Choosing a Telescope for Astronomy - Advanced

Given the bewildering array of telescopes on the market, how do you choose the right one?  To answer this question it is first necessary to explain the differences between telescope types, but for that discussion to be meaningful it is important first to understand some very basic points about astronomical telescopes in general.

Aperture is the Most Important Factor
The single most important specification for any astronomical telescope is its aperture.  This term refers to the diameter of the telescopeís main optical element, be it a lens or a mirror.  The aperture of a telescope relates directly to the two vital aspects of the scopeís performance: its light-gathering power (which determines how bright objects viewed in the scope will appear), and its maximum resolving power (how much fine detail it can reveal).  There are other criteria to be considered in selecting a telescope, but if you learn only one thing from this explanation, let it be this: the larger a telescopeís aperture (i.e., the fatter it is), the more you will see.

Donít Get Hung Up on Power
Unfortunately, the first question most beginners ask is not "What is this telescopeís aperture?" but "What is its magnifying power?" (Actually, when we are out at public observing sessions with our 14" Celestron Schmidt-Cassegrain telescope the two main questions I get are, "What did it cost?" and "How heavy is it?"  I usually avoid direct answers to either question).  The truth is, any telescope can be made to provide almost any magnification, depending on what eyepiece is used (there is a direct relationship between the focal length of a telescope and the focal length of any eyepiece you use).  The factors which limit the highest power that can be used effectively on any given scope are its aperture and the optical quality of its components.  In the case of aperture limitations, as magnification is increased and the image in the scope grows larger, the light gathered by the telescope is spread over a larger area, so the image is dimmed.  For optical quality of the components, poor components will yield a poor quality image, and magnifying that image will simply magnify the blurring in the image.  There is also an absolute limit, determined by the physical properties of light, to the image resolution which is possible for any given aperture.  As the magnification is pushed beyond that limit the image fails to reveal any additional detail and gradually breaks down into a dim, fuzzy blob.

The maximum useful magnification for any telescope is about 50 times the aperture in inches, or two times the aperture in millimetres.  This equates to about 100x to 120x with the smallest telescopes, which is enough to see the rings of Saturn and the cloud bands and moons of Jupiter.  The 2x per millimetre figure is a rule of thumb, and can vary up or down somewhat depending on the optical quality of the scope in question and the vision of the individual observer.  Experienced observers usually use much less power; 0.5x to 1x per millimetre is more appropriate for most objects.  So, any manufacturer claiming that their 60mm scope can provide good views at 450x (7.5 times the aperture in millimetres) is trying either to pull your leg or pick your pocket!

Bigger is Better, But...
While aperture is the most important specification of any telescope, there are exceptions to the rule that "bigger is better."  One is obvious: the need for portability.  The largest amateur telescopes are very big indeed, and demand either housing in a permanent observatory or possession of a strong back, a truck, and a gang of muscular and motivated observing buddies!  There is a line to be drawn between performance and portability, and where the line is drawn varies with the individual and his or her capacity for storage and portage.  Beginners are encouraged to start out with a scope of sufficient aperture to feed their interest, but of a size that they can manage easily.  Avoid succumbing to "aperture fever."  Victims of aperture fever choose the largest telescope they can afford without regard to portability.  Their monster scopes soon gather dust in the garage, unused because they are too heavy and bulky to bother with, while the once enthusiastic would-be astronomer loses interest in the hobby, or is frustrated by their inability to haul the beast around.

The Sky IS the Limit
The second limitation on very large telescopes is less obvious, but becomes apparent after the first couple of observing sessions: the Earthís atmosphere limits how much we can see.  Stars and planets viewed through a telescope appear to shimmer and shake, as their light passes through the air and is distorted.  This effect is known to astronomers as seeing, and it becomes more noticeable and bothersome as telescope aperture increases.  It especially affects observations of the Moon and planets, where high power applied to reveal fine details also magnifies the air turbulence.  For this reason, visitors to the George Observatory often comment that the image of Saturn which they see through my 14 inch scope is superior to the image through the 36 inch research telescope in the large dome.  Itís hard to explain to them that, in this case, for this type of object, the effect is not surprising.

The amount of distortion due to seeing varies, depending on the behaviour of air currents in the upper atmosphere, and to a lesser extent upon the altitude and topography of the observing site.  The recent passage of a weather front for instance, can produce poor seeing for several hours afterwards.  But on an average night, at an average site, air turbulence will limit useful magnification to 250x or 300x, and prevent telescopes larger than about 8" or 10" aperture from achieving their full potential for high-powered viewing.  Telescopes larger than 10" are most often chosen by observers who want to gather as much light as possible for viewing dim galaxies, nebulas, and star clusters.  These "deep sky" objects, affectionately called "faint fuzzies," are most often viewed at much lower power than the planets, so seeing is less of a problem.

