Astronomical Terms

Airy Disk

Because light behaves in some ways like a wave, it is bent or "diffracted" by a telescope's structure (the edges of the optical tube, for example). This happens in the same way that ocean waves are partially bent or turned in a new direction as they pass a dock piling or the corner of a jetty. This scattered light prevents a star from being focused to a point. Telescopes always show stars as small disks of light (called Airy disks, after British Astronomer Royal Sir George Airy, 1835-1892). The disks are surrounded by faint rings of light called diffraction rings. The size of the Airy disk is determined by the aperture of the telescope - the larger the aperture, the smaller the Airy disk. The illustration shows the simulated Airy disk of a star seen through an unobstructed refractor. The brightness of the first and second diffraction rings has been emphasized for clarity.

Altazimuth Mount

This is the simplest type of telescope mount. It has only up/down (altitude) and left/right (azimuth) motions. It is used primarily for low to medium power Dobsonian telescopes and terrestrial spotting scopes, as it cannot easily follow at high powers the seemingly curved paths taken by celestial objects as they cross the sky. A telescope on this type of mount generally cannot be used for astrophotography.

Aperture

Large telescopes often have a problem when looking at the Moon or planets. They gather too much light. The Moon and planets are bright enough on their own. You don't need to gather a lot of light to see them.

If an image is too bright, a property called irradiation comes into play within your eye. Irradiation causes bright areas to bleed over into adjoining dark areas and soften or eliminate the border between them. For example, when two bright areas are separated by a thin dark area, such as a narrow rille on the Moon, the irradiation of the two bright areas can cause the thin dark rille to disappear. With a big scope, then, trying to see faint details in a planet's atmosphere or on its surface is often like trying to read the brand name on a car headlight at night when it's lit. There's just too much glare to see any detail. In addition, big scopes look through more of our atmosphere than small scopes and are often limited in their ability to resolve small details by conditions in our turbulent atmosphere.

An aperture mask is a circular piece of cardboard or metal the same size as the front cell of your telescope. A 3" to 4" circular hole is cut into the mask off to one side, not directly in the center of the cardboard circle. The mask is painted flat black and taped over the front of your scope. If properly sized and positioned, the circular hole will line up with an unobstructed portion of your scope's optical tube. It will not be blocked by the edge of the tube or the secondary mirror, and none of the secondary mirror's spider vanes or support structure will cut across the circular hole. By masking down the aperture of a large telescope you reduce the brightness and glare, reducing irradiation, You eliminate the diffraction effects of the secondary mirror and its supports, improving contrast. And you reduce the image-blurring effects of atmospheric turbulence, sharpening the image.

An aperture mask reduces the resolution of a large scope to that of an unobstructed refractor with the same aperture as the size of the hole cut in the mask. However, most observers would rather have a sharp steady image with reduced resolution than a blindingly bright image that's too fuzzy and washed out to see any details. An aperture mask gives them a sharp steady image with a big scope.

A neutral density eyepiece filter will also reduce the image brightness, reducing irradiation. However, it does not reduce the diffraction effects of spider vanes or the secondary mirror and does not reduce the effect of atmospheric turbulence. A filter can also reduce the resolution slightly at high powers.

A refractor telescope with a two to four lens optical system that uses one or more elements of costly ED (Extra-low Dispersion) or SD (Special Dispersion) glass and/or calcium fluorite crystal to virtually eliminate chromatic aberration. Technically, any lens system that is corrected for spherical aberration at two wavelengths, or colors of light, and for chromatic aberration at three.

A binocular or spotting scope whose body is clad in rubber or polyurethane armor is said to be armored. Armor can be applied for looks, a better grip, noise-proofing, etc. An armored body does not guarantee that a binocular or spotting scope is waterproof, although most waterproof optics are armored.

For the faint fuzzies outside the solar system, large aperture binoculars offer something a telescope can almost never give you -- a wide field of view. They let you see the big objects all at once, such as the three degree width of the Andromeda galaxy that has to be seen in segments in the one degree field of the average telescope. Generally speaking, binoculars with apertures of 50mm and larger, and magnifications of 10x and higher, are best suited for astronomical observing outside the solar system. 50mm binoculars have the same light gathering capacity as a 70mm refractor at the same power. 80mm binoculars have the same light gathering as a 4.5" reflector. You should aim for a binocular exit pupil of 4mm to 5mm if you're in your 40's or older (10 x 50mm, 16 x 70mm, or 20 x 80mm binoculars, for example). Exit pupil is found by dividing the binocular aperture by the magnification. Binoculars with 6mm to 7mm exit pupils (a 7 x 50mm or 8 x 50mm binocular, for example) are best suited for teenagers and those in their early 20's, who have eyes with pupils that can still expand wide enough in dark skies to take in all the light of the large binocular exit pupils.

