This guide is meant to be your bridge over the tumultuous river of astronomical terminology that you will face if you search for telescopes in either ads or in the marketplace.  Here, you will find key terms defined for your aid.  If you have further questions, or would like some personal advice before making a telescope purchase, please feel free to use the "Ask the Astronomers" form.


General Telescope Terminology
Eyepieces and Accessory Lenses--Types and Functions

Types of Telescopes



            The first thing that a salesman will hawk will be "power."  This will be represented by a large number, such as "200," and followed by an "X."  This indicates that when you marry an eyepiece to its holder (at the other end of the tube from the "main objective"--the big lens at the front or the light-gathering mirror in the back), you will get that much magnification...everything will be 200 times larger in the "field of view."  That sounds wonderful, and very powerful, but unless you have the resources of Mount Palomar or the Kit Peak Observatory, you may be quite disappointed in the results.

            In order to get a highly magnified image, the lens has to focus in, much like a magnifying glass does.  For example, if you look at your hand, you will see not only your hand, but your fingers, your arm, and everything around your hand.  Then, if you take a strong magnifying glass and use it to look at your hand, you will lose all imaging except for a very small area that shows either ridges and whorls, or a cracked landscape with branchless trees. 
            Telescope magnification works much the same way, except that the objects that you are observing are much more distant, and often suffer from atmospheric distortion.  When you use high magnification, the distortion is also magnified.   That is why "high powered lenses" are not the best selling point for a telescope, although that is the only thing a salesman is likely to know about the merchandise.
            A smart shopper will check to find out which good "low power" eyepieces come with the telescope.  It is much easier to find something if you have a larger "field of view."  You can always zoom in later, after you have the image centered.  Low powered eyepieces give you enough sky to work with to find your way from star... to star... to desired object.  They also give you a crisper, clearer image, because distortions are not magnified as much as they will be with a high powered lens.
            Back to analogies....if someone told you that they thought they felt something sting their back and wanted you to find the site, you could easily "eyeball scan" their back for a tiny red dot.  However, if they gave you a strong microscope lens, and you had to scan using only the  lens--one tiny portion of skin at a time, you can see how frustrated you might get looking over an entire back for a tiny red speck.  That is how you might feel if you depend only on all that "power" hawked by the salesman.  First, you'd want to "eyeball" the area until you found the spot, and then you would get out your magnifier.   That is why you will check for a good set of low powered eyepieces...they will allow you to "get there."  If you want to magnify further, you may then do so.

            A series of good low power eyepieces to choose from includes:   25mm, 30 mm, 32mm, 35mm or 40 mm focal length with a 1 1/4" barrel.   Kellner is the least expensive, and is acceptable in performance.  Erfles and Plossls cost considerably more, but offer even wider fields of view.  Do not get Huygens or Ramsden...they have a very narrow field of view, and irritating color distortions.

            A series of good mid-power eyepieces to choose from includes:   18 mm, 16 mm, 15 mm, 13 mm, and 12 mm.  The lower the number, the higher the magnification you will get.  Orthoscopic is the best mid-range eyepiece obtainable at a reasonable price.  Plossl and TeleVue designs are more expensive, but their performance matches their price. 

            High power eyepieces to choose from include:  10.5 mm, 10 mm, 9 mm, 8 mm, 7 mm, 6 mm, 5 mm, 4 mm, or 3.5 mm.  Rarely will atmospheric conditions be stable enough to push for the highest magnification (3.5 - 5).   Orthoscopic, again has the best high power eyepiece obtainable at a reasonable price.  Plossl and TeleVue designs are more expensive, but again, their performance matches their price.  

Field of View

            The "field of view" is the amount of area that you can see through any given eyepiece.  The field of view varies with different eyepieces...low to medium power eyepieces give you a broader field of view than high power eyepieces.

            The Huygens and Ramsden eyepieces have the narrowest field of view, and are not recommended for use.  They are quite inexpensive, but you will be disappointed in the performance and your 'scope will quickly end up in a yard sale where experienced amateurs will pick it up for a song and add some quality eyepieces. 

            Kellner and Orthoscopic eyepieces have moderate fields of view, and very good color correction--they yield true colors. 

            The Erfle has a very wide field of view, but has some distortion at the outside edges of the field of view.

            Plossl and TeleVue offer wide fields of view without the edge distortion.