Telescope Mounts
The last important topic to cover before delving into optical designs is that of mounts. Telescopes are offered on either altitude-azimuth (or altaz) mounts, which move up-down (altitude), left-right (azimuth), or equatorial mounts, which are tilted to align with the rotational (polar) axis of the Earth.

Altaz mounts are generally lighter and simpler to use, and are preferred if the telescope is to be used both for astronomy and daytime observing (or for daytime observing only).  The better ones offer slow-motion controls to aid in moving the scope by small increments, and are useful for powers up to about 150x.  The Dobsonian mount is a variation on the altaz mount.  It employs unconventional (for telescopes) materials like plywood and Teflon in a compact mounting that moves easily, is extremely stable, and can adequately support large telescopes at a very low cost.  Though there are no mechanical slow-motions or electric drives on a Dobsonian, a well-made example glides so smoothly on the Teflon bearings that with a little practice it is quite easy to track objects manually at 200x or more!

Equatorial mounts are designed specially for astronomy, and are not recommended for terrestrial viewing.  Their advantage is that they allow easier tracking of the stars across the sky.  This motion can be achieved with either a single manual slow-motion control or an electric motor drive (or clock drive).  The easier viewing they provide at high power makes equatorials preferred by observers who are most interested in the Moon and planets.  Also, youíll need an equatorial mount if you want to do astrophotography.  Please note that, due to rotational effects of objects as they cross the sky, it is not possible to do astro-photography using an altaz mount (for more information on this point see the relevant section on astro-imaging on this web site).

Different Scopes for Different Folks
Now that we understand these basic points of telescope performance and mounting, we can discuss the three basic optical designs of telescopes: the refractor, the reflector, and the compound (or catadioptric) telescope.

THE REFRACTOR

A refractor is what most non-astronomers think of when they hear the word "telescope."  Its tube is most often long and skinny, mounted on a tripod, with a lens at one end and the eyepiece at the other.  Refractors were the first type of telescope invented, and the finest refractors still provide the best images of any design for a given aperture.  They are often chosen by observers with a dominant interest in the planets and Moon, because they can provide sharp, high-contrast views at high magnification and are less bothered by atmospheric "seeing" than the other designs.  They also require less maintenance than reflectors or compound scopes, and are therefore popular with beginners.  The refractorís good performance at high power and relative insensitivity to light pollution makes it a good choice for a city-based observer, as the design performs best on the objects that are most easily seen from urban or suburban locations.  These advantages do not come without a price ó literally: refractors are the most expensive telescopes per inch of aperture.  Big refractors can cost several thousand dollars, and still are considered too small in aperture for serious deep-sky observing.  The long focal length of most refractors restricts the field of view, making it difficult to take in large extended objects like some clusters of stars.  And the long tube, with the eyepiece located at the back end, requires a tall tripod, which, if poorly made, can allow the scope to shake in the breeze, rendering high-powered observing difficult.  Also it should be noted that the highest quality "apochromatic" refractors (e.g. Takahashi brand name) do not use silica glass for their objective lens, but instead use a multi-element calcium fluorite lens.  This dramatically improves the colour correctness of the lens and produces pin-sharp stars - at a price!
THE REFLECTOR

The reflector uses a mirror, rather than a lens, to gather and focus light.  By far the most common design is the Newtonian reflector, which places a concave (dish-shaped) primary mirror at the bottom end of the telescope tube.  A small, flat secondary mirror at the other end directs the focused light out the side of the tube and into the eyepiece.  Newtonians offer the largest aperture available at given price, and when well made, they can provide sharp, high contrast views which rival all but the finest refractors.  A Newtonianís low centre of gravity and eyepiece location at the top of the tube allow for comfortable viewing with a more compact mounting, which can be made stable with much less bulk and cost than the tall mounting required by a refractor of similar aperture.  Big reflectors of 10" aperture and larger on Dobsonian mountings are the most popular telescopes for astronomers who seek to gather "buckets of light" for deep-sky observing.  These giant scopes perform best at remote dark sky sites, away from the glare of city lights.  The value and versatility of the smaller 4.5" to 8" Newtonians, mounted either equatorially or as Dobsonians, makes them a fine choice for the beginner with general interests.  Newtonian reflectors require occasional maintenance.  Unlike the lenses in a refractor, the mirrors in a reflector need periodic alignment, or collimation, for best performance.  While many beginners seem intimidated by collimation, itís really not difficult, and takes only a few minutes once you get the hang of it.  A reflectorís tube is also more open to air and humidity than that of a refractor, and if left uncovered the mirrors can accumulate dust and grime, which will require occasional cleaning.  While these maintenance concerns are often overstated, a Newtonian may not be the right choice for someone who finds the prospect of occasional tinkering with the telescope unappealing.
THE CATADIOPTRIC