An optical defect in which star images are elongated into ovals which change from a radial orientation (pointing towards the center of the field) to a tangential one (at right angles to the center) as the observer moves the telescope focuser from one side of the best focus to the other. Should not be confused with the star images of an out-of-collimation telescope, which may also be oval but which will not change orientation as the observer passes through best focus.

A CCD or CMOS camera records images by converting photons of light into electrons. The electrons are temporarily stored in individual picture elements (pixels) on a photosensitive detector or chip. At the end of an exposure, the accumulated charges are read off the chip and sent to your computer for conversion to an image.

During the long exposures needed to record faint objects, bright stars in the field can exceed the full well capacity (the electron-holding capacity) of the pixels on which they are being recorded. As a light-gathering pixel exceeds its capacity to hold captured photons, the excess energy spills over into the adjacent pixel (or pixels, if the second pixel also fills to its capacity). This spillover, called "blooming," produces a spike of light on every bright star in the image. It is often seen in CCD pictures taken with cameras without antiblooming, as shown in the image.

An antiblooming grid (ABG) is a metallic grid placed over the imaging chip. The metallic antiblooming grid drains off the excess energy from each pixel before that energy can spill over into the adjacent pixel. It allows you to take high-quality images without the danger of bright stars blooming before fainter objects are fully imaged.

The drawback of this system is that the metallic antiblooming grid covers a portion of each pixel, reducing the pixel's capacity to collect light. Scientific measurements can be compromised by this reduced capacity, as the response of the detector is no longer linear. A hundred thousand units of light coming in during a long exposure may result in only fifty thousand units of light being recorded by a chip with an antiblooming grid. This will make a particular object look only half as bright as it actually is. This non-linearity is unacceptable for accurate scientific measurements, such as spectroscopy and photometry, but is perfectly acceptable for purely imaging purposes.

If you plan on doing any measurements with your CCD camera, a non-antiblooming camera is the best choice because of the higher accuracy possible without ABG. Casual astrophotographers will have to take a little more care in setting exposure times with a non-antiblooming camera, as they will not have the cosmetic protection of the ABG's arbitrary bright signal cutoff. That being said, it should be noted that non-antiblooming chip cameras are the most popular with casual imagers and scientists alike.

Any telescope that folds the light path and directs it through a hole in the center of the primary mirror (called the Cassegrain focus) at the bottom of the telescope.

A telescope that uses a combination of mirrors and lenses to increase the effective focal length of the telescope while allowing it to be folded into a more convenient and compact size. The use of a full-aperture correcting lens in these scopes virtually eliminates spherical aberration, chromatic aberration, and coma. The word catadioptric is derived by combining the term for an optical system that forms images by using mirrors (catoptric) with the one for a system that uses lenses (dioptric). The most popular catadioptric designs are the Schmidt-Cassegrain and Maksutov-Cassegrain.

Kodak assigns a cosmetics "class" to their CCD monochrome imaging detectors based on the number and type of "defects" found on the detector chip. Defects are CCD camera pixels that usually still function but respond to light slightly differently than their neighboring pixels (the surrounding 128 x 128 pixels or +/- 64 columns/rows). The "defects" look lighter or darker than the rest of the pixels that surround them. The amount of difference from its neighbors that classifies a pixel as defective depends on the model of the CCD detector or chip. Defects are of three types: point, cluster, or column.

Point defects: These can be classified as either "dark" or "bright" pixels. A "dark" pixel is one that is more than 6% dimmer than its neighboring pixels when illuminated to 70% of saturation. However, some chip models allow a difference of up to 20% before a pixel is considered dark. A "bright" pixel is one whose dark current exceeds some specified value. This value can range from >3000e/pixel/sec at 25° C up to >5000e/pixel/sec, depending on the chip model.

Cluster defect: This is a grouping of not more than 5 adjacent point defects.

Column defect: This is a grouping of >5 contiguous point defects along a single column, OR a column containing a pixel with dark current >6,000e/pixel/sec (up to >150,000e/pixel/sec with some chips), OR a column that does not meet the minimum vertical CCD charge capacity, OR a column which loses more than 250e under 2Ke illumination (more than 500e with some chips).

