This article will help the reader understand how telescopes work and how to best pursue the decision making process when buying an amateur telescope. While it is geared more towards astronomy, the many of the basic principles apply to birding or nature watching telescopes too.

A suitable astronomical telescope can cost as little as a few hundred dollars for a youngster, or hundreds, or thousands of dollars for an teenager or an adult depending on their interests. One should think carefully before buying a telescope as a gift for anyone else particularly if you are uncertain about what the person may wish to do with a telescope. Also keep it in mind that when buying a telescope for a youngster, the chance of true success will be improved if someone can provide initial instruction about how to use the telescope, with some close guidance on finding one's way around the night sky. A telescope can be a great gift for a young child if that child has demonstrated some sustained interest in astronomy or related subjects but, consider that very young children go through phases that often pass in a matter of weeks, days or hours and you should so balance the desire and expense carefully.

After you buy: for reasons including permitted access to observing sites, for security in numbers at night, and to obtain help in learning the hobby we urge you to participate in astronomy club events, especially their star watch activities which are usually open to members and the public for free. And to learn more about what can be seen in the night sky, and how to observe you may refer to many on line sources including our own observing planning aids page, or you can perform a search by entering your state and "astronomy club" in an Internet search engine.

Table of Contents:
1. Introduction: Telescopes Are Tools
   
2. The "Test Telescope"
   
3. How Telescopes Work
   
4. What Parts Make a Telescope?
   
5. Earth's Motion and the Equatorial Mount
   
6. Fork Equatorial Mounts
   
7. German Equatorial Mounts
   
8. Computer Controlled Fork Mounts
   
9. How To Weigh What Will be Most Important to You
   
10. Refracting Telescopes
    
11. Reflecting Telescopes
    
12. Catadioptric Telescopes
    
13. Accessories: The Eyepiece
    
14. How and Where Telescopes are Sold
    
15. How To Get Started and Avoid Failure

Company Seven's staff is extraordinary qualified to assist you. We are public service oriented and have decades of experience working with amateur and professional astronomers. We provide an uncommonly fascinating showroom open for more than twenty five years. While our customer satisfaction rate is uncommonly high, we have noted the majority of people who purchase a telescope from most other retailers tend to become bored or frustrated with the instrument. The frustrations tend to originate with:

a. a poor choice of instrument and or accessories,
b. the poor quality of manufacture: materials, or design, or assembly and
c. the seller and the new owner lack the understanding of how and when to use the telescope.

Not only will the loss of interest in the instrument result in the loss of money, but it is far worse when any fascination that one might have ever had about this hobby is destroyed.

1. Introduction: Telescopes Are Tools Our first contact with a newcomer calling or visiting our showroom often starts out with them introducing themselves with something like "I want to buy a beginner telescope" or "I want to buy a telescope for $XXX, what do you have?" While most of us who work for Company Seven probably started into the hobby years ago with asking similar questions, now we relate that approach akin to greeting the staff in a hardware store with "Hi, I want to buy a tool and I have $50 to spend - what do you recommend?" That approach reveals nothing about what one hopes to accomplish, and it may arbitrarily limit what you are offered by most salesmen elsewhere.

The approach that is more likely to produce a good result requires one to start thinking of telescopes as tools, with each having specific strengths and weakness. There is no one instrument that can do all that can be done in astronomy or for nature watching; even the Hubble Space Telescope has it's strengths and weaknesses. And so, the approach used when buying a telescope should be similar to that decision making process when one buys a tool. And at least initially do not be too preoccupied with budget while you are learning - learning is free anyway. And just like picking a tool when your task requires a $100 table saw then you should buy (or rent) that table saw; you would not buy a $5 screwdriver instead just because that is all you have to spend: without first knowing what the tool is expected to do it makes little sense to buy one solely because it is within a preconceived budget.

