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3 December 2008 18:12
- one in a series of shots taken of an astronomical target in digital astrophotography
- sometimes "sub exposure" or most often just "sub"
- long exposures are needed to capture faint deep space objects, but practical limitations of extremely long exposures (e.g., tracking error, camera sensor noise, light pollution, wives with flashlights, and airplanes) are overcome by capturing multiple shorter subexposures
- subexposures are typically combined (or "stacked") by purpose-written software (such as DeepSkyStacker, ImagesPlus, or Nebulosity)
- adding subs increases the signal-to-noise ratio resulting in images more free of noise than otherwise possible (noise is inherent in digital astrophotography)
- often denoted in the format AxB where A is the number of subs taken, and B is the time of each sub, e.g., 30x120s means 30 subs of 120 seconds each
- stacking software will align the stars in each sub to minimize or eliminate the effect of tracking errors and field rotation in a process called registration
- "lights" or "light subs" are subexposures of the actual target - other types of exposures taken in astrophotography include flats, darks, dark flats, and bias frames
- finished images are often combinations of multiple stacks of subs, with each sub in a given stack being the same length; for example, M42 (the Great Nebula in Orion) is often a combination of stacks of short subs to capture the stars of the Trapezium, and longer stacks to capture the faint outlying nebulosity
- not to be confused with exposure, which is a dangerous medical condition resulting from lack of protection over prolonged periods to extreme temperatures - like those faced by astrophotographers out all night taking lots of subs
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Orion Telescopes
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