In most cases, defects can be removed by software when processing the image. For casual imaging purposes, any class of detector will generally be acceptable. However, the more defects you have to begin with, the more processing time will be required to remove them. For scientific work, however, defects can affect the accuracy of astrometric and photometric measurements, should the object of interest inadvertently fall on a defect. If you are interested in doing this type of work, the better the class of chip, the more accurate your results will be.

Some Kodak multi-megapixel imaging detectors are considered "single class" detectors and are not divided into classes based on the number of defects they might have. In addition, the defect specifications are a bit looser, and tested at 80% saturation. Dark pixels are classified as either major or minor, depending on their departure from the nominal response. Cluster defects can be a group of two to ten contiguous defective pixels, but with no more than two adjacent defects horizontally. Column defects are a group of more than ten contiguous major defective pixels along a single column. In all cases, the specs require that there be at least two non-defective pixels separating any two major defective pixels to facilitate the removal of the defective pixels by your software during image processing.

 How close you can get to an object and still see a sharp image of it in your binocular or spotting scope is called the "close" or "near" focus.

The alignment of the optical elements of a telescope at the correct angles to the light path. If not properly collimated, a telescope will deliver distorted images ( lopsided or elongated stars, hazy planetary images, an inability to split close binary stars, etc.) Atmospheric turbulence ( seeing ), thermal currents within the telescope, and dirty or defective eyepieces can often mimic poor collimation. The illustrations below simulate the Airy disks and diffraction rings of a properly collimated scope as well as those of a mis-aligned scope.

An optical defect in reflector telescopes in which in-focus star images appear progressively more triangular or comet-like the closer they get to the edge of the field of view. The faster the focal ratio, the more prominent the coma. The visually coma-free field of a telescope in millimeters is roughly equal to the square of the scope's focal ratio - an f/5 focal ratio scope has a 25mm field (5 squared = 25), an f/6 scope has a 36mm field (6 squared = 36), etc. Since a 1.25" eyepiece barrel only about 29mm in internal diameter, and a 35mm film negative or slide measures 44mm across its diagonal, it can be seen that even a 25mm coma-free field is more apparent in photos than it is in most visual observing. Coma can superficially appear similar to a star's image in a poorly collimated telescope. With coma, however, the brightest portion of the comatic wedge (actually the Airy disk) always points toward the center of the field. This differs from an out-of-collimation telescope, where the Airy disks are all offset to the same side of the diffraction rings, no matter where in the field the star image is located.

The Crayford Eyepiece Mount (CEM) or Crayford focuser was invented by John ("Jack") Wall in England. The name "Crayford" comes from the Crayford Manor House Astronomical Society (UK) to which he belongs.

The Crayford design moves the focuser drawtube by applying high pressure on a metal drive shaft that in turn presses against a flat surface machined into the metal focuser drawtube. The drawtube is held in place by sets of bearings on the opposite side of the drawtube from the drive shaft. This direct metal-to-metal and surface-to-surface drive mechanism eliminates the problems found in the rack-and-pinion focusers found on many telescopes, such as backlash, gear slop, and side to side shifting of the drawtube as the focus knob is turned.

The Crayford design allows for very fine adjustments, with tolerances up to 100 times better than conventional rack-and-pinion focusers. Its zero image shift and zero backlash makes it outstanding for visual and photographic work and a must for CCD imaging.

JMI recognized the benefits of this design and was the first company to bring it to the amateur market in a commercial product. Many telescope accessory companies have used the design for add-on focusers since JMI first adopted it and it is now being incorporated into many telescopes as standard equipment, as well.

The illustration shows one of Jack Wall's original design drawings for the first Crayford focuser.

An optical defect in which objects at the edge of the field of view can't be brought into sharp focus at the same time as objects in the center, and vice versa.

If two equally-bright stars are so close together that their Airy disks overlap, they will be seen as a single star, although perhaps as an elongated one. If, however, the Airy disk of one star falls in the first dark diffraction ring of the second, each star can be seen separately - not as two distinct points, but as two small disks of light touching and forming a Figure 8, in which the intensity of light between the two touching disks drops by a clearly visible 30%.

English astronomer William R. Dawes (1799-1868, and known as the "eagle-eyed" for his acute vision) determined that the smallest separation between two stars which shows this 30% drop is equal to 4.56 arc seconds divided by the aperture of the telescope in inches. The larger the telescope aperture, the smaller the separation that can be resolved.

