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Background Photo: 
Open Star Cluster M41
in Canis Major

 

 

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Glossary of Terms
   Found on this website


A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Addition Combination - method of assembling a collection of image files whereby the pixel values of the same pixel location in each file are mathematically added together, increasing the overall brightness of the single resulting image file.

Asterism - a grouping of stars, not recognized as a whole constellation, that still appear to form some recognizable object.  Well-known asterisms include the Big Dipper (in the constellation Ursa Major), the Little Dipper (in the constellation Ursa Minor), the Northern Cross (part of the constellation Cygnus), and the Summer Triangle (three very widely separated stars that span several constellations).

Aurora - also known  as the Northern Lights (or Aurora Borealis), or the Southern Lights (or Aurora Australis).  An aurora consists of light generated by electromagnetic excitation of certain molecules in the earth's upper atmosphere.  The colors within an aurora are  indicative of different elemental molecules.  For instance, the red color in an aurora is the result of the excitation of oxygen or nitrogen molecules, while green is usually seen when oxygen molecules are excited.  Auroras are common at the earth's polar regions and occasionally reach the middle latitudes.  Auroras are caused chiefly by electromagnetic effects of the sun (such as solar winds), and are particularly common (and more likely to reach middle latitudes) during peak sunspot cycles.

Autoguider - a specialized auxiliary camera that is used to cause fine adjustments to be made in a telescope's tracking motors during long-exposure astrophotography.  By doing so, the intent is to keep the image being captured by the main camera in exactly the same location for the duration of the main camera's exposure, thereby ensuring the sharpest and roundest possible stars in the resulting image being exposed simultaneously by the main camera.  The autoguider is optically coupled to the telescope in one of several configurations, taking the place of an eyepiece, where it reads brightness of stars within its image frame when it is properly focused.  Via software, the autoguider maintains a row-column pixel  or pixel array location of a specific star on the image sensor, and sends fine adjustment commands to the telescope (via a special electronic connection - a cable) in order to quickly return that star to the same pixel or pixel array if it should drift away from that pixel or pixel array location.

Average Combination - method of assembling a collection of image files whereby the pixel values of the same pixel location in each file are mathematically averaged together, thereby reducing the overall effect of extreme brightness values in any single pixel in any one image file, in the single resulting image file.  This technique is used to reduce random pixel "noise" (non-image data) occurring in any single image file while improving the overall strength of the image data across all of the image files, at the same pixel locations.


B

Barred-Spiral Galaxy - a galaxy that has a definite spiral shape, with spiral arms radiating not from the galaxy's center but from the ends of an elongated "bar" that passes through the center of the galaxy.  Classic examples of barred-spiral galaxies include M83 and M95.

Bias Frame - data recorded in special image files for the purpose of eliminating one of two types of fixed-pattern noise in a "raw" image file of an astronomical object.  The Bias Frame is typically an averaged combination of several image captures,  All of these captures are made with the light path tightly capped, and at the very shortest exposure length possible (for example, 1/8000), but also made at the same ISO setting as the original "raw" astronomical image and with the same environmental conditions.  The result is a nearly-black image, except for the electronic "noise".  These exposures are typically made at approximately the same time as the original "raw" astronomical image data is captured, in order to best match the environmental conditions such as temperature and humidity.  The Bias Frame that results from averaging several frames thus captured will contain a good representation of the electronic "noise" (visually represented by hot pixels and / or speckles) that the camera's image sensor inherently contains even with the shortest exposure length.  This "noise" is typically fixed in pattern and will therefore appear the same in every image captured with that camera and with the environmental conditions at the time of capture.  Once captured, this noise can be removed via software such as ImagesPlus from the original astronomical image files in order to improve the quality of those images.  See also Calibration, Dark Frame, and Flat Frame.

