Knowledge Base

Importance of coatings

When light enters or leaves a lens, there is a loss of some transmitted light due to reflection. By applying a surface coating of an antireflective material such as magnesium fluoride, the transmission can be greatly increased and internal flare can be reduced. When all lens surfaces have been coated they are said to be fully-coated and when the surfaces are coated with multiple layers to maximize transmission, the optics are said to be multi-coated. Coatings also play a big part in the performance of reflectors because not all of the light is reflected; there is a small loss at each mirror surface. Today's reflectors usually have a thin coat of aluminum as the mirror and then an overcoat of silicon monoxide or silicon dioxide to protect it. Silicon dioxide produces a more durable coat than silicon monoxide but requires specialized equipment to apply it and is therefore more expensive. Protection is needed because, in most reflectors, the mirror is open to the elements and deterioration of the reflective layer reduces the resolution of the telescope. All Sky-Watcher reflectors are multi-coated with silicon dioxide for more durability.


Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore reducing the quality of the image.

Absolute Magnitude

The apparent brightness a star would have if placed at a distance of 10 parsecs from the earth.

Achromatic Lens

A refractor lens, made of two or sometimes three separate lenses, which has the effect of bringing most of the viewed colours to a sharp focus, thus reducing chromatic aberration.


A simple mount that allows movement in altitude (up and down) and in azimuth (side to side).

Antireflection Coating

A thin layer of film applied to an optical surface that reduces the loss of transmission of light.


The diameter of the primary mirror or lens.


A group of stars that appear to make an easily recognized shape, such as the "Big Dipper" or the "Coathanger".

Barlow Lens

A "negative" lens which, when placed in front of the eyepiece, increases the focal length and magnification and decreases the field.

Catadioptric System

A system using both lens and mirror components to produce an image, allowing these telescopes to be more compact than other designs.

Celestial Sphere

An imaginary ball with the earth at its centre. All astronomical bodies, disregarding their true distance, are assigned a two-dimensional location on the surface of this ball.

Chromatic Aberration

The tendency of a lens to bend light of different colours by unequal amounts. It can produce nasty haloes around bright objects. A well-made achromatic lens reduces this problem.

Coated Optics

In lenses this is an antireflection coating. In mirrors a coating is applied that preserves the aluminum mirror surface.


The process of aligning all the elements of an optical system. Collimation is routinely needed in reflectors, often in Catadioptric systems but seldom in refractors.


Similar to Latitude on the Earth's surface, it is the distance in degrees North or South of the Celestial Equator (the projection of the Earth's Equator onto the Celestial Sphere). The degrees can be sub-divided into minutes and seconds.

Deep Sky

A name given by amateur astronomers to objects beyond our Sun and its planets.

Dew Cap

A tube extending forward from the front lens of a telescope. It prevents dew from forming on the lens as it cools down, and acts as a sunshade to reduce reflections during the day.


A mirror or prism system which changes the angle and orientation of the light rays coming from the telescope to the eyepiece.

Double Star

Two or more stars that appear very close in position. True double stars are in orbit about one another, while optical doubles simply seem close from our point of view.


The blocking of one astronomical body by another as seen from the earth. The most common of these events are Solar and Lunar eclipses.

Equatorial Mount

A telescope mount with an axis parallel to the axis of the earth. This provides easy tracking of sky objects and for photography when combined with a clock drive.


Also called an ocular. This is a small tube that contains the lenses needed to bring a telescope's focus to a final image in the eye. Telescopes usually come with at least two eyepieces: one for low power and a second for a higher power view.

Eye Relief

The distance between the eyepiece lens and the position in which the eye must be placed to see through the telescope. Telescope users who wear eyeglasses while observing, appreciate the benefits of longer eye relief.

Exit Pupil

This is the diameter of the beam of light from the eyepiece which reaches the pupil of the eye. It is usually expressed in mm, and determined by dividing the diameter of the primary (in mm) by the Magnification. Knowing this value and the diameter of your dilated pupil allows you to choose the eyepieces which will work best for you with a specific telescope.

Field of View

The maximum view angle of an optical instrument. The number, in degrees, supplied by the manufacturer is the Apparent Field of View. To find the True Field of View (also known as the Actual Field of View), divide the Apparent Field of View by the Magnification.


This is usually a disk of coloured glass or film that sits in front of the telescope eyepiece or objective. It transmits only certain wavelengths of light while rejecting others. (It is important to remember that a Solar filter must always be placed in front of the objective.)


A low power telescope attached parallel to the main instrument which provides easy object locating and telescope aiming.

Focal Length

The distance of the light path from the objective (primary lens or mirror) to the convergence of the beam. The convergent spot is called the Focus or Focal Point.