When most people visit a hardware store they do tend to understand which tool they need. When they contact a telescope store, then the matter of choosing a telescope is complicated by the fact that few of those who are new to the hobby have an understanding of what they may expect to do with the various available telescopes. For example, when we ask visitors to our shop what they wish to see, they often answer "I want to see stars". Well, one can see stars with the naked eye. And a star viewed through a telescope still looks like a pinpoint of light (in some telescopes or on turbulent nights a star may appear more like a "blob"). And so we at Company Seven first work to encourage to the novice, explaining what they may realistically expect to observe with a telescope. We do this by discussion, by showing drawings and photos on display in our showroom, by offering well though out introductory books, and providing opportunities to attend observing sessions with astronomy clubs.

2. The "Test Telescope" We hear many newcomers reasoning "I just want to get myself (or him or her) a cheap telescope to see if we are interested in the hobby". This rationale is rarely successful because the cheap telescopes are quite limited in what they can show. And so the novelty of most modestly priced telescopes usually wears off soon as the user will almost always quickly tire of going out only to observe an object with no detail, or changing features. Imagine going out night after night only to see Saturn appear as a "BB within a washer". The worst failure is when a poor choice of telescope destroys any budding interest there may have been in the hobby of astronomy. On the other hand, if one does remain at all interested in the hobby then that modest first telescope is likely to be outgrown so fast that that the customer will be back soon for a better telescope, and the $100 or more invested in the first telescope will likewise have been wasted. And since the accessories provided with the common department store telescope are usually not upwardly compatible (if at all desirable) for use with a better quality telescope, there will be nothing to salvage from the first telescope to help growth into another more advanced model. We suggest you save and buy something that has a better chance of making you want to go out night after night.

God did not make all telescopes equal: telescopes do vary in design, aperture, physical dimension (length, width, and weight), and in terms of their degree of excellence. And while two telescopes may appear to be identical in their advertising claims and by their appearance, one telescope may provide a great experience while the other becomes a source of frustration. Unlike other commodities such as cars where there are standards (every car has headlights, tail lights, can do 55 mph, etc.), or in foods where guidelines define what claims may be made, there is no regulatory authority governing the advertising of telescopes. Every telescope therefore brings some compromises. The goal of the buyer should be to make the most reasonable "compromise" possible after considering your particular circumstance and observing (or photography) goals.

3. How Telescopes Work a telescope can be thought of as a "Light Funnel". The telescope gathers light that enters through a relatively large opening (the Aperture). Then the telescope optical components (reflecting mirrors, refractive lenses, or both) will then bend that light gradually condensing it to form one "virtual" image at a point (Focal Plane). The Focal Plane of a visual telescope is located at a point just beyond the telescope Focuser. The Focuser holds an Eyepiece, which magnifies the image and makes it visible to the eye.

A good telescope improves on the naked eye in two aspects:

Light Gathering Power: to see very faint objects - making the invisible obvious, and

Resolving Power: reveal fine detail on a distant object, or those smaller than the eye alone can perceive.

If one were to hold a thin sheet of plain or wax paper just beyond the telescope focuser and move it in or away from the telescope until the image is most clear then one would observe the image formed at the Focal Plane. Furthermore, as light is bent and shaped on its way through the telescope the image will be reversed; this can be seen in the illustration below where the image formed is upside down and reversed left to right.

Above: How light passes through a telescope to form an image at the Focal Plane. Refractor portrayed with Objective Lens at right. (7,433 bytes)

Terrestrial (or "spotting") telescopes differ from astronomical telescopes primarily in the facts that spotting telescopes are generally more compact and rugged for extreme ease of transport in the field, and spotting telescopes incorporate an image erecting prism to present an image that appears right side up and correct left to right. While the image from most astronomical telescopes may appear upside down, and or inverted left to right. This is not a great concern because astronomers care more about the quality of an image than it's orientation. And after all, how many people would even know they were looking at an Jupiter upside down? The eyepiece is at a convenient position on most Newtonian telescopes, but it is at the rear of the tube of Refracting and Catadioptric telescopes. So to provide more comfortable observing through Refracting and Catadioptric telescopes these are usually furnished with a 90 degree angled mirror or prism accessory ("Diagonal" or "Zenith Prism") installed between the focuser of a telescope and an eyepiece. The Diagonal diverts the cone of light coming out of the of the telescope to a more comfortable viewing position, this is especially helpful if you are looking at an object overhead in the sky with these telescopes. A good quality Diagonal may account for some barely perceptible light loss, but the convenience provided by it will be well worth it. The Diagonal accessory is not compatible with most Newtonian telescopes since the Newtonian lacks the back focus distance needed to pass the light through the extra distance taken up by the Diagonal and reach the eyepiece.

