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4 December 2008 17:00
- using a system to automatically correct the position of a telescope's mount to keep the telescope pointed precisely at a given target
- autoguiding systems work by sending signals to the mount to correct the
mount's position when a given star (the "guide star") moves from its
starting position
- autoguiding systems typically consist of a either a separate guidescope and camera, a camera mounted in an off-axis guider
- some imaging cameras have dedicated autoguiding chips built-in
- most systems utilize a camera, software and a computer, but there are some self-contained autoguiders (the SBIG ST-4 being the most well-known)
- cameras used include repurposed imaging cameras (the Meade DSI series, for example), or webcams like the Philips SPC900
- popular free autoguiding software includes PHD, GuideDog, and MetaGuide
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
25 November 2008 23:38
- a specialized type of photography targeting celestial bodies - including the stars, star trails, constellations, galaxies, nebulae, the moon, planets, asteroids, meteorites, comets, and even man-made satellites
- the first astrophotograph was a picture of the moon taken by John William Draper in 1840
- astrophotographs can be taken with film cameras, digital point and shoot cameras, digital SLRs, modified webcams, or dedicated astronomical CCDs
- cameras can be mounted on tripods for wide-field images of the stars (short exposures) or star trails (long-exposures), or mounted on sophisticated tracking mounts to counteract the rotation of the earth, allowing for multiple long exposures of the same spot in the sky
- images can be taken with camera lenses, or through telescopes
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Orion Telescopes
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