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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
2 December 2008 16:44
- when one celestial object completely hides another celestial object from the point of view of an observer
- from the Latin occultus (clandestine, hidden, secret), which
doesn't help matters when amateur astronomers have to explain
(repeatedly) that our hobby is not astrology
- contrast with transit, where a celestial object moves in front of another object, yet both remain visible
- related to an eclipse, where a celestial object moves into the shadow of another celestial object - pedants will point out that a solar eclipse is actually an occultation of the sun by the moon, and a partial eclipse of the earth by the moon's shadow
- the size and shape of asteroids can be determined by accurately timing the occultation of a given star from many vantage points
- timing lunar-grazing occultations can help refine information about the topography of the moon (among other things) - stars will appear to wink on and off as lunar features pass in front of them
- the International Occultation Timing Association (www.occultations.org or www.lunar-occultations.com/iota/iotandx.htm) provides information about and conducts research related to occultations; they encourage participation from amateur astronomers
1 December 2008 15:21
- the part of a telescope (or binoculars) into which you (typically) look
- a.k.a. "ocular" or (less commonly) "ocular lens;" often abbreviated EP
- telescope eyepieces are typically removable, binocular eyepieces typically are not
- many telescopes (refractors, Newtonian reflectors) achieve focus by moving the eyepiece by means of a mechanical focuser forwards or backwards in the light cone near the focal point of the telescope's objective
- on some binoculars, both eyepieces are focusable via helical focusers, on others only one eyepiece can be adjusted (a diopter adjustment)
- common eyepiece designs include: Plössl, orthoscopic (Abbe), Kellner, and Erfle
- some eyepiece designs are patented and/or the name is trademarked, e.g., TeleVue's Radian, Panoptic, Nagler, and Ethos lines
- the focal length (FL) of the eyepiece determines the magnification of the view through a given telescope; the lower the eyepiece FL, the higher the magnification: magnification = telescope's FL / eyepiece's FL
- for better viewing positions, eyepieces are generally placed in diagonals on cadioptric and refractor telescopes
- common barrel sizes for eyepieces are 1.25 inches and 2 inches. Less common (and typically now only found on lower quality scopes) are 0.965 inch barrels.
- some eyepiece barrels have "safety cutouts" so set screws will hold them more securely; they can be a liability for people who have accessories (diagonals/focusers/visual backs) with compression rings as the cutouts sometimes "hang" on the compression rings
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Orion Telescopes
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