The image from a Refracting telescope with a Diagonal installed will appear right side up, but backwards left to right. Examine the following illustration to see how the accessories affect the perspective.

Above: How an image will appear at the Focal Plane of most telescopes. Top Left: Normal view. Top Right: Erect and Reversed - typical in an astronomical refracting or Catadioptric telescope with Diagonal, Bottom Center: Inverted and Reversed - typical of a Newtonian or other telescope with no accessory. (5,849 bytes)

Accessories can affect the quality of the image from a telescope. For example, a poor quality Diagonal will introduce problems which may appear as a blurring, loss of brightness or subtle details. This is why astronomers do not mind seeing an image that is reversed, in fact it is not uncommon to find Sky Atlas or Moon chart printed to match their orientation as observed through a telescope. For viewing the brighter large objects such as the moon, or for day time use looking at terrestrial objects then an image erecting prism may be employed with very little noticeable degrading of the image.

4. What Parts Make a Telescope?

Right: Astro-Physics 92mmf7 "Stowaway" Apochromat Telescope with retracting Lens Shade extended, Lens Cover removed. Telescope is shown on optional TeleVue "Telepod" Alt-Azimuth Head, with optional Astro-Physics 2 inch Maxbright Mirror Diagonal and TeleVue 35mm Panoptic eyepiece attached (209,055 bytes).
Click on image to see enlarged view The astronomical telescope should be thought of as a system made up of two primary components, and accessories:

A) the Optical Tube Assembly (OTA) - includes the optics of the telescope, mechanisms that precisely hold the optics in place, and a focuser which supports an eyepiece or camera. It is the OTA which will determine what and how well you will be able to see or photograph, and how bulky the system will be. The OTA design and quality of manufacture also determine how maintenance free, or durable the telescope will be.

The telescope optical tube is characterized by it's 1) Design (see below), 2) Aperture the diameter of the primary mirror or objective refractive lens (expressed in millimeters or inches), and 3) effective Focal Length - essentially a measure of how much magnification the basic telescope optical system produces; this too is expressed in millimeters or inches. Another term that you may come across is Focal Ratio.

So when you read the description of a telescope "60mm x 700mm" this indicates an aperture of 60mm (2.4 inches) and Focal Length of 700mm. The Focal Ratio of aperture to Focal Length would be 700 divided by 60 or f/11.7. This is not a true measure of relative brightness since this formula does not consider the efficiency of the system which may include effects of: central obstruction, quality of optical figure, reflectance of mirrors, throughput of refractive elements. A 4 inch f10 aperture refractor can provide a notably brighter image than a 4 inch aperture f10 obstructed reflecting or Catadioptric telescope...more on these later.

B) the Mount: includes either a Tripod or Pier, and a Head. These are the components which will determine how one can employ that telescope OTA, how complicated or simple the system will be to operate, the degree of convenience or ability to share the telescope with others, and how bulky or heavy and quick to set up the overall system will be.

A poor mount supporting a superb OTA produces a poor telescope. One good test of a telescope mount is to tap on the telescope optical tube lightly then observe if the telescope jitters for more than a brief moment. This is not to say that astronomers strike the telescope while observing, but imagine how a slight breeze or the touch of a hand reaching to focus that telescope will cause the image in the eyepiece to blurr at 100X or more when even a slight tap causes the telescope to vibrate for several seconds!