Focal Ratio

This is found by dividing an optical system's Focal Length by its Aperture. The resulting value is sometimes called the system's "speed".


A device which brings the light rays in a telescope to a precise focus. Common designs include geared (rack-and-pinion), gearless (Crayford-style) and helical.

Galactic Coordinates

A system of latitude and longitude defined by the plane of our galaxy rather than the equatorial system (RA and DEC) based on the celestial equator. Coordinates can also be specified locally, for example by Altitude and Azimuth.

Globular Cluster

A very old, large, dense cluster of stars, bound by gravity. Many form spherical clouds around galaxies. Our galaxy is surrounded by at least 130 globular clusters.


A transparent optical element consisting of one or more pieces of glass. A lens has curved surfaces that bring distant light to a focus.

Magnifying Power

The amount by which a system increases the apparent size of objects. Magnification is determined by dividing the Focal Length of the telescope by the Focal Length of the eyepiece.


The bright flash of light seen when a piece of material from space (a meteoroid) burns up in the earth's atmosphere. A piece of this material which reaches the ground, is called a meteorite.


In a telescope, it is a highly polished surface made to reflect light. Primary mirrors are usually made spherical or paraboloidal (parabolic) to focus the light rays.


A cloudy object composed of gas and dust which glows with its own light is called an emission nebula while one illuminated by the starlight of nearby bright stars is a reflection nebula. A cloud of dust which blocks light from star fields or bright nebulae beyond it is a dark nebula.


The primary or largest element in an optical system; sometimes called the "fixed optics."

Open Cluster

A group of stars, normally resolvable, which are bound together gravitationally. They are usually about the same age, having being born together from a collapsing nebula.

Optical Tube Assembly

The housing and optical train of a telescope; not including the mount, diagonal, eyepiece or accessories.

Parabolic Mirror

A parabolic or more accurately a "paraboloidal" mirror, is ground to a shape which brings all incoming light rays to a perfect focus, on axis.

Planetary Nebula

A circular or oblong region of gas that has been thrown off by a central star. Its name comes from its apparent similarity to the disk of a planet seen in a very small telescope.

Polar Axis

A telescope mount's axis that is parallel with the earth's axis. With a drive motor, the motion of stars due to the earth's movement can be counteracted so that they remain in the field.


See Magnifying Power.

Prime Focus

The focal point of the objective mirror or lens.


The ability of an optical system to reveal details.

Resolving Power

The ability of a telescope to separate closely positioned points.

Right Ascension

Similar to but not the same as Latitude on the Earth's surface. It is the position eastwards from the Vernal Equinox, in 24 one-hour units. The hours can be sub-divided into minutes and seconds.

Setting Circles

Circular scales attached to the telescope. They are marked off in degrees of Declination and hours of Right Ascension. Together, the circles allow the position of a known object to be found by setting the dials to the equatorial coordinates.

Spherical Aberration

A blurring of the image caused by the inability of a spherical mirror to focus all light from infinity to one focal point. Light rays from the edge of the spherical mirror focus to different points than those from the centre.

Star Cluster

A group of stars that travel together through space. See Globular Cluster and Open Cluster.

True Field

How much sky, in angular measure, is available at the eyepiece. It is contrasted with Apparent Field, which measures the field of the eyepiece alone.

Widefield Eyepiece

An eyepiece with an Apparent field of more than 50 degrees.


The point in the sky directly overhead.

Zoom Lens

An optical system which provides a variable focal length.

Amateur Astronomy

Beginner's Guide to Amateur Astronomy: An Owner's Manual for the Night Sky by David J. Eicher and, Michael Emmerich (Kalmbach Publishing Co., Books Division, Waukesha, WI, 1993). NightWatch: A Practical Guide to Viewing the Universe by Terence Dickinson, (Firefly Books, Willowdale, ON, Canada, 3rd edition, 1999). Star Testing Astronomical Telescopes by Harold Richard Suiter, (Willmann-Bell, Inc., Richmond, VA, 1994). Star Ware: The Amateur Astronomer's Ultimate Guide to Choosing, Buying, and Using Telescopes and Accessories by Philip S. Harrington (John Wiley & Sons, New York, 1998 ). The Backyard Astronomer's Guide by Terence Dickinson and Alan Dyer (Firefly Books Ltd., Willowdale, ON, Canada, revised edition, 1994). The Beginner's Observing Guide: An Introduction to the Night Sky for the Novice Stargazer by Leo Enright, (The Royal Astronomical Society of Canada, Toronto, ON, Canada, 1999). The Deep Sky: An Introduction by Philip S. Harrington (Sky Publishing Corporation, Cambridge, MA, Sky & Telescope Observer's Guides Series, ed. Leif J. Robinson, 1997). The Universe from Your Backyard: A Guide to Deep Sky Objects by David J. Eicher (Kalmbach Publishing Co., Books Division, Waukesha, WI, 1988). Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope--and how to Find Them by Guy J. Consolmagno and Dan M. Davis, (Cambridge University Press, New York, 3rd edition, 2000)


