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Correct exposure time is dependent on many factors, including the chosen object, telescope aperture, telescope focal length, mount tracking ability, camera type and light pollution. As you would expect, brighter objects require shorter exposures and dimmer objects require longer exposures to capture detail, but also consider your camera’s sensitivity. DSLRs and one-shot-colour cameras are less sensitive than monochrome CCD cameras.
Most deep-sky images are produced from multiple images that have been stacked to produce a single image, as this is a great way of increasing the signal-to-noise ratio in your images. Lots of shorter images stacked together go some way towards matching a single exposure of the same total time, but the single exposure will be deeper and contain more detail. However, the stack of shorter exposures will be less noisy so the aim is to take the longest exposures you can and stack as many as possible to get the best of both worlds.
Light pollution places a ceiling on exposure length as this can produce ‘fogging’, where its orange glow overwhelms the image. It is important to keep exposures below this threshold, which requires some trial and error. For star clusters and globular clusters, 120-180 seconds is a good start. For galaxies and nebulae, 300-600 seconds is a good base if your tracking will allow it. Do remember that these are only rough guides.
Capturing a deep-sky image like this is down to a multitude of factors – light pollution is only one