Common photographic camera heads and tripods are inadequate for holding astronomical telescopes since tracking smoothly while maintaining proper balance become problematic. Most of these tripods will "shake, rattle and roll" with even the slightest touch of a guiding hand, or the pressure from a stiff breeze. If you adjust the head lock so that the motions are smooth and easy, then as the telescope is pointed higher and higher into the sky the center of gravity shifts to the observer's side of the mount head and the weight of that telescope will cause it to slip and gradually slide. If you tighten the head lock to support the telescope then there is enough drag so that one can not move the telescopes smoothly - a move to just slightly center a planet in the field of view is likely to produce a "jerk" that shifts the object right out of the field of view. Even Fluid Heads which move more smoothly rarely can reach zenith (overhead), and they tend to sag when set onto a target - this is not a problem at low magnifications with camera lenses, but is pronounced at higher magnifications.

5. Earth's Motion and the Equatorial Mount Stop the World, I Want To Observe! The relatively simple Mount Head that permits you to move the telescope smoothly left to right or up and down (Alt-Azimuth Mount - as shown above to the right) may be quite suitable for some casual astronomy, and terrestrial uses. This mount may also be suitable for a short exposure film or video photography of the Moon, or with a safe solar filter the Sun, or terrestrial objects and wildlife. But keep in mind the Earth rotates at a rate of about one revolution every 24 hours or so, and celestial objects seem to move across the sky in arcs as the Earth turns. On a clear night one can observe the Moon rising through the trees at only a magnification of 1X (naked eye). A view of the full Moon will require a telescope operating at a magnification of about 80X or more. One has to move a manual Alt-Az mount very carefully up and to the right as the Moon rises from the East peaking in the Southern sky, and then continue moving the telescope down and right as the Moon sets in the West. So now speed up that motion of "up, right, etc." with the Moon moving across the field of view of an eyepiece at a rate about 80 times faster than when you observed it with the naked eye...get the picture? Now try following a Planet at 200X or more!

The Earth rotates about its axis once full turn over a period of just under a day. This period of 23 hours 56 minutes 04.09053 seconds is called the sidereal period, or the period relative to position of the stars overhead. The sidereal period is not exactly equal to one 24 hour day because by the time the Earth has rotated once on its axis, it has also moved along the path of its orbit around the Sun, because of this the Earth has to keep rotating for about another 4 minutes before the Sun seems to be back in the same place in the sky that it was in exactly a day before. As the Earth rotates in space the celestial objects overhead appear to move in an arc across the sky.

Right: Click image if you wish to see an animated movie of the Earth Rotating.
This is a Quicktime presentation courtesy of NASA, and so your Web Browser should be enabled to view it (3,491,793 bytes).

For an observer located in the Northern Hemisphere objects rise in the Eastern horizon, gradually moving in an arc up and right across the sky towards the South, and then continue moving to the right and down towards the Western horizon. This motion can be observed with the naked eye if one carefully observes the limb of the Moon as it rises or sets behind trees or some other fixed object in the distance, the motion though slow is perceptible. A telescope provides magnifications far beyond that of the human eye, as the magnification of a telescope in increased (25X to 50X, 50x to 100X, etc.) then this motion of an object drifting across the sky will appear accelerated. With a very well designed Alt-Azimuth mount it is possible for an observer to track objects by manually guiding the telescope at magnifications of 200X or more, and some people are comfortable at magnifications in excess or 300X.

The solution to the limits imposed by the manually guided Alt-Azimuth mount is the Equatorial Mount. There are a number of designs of Equatorial mount but those which the amateur is most likely to encounter are the Equatorial Fork and the less intuitive German Equatorial arrangement. Computer Controlled Fork Alt Azimuth mounts are coming available too where the mount moves simultaneously in two axes to stay on target. Either mount head is attached to a suitable field Tripod (usually made of wood or aluminum), or onto a portable or installed Pier usually made of aluminum or steel.

The Equatorial Mount permits one to turn a telescope in the opposite direction from that which the Earth is rotating, at the same apparent speed as the Celestial objects are moving. This permits tracking of celestial objects by moving the telescope clockwise in an arc sweeping in one smooth movement from East to West across the sky in the "Right Ascension" (R.A.) direction. To track precisely, the rotating Right Ascension Axis of the mount is adjusted up or down and left or right by the user so that this axis of the mount is parallel to the rotational axis of the Earth. Depending on the telescope, an equatorial mount will provide acceptable results for visual uses even if the mount is aligned to only within about a few degrees of the Celestial Pole. But as you align the Mount more and more accurately then the need to periodically make minor adjustments to center a target in the eyepiece becomes less frequent. A very accurate alignment of the mount to the Celestial Pole is necessary for those persons who intend to attempt long exposure astrophotography or CCD imaging.