 

C

Calibration - the process of adjusting the "raw" image data recorded in an astronomical image file, in order to reduce or eliminate electronic "noise", as well as uneven illumination and other optical imperfections across the frame of that astronomical image that originate in the optical path itself.  Image calibration is performed by applying, via software such as ImagesPlus, special image data that is captured for this purpose.  This special data typically includes bias frames, dark frames, and flat frames.  By applying bias and dark frames to the original "raw" image data, fixed-pattern electronic noise can be removed from that original image.  The purpose of the flat frame is to record the exact optical-path imperfections and with the same software, the original image can be further adjusted to reduce or eliminate these imperfections by applying the flat frame.  See also Bias Frame, Dark Frame, and Flat Frame.

Cluster - see "star cluster".

Combination - by using many exposures of the same object, properly aligned on the same points (typically stars), it is possible to reduce electronic "noise" that occurs in each image file.  Since much of this "noise" is random in nature, combining several image files using any of several pixel-location combination methods (such as average or median) will have the effect of improving the signal-to-noise ratio (SNR) of the image's pixel values by emphasizing the "true" pixel values of the real image data, while de-emphasizing the random pixel values associated with non-image data that could occur in any one image file.  The SNR rapidly improves with quantities of image files up to about 10 or 12, and after that point the improvement, which still occurs, rapidly declines.

Comet - a comet is a relatively small (1km or so at most) ball of dirt, rock, and ice that orbits the sun in a highly elliptical orbit.  Most comets have orbital periods that are hundreds or thousands of years long, although some comets have much shorter orbits.  When a comet begins to approach the sun more closely (due to the elliptical nature of its orbit, comets spend most of their orbits far away from the sun), the solar winds will cause the ice to vaporize and form a tail for which comets are known.  Some tails can extend many thousands or millions of miles out from the comet's head, or nucleus.  A comet's tail always points away from the sun due to the solar winds that form it, and so sometimes the comet's tail is trailing out behind it (before it reaches its closest point to the sun), and then later (as and after it passes around the sun) it will actually be preceding the comet's nucleus.  Every year, several comets are discovered, although the vast majority are small, faint, and unimpressive even in a telescope.  However, every few years a comet will approach the sun and become bright enough that it is visible to the naked eye and often quite spectacular when viewed through binoculars.  Most of these brighter comets will also display a tail, also visible to the naked eye.  The most famous comet is Comet Halley, which has an orbital period of about 76 years and made its last close pass to the sun in 1986.  Another famous comet is Comet Shoemaker-Levy 9, which broke into pieces and plunged into Jupiter's atmosphere spectacularly in 1994.

 

D

Dark Frame - data recorded in special image files for the purpose of eliminating one of two types of fixed-pattern noise in a "raw" image file of an astronomical object.  The Dark Frame is typically an averaged combination of several image captures,  All of these captures are made with the light path tightly capped, and at the exact same exposure length (for example, 600 seconds), at the same ISO setting, and with the same environmental conditions as the original "raw" astronomical image .  The result is a nearly-black image, except for the electronic "noise".  These exposures are typically made at approximately the same time as the original "raw" astronomical image data is captured, in order to best match the environmental conditions such as temperature and humidity.  The Dark Frame that results from averaging several frames thus captured will contain a good representation of the electronic "noise" (visually represented by hot pixels and / or speckles) that the camera's image sensor contains at that exposure length.  This "noise" is typically fixed in pattern (but will vary by exposure length, ISO setting, and ambient temperature) and will therefore appear the same in every image captured with that camera at that exposure length and ISO setting, and with the same environmental conditions at the time of capture.  Once captured, this noise can be removed via software such as ImagesPlus from the original astronomical image files in order to improve the quality of those images.  See also Calibration, Bias Frame, and Flat Frame.

Dark Nebula - a cloud of dust and gas in space that neither emits its own light nor reflects light from a nearby light source like a star.  Also called an "absorption nebula" .  As a result, dark nebulae often appear as voids in star fields because they block the light from stars behind them in the star field.  Classic examples of dark nebulae include Barnard 93, which blocks light from a part of the Sagittarius Star Cloud (M24).  See also, Nebula.

