Glossary of Terms

 Airy disk-


A diffraction pattern is the physical term for what you see when looking at a star through a telescope at high power under good seeing conditions (steady atmosphere with little air turbulence). Simply put, it is the concentric bull’s-eye pattern of the stellar image you see in your high-magnification eyepiece.

Because even the closest stars are so very far away, they will never show a real disk or ball shape in a telescope. On steady nights you can see the star as a tiny disk surrounded by a concentric ring or rings. (assuming quality optics that are properly collimated or optically aligned). This bull's-eye is called the diffraction pattern of the star; it's created by the interaction of light waves from the star with the circular edge (aperture) of your lens or mirror. The central bright region is the Airy disk and the surrounding bright circle is the diffraction ring.

The Airy disk becomes smaller as the aperture of the telescope gets larger. In theory, when you double aperture, you also double resolving power, since the Airy disk is only half as large. Two points of light--a double star--can be distinguished more closely with the larger scope. Since the disk is only half the diameter, it has 1/4 the area of the disk seen through the smaller telescope. In addition, when you doubled the size of the mirror or lens, you increased the area and light-gathering power of the scope by four times. So four times the light is in an area 1/4 as big, so the star appears 16 times as bright. This is why fine details in large telescopes jump out compared to the same view with a smaller telescope.





The diameter of the lens or mirror, is the single most important factor determining the performance of a telescope. The larger the aperture, the more light your scope gathers and the higher resolution (ability to see fine detail) it has. 

The clear aperture of a telescope is the diameter of the objective lens or primary mirror, specified in either inches or millimeters. Doubling the aperture means doubling resolution and quadrupling light gathering power. This means that an 8-inch scope can see things that are only one-fourth as bright as the limit of a 4-inch scope and details that are only half as big as the best that the smaller scope can resolve. 

Larger scopes also have longer focal lengths, meaning greater magnifications and image sizes are possible with both the eye and cameras.

Greater detail and image clarity will be apparent as aperture increases. For example, a globular star cluster such as M13 is nearly unresolved through a 4 in aperture telescope at 150x. However, with an 8 in aperture telescope at the same power, the star cluster is 16 times more brilliant, stars are separated into distinct points, and the cluster itself is resolved to the core.

The following photos of Saturn demonstrate what increasing aperture will give you: higher contrast, better resolution, and a brighter image. Top to bottom with Celestron telescopes: C5 (5 in aperture), C8 (8 in aperture) and C14 (14 in aperture). All were taken using eyepiece projection photography at a focal ratio of f/90. The effects of aperture are even more pronounced during visual observation.




Contrast is the difference in brightness between the bright objects in your field of view and the background (when compared to the background). Good contrast is needed for seeing fainter objects and discerning subtle visual details.

Maximum image contrast is desired for viewing low-contrast objects such as the Moon and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. This degradation is only significant if more than 25% obstruction is present.

To calculate the secondary obstruction, use the formula ?R2 to calculate the area of the secondary  and primary mirrors, then divide to find the percentage of obstruction. For example, an 8 in telescope with a 2.16 in secondary obstruction has a 7.3% secondary obstruction:
Primary radius is 4”:  ?R= 50.27
Secondary radius is 1.08”: ?R= 3.66
Percentage = 3.66/50.27 or 7.3%

For a given object, a telescope’s design (including central obstruction), coatings, optical quality, cleanliness, and collimation affect contrast. Important external factors can also affect contrast, including seeing (air turbulence) and air quality.


Exit Pupil-


The exit pupil is width of the beam of light leaving the eyepiece. It’s usually measured in millimeters. A large exit pupil is advantageous under low light conditions and at night because the larger the exit pupil, the brighter the image. For astronomical applications, the exit pupil of the telescope plus eyepiece should correspond with the amount of dilation of your eye's pupil after it is fully dark-adapted. This number will be between 5mm and 9mm. 9mm of dilation is the maximum amount for the human eye. Maximum dilation tends to decrease with age. By age 50, the exit pupil may be close to 5mm. An exit pupil larger than your dilation just wastes light from the objective, since the outside of the beam just falls on your iris and doesn't go into your eye.