Equatorial mounts may be equipped with hand driven gear controls for fine adjustment of the Right Ascension axis, and usually also for the other Declination axis too. The controls of some German Equatorial Mounts are provided with long, dangling flexible cables to make it possible to control the telescope while at the eyepiece. The motion is so slow that few people can observe the motion, this rotation is only on the order of one revolution per 24 hours! For convenient observing, or for astrophotography many mounts will accept the option of (or may include) a battery powered drive motor to permit automatic rotation of the telescope. The drive is sometimes referred to as a "Clock Drive" since the earliest mechanical telescope drives were clock mechanisms modified to rotate only once every 24 hours. With this accessory you could walk away from the telescope and come back later to find the telescope still on the target. For those persons "down under" in the Southern Hemisphere, the motion turns the telescope in the opposite direction and so most of the better motorized telescopes have a switch to permit the operator to select a clockwise or counter clockwise motion.

When properly set up the Equatorial Mount (do not panic, the staff at Company Seven will teach you how to do it) These mounts may be equipped with hand driven gear controls with long, dangling flexible cables. Many mounts will accept the option of (or may include) a battery powered motor drive, in such a case you could walk away from the telescope and come back later to find the telescope still on the target. The drive is sometimes referred to as a "Clock Drive" since the earliest mechanical telescope drives were clock mechanisms modified to rotate only once every 24 hours. The type, and quality of the mount and its drive system will determine what if any distractions from observing there may be, and if one will be provided the ability to become involved with astrophotography.

6. Fork Equatorial Mounts are so named because of their resemblance to the tines of a fork. The Fork Mount is a very easy to understand device being the most intuitive telescope mount they have become ubiquitous. Fork Mounts can be the quickest Equatorial platform to set up since as few as two components (Telescope in the Fork, and Wedge and Field Tripod) may be all that comprise the basic telescope. The Fork Mount is basically an Alt-Azimuth mount, it moves up and down left and right, and so it can easily be used for terrestrial applications. For tracking in astronomical uses, the Fork Mount is bolted onto an Equatorial Wedge. The Wedge has an adjustable Tilt Plate which is raised or lowered so that the R.A. rotational axis of the Fork can be aligned to the rotational axis of Earth.

The Fork Mount usually includes a clock drive in the R.A. axis, and manual over ride fine geared controls. More recent computer controlled telescopes may include no manual controls relying instead on the electronics to operate all functions of the mount.

Left: Meade Instruments Model 2080, 8"f10 Schmidt-Cassegrain Telescope with Equatorial Fork Mount, 1980 (74,655 bytes)

The best known Fork Mount telescopes are the Questar 3-1/2" Maksutov-Cassegrain, the Ferrari of ultra compact, user friendly telescopes introduced in 1954 and still in production. But the most popular are the versatile and relatively compact Schmidt Cassegrain telescopes (SCT) pioneered by Celestron in 1970, but since 1980 also made by Meade Instruments. The biggest sellers are the 8" (20cm) models but Fork Mount SCT's are made as large as the Meade 16" observatory telescope. But the largest of these telescopes that can be managed by the average adult is the Celestron 11" (28cm) or the Meade 12" LX200GPS (30cm); each of these larger telescopes will exceed 70 lbs. (32kg) for OTA and Fork alone!

7. The German Equatorial Mount is selected for use with telescopes of relatively longer physical lengths, or for improved portability with some mid sized and larger telescopes, or to increase versatility. The German Equatorial Mount is the design preferred for the physically longer telescopes including the Newtonian Reflector and Refracting telescopes. Incidentally, Josef von Fraunhofer invented the mount, and in Germany the German Equatorial mount is called a "deutsche Montierung" (thanks Georg).

Right: Click image if you wish to take a 360 degree tour of the SkyView Pro™ 8 telescope which includes the German Equatorial mount shown at right.
This is a Macromedia Flash presentation, and so your Web Browser should be enabled to view it (163,690 bytes).