DSI, DSI II - Deep-Sky Imager, made by Meade Corporation.  This astronomical camera can be used as a basic , non-cooled imager or as an autoguider.  The original DSI was produced in a color-image model only, with a better-performing monochrome version added shortly afterwards.  Later on came the DSI II, in both color and monochrome versions, sporting better sensitivity, much lower electronic noise on their image sensors, and slightly larger image sensors.  The DSI or DSI II, coupled with appropriate software such as Envisage, makes a very capable and easy-to-use autoguider, although their primary intent is to function as an inexpensive astronomical imaging camera.

DSLR - Digital Single-Lens Reflex camera.  The digital equivalent of the 35mm film camera.  Known as "single-lens reflex" because the viewfinder and image sensor both use light that comes through the same lens, although not at the same time.  For this reason, a DSLR has a mirror to direct the light from the lens to the appropriate location within the camera based on its mode at any given time.  That mirror flips up out of the way of the image sensor when an image is being exposed there, blocking light to the viewfinder, while the mirror flips back down when no image is being exposed, directing light to the viewfinder instead for image focusing and image composition.

Double Star - a double star is a pair of stars that are gravitationally bound together and orbit around a common center of gravity.  In situations where one of the two stars is far larger than the other, that center of gravity is typically at or near the center of the larger star.  However, some double star pairs that are more close in size to one another may actually revolve around a point in space between them.  Some star pairs are actually not related to one another and in fact are not even close together, but appear to be double stars simply due to our perspective upon them.   Well-known double stars include Alberio (in Cygnus), Rigel (in Orion), Polaris (in Ursa Minor and also called the North Star), and the famous "Double Double", Epsilon Lyrae (in Lyra), which is actually a pair (double) of double stars.

 

E

Edge-On Galaxy -  a galaxy that, viewed from earth, appears long and thin because from our perspective only the side, or edge of the galaxy is visible.  Edge-on galaxies often display dark dust lanes across their mid-sections.  Classic examples of edge-on galaxies include NGC4565 and NGC891.

Elliptical Galaxy - a galaxy that appears as a diffuse, featureless ball or oval, mostly or totally devoid of dust lanes or spiral arms.  Classic examples of elliptical galaxies are M84 and M86.

Emission Nebula - an emission nebula is a cloud of dust and gas that generates ("emits") light due to the atomic reactions taking place within it.  Classic examples of emission nebulae include NGC1499 (The California Nebula) and M8 (The Lagoon Nebula).  See also, Nebula

Envisage - the software bundled with the Meade DSI / DSI II series of low-cost imagers.  This software makes it possible to operate the DSI or DSI II camera to capture astronomical images.  It also includes the capability to drive the camera as an autoguider, including direct telescope control. 

 

F

Flat Frame - data recorded in special image files for the purpose of eliminating optical-path defects in a "raw" image file of an astronomical object.  These optical-path imperfections include dust specks (usually out-of-focus and often as a result quite large but also softly diffused) as well as unevenly-illuminated areas of the image frame (typically the corners and maybe the edges), also called "vignetting".  Vignetting occurs normally in telescopic optical paths and is an artifact of the way the light cone is architected to reach the camera's image  sensor.  Extreme vignetting, if not removed from the "raw" original images, will appear as dark corners and edges, which can make an image appear unnatural.  The Flat Frame is typically an averaged combination of several image captures.  All of these captures are made at the same ISO setting, and with the same environmental conditions as the original "raw" astronomical image .  The other critical element in capturing successful Flat Frames is that the original optical path must be undisturbed, meaning that the camera must remain exactly as it was during the capture of the "raw" original astronomical image and the focus must remain precisely as it was at that time as well.  Unlike Dark Frames and Bias Frames, however, the Flat Frame is captured with an artificial light source illuminating the optical path with extremely even, or "flat" light.  This is often accomplished by placing a specially-constructed lightweight box, with a light-diffusing panel on one end and soft, indirect lighting inside, over the end of the telescope.   Exposure lengths vary by ISO and by optical path and are timed to provide the best exposure data, neither overly bright nor overly dark.  The resulting "flat field" image thus captured is a faithful representation of the imperfections in the optical path at the time that the "raw" original astronomical image's was recorded.  Once captured, these optical-path imperfections can be removed via software such as ImagesPlus from the original astronomical image files in order to improve the quality of those images.  See also Calibration, Dark Frame, and Bias Frame.

