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Aperture Is King!

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Aperture Is King! Spectrum Scientifics Telescope Buyers Guide There are several telescope buyers guides available on the Internet, some good, some not so good. At Spectrum we are writing from our experience with customers and hope to make this simple and helpful. Towards that end, the first and in some ways only rule of telescopes is: Aperture is King! Aperture is the diameter of the main lens or mirror of the telescope. The bigger it is, the more light the telescope gathers. Do not judge a telescope by its magnification, and stay away from any brand of telescope that sells itself on excess magnification claims (300x!, 600x!, etc.). This is sure sign of poor quality. More light gathering means better, brighter images, assuming all other things being equal. Decent commercially sold telescopes usually start about 60mm in size (about 2.3”) and go to 20” diameter or more. Roughly speaking, every 2 extra inches of aperture doubles the light gathering capacity of the telescope. The big problem with getting more aperture is that it increases the size and weight of the telescope. Having a huge, giant telescope with lots of light gathering power has little benefit if it is so heavy you never want to take it out and use it! A minor, but critical caveat to the 'Aperture is King' rule is that the small, portable telescope that gets used all the time is more powerful than the giant telescope that never gets moved out of the garage. What Kind of Telescope? There are three types of telescope: Reflector, Refractor, and Cassegrain. For beginners purposes, only the first two should be seriously considered. Cassegrain telescope are very nice, but are a bit advanced for first time scope buyers. Reflector telescopes use parabolic or spherical curved mirrors to gather and concentrate light. The advantage of reflector telescopes is that they are the most economical for larger sizes. The disadvantages are: in inverted image (meaning a reflector telescope cannot be used for looking down the street) and a need for occasional maintenance: the mirror must occasionally be aligned, or collimated to ensure the telescope is working at its best. Refractor telescopes use two or more lenses to gather and bend (or refract) light. The advantage for refractors is that, at equal sizes, they provide a more crisp image of the object being view versus a reflector telescope, refractors also can be used for terrestrial viewing (i.e. Looking down the street again), and they do not need to be collimated like reflector telescopes. The disadvantages to refractor telescopes is that as refractor telescopes get larger, they increase in price at a much faster rate than reflectors. At smaller sizes, say 2-3” in diameter, the prices are roughly equal for reflectors and refractors. But by the time you reach a 5” aperture, the price of the refractor will be at least double that of the reflector. Due to the difficulty of grinding larger lenses, the weight of those lenses, and an optical effect called chromatic aberration (where the light is broken up as it travels through the refractors lens in a manner similar to prisms) refractors generally are not made larger than 5-6” in diameter. What Kind of Mount? Any telescope is going to need a mount! There are three different mount designs to consider: altazimuth, equatorial, and dobsonian. Whatever mount you decide on, it should be strong enough to hold the optical tube without wobbling. Nothing is more annoying than trying to view an object in the sky, only to have it bounce around and be unwatchable because of a poor mount. Altazimuth Mounts: Altazimuth mounts are simple mounts designed to help aim the telescope in simple up/down (altitude) and left/right (azimuth). Altazimuth mounts are simple and intuitive, and work well for beginners. They are also useful if you wish to use your telescope for terrestrial viewing. The problem with altazimuth mounts is this: objects in the sky do not move in convenient up/down, left/right motion. They move through the sky in an arc (or at least it seems that way to us!). This means that trying to track celestial objects using an altazimuth mount can be like drawing a curve with an etch-a-sketch! For most beginner viewing, this is not an issue, and one can always reacquire an object that moves out of the field of view. But it does mean that if you find a nice object with your telescope, and leave to go let your friends know, it will likely move out of the eyepiece view by the time you come back! Equatorial Mounts: Sometimes called German equatorial mounts, are distinguished by their counterweights that are needed to keep the telescope properly balanced. Equatorial mounts require more setup than altazimuth mounts as they must be adjusted to your latitude and aimed North. They are also not as intuitive to aim as altazimuth mounts as they do not follow left/right up/down motions but instead move along declination and right ascension. This follows the path of stars, planets, and deep space objects, but takes some getting used to. The advantages of equatorial mounts are that they can track objects with a turn of a knob, or they can even be motorized. The other advantage is that with some study, the equatorial mount's setting circles can be used to actually find objects in the night sky! Equatorial mounts are also required for any type of astrophotography, but for beginners this should not be a great concern. Dobsonian Mounts: Some consider the Dobsonian to be just variant of the altazimuth mount, and they are not completely wrong. Dobsonians have the same advantages and disadvantages of altazimuth mounts: intuitive movement, no tracking, etc. But the difference is that a Dobsonian mount uses a lazy-susan style platform to move in azimuth and usually some form of hubs to move in altitude. The result is that a Dobsonian mount can handle much, much heavier optical tubes than most altazimuth tripod mounts are capable of handling. Thanks to several improvements in design, Dobsonian mounts have become more and more popular as they are one of the most economical telescope designs on the market today. Other Considerations: The optical tube and mount are major concerns, but they are not the only things one should consider when buying a telescope: Eyepieces: Eyepieces are often overlooked when buying a telescope, but they should be considered seriously by the beginner as they are 50% of the overall optical system. Almost all telescopes include 2 eyepieces, but by no means are all eyepieces created equal. Cheap telescopes usually include old, cheap eyepiece designs such as Ramsden or Huygenian designs that actually can make the image worse. The telescope you buy should come with eyepieces that, at a minimum, are Kellner or preferably Plossl design. These eyepiece designs are considered the standard for decent eyepieces. Finder Scope: Every telescope needs a finder scope, a small telescope that sits on top of your main optical tube and aids in aiming the telescope. Most lower end telescopes these days use a reflex finder which projects a red dot onto an optical window to show where the telescope is pointing. These reflex finders are actually easier to use than a cheap finder scope would be. However, for larger telescopes a 6x30 (which stands for: 6 magnification, 30mm aperture) finder scope is much more appropriate. Larger telescopes may also have even larger finder scopes. Avoid telescopes with old 5x finder scopes, or at least be willing to try and attach some sort of reflex finder in its place. Optional Accessories: Not everything you need for observing the night sky will come with your telescope (and if it does, beware, some companies gussy up cheap scopes with cheap accessories!). There are some things that should be in any astronomer's 'kit'. Such as: – A Planisphere Make certain this is one you can read easily at night with your red flashlight! – A Red Light Flashlight A red flashlight prevents you from losing your night vision the way a regular (white) flashlight would. – A Barlow Lens A Barlow lens is a lens you slip your eyepieces into that then doubles or triples their magnification. Having a barlow is like doubling the number of eyepieces you have. Make sure you have a barlow before you go buying additional eyepieces. – A decent Astronomy book Don't just buy a book with pretty pictures. Make certain it is a useful book that gives helpful instructions and advice on how to use your telescope, find& observe night-sky objects and other hints. Make certain to read it fully before you go observing, then refer to it during your observation session. – Time and good weather Make certain that you have time for your new hobby. It takes some commitment for even casual viewing. Also, make certain that before you go observing that the weather is decent for viewing. There's no point in going observing on a night where haze clouds everything in view! – Warm clothing OK, you should really have this stuff already. But it is important to know. Even in warm summer months the temperature can get surprisingly cool at night. Be certain that you are ready for the weather, wherever you are and whenever you observe. Other accessories you should consider, but are not as crucial as the above items are: – Filters There are a lot of filters available, and they all help with viewing certain objects. Moon filters cut down on the bright moon (which can actually be painful to view through a large telescope!).
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