This "Dawes' limit" (which he determined empirically simply by testing the resolving ability of many observers on white star pairs of equal magnitude 6 brightness) only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini's Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5" telescope - which has a Dawes' limit of 1.8 arc seconds!

The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer's visual acuity, than it is by the optical quality of the telescope.

The resolving power of each of the telescopes on our website is shown as determined by the Dawes' limit formula, as this is the standard measurement that all manufacturers use to specify the resolving powers of their telescopes.

 The angular distance of a celestial object north or south of the celestial equator, measured in degrees. One of the two coordinates (right ascension is the other) that let you find celestial objects with the aid of a star chart and telescope setting circles. Called declination because stellar positions in degrees "decline" or decrease in numerical value from 90 degrees at the north and south celestial poles (around which everything in the sky appears to rotate) down to zero degrees at the plane of the celestial equator. Declination is in positive degrees if the object is between the celestial equator and the north celestial pole, and in negative degrees if it is between the celestial equator and the south celestial pole.

A conventional Newtonian reflector optical tube on an inexpensive plywood or fiberboard altazimuth mount. Nylon or Teflon bearings allow smooth telescope motion at a finger's touch, with no vibration or unsteadiness. The scope is moved by hand from object to object (there are no manual slow motion controls or motor drives) using a technique called star-hopping to locate objects. Usually it's a large aperture, fast focal ratio scope designed for visual deep space observing - although 6" and 8" medium f/ratio Dobsonians also suitable for planetary observing are becoming increasingly popular. Cannot be used for astrophotography. The Dobsonian is an economical way to get into large aperture astronomy at a fraction of the cost of an equatorially-mounted scope.

A telescope mount designed for astronomical use. It aligns the axis of rotation of a telescope with the axis of the Earth, allowing the scope to follow the seemingly curved paths taken by the stars and planets. When equipped with a motor drive, it automatically tracks celestial objects without the need for constant manual corrections, as is the case with an altazimuth or Dobsonian mount. This is particularly important at high magnification, where objects drift across the field of an unmoving scope in a minute or less. Usually supplied with setting circles that help locate objects by their right ascension and declination coordinates. Convenient for visual observing and essential for astrophotography. Two types are commonly available with commercially-made amateur telescopes -- the German equatorial and the fork mount.

Eye relief is the distance from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the circular image is formed, sometimes called the "Ramsden Disk." This is where your eye should be positioned to see the full field of view of the eyepiece. If you must wear glasses because of astigmatism, you'll usually need at least 15mm of eye relief or longer if you want to see the full field of view with your glasses on.

A note on our eye relief figures: Quite often, our eye relief figures will differ from those of the manufacturer. This is because we measure the "usable" eye relief, while the manufacturers specify their usually-longer (but technically correct) "designed" eye relief.

The eye lens of the eyepiece is normally recessed below the rubber eyeguard or rubber rim of the eyepiece to keep the lens from being scratched during use. An eyepiece might have a "designed" eye relief of 15mm (and the eye relief will truly measure 15mm from the eye lens to where the image forms). However, if the eye lens is recessed 3mm below the eye guard, the Ramsden Disk forms only 12mm above the eyepiece body (the 15mm "designed" eye relief, less the 3mm of eye relief made unusable by having the eye lens recessed into the body of the eyepiece). This "usable" eye relief of 12mm (measured from the rolled-down eyeguard - the closest point you can get your eye to the eye lens - to where the image forms) is the eye relief figure we would measure and list in this website.

Why is it important to list the "usable" eye relief? For those people who don't wear eyeglasses while observing, a few mm difference between the eye relief they expect from the manufacturer's literature and the shorter eye relief they actually get in real life doesn't mean a lot. They can simply move a little closer to the eyepiece to see the full field, and never realize that the eye relief is a little shorter than they expected. However, some people must wear eyeglasses while observing, because of severe astigmatism. These observers can't move closer to the eyepiece if the eye relief is shorter than expected because their glasses get in the way. For these people, the real life "usable" eye relief is more important than the technically correct but sometimes not fully usable "designed" eye relief. We measure and list the actual usable eye relief so that people in the real world can pick the eyepieces that will work best for them.