            Resolution is the ability to detect fine detail.  The larger the objective (large lens or light-gathering mirror), the better its ability to resolve detail.  This is very important with high magnification viewing of the moon, the planets, open or globular clusters, and double stars.


            This is the proper alignment of the optics in any optical device.   With refractors, this is often fixed, but in the various forms of reflector, the mirrors may at times need adjustment in order to line up in order to properly focus the light.  This is critical to good telescope performance.  The more often you move a reflector telescope, the more frequently you will have to collimate it.

Light Grasp

            The amount of light that an objective collects is proportional to its surface area, or the square of its radius.  This is why larger telescopes allow you to view much fainter objects.

Right Ascension & Declination

            If you consider it as terrestrial latitude and longitude extended into space, declination corresponds to latitude (north/south axis), while right ascension corresponds to longitude (east/west axis).  Many telescopes have "setting circles" with the degrees measured off in order for you to find an object by its celestial coordinates.  Like latitude, declination is measured in degrees north or south of the equator.  Right ascension, however, is a 24-hour clock face with 0 (zero hours) automatically set for the vernal equinox (the point that the sun crosses the equator as it heads north to begin spring).  In that sense, right ascension is similar to the 24-hour time zones that cross the globe.

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Eyepieces and Accessory Lenses--Types & Functions

This is a double lens used as a refractor objective to give better color-correct images.

This is a negative lens that you place in the eyepiece holder.  It holds the eyepiece.  Barlow lenses extend the focal length (the distance between the lens or mirror, and where it focuses light to a point--the greater the focal length, the higher the magnification) of the main objective, which, in effect, causes the eyepiece to yield a higher magnification...generally 2-3 times greater than the original power of the eyepiece.

ERFLE LENS (ur' full)
This is an expensive wide-angle lens that is very is a five-element lens (it uses five different pieces of glass).  It gives you a very wide field of view, but it does have some distortion at the edges of the field of view.  This is your best low power lens.  The wide field of view makes it very easy for you to locate objects.

HUYGENS LENS (hi' gens)
This is a two-element lens that has a very narrow field of view, making it difficult to find any object in any magnification range.  It also suffers from color distortion.   This cheap eyepiece comes as standard equipment on most Japanese telescopes (Tasco/Jason/Swift), and should be discarded as soon as possible.  Get a Kellner lens instead, if you want to actually use the telescope for viewing anything.  Telescopes used as decorative items, however, may keep the original lens.

If you can't afford an Erfle, this is the best value for your low and medium power lenses.  It is a three-element lens (three pieces of glass) that gives a flatter field of view than the Erfle (very little distortion at the edges), but not as wide. 

A four-element lens, this lens gives very good images at high power (9mm; 6mm; 4mm--smaller focal length=higher power/magnification in eyepieces), although you can get low power Orthoscopic lenses (30mm to 20mm), and medium power (18mm-12mm).  The low and medium powered Orthos are not superior to the lower priced Kellners, but the high power Ortho lenses are the cream of the crop in high power lenses.

PLOSSL LENS (just like it looks...ploss' ul)
This is a four-element eyepiece that has a wide, flat field of view.  It is a good general purpose eyepiece in all magnifications...more expensive than a Kellner but less expensive than Orthoscopics and Erfles.  It doesn't have as wide a field of view as the Erfle, but they do not have edge or color distortions, either.

This is the other lens that comes with your Japanese low cost telescopes (Jason/Tasco/Swift).  It is the high power lens that has a very narrow field of view and produces a very dim image.  This lens is probably the main reason you can find soooo many telescopes at yard sales.  Also known as the SR-4, this lens will frustrate practiced observers and drive novices into easier pastimes such as calculus.   Get a good high powered Kellner or, ideally, an Orthoscopic lens instead.  The main objective of the Japanese 'scopes are fine.  The problem is with the eyepieces.

STAR DIAGONAL (link to construction of a star-diagonal)
This accessory for refractors (in any form) and Cassegrain reflectors allows the observer to look through the eyepiece without straining his neck or getting down on his knees.  The image will be erect instead of being inverted, as usual, but the image is a mirror image with left and right reversed.

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Types of Telescopes

The refractor takes in light through the main objective lens (the large lens at the front), and delivers it straight to the eyepiece.  The old ship's spyglass is a good example of this type of telescope.

Light enters the tube and is collected by a mirror which reflects it back to a focuser via a secondary mirror....the eyepiece is set on the side instead of directly at the end of the tube.