Face-On Galaxy - a galaxy that, viewed from earth, appears wide and round (or oval) because from our perspective, the galaxy is being seen from directly above or below, and as a result is showing the maximum dimensions possible.  Classic examples of face-on galaxies include M51, M33, and M101.

 

G

Galaxy - a large system of stars, star clusters, and nebulae that are gravitationally bound together.  Where visible at all, the individual stars, star clusters, and nebulae within galaxies are tiny, while the stars, star clusters and nebulae that are generally visible in the night sky from earth (which are part of our own Milky Way Galaxy) are far larger.   An appropriate analogy would be to consider the view through a window after a rain shower, looking across a field at a distant house.  In this analogy, consider that the distant house represents another galaxy, while the house from which the view is had would be the Milky Way, and the raindrops left on the window from the shower represent all the stars, star clusters, and nebulae in our own galaxy that would be seen when looking in the direction of the other, distant galaxy.  In other words, we must look through the stars and nebulae from our own Milky Way Galaxy that are readily visible in the night sky, in order to view other galaxies.  Those other galaxies have their own stars and nebulae, but because the galaxies that contain them are so distant, they appear as a tiny part of those galaxies.   Galaxies are some of the most distant objects from us.

Globular Star Cluster - (See also, Star Cluster.)  A Globular star cluster is a collection of many (tens of thousands to millions) stars that are gravitationally bound together, arranged in a very dense sphere, or "globe" structure.  Typical globular star clusters include M13, M92, M22, and NGC5128 (Omega Centauri).

Guided - term referring to long-exposure photography of astronomical subjects taken through a telescope or other optical setup that is not only tracking the earth's rotation but is also subjected to continuous, fine adjustments to that tracking.  The classic, old-school way of guiding a long-exposure photograph is manually, where a person monitors a star in a special eyepiece with illuminated cross-hairs displayed, and makes manual adjustments to the telescope's tracking based on the star's drift, in order to keep the star as precisely in the same exact place in that eyepiece (typically at the center of the cross-hairs).  The more modern (and far less fatiguing!) manner of guiding long-exposure photography is with an autoguider, which is a smaller, auxiliary camera that replaces the eyepiece and the person.  With an autoguider, a star is monitored anyplace on that guider's image sensor, and through software, fine adjustments are sent to the telescope's tracking drive.  The entire purpose behind guiding long-exposure photography is that by keeping this "guide star" precisely positioned for the duration of the exposure, the resulting image captured by the main camera will be sharp, with perfectly round, pinpoint stars.  See also, Autoguider.

 

I

ImagesPlus - astronomical image-capture and image-manipulation software written by Mike Unsold, particularly for use with modern DSLR cameras.  ImagesPlus can control the camera directly for real-time capture of "raw" astronomical image data, as well as Dark Frames, Bias Frames, and Flat Frames.  This software further facilitates the post-capture Calibration of all of these different image frames in order to produce the best, cleanest astronomical image.  ImagesPlus also provides several methods of combining a collection of identical, fully Calibrated astronomical images, such as Average,  Median, and Maximum, in order to produce one astronomical image with the best signal-to-noise ratio.  Finally, ImagesPlus provides functionality for basic and advanced image processing so that these images can be completely processed, from capture to completion, within one software package if so desired, as opposed to using different image-processing software (for example, Photoshop) for final image processing once an image has been captured, Calibrated, and combined in ImagesPlus.

Irregular Galaxy - a galaxy that displays no definite structure while still containing obvious dust lanes or other characteristics.  Classic examples of irregular galaxies include M82 and NGC5128.  Irregular galaxies differ from elliptical galaxies, which also have very little defined structure, in that elliptical galaxies are obvious but diffuse, featureless balls or ellipses, while irregular galaxies typically have some obvious feature like dust lanes.

 

M

Maximum Combination - method of assembling a collection of image files whereby the pixel values of the same pixel location in each file are examined for the highest value in the image file set, which then becomes the pixel value for that pixel location in the single resulting image file.  This technique makes it possible for an image set of wide-field, short exposures of the night sky to be assembled into a photo of star trails. 