To calculate the exit pupil, simply divide the aperture of the scope’s objective mirror or lens by the magnification of the eyepiece. Or divide the eyepiece focal length by the f/ratio of the scope. Example: an 8-inch (203.2mm) telescope with a focal length of 2032mm is used with a 20mm eyepiece. The exit pupil of this combination is 2mm.
2032/20 = 102x
203.2/102 = 2mm exit pupil

20/10 = 2mm exit pupil


Eye Relief-


The proper way to look through your telescope is by placing your eye just behind the eyepiece to take advantage of the eyepiece’s eye relief.

Eye relief measures the spacing 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 are visible. (The eye lens is the lens of the eyepiece closest to your eye when the eyepiece is inserted correctly into the focusing drawtube.) Your eye should be positioned here to experience the full field of view of the eyepiece. 
You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. On the other hand, getting too close will prevent you from blinking and also cause a black ring to appear around the field of view.

Eye relief should be at least 15 mm for the best comfort, but it may be more if you wear eyeglasses. 

Longer-focal-length eyepieces generally have longer eye relief. Using a Barlow lens with them to increase magnification will allow more comfortable high-power viewing because the Barlow will keep the eye relief constant.

Eyepieces with long eye relief need rubber or plastic eyecups to shield your view from extraneous glare.


Field of View-


The angular size of the sky you can view through a telescope is called the real (true) field of view; it is measured in degrees of arc. The larger the field of view, the larger the area of the sky you can see.

Field of view is calculated by dividing the apparent field of view (in degrees) of the eyepiece by the magnification. If you are using an eyepiece with a 50 degree apparent field, and the power of the telescope with this eyepiece is 100x, then the field of view would be 0.5 degrees (50/100 = 0.5).

Manufacturers normally specify the apparent field (in degrees) of their eyepiece designs. For a given focal length eyepiece with a given scope, the larger the apparent field of the eyepiece, the larger the real field of view and thus the more sky you can see. Likewise, lower powers used on a telescope allow much wider fields of view than do higher powers. 


Focal Length-


Focal length is the distance from a telescope’s objective element (lens or primary mirror) to the point where rays of light from the objective converge to a focus. It’s measured in inches or millimeters. 

Longer focal lengths will have more capacity for high magnification but narrower fields of view than shorter focal lengths. For example, a telescope with a focal length of 2000 mm has twice the power and half the field of view of a 1000 mm telescope when using the same eyepiece.

Most manufacturers specify the focal length of their scopes. If it‘s unknown and you know the focal ratio, you can use the following formula to calculate focal length. Focal length is the aperture (in mm) times the focal ratio. Example, the focal length of an 8 in (203.2 mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032 mm.


Focal Ratio-


The focal ratio is the ratio of the focal length of the telescope related to its aperture. It’s calculated by dividing the focal length by the aperture (both in the same units). For example, a telescope with a 2032 mm focal length and an aperture of 8 in (203.2 mm) has a focal ratio of 10 (2032/203.2 = 10) or f/10.

It’s variously abbreviated as f-stop, f/stop f-ratio, f/ratio, f-number, f/number, f/no., etc.

Smaller f-numbers will give brighter photographic images and the option to use shorter exposures. An f/4 system requires only ¼ the exposure time of an f/8 system. Thus, small focal ratio lenses or scopes are called “fast” and larger f/numbers are called “slow.” Fast focal ratios of telescopes are f/3.5 to f/6, medium are f/7 to f/11, and slow are f/12 and longer. 

Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture--the larger the aperture, the brighter the images. Extended objects will always appear brighter at lower magnifications. The main advantage of having a fast focal ratio with a visual telescope is that it will deliver a wider field of view than slower f-numbers.


Light Gathering power/ Magnitude limit-


Light gathering power is a telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture. 