SCHMIDT-CASSEGRAIN (shmit' - cass' uh grin)
This is a folded version of telescope that combines the best of a refractor with a reflector....the short, folded optical design focuses light through a hole in the main mirror.  The eyepiece is placed at the end just like the refractor.

MAKSUTOV (mack' suitoff)
Like the Schmidt-Cassegrain, this is a folded-optical design, but with even better images.   It is the most expensive telescope that you can buy, but worth it if you are a rabid amateur or professional astronomer.

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Telescope Parts

ALT-AZIMUTH MOUNT (alt- az' u muth)
This simple mount, such as a camera tripod, moves vertically and horizontally with manual control.  It comes on the least expensive telescopes, but it requires constant adjustments of both axes to track objects in the sky as the Earth rotates.  For astronomy, therefore, the equatorial mount is far superior.

A drive mechanism is an electrical driver that, once your equatorial mount is aligned on Polaris, keeps your telescope aligned by physically turning an equatorial mount's polar axis westward to match Earth's rotation.   There are controls for speed (+ & -), so you can set the telescope for accurate tracking before you begin an observing or photo session.  You must have either an electrical outlet nearby, or an auto adapter to use this equipment.  Some amateurs carry small generators with them to the field so that they can hook up. 

This is a form of tripod for mounting the telescope.  It has a polar axis, which must be aligned to Polaris (the North Star), and a SLOW MOTION CONTROL GEAR (a turnbuckle of some sort) that allows you to gradually move the telescope westward in order to track an object.  As you have observed the sun, moon, planets and stars rising and setting, they do so ever much faster when magnified in the lens of a telescope.   By tracking, you can keep an image in the eyepiece long enough for observation.   Often, the declination axis has a similar slow motion control so that you can make minute north/south adjustments.  (Slow motion controls make telescopes much easier to use).

The finder scope is the miniature telescope often found attached to the body of a larger telescope.  The eyepiece often has crosshairs.  The purpose of this 'scope is to help you:  1) focus on a particular image (it is much easier to eyeball the image as well as getting it in the finder scope than trying to find the image through a telescope eyepiece with the bulk of the telescope blocking your view of the object), and 2) as a guide during astrophoto sessions which allows you to position a "guide star" in a certain relationship to the crosshairs, which position you then maintain--either by clock drive or manual guidance.

SIDEREAL DRIVE (sid ear' ee ul)
The sidereal drive is a clock drive that is set to the actual motion of the stars, rather than the rotation of the Earth.  It is set for 360 degrees (normally, our 24-hour day) to be complete every 23 hours and 56 minutes.

On a large equatorial mount tripod, this is the actual cradle that holds the telescope body.  It can be set to point the polar axis at Polaris, and thus match the observer's latitude.  As long as you stay in the same place, you do not have to change it.  If you move north or south, it must be adjusted accordingly.

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These are simple color filters used for different effects in observation or photography.   Red allows you to view more clearly through haze.  Blue tends to show cloud formations on Mars and Venus well. 

This cuts down on the brightness of the full moon in order for you to observe contrasts better.

Tuned to particular atoms, this filter cuts down on sky pollution while still allowing the bright emission lines (the spectrum of just those atoms, such as hydrogen and oxygen) to show through.

SOLAR FILTERS--All should be placed in front of the telescope main objective...NOT at the eyepiece!
These filters cut the sun's light down about 10,000 times to safe viewing levels.

MYLAR FILTERS give you a bluish solar image.  This is usually purchased in the form of a flexible film.

ICONEL FILTERS give you a nice, orangey solar blue images, but is very expensive.  This is a hard filter composed of a thin metal film on glass.

THOUSAND OAKS POLYMER PLUS  gives you an orange solar image.  This is usually purchased in the form of a flexible film similar to Baader Solar Filter Material, but is thicker. 

BAADER SOLAR FILTER MATERIAL  give you a white solar image with a black, contrasting background.  This flexible film is the best solar filter material now in production.  It comes in Neutral Density-3 for solar photography, or Neutral Density-5 for visual observation.  Draco Productions only sells ND-5, which is not only safe for visual observation, but which is also excellent for using as a filter for solar photography when cells are constructed for your telephoto lens or telescope.
Baader material gives you the best resolution and best contrast (according to the latest NASA tests) of any of the currently available solar filters.

Good for daytime observation, when properly turned, it makes the half-moon stand out against a much darker blue contrasting sky.

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