New! The Great Atlas of the Stars by Serge Brunier, Constellation photography by Akira Fujii (Firefly Books; Willowdale, ON, Canada 2001). A Manual Of Advanced Celestial Photography by Brad D. Wallis and Robert W. Provin (Cambridge University Press; New York; 1984). Astrophotography An Introduction by H.J.P. Arnold (Sky Publishing Corp., Cambridge, MA,Sky & Telescope Observer's Guides Series, ed. Leif J. Robinson, 1995). Astrophotography for the Amateur by Michael Covington (Cambridge University Press, Cambridge, UK, 2nd edition,1999). Splendors of the Universe: A Practical Guide to Photographing the Night Sky by Terence Dickinson and Jack Newton (Firefly Books, Willowdale, ON, Canada, 1997). Wide-Field Astrophotography by Robert Reeves (Willmann-Bell, Inc., Richmond, VA, 2000).

Observational References

A Field Guide to the Stars and Planets by Jay M. Pasachoff, (Houghton Mifflin Company, 1999). Atlas of the Moon by Anton


Astronomy Magazine (Kalmbach Publishing Co., Waukesha, WI) Sky & Telescope Magazine (Sky Publishing Corp., Cambridge, MA) SkyNews Magazine: The Canadian Magazine of Astronomy & Stargazing (SkyNews Inc., Yarker, ON, Canada)

What is an eyepiece?

An eyepiece is a magnifier, much like a high power magnifying glass. When placed at the real image made by the lens or mirror of a telescope, the eyepiece projects a virtual image into your eye, enabling you to see the target.

What is a Barlow Lens

A barlow lens has a negative focal length which increases the effective focal length (E.F.L) of the objective lens or mirror of the telescope. It is always placed between the objective and the eyepiece and results in increased magnification and decreased field of view.

Will a telescope work without an eyepiece?

Not for visual purposes, as the eye cannot process the real image made by the objective. The telescope may be used without an eyepiece for camera and other instruments.

How much magnification can I use with my telescope?

Every telescope is different, but a rough rule of thumb is 30-50X per inch diameter of the objective. A good refractor may, however, use 100X/inch on bright objects, so this is not a hard rule. You can always increase the magnification above these limits, but it is pointless if you're not seeing more. This rule breaks down for larger instruments, as the distortion of the atmosphere limits practical magnification to 300X. See Usable Magnifications.

What is Apparent Field?

Apparent Field (A.F.) is the angle viewed by the eye when looking into the eyepiece. An eye by itself has an A.F. of about 100 degrees, so any well corrected design up to this value would be a benefit.

What is Eye Relief?

Eye relief is maximum distance between the eye and the eye lens of the eyepiece to see the eyepiece's field stop. (The field stop is the baffle at the image plane that produces the field edge.) Adequate eye relief is a very important factor for comfortable viewing.

The image with my low power eyepiece is clear, but my high power is fuzzy. What's wrong with it?

There's probably nothing wrong with the eyepiece: you have probably exceeded the resolving power of your telescope. A television set looks clear 10 metres away, but up close you can see the imperfections.

Which eyepiece design is best?

This often asked question is quite irrelevant, as different design's performance varies with different telescopes. Different eyepiece designs have various characteristics. For example, and expensive widefield design is not required for planetary viewing, where the only important thing is maximum contrast. A Plossl or Orthoscopic would probably be best, but almost all design s are good performers on-axis for any f/ratio. Telescopes with F/ratios>10 are quite tolerant of simple low element eyepieces up to 55 deg. A.F., but telescopes <6 are a different matter. Off-axis performance requires powerful correction to properly image the highly convergent beam. Each eyepiece and telescope performs as a system, and their image can only be evaluated as much.

What is Exit Pupil?

Exit Pupil is the size of the light beam the eyepiece projects into your eye. Exit pupil can be calculated as follows:

Most night adapted eyes open to 5-7mm, so it's not a good idea to use eyepieces which give an exit pupil much larger than this, as the beam won't fit into entrance pupil of your eye.

Is high magnification better?

Only for some objects, although undermagnification is often a problem, even for experienced observers. The penalty for increased magnification is reduced field of view and brightness; faint objects grow fainter as the magnification is increased This is why larger aperture telescopes are so effective on faint objects; they provide enough light to stimulate the eye at high magnifications. For example, a 4-inch telescope will only view a globular cluster effectively at 80X, and it will appear as a blob. A 6-inch will resolve the outer stars at 130X, an 8-inch will resolve further in at 200X. 10 and 12.5-inch telescopes will make them glitter to the core at 300 and 400X.