The most modern of the three common designs for amateur telescopes is the compound, or catadioptric type, which uses a combination of lenses and mirrors to gather and focus light.  The greatest advantage of this design is its compactness: the lenses and mirrors "fold up" the light path inside the telescope, reducing large-aperture scopes to a manageable size.  If an equatorial mounting is desired, the smaller tube can be carried on lighter and more economical mounts than that required by a Newtonian of the same size.  Compound telescopes are most popular with observers who desire both generous aperture and an equatorial mounting in a transportable package.  The names Schmidt-Cassegrain, Maksutov-Cassegrain and Dall-Kerkham refer to specific designs of compound telescopes, which use differently shaped lenses and mirrors to achieve a similar result.  The Maksutov is often cited as offering better image quality, though there is little in the way of optical theory to support this opinion.  Most probably the Maksutov has developed its reputation as the superior catadioptric design because its spherical optical surfaces are easier to make to very high precision than the more complex shapes demanded by the Schmidt.  As a result, if a telescope maker practices anything less than the strictest quality control, their "average" Maksutov will outperform their "average" Schmidt.  In top-quality telescopes from careful manufacturers, both designs can yield excellent images.  There are a few drawbacks to all compound designs.  As in any telescope which employs mirrors, occasional alignment is required for peak performance.  The cost of a compound telescope is higher than that of a Newtonian of the same aperture, though still lower than the cost of a comparably sized refractor.  Most significantly for the planetary observer, the secondary mirror in a compound is much larger than that in a Newtonian, and its presence in the light path of the scope reduces contrast somewhat for high-powered viewing.  In general, astronomers who desire a highly capable, easily transportable telescope find these worthwhile compromises, and have made the compound scopes very popular.

Price is a Consideration
Budget is a factor in almost every telescope purchase decision, but there are at least three major price-related pitfalls to be avoided.

  1. DO NOT buy a flimsy, cheap scope at the mall with the intention of getting a taste of the sky and upgrading later.  Many of those scopes are so poor-quality and frustrating that they can turn budding astronomers off the hobby for good!
  2. On the other hand, donít give up on astronomy if the scope of your dreams is financially out of reach at this moment.  There are many reasonably priced, high-quality beginnerís scopes that can reveal incredible wonders, while helping a novice define his or her particular observing interest.
  3. Finally, if you are one of the fortunate few for whom price represents little obstacle, think twice before buying the biggest, most expensive telescope in stock.  Many of the large, fully featured scopes favoured by experienced observers are also the most complicated, and are too much to grasp for someone still trying to find the Big Dipper!

What About Astrophotography?
Before concluding, hereís a quick word for the beginner who wants to jump right into astrophotography through their new telescope: Donít!  At least, not until you have taken some time to learn the sky and become familiar with operating your scope.  Photography of the heavens can be a wonderfully rewarding pastime, but it is hugely more difficult and frustrating to do than you can imagine.  It is a combination of art and science with a steep learning curve which can thoroughly put you off it forever if you try to take on too much at once.  Of course, if astrophotography is a primary interest there is nothing wrong with selecting a first scope based on its easy adaptability to camera work in the future.  While most telescopes can be used for picture-taking (with varying prospects for success), the most important qualifications for a photographic instrument are a rock-solid equatorial mounting, an excellent tracking mechanism and ease of attaching a camera so that it can be focused.  For a variety of technical and economic reasons, compound telescopes of 8" aperture and larger are most popular for photography.  They also make fine instruments for general observing.

In Summary
Which, then, is the right telescope?  Thatís a decision that must be made individually, but the three best pieces of advice are:

  1. The best telescope for you is the one you will use most often.  A huge, optically wonderful scope will bring no joy if it is consigned to the cupboard or closet!
  2. All else being equal, a larger-aperture (diameter) telescope will reveal more in the night sky than a smaller one (Seems obvious, but needs to be said.)
  3. Buy from a company thatís knowledgeable about telescopes and astronomy, and who will support you even after your purchase (since you will likely have questions).

My final advice would be to select a well-made telescope, of a design matched as well as possible to your primary observing interest and most frequent observing site.  Make sure itís a size that you can handle easily (by your standards and no one elseís) and will use often.  If you do these things, then you should enjoy a lifetime of satisfied observing!

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