A telescope collects light and forms a small fixed-size image at a point (called the prime focus) that's determined by the focal length of the optical system. You can see this image by aiming your telescope at something bright, such as the Moon, taking out the eyepiece and star diagonal, and holding a piece of paper behind the focuser. Move the paper back and forth. At some point, you will find a small, but sharp, image of the Moon projected onto the paper. This is the prime focus image formed by the telescope. Unfortunately, human eyes typically cannot focus sharply on an image unless it's more than eight inches from the eye. This makes it difficult to see detail in the small prime focus image formed by the telescope if it's examined solely with the unaided eye. An eyepiece is a small microscope that allows you to get closer than eight inches from that small fixed-focus image -- and the closer you can get to an object, the bigger it appears. A 25mm eyepiece, for example, lets you focus on the scope's prime focus image from an effective distance of only 25mm (one inch away from your eye); a 12mm eyepiece puts you half an inch away; etc. The magnification of an eyepiece is found by dividing the telescope focal length by the eyepiece focal length. A 25mm eyepiece used with a 2000mm focal length scope therefore provides 80 power (2000 / 25 = 80x), making objects appear 80 times larger than they do to the bare eye (or 80 times closer, to put it another way).

 An inability to bring the center and edge of the field into focus at the same time, with the edge out of focus when the center is sharply focused and vice-versa.

This is the length of the effective optical path of a telescope or eyepiece (the distance from the main mirror or lens where the light is gathered to the point where the prime focus image is formed). Focal length is typically expressed in millimeters.

The longer the focal length, the higher the magnification and the narrower the field of view with any given eyepiece. The shorter the focal length, the lower the magnification and the wider the field of view with the same eyepiece.

This is the 'speed' of a telescope's optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera.

Fast f/4 to f/5 focal ratios are generally best for lower power wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either.

An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio - so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

A type of equatorial mount used on short tube catadioptric telescopes in which the telescope tube is mounted between two arms connected to a motor drive. It does not need a counterweight to balance the tube, as with a German equatorial mount. An equatorial wedge and field tripod are used tilt the scope over to align it on the celestial pole for proper tracking. Setting circles are provided to locate celestial objects by their right ascension and declination coordinates. The r. a. setting circle is usually driven by the scope's motor drive to move across the sky at the same speed as the stars, following their apparent motion. This makes fork mount setting circles more convenient to use than the unpowered circles on most German equatorial mounts, as the latter must be readjusted periodically to keep pace with the motion of the stars. Photography near the north celestial pole is difficult with a fork mount.

A mount that works well visually, but is especially suited for astrophotography. A counterweight on one side of the polar axis balances the weight of the optical tube on the other. Not as convenient as a fork mount when sweeping from horizon to horizon, as the tube can bump the legs or pedestal mount as the scope passes the zenith, requiring that the tube be "tumbled" or rotated 180° to continue its tracking of objects down to the western horizon. Its setting circles usually are operated manually. Somewhat more difficult to use and transport than a fork mount telescope, but stable, relatively inexpensive, durable, and capable of astrophotography near the celestial pole.

This is the highest visual power a telescope can achieve before the image becomes too dim for useful observing (generally at about 50x to 60x per inch of telescope aperture). However, this power is very often unreachable due to turbulence in our atmosphere that makes the image too blurry and unstable to see any detail.

On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture or less) is usually more enjoyable than fruitlessly attempting to push a telescope's magnification to its theoretical limits. Very high powers are generally best reserved for planetary observations and binary star splitting.

Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.

The distance between the pupils of your eyes, measured from center to center when your eyes are focused on infinity, is called your interpupillary distance. You interpupillary distance will be smaller when looking at something nearby. Interpupillary distance is also the distance between the centers of a binocular's exit pupils.

This is the magnitude (or brightness) of the faintest star that can be seen with a telescope. The larger the number, the fainter the star that can be seen. An approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm).

This is the formula that we use with all of the telescopes we carry, so that our published specs will be consistent from aperture to aperture, from manufacturer to manufacturer. Some telescope makers may use other unspecified methods to determine the limiting magnitude, so their published figures may differ from ours.

Keep in mind that this formula does not take into account light loss within the scope, seeing conditions, the observer's age (visual performance decreases as we get older), the telescope's age (the reflectivity of telescope mirrors decreases as they get older), etc. The limiting magnitudes specified by manufacturers for their telescopes assume very dark skies, trained observers, and excellent atmospheric transparency - and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

Magnification is the ability of a telescope to make a small, distant object large enough to examine in detail. If you look at the Moon (250,000 miles away) with a 125 power (125x) telescope, it's essentially the same as looking at it with your bare eyes from 2000 miles away (250,000 ÷ 125 = 2000). The same telescope used terrestrially will make an object one mile away appear to be only 42 feet away (5280 feet ÷ 125 = 42).