Median Combination - method of assembling a collection of image files whereby the pixel values of the same pixel location in each file are mathematically averaged together with a median, thereby reducing the overall effect of extreme brightness values in any single pixel in any one image file, and entirely eliminating outliers, in the single resulting image file.  This technique makes it possible to include an otherwise unusable image that has an airplane or satellite streak in it, without ruining the single resulting image.

Meteor - a brief, visible streak of light in the sky caused by a (typically small) piece of debris burning up in the earth's atmosphere.  Most of the meteors seen in the night sky are at most a few millimeters in size and the vast majority burn up in the earth's upper atmosphere before ever reaching the earth's surface. Before a meteor strikes the earth's atmosphere, it is known as a meteoroid, and meteoroids are typically stony or metallic (or both) in compostion.   If the meteor does not actually burn up and strikes the earth, it is known as a meteorite.  Many meteors are the result of random debris striking the earth's atmosphere as the earth revolves around the sun, but many others are caused by debris left behind by comets as they approach the sun and leave debris fields in the path of the earth's orbit.  These meteors tend to occur in peaks occurring at the same times every year, known as "meteor showers".  There are several meteor showers that occur at the same times each year.  One of the most impressive and predictable meteor showers is the Perseids meteor shower, which builds in frequency over several days in early August each year, reaching a peak around August 12, then rapidly falling off in frequency over the next few days.  At its peak, the Perseids meteor shower can display almost 100 meteors in an hour.  Most meteor showers are best visible in the hours after midnight due to the angle at which the earth encounters the debris field.  Meteor showers are named for the constellation in which the meteors appear to originate, or "radiate".  For example, the Leonids meteor shower in November each year is named for the constellation Leo, because meteors associated with the Leonids meteor shower all appear to radiate from a spot near the "head" of Leo, the Lion.  Although meteors associated with a named meteor shower could easily be seen in any part of the sky, traveling in any direction, by tracing the path of the meteor's streak backward, it will always appear to originate from the same spot in the constellation for which the meteor shower is named.

 

N

Nebula - A nebula (plural form nebulae) is a cloud of dust and gas in space.  All of the most impressive nebulae are contained within our own Milky Way Galaxy.  Other galaxies also contain nebulae, but those nebulae are very far away and appear very tiny.  There are several types of nebulae.  An emission nebula generates ("emits") its own light, due to the atomic reaction taking place within it.  Classic examples of emission nebulae include NGC7000 (The North America Nebula) and M17 (The Swan Nebula).  A reflection nebula does not generate light, but is visible and appears because it is being illuminated by some nearby light source, usually a star.  Classic examples of reflection nebulae include IC2118 (The Witch Head Nebula), which is being illuminated by the star Rigel, and the gas around M45, the Pleiades star cluster.  Most (but not all) reflection nebulae appear blue due to the fact that they scatter blue wavelengths of light more efficiently than other wavelengths.  Some nebulae contain both areas that emit their own light and other areas that merely reflect light.  Classic examples of emission/reflection nebulae include M45 (The Orion Nebula) and M20 (The Trifid Nebula).  Another type of nebula is a dark nebula, also known as an "absorption nebula", which is simply the same as a reflection nebula but appears dark because there is no nearby light source to illuminate it.  Dark nebulae will often appear as dark lanes or voids in a star field because they block or obscure the light from the stars that would otherwise appear behind it from our perspective on earth.  Classic examples of dark nebulae include Barnard 33 (The Horsehead Nebula) and Barnard 72, the Snake Nebula.  Another class of nebula is the planetary nebula, so called because they often appear as small spheres or disks, resembling planets.  Planetary nebulae are the result of stars having ejected layers of matter through violent means.  Classic examples of planetary nebulae include M57 (The Ring Nebula) and NGC6826 (The Blinking Planetary). 

