily speckled and gritty, the latter smooth and with a wealth of subtle detail. The reason is, of course, that all-important signal-to-noise ratio.
The signal part of S/N is the easiest to understand and is simply the number of photons recorded by the photosite or pixel. Noise is not quite as easy to grasp.
Here something called "uncertainty" rears its head. Detecting photons has an inevitable randomness - repeating the identical observation will not produce exactly the same numbers. This unpredictability, which can never be totally removed from a signal, is called noise. Note this subtle definition. An unwanted signal that can be removed is not noise. Dark current is therefore not noise - but the random element embedded in it is! This is a common misconception, which you will often see repeated. Now the good news: If we increase the signal by whatever means, such as a longer single exposure or multiple exposures, then the signal increases faster than the noise. We can fight back.
So what are the causes of noise in our image? We have already seen that just detecting photons has a randomness that creates noise. This noise is proportional to the square root of the signal. This means that as the signal increases the noise only increases as its square root. Therefore long exposures are best. It is hardly a new concept but if you want good images there is no short cut to long exposures. There are other sources of noise, which unfortunately further degrade our image, and we must be aware of and attempt to reduce them too.
The first is a part of the dark current count. Even when the telescope is capped, our CCD camera will produce electrons. This count is produced in proportion to the temperature of the CCD. The lower its temperature the lower the dark count. Remember the definition of noise - it is the unpredictable element that is the noise not the part that can be removed. For dark current the noise is again proportional to the square root of the count. This amount cannot be totally removed and it is therefore noise we must attempt to reduce. That is why astronomical cameras are cooled. Professional cameras are cooled to incredibly low temperatures in an attempt to get this as near zero as possible.
The second source of noise, important for us amateurs whose exposures tend to be short, is readout noise. Every time we read out an image from the CCD, that uncertainty rears its head again producing another source of noise. The good news is that modern cameras with their sophisticated readout software have reduced this considerably compared to several years ago. It is sensible to select a camera with a low readout noise so check those specifications. Because readout noise is not related to pixel size, larger pixels are actually better (because the signal will be higher), but of course we need to avoid undersampling. However, for amateur observers, readout noise is not usually the limiting factor unless very short exposures of faint objects are contemplated. The limiting factor is usually the next source - the curse of modern society!
Sky brightness is the most annoying source of noise. Just like dark current it produces an extra signal and one with an uncertainty. We can subtract an average value but we cannot subtract that uncertainty, which is again proportional to the square root of the signal. It gets worse. Sky brightness eats into the dynamic range of our camera. Each pixel has a maximum limit to its number of counts (full well capacity) and sky brightness added to the signal means that this limit will be reached much sooner. Again we get a commonsense result. Dark skies are really the best. Alternatively we can use filters that reduce sky brightness.
Finally there is image processing noise. It may come as a surprise but taking dark frames, bias frames and flat fields and applying them to our image all introduce noise. I am of course not advocating not taking them! Just be aware that we must take as much care over these frames as those of the actual image. A flat field from a month or two ago or a single dark frame rather than a median of many could actually make things worse. Take many calibration frames and average them to minimize that noise.
In summary, to maximize the S/N in our image:
• Maximize the signal. Longer exposures are an obvious solution but providing readout noise is not the limiting factor, so summing many exposures can be equally beneficial. A higher QE CCD such as a back-illuminated type is also best. Other helping factors are better tracking and focusing to concentrate those photons. A telescope with a bigger aperture or faster focal ratio helps. So does keeping the optics clean.
• Choose a camera with low readout noise. Readout noise is also minimized by longer exposures. If summing many exposures, these must not be so short that readout noise is the limiting factor. Again fast optics helps with this, as does bigger pixels.
• Select a camera with low dark current. Look for good cooling and perhaps the option of water cooling for even lower temperatures.
• Reduce the effect of sky brightness. Observe from a dark sky. If this is not possible use a filter. Look at shielding the telescope from direct light and minimize internal reflections. These don't stop sky brightness but if ignored would make the S/N even worse.
• Select a CCD that has pixels consistent with proper sampling. Remember small pixels can be binned to form larger pixels. This will improve the S/N.
• Take many dark, bias and flat field frames and average them to produce master frames. Regard 10 of each as an absolute minimum but the more the better so why not take 30 each!
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