The magnification of a telescope is determined by dividing the focal length of the telescope (usually in millimeters) by the focal length of the eyepiece used (again, usually in millimeters; but in all cases by the same unit of measurement used for the telescope focal length). For example, a 2000mm focal length telescope and a 10mm focal length eyepiece will give you a magnification of 200 power (2000 ÷ 10 = 200). The same 2000mm telescope with a 20mm eyepiece will give you 100x (2000 ÷ 20 = 100).

 A number indicating the brightness of a star or extended object. The larger the positive number, the fainter the star or object; while the larger the negative number, the brighter the star or object. A one digit magnitude change indicates a 256% difference in brightness. 4th magnitude stars are often the faintest visible to the naked eye from a light-polluted suburb. 14th magnitude stars, by comparison, are a mere 1/10,000th as bright! 6th magnitude stars are typically the faintest naked eye stars visible from a reasonably dark sky observing site. The Sun has an apparent magnitude of -26.5.

On extended objects (galaxies and nebulas), the magnitude is the one the object would have if all its light was gathered into a single point, like a star. A 9th magnitude galaxy, therefore, will
appear dimmer than a 9th magnitude star because its light is spread over a larger area than the star. A good example is M33, the face-on spiral galaxy in Triangulum. It's a 6th magnitude object, but is often difficult to see in even an 8" telescope (whose visual limiting magnitude is 14), because its mag 6 brightness is spread over nearly one square degree of sky. Such an object is said to have low surface brightness and is quite often masked by light pollution when observing from city or suburban sites.

 A catadioptric telescope that uses a thick and deeply-dished spherical corrector lens to correct for the spherical aberration of its spherical primary mirror - an all-spherical design that keeps its collimation virtually indefinitely. Its typically long focal ratio and small secondary obstruction yield higher contrast and resolution than any other catadioptric or reflector design.

Newtonian Reflector

This classic 300-year old Sir Isaac Newton design uses a large primary mirror at the bottom of the telescope tube, with a flat diagonal mirror at the top that brings the light out to the Newtonian focus at the side of the tube. Totally color-free, for excellent planetary observing. Offers more light-gathering aperture per dollar than any other telescope design, as well, for very good deep space performance.

Number of Optical Elements

Objective

The main light-gathering lens or mirror of a telescope.

Optical Type

An optical technique used with roof prism binoculars to increase color fidelity.

Due to a roof prism's optical design, the light entering a binocular's image-erecting roof prism is split in two. The two halves travel through the prism independently and are rejoined before entering the eyepiece. Because the two light paths are slightly different lengths, one half of the light takes a little longer to travel through the prism than the other. When the two halves of the image are rejoined, the longer light path half is slightly out of phase with the light that took the shorter route. This can reinforce some colors of light and cancel out others, affecting the color balance and fidelity.

Phase correcting coatings are optical coatings that are applied to one surface of the shorter light path half of the prism. The coating slightly slows down the short light path half of the incoming light that passes through that surface, causing it to once again be in phase with the light that traveled the longer path when they halves are rejoined.

With phase-corrected prisms, no colors are reinforced or cancelled, giving a more accurate color reproduction. The effect is particularly visible when looking sunward at a back-lit or silhouetted bird, where more color and detail can clearly be seen in the shadowed areas of the bird.

 The effective focal length of a spotting scope/camera adapter combination when the scope is used as a telephoto lens. The photographic focal length divided by 50 will give you the magnification of the combination compared to your standard camera lens.

A telescope that uses two or three lenses to bring light to a focus at the end of a long tube.

This is the ability of a telescope to separate closely-spaced binary stars into two distinct objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. The resolution values on our website are derived using the Dawes' limit formula.

Dawes' limit only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini's Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5" telescope - which has a Dawes' limit of 1.8 arc seconds!

The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer's visual acuity, than it is by the optical quality of the telescope.

The Ritchey-Chrétien is a Cassegrain-type telescope design that uses hyperbolic primary and secondary mirrors to provide images that are free from spherical aberration and coma over a very wide field.

Because of its wide field and relatively fast focal ratio (often f/8 or faster), the RC design is better suited to wide field astrophotography and visual observing than optically slower classical Cassegrain, Schmidt-Cassegrain, and Maksutov-Cassegrain optical designs.

Many professional reflector telescopes in the world's observatories are Ritchey-Chrétiens, including the twin 10-meter Keck telescopes in Hawaii and the Hubble Space Telescope.

A catadioptric telescope that uses a thin aspheric corrector lens to compensate for the spherical aberration of its primary mirror.