 

O

Open Star Cluster - (See also, Star Cluster.)  An Open Star Cluster is a collection of stars that are gravitationally bound together.  Open star clusters contain a few tens or hundreds of stars and are arranged very sparsely, in no typical structure.  Most open star clusters have no recognizable structure at all.  Some open star clusters can cover a relatively large area of the sky, perhaps even similar to that covered by the full moon.  Typical large open star clusters include M44 and M45.  Typical open star clusters, but smaller, include M11 and M52.

 

P

Photoshop - image-processing software made by Adobe, Inc., and used for final processing of astronomical images.  Although Photoshop is a general-purpose image-processing package, intended for any kind of image data (including astronomical images), it is extremely powerful and offers many advanced image-processing functions.  Photoshop is considered the de facto standard for image processing of virtually any kind of image data.   It can be used, typically, to improve the contrast, brightness, and color present in an astronomical image.  Photoshop also contains many advanced image-data functions, such as filters, that can alter the image data, (hopefully) improving its appearance in the final image.  It also has advanced workflow capabilities for those who wish to use them.  Because of its power, Photoshop is often the software of choice for final image processing of astronomical images.

Planetary Nebula - A planetary nebula is a cloud of gas and plasma surrounding a star and is the result of that star violently shedding layers of itself.  A planetary nebula is so called because these nebulae very often resemble planets due to their spherical or disc-like shape.  Classic examples of planetary nebulae include NGC3242 (The Ghost of Jupiter) and NGC7293 (The Helix Nebula).  See also, Nebula.

 

R

Reflection Nebula - a cloud of dust and gas in space that is visible due solely to the fact that some nearby light source (typically a star) illuminates it, causing it to reflect that light.  Most reflection nebulae scatter light in the blue part of the visible spectrum more efficiently than other colors, and so appear blue.  Classic examples of reflection nebula include NGC1977 (The Running Man Nebula).  See also, Nebula.

Reflector - a telescope design that uses a primary mirror that reflects the light it receives to an eyepiece or camera.  Most reflecting telescopes are variations on the "newtonian" construction, named after Sir Isaac Newton who first popularized the design.  Reflecting telescopes can have very large primary mirrors, and all large amateur telescopes are of this design.  A very popular newtonian reflector design variation is the Dobsonian telescope, so called because of its mount, which is a box that is low to the ground, contains the primary mirror, and has the two axes of pivot integral to the mirror's box.  The Dobsonian mount is popular because of its simplicity and low cost as compared to an equatorial mount, which is more complex and costly.  Reflectors almost always have a small secondary mirror, to which the light is bounced from the primary mirror, and from which the light exits the telescope's optical tube to the eyepiece or camera.  This secondary mirror is actually in the path of the light entering the reflector's tube, and as such blocks a small amount of the light and scatters an even smaller amount of the light coming into the optical path.  As a result of this "central obstruction" (the secondary mirror), reflectors tend to lose some contrast in the resulting image as compared to a refractor, which has no central obstruction. 

Refractor - a telescope design that uses only one or more lenses to capture the incoming light and present an image at the eyepiece or camera.  Refractors are limited to a relatively small diameter ("aperture") because of the inherent difficulties and cost in making large lens systems.  Premium refractors have very complex and exotic glass and coatings to ensure that the light focused at the eyepiece and camera is the sharpest and that as much of the light wavelengths as possible reach focus at the same point.  Less-expensive refractors do not use these technologies and thus suffer from several optical phenomena, notably color fringing (typically blue or purple) around the edges of bright stars, the full moon, and planets.  Per inch of aperture, the refractor is typically the most expensive design, particularly when compared to reflectors.  However, refractors have no secondary mirror (indeed, no mirrors at all) to obstruct light (reflectors do) and often have superior images to reflectors at identical apertures.

