All images are made up of two things - signals and noise. Signal Basically, the signal comes from the light of the astronomical object that we are shooting. Everything else can pretty much be considered noise. Technically, the above statement is not quite accurate, but it is the way most astrophotographers look at things. Signal is anything that consistently repeats in an image. We can have signal from other things, such as light pollution, that we don't really care about, so we consider it noise. In reality, noise does not repeat, it is random. That is why it is so hard to deal with. After you have dealt with the basics of focus and tracking, making really good astronomical images of deep-sky objects is really all about signals and noise. The relationship between signals and noise, is called the signal-to-noise ratio. This is simply the amount of signal we have compared to the amount of noise we have. The higher the signal-to-noise ratio, the better an image will be. It's that simple. We can't generally do much about the noise because, by its nature, it is random. But we can always improve the image by collecting more signal. With digital cameras, there are two strategies for collecting more signal:
Exposing a single frame for a long time can run into several problems. A single exposure with a lot of valuable photons collected in it, and a lot of time and effort invested, can be ruined by bad tracking, or if an airplane flies through it, or if someone accidentally bumps the telescope, or shines a light into the lens. At high ambient temperatures, the length of a single frame will ultimately be limited by the thermal signal. Long exposures will also start to overexpose the bright parts of the image, and no detail will be recorded there. For these reasons, it is best to shoot a lot of shorter exposures and stack them to equal the signal-to-noise ratio of a longer exposure. If a single short frame is ruined by poor tracking or an airplane, it is not a terrible loss. With single-frame images that you will shoot as a beginner, you will one day reach the point where you run into difficulty when you start shooting fainter objects. Because they are fainter, they put out less photons, and this produces less signal. To improve your images of these fainter objects, you will have to start collecting more signal by shooting multiple frames and stacking them. Shooting faint deep-sky objects under a bright sky also produces a lower signal-to-noise ratio. The signal is the same from the object whether the sky is dark or bright, but the contrast between the object and the sky background is higher at a dark-sky site. This can be considered a kind of signal-to-noise ratio also, with the sky background being the noise. The ways to deal with this problem of a bright sky background is to use a filter, gather more signal, or shoot from a darker location. Noise Every camera has some noise. Camera electronics are not perfect even though the latest DSLR cameras have excellent noise characteristics. Since noise is random, and there isn't much we can do about it, the best way to deal with it is by collecting more signal to improve the signal-to-noise ratio. Paradoxically, even signal has noise associated with it. Because of the quantum nature of light, photons do not arrive with precise regularity. If you take a number of fixed exposures, each will have some variation in the number of photons recorded. There will be some variation and uncertainty involved. This uncertainty is noise. If you don't know for sure if it is signal, then it is noise. This amount of noise is equal to the square root of the signal. So if you record 100 photons, you will have 10 photons of noise since 10 is the square root of 100. This is called Photon Noise. It's not a big deal though for the signal from the deep-sky object that we are interested in because the photon signal will always increase more than the photon noise when we extend the exposure, or stack frames to record more signal. Noise is also introduced in the electron counting process in the camera, and in the conversion of the analog signal to a digital number. There really isn't anything we can do about these sources of noise. However, we can reduce its impact in our final images by increasing the signal and improving the signal-to-noise ratio by stacking frames. The example below provides visual evidence of how stacking multiple exposures improves the signal-to-noise ratio in the image. This is an image of the Veil Nebula, cropped and enlarged 100 percent from the original to make the noise more visible.
Thermal Signal Remember, as we discussed in section 308, Thermal Signal, what appears to be noise from heat in the camera is actually signal. Because it is signal, it repeats in the same place every frame. Therefore, if we use dark frames, we can subtract this thermal signal and remove it from our images, and greatly improve their appearance. We'll discuss this in detail in the section on image calibration.
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