Which eyepiece should I choose?

If brightness is not a factor, choose the eyepiece that will encompass the object, then allow for a suitable backdrop. If you want to know the actual field on view the eyepiece will give (True Field), this can be calculated as:

Why are some eyepiece more expensive than others?

When you pay more for an eyepiece you are usually paying for: Field of view: Eyepieces that have many lenses to correct for the five major aberration (these aberrations give increasingly worse, the lower the focal ratio of the telescope) have obviously higher costs in lenses and coatings. Eye relief: Using larger, more expensive elements in eyepieces allows for a greater distance between the eyes and eyepiece. Coatings: 2-layer multicoatings on both faces of all lenses will typically add 25% to the cost of an eyepiece, but this is absolutely necessary to preserve the contrast of the image when the light has to go through 7-9 lenses. Advertising: Those ads aren't free.

Why is the image better at the center of the field?

All commercial eyepieces are made with spherical elements, as these are the only type that are easily mass produced. These naturally produce aberrations, which become much worse in highly convergent light beams. There is no way to avoid all aberrations when using spherical elements. Clever eyepiece designer can, however, minimize the objectionable ones and cause others to manifest themselves in an acceptable form.

What is the black spot I see in a low powered eyepiece in my reflector during daylight?

A low powered eyepiece in a reflector produces a large exit pupil with a large image of the secondary mirror obstruction. During the day, when the pupil of the eye is small, if the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night, when the pupil of the eye is large, the darkened region is not noticed.

What is the "Kidney Bean Effect"?

The "Kidney Bean Effect" is not the same phenomenon as the before mentioned "black spot". In some long f.l or wide angle eyepieces, it is sometimes necessary to move the eye closer to the eyepiece in order to see the edge of the field. Sometimes, when this occurs, parts of the field between the centre and the edge are cut off, as part of the quickly converging beam misses the eye's pupil. This appears to the observer as a giant kidney bean shaped dark region that meandors around the field as head moves.

Which works better? An eyepiece or a Barlow+eyepiece giving the same magnification?

The only time the eyepiece alone may perform as well, is on-axis, in a high-contrast application, as the extra optics of the barlow may cause a slight depreciation. Optically, for all other sues, the eyepice+barlow outperforms the eyepiece working alone. The reason? Most of the aberrations caused by positive spherical lenses (Coma, Astigmatism, Curvature of Field and Spherical Aberration) can be reduced and sometimes almost eliminated by introducing a negative system (barlow) which has the same aberrations in negative quantities! Spherical aberration of the system is reduced as the positive spherical aberration of the eyepiece is cancelled by the negative spherical aberration of the barlow. The other aberrations cancel in a similar way! This is one of the eyepiece designer's most powerful weapons, and it is used in most of the shorter focal length ultra-wide designs. Another great benefit of this idea is that the longer eye relief of the longer f.l. eyepiece used with the barlow is retained.

How important it is to get a parfocal series of eyepiece?

Parfocal eyepiece sets reduce the amount of refocusing when changing powers, but it is rare when no refocusing is required. Parfocallizing of eyepiece sets is a non-performance factor when choosing oculars.

Do anit-reflection coatings improve light transmission?

Yes. Conventional thought seems to be that all the light not reflected is transmited through to the next medium. This is critial to the performance of high-element wide angle designs with many refractive surfaces.

When I'm observing a bright object like a planet, I see an opposing ghost image. What causes that?

The ghost image, and it's evil twin, the out-of-focus ghost is caused by internal reflections inside the eyepiece. The only way to eliminate these is to eliminate air-spaces in the eyepieces, as the ghost is caused by a double bounce between two lenses in close proximity. While the ghost is an annoyance, the out of focus ghost is more of an enemy, as it reduces overall contrast of the image, which determines how much detail you'll be able to see. The treatment, if not eh cure, is di-electric multicoating of the lens-facing surfaces inside the eyepiece.

How many eyepieces should I have?

Eyepieces are the most critical factor concerning the performance of your telescope, excepting a dark sky. Eyepieces create the image your eye will see, and the right ones will give you the experience that makes amateur astronomy so rewarding. Even the best instrument will never perform to it's potential visually with poor oculars. Since most manufacturers sell their telescopes with inexpensive ones, and since most people selling a telescope keep their good eyepieces, the aftermarket is your best source. Borrow as many as you can and try them out; for every object there will be an eyepiece that works best with your particular telescope. You'll probably be satisfied with 5-8 good eyepieces; and you'll use your telescope much more often with good ones.