You can calculate this by first dividing the aperture of the telescope (in mm) by 7 mm (dilated eye for a young person) and then squaring this result. For example, an 8 in telescope has a light gathering power of 843: (203.2/7)2 =  843.

The faintest star you can see with a telescope (under excellent seeing conditions) is the telescope’s limiting magnitude. It’s another way to compare telescopes of different apertures to each other and to the eye. It’s directly related to aperture. Larger apertures allow you to see fainter stars. A rough formula for calculating visual limiting magnitude is: 7.7 + 5 LOG (aperture in cm). For example, the limiting magnitude of an 8 in aperture telescope is 14.2: 7.7 + 5 LOG 20.32 = 7.7 + (5x1.3) = 14.2 

Atmospheric conditions and the visual acuity of the observer will often reduce limiting magnitude. The unaided or naked-eye magnitude limit is usually considered as 6.0. With a given scope, photographic limiting magnitude is often two or more magnitudes fainter than visual limiting magnitude.


 Aperture  Magnitude limit
 3.1 in (80 mm)
 4 in (100 mm)  12.7
 5 in (125 mm)
 6 in (150 mm)
 8 in (200 mm)
 10 in (250 mm)
 12.5 in (320 mm)
 14 in (355 mm)
 16 in (400 mm)
 20 in (500 mm)  16.2




Magnification of a telescope is actually a relationship between two independent optical systems: the telescope itself and the eyepiece you are using. To determinepower, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. A 20 mm eyepiece used on a 1000 mm focal-length telescope would yield a power of 50x (1000/20 = 50), while a 10 mm eyepiece used on the same instrument would yield a power of 100x (1000/10 = 100). Since eyepieces are interchangeable, a telescope can be used at a variety of powers.

There are practical limits of magnification for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50-60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower-contrast image. For example, the maximum power range on a 90 mm telescope (3.6 in aperture) is 180x-216x. As power increases, the sharpness and detail seen will be diminished. Higher powers are mainly used for lunar, planetary, and binary star observations. 

Most of your observing will be done with lower powers (6 to 25 times the aperture of the telescope in inches). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.

A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece to double the magnification of any existing 


Near Focus-


The near focus is simply the closest distance you can focus a telescope. It’s a good number to know when using your scope for terrestrial visual or photographic purposes.

Different types of telescopes will have greatly differing near focus capabilities. In general, catadioptric designs like SCTs and Maks are superior, with small near focus distances. Refractors and Newtonian reflectors will have much larger near focus distances and may not focus closer than several hundred feet (50-100 m).

Most of Celestron's iconic SCTs can focus as close as 20 ft (6 m). Older models of the C-90 MAK can focus as close as 6 ft (2 m).


Resolution/ Resolving Power-


Resolution is the ability of a telescope to render detail, while resolving power is a direct function of aperture. The larger a telescope’s aperture, the higher its resolving power and resolution, enabling it to see finer detail.

For a telescope, resolution is often measured as the smallest angle that can be cleanly “split” or resolved between two stars. This number is called the Dawes limit. To determine this limit for a telescope, divide 4.56 by the aperture of the telescope (in inches). For example, the resolving power of an 8 in aperture telescope is 0.6 seconds of arc (4.56 divided by 8 = 0.6).




Vignetting is the term used to describe a situation in an optical system where the field of view is not fully illuminated at the edges. In real-world optics, this is actually very common. Fortunately, it’s not always a problem with visual or photographic observations. Vignetting is caused by a reduction of light through the optics from rays of light that don’t come straight in through your scope. These off-axis rays are either absorbed or bent away from the optical axis and/or miss a secondary mirror, dimming the image at its edges. It can also be caused by baffling or field stops inside the optical tube actually blocking the light from other parts of the optical system. Eyepiece drawtubes can also cause vignetting.

For visual work, vignetting is typically noticeable when using wide-angle eyepieces and low magnifications; it manifests as a dimming around the outer edge of the field. In photographs, prime focus imaging with full-frame 35 mm or larger films or large-format CCD chips will usually show darkening at the corners.