Easy to Use? The Equatorial Mount is not the most intuitive arrangement to the novice. But once the telescope is slipped into place, with counterweights added and balancing done in both axes done, and the mount Pole Aligned, with a little practice the system is then easy to use. The German Mount has two perpendicular axes called the Right Ascension (RA, or Polar) and Declination (Dec) axes. When the RA axis is aligned parallel with the Earth's rotational axis, objects can be easily "tracked" as they drift across the sky (due to Earth's rotation) by turning just one axis (RA) instead of two, as is required with an alt azimuth mount. You point the telescope onto an object by simply nudging the tube up or down, and left or right by hand. Once centered onto an object an electronic "Clock Drive" can keep the telescope moving with the object across the sky. And with the ability to track in Right Ascension comes the electronic capability to make fine adjustments in Declination too, since this is a "dual axis" drive system.

When mated to a suitable telescope, the German Equatorial Mount provides:

• The capability to easily center and then smoothly track celestial objects as they drift across the sky. This becomes particularly desirable when operating at those higher magnifications necessary to observe some of the brighter deep sky objects, and the more subtle features on the planets.
  

• Capability to accept electronic single or dual axis tracking drive. This can make the observing session more pleasing for an individual or for any group of people: a family, or with young children, or when sharing with groups of persons who may lack experience using a telescope.
  

• The German Equatorial Mount facilitates teaching a number concepts in astronomy including the Coordinate System used for Celestial Navigation.

The German mount arrangement has advantages over the fork mounted designs in that:

• the better German mounts are less sensitive to the balance needs of the fork mount

• when working towards the celestial pole while using accessories such as CCD cameras with flip mirrors, etc. fork mounts can bind the telescope and restrict movements

• German mounts allow one to quickly interchange telescopes. One night you might use a larger "light bucket" for small faint objects, then later slide off the light bucket and slip on another high resolution telescope for even better views of planets, or for astrophotographic coverage of wide fields of view.

• the fork mount arrangement can become heavy and unwieldy before the limit of practicality is reached with the German Mount. It is usually quite easy to remove an optical tube from a German Mount and then disassemble the mount too into smaller, lighter weight components for travel.

The telescope will swing from one side of the mount to another as once proceeds to explore the four quadrants of the hemisphere. When using a German Mount with a Newtonian telescope this may at times put the eyepiece at an uncomfortable position however, most Newtonians we offer will come with or have available as options a pair of clamp style Mounting Rings can be loosened so that one may rotate the optical tube to position the Focuser and Eyepiece at more convenient positions.

8. Computer Controlled Fork Mounts when introduced in 1985 by Celestron International the "Compustar" telescope became the first consumer oriented computer-driven telescope sold. The consumer oriented computer-driven Fork Mounted telescope has since evolved over the past decades to offer a good degree of simplicity and user friendliness, and Celestron and Meade Instruments have come to dominate the consumer markets for these products.

Left: Celestron International NexStar 11 GPS, 11"f10 Schmidt-Cassegrain Telescope with Computer Controlled Alt-Az (or Equatorial) Fork Mount, (85,089 bytes)

Computer control telescope mount systems require the careful integration of at least two basic sub systems: 1. a dual axis drive or telescope positioning system to move the telescope to a specified position, and 2. dual axis position feed back systems to keep the positioning system informed of where the telescope is pointing to.

No Pole Alignment is necessary to operate these telescopes, and set up may be as simple as turning on a power switch since the latest generation of GPS telescopes can determine their location, and with very little assistance from the operator the telescope may become operational. These Fork Mounts dispense with the weight and cost of the Equatorial Wedge, unless one wishes to become involved in time exposure astrophotography. But there are certainly some caveats when buying a consumer grade computer controlled telescope.

The Meade and Celestron computer controlled telescopes have approximately the following features: altazimuth operation, incredible 10 degrees per second or faster fast slewing speed, a selection of built-in tracking rates which may include: sidereal, King, lunar and solar, fully enclosed optical encoders for feedback of position location, standard 12 volt DC systems which can operate from batteries at a site almost anywhere in the world.