 

S

SCT - Schmidt-Cassegrain Telescope.  Sometimes called a "compound telescope".  A telescope design that employs botha lens and mirrors to capture and present an image to the eyepiece or camera.  The SCT "folds" the light once on its way to the eyepiece or camera and as such results in a very compact (but optically comparatively slow, typically f/10) optical design as compared to a reflector of the same diameter (aperture), which is typically in the f/4 to f/6 range and physically much larger.  The SCT is very popular among amateurs, particularly beginners, because it represents a superior compromise between light gathering ability, image quality, size/compactness, viewing comfort, mount convenience and stability, and automation.  SCTs are often found with advanced, computerized mounts (usually fork-style) that afford the user the ability to view tens of thousands of objects from their computer databases.  SCTs are typically nowadays also produced with "go to" mounts that drive the telescope quickly to the object desired (selected from the computer's database) and track the object once it is reached.  The basic construction of the SCT includes a "corrector" lens at the front of the optical path, with a complex pattern ground in the glass to pass the incoming light around the secondary mirror that is mounted at the back of the corrector lens.  The incoming light next strikes the primary mirror at the bottom of the telescope's tube.  The light reflects from the primary mirror, back to the secondary mirror at the back of the corrector lens assembly (at the front of the telescope's tube), which then reflects the light back through a hole in the primary mirror and out through that hole to the eyepiece or camera at the bottom of the telescope.  As such, the light can be said to have been "folded" once on its way in to the focused image.  Unlike reflectors or refractors, which provide focus adjustment via an assembly that moves the eyepiece or camera in or out, the SCT focus mechanism moves the primary mirror while the eyepiece or camera remains fixed in place.    

Spiral Galaxy - a galaxy that has a definite spiral shape, with spiral arms radiating from the galaxy's center instead of from the ends of an elongated "bar" that passes through the center of the galaxy as in a barred-spiral galaxy.  Classic examples of spiral galaxies include M51 and M101.

Star Cluster - a collection of stars that are gravitationally bound together and probably formed from the same gas cloud.  The two types of star clusters are the "globular" star cluster, and the "open" star cluster.  Globular star clusters can contain many thousands (even millions) of stars, formed in a tightly-packed ball, or "globe".  Open star clusters typically only contain tens or hundreds of stars, and are much more sparsely organized, without any defined structure.  See also, Globular Star Cluster and Open Star Cluster.

Star Trails - an image taken of the night sky that shows the earth's rotation over some period of time.  The effect of the earth's rotation is to make the stars in the picture appear to move, which appears in the photograph as an arc of light against the dark sky.  Photographs taken with the camera pointing north (northern hemisphere) will dramatically show that the North Star (Polaris) is almost exactly the center of the rotation of the earth, if the earth's axis were to be projected out into space.  The arcs of light showing the stars' paths will be relatively tight and if Polaris is in the photograph, these arcs will describe a circle around it.  Polaris will appear in the photograph at a vertical height above the horizon that equates to the latitude of the location from which the photograph is taken.  Star trails photographs taken with the camera pointing south (northern hemisphere) will show much broader arcs of light from the stars, because the center of rotation is well below the horizon.  Star trails photographs taken with the camera pointing east or west will show, if the field of view is broad enough, very broad, gentle arcs at both sides, curving in opposite directions from the other side, with perfectly straight lines instead of arcs at some point between them.  These star trails will be tilted from vertical at an angle approximating the latitude from which the photograph is taken.  Typically, star trails photographs are taken with a wide-field or normal lens focal length, or the equivalent of 28mm-75mm, in order to capture enough of the night sky that the resulting trails of the stars across it are aesthetically interesting. 

ST-4 - a particular autoguider that has long been popular with astrophotographers due to its very low cost and simple, no-computer-required operation.  The ST-4 was made by the Santa Barbara Instrument Group (SBIG) but has been out of production for many years, although it is still used by many astrophotographers.  Newer-technology autoguiders have since overtaken the venerable ST-4 in terms of image sensor size and sensitivity, sophistication and flexibility, and accuracy, although most are far more costly than the ST-4.

 

U

Unguided - term referring to long-exposure photography of astronomical subjects taken through a telescope or other optical setup that is merely tracking the earth's rotation and not subjected to continuous, fine adjustments to that tracking like a guided exposure would be.  Unguided exposures are typically very short, a few seconds at most, because most telescope tracking will cause slight drift after a few seconds, causing blurred or streaked stars in the resulting astronomical image.