However, in part by the manner in which some of these telescopes are marketed some people have come to believe that these telescopes do everything for you, completely automatically, this is not true. Most of the consumer consumer oriented computer-driven still require the operate to have at least some knowledge of the night when setting up the telescope. When the telescope mount is properly set up, it can move the telescope from one object to another across the sky. If there are any errors in the system due to even a slight inaccuracy of initial set up, or slop in the optical system or optical tube mounting hardware, then the pointing errors can accumulate reducing the pointing accuracy to a point that the observing session may have to pause as the operate reindexes the mount precisely onto a known star or other reference point in the sky.

Also keep in mind the telescope's computer is not a substitute for observing experience. The telescope can not tell you when a night is clear and steady enough to see an object in its database clearly. And the telescope electronics can not tell you which accessory to use or not on any particular target of the observing session.

9. How To Weigh What Will be Most Important to You

There are many factors to consider when starting the process of choosing a telescope. One should be considering these thoughts:

a. What objects can you realistically expect to see from where you will observe?
• will you choose a high contrast telescope design that is more likely to show those objects you can see from an urban or suburban environment?

• are you prepared to take the telescope away from the city to darker skies? If so then you will be able to use it to observe the fainter deep-sky objects and so you may be more inclined to buy a larger telescope.

b. How will the telescope fit into the users lifestyle?

• how large a telescope will you feel comfortable carrying in and out of your home? If you do want a larger aperture but can not manage the weight of a fork mounted 11 or 12 inch SCT, then maybe you will consider a more modular German Mount arrangement.

• how large a telescope can you fit in your automobile? Will the wife, kids, dog, tent and cooler also fit or must they stay home?
  (Depending how you answer this may be followed by do you know a good divorce lawyer).

1. Telescope Aperture: the effective diameter of primary mirror or refractive objective lens.

2. Telescope Design: affecting telescope focal length and ratio, physical size and weight. Some designs are better suited for some activities (and the users' lifestyle) than others.

3. Telescope Quality: how well made it is; to some degree a superb small telescope can outperform a mediocre larger telescope - size does not always matter.

Left: "A Telescope Optics Selection Matrix" (9,638 bytes).

Since no one telescope is "perfect", that is capable of doing everything exceptionally well, each is a compromise of sorts. During the process of choosing a telescope based on performance the importance of the three variables to the buyer are balanced, one against the other. Observe how the "meeting points" originate at the corners and meet somewhere in between reflect potential compromises.

This is not to say that one can not find all three good qualities in one telescope. For an example of how this may work consider:

Telescope #1 is of Grade A quality (the best); it has a Grade A design, and it is of a relatively small Grade D aperture. Telescope #2 has a large Grade A aperture, with a Grade C or D in areas of Quality of excellence and Design. Depending on your priorities Telescope #1 may yet actually be a better choice Telescope #2.

The user may determine that telescope #1 is the best choice in terms of performance and portability reasons. Or, if the cost of telescope #1 is too high, then possibly the buyer may be able to find another telescope with some compromise of Quality or Design to a degree that it becomes a reasonable acquisition.

One could extend the number of variables if so desired, possibly factoring in usefulness for astrophotography. A spreadsheet could be written, with a column for each aspect that is considered important, and each with a value assigned - possibly using a 1 to 10 scale. After values and priorities are assigned, the values could be totaled up; this can help one to reach more objective understanding of the possible choices. Some spreadsheets can graph the data revealing something like a matrix illustrated above. A similar approach could be employed to help with the selection of a suitable mount. One can complicate the process of selection, or simplify it to many degrees in fact, we have encountered some persons who seemed to prefer doing the research for the purchase as much or more than the actual use of the telescope!

The consumer is likely to run into only a few basic telescope optical arrangements:  

10. Refracting Telescopes Left: Vixen 102FL 4 Inch f9 Fluorite Apochromatic refractor telescope shown on optional Vixen GP German Equatorial Mount and Tripod. This is one of the first relatively "affordable" high performance 4 inch apochromats made available to the amateur. (13,789 bytes).