Lawnchair Telescope
FAQ

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Gerry Descoteaux
The
Lawnchair
Astronomer
 











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Telescopes - Bad Gift Idea!

     So, Santa brought you a shiny brand new telescope for Christmas. Boy, are we gonna have some fun now! What? You don't know the first thing about telescopes - even less about the night sky and astronomy in general. Well, it's a mighty good thing that you're reading this column.

     I hate to bring this up again, but I get a ton of E-mail on a regular basis. And I answer every inquiry as best I can. Typically, it's from beginners and novice Astronomy enthusiasts looking for some advice on everything from how to "turn on" their telescopes, to what it was they saw in the sky last night. During the past few months, I've been answering a lot of questions about meteor showers and the like. Most recently, however, the Christmas Gift questions have been prevalent. Here's a typical request.

Dear Mr. StarMan, (my online persona)

     I'd like to buy my son (daughter, husband, wife, uncle, some guy down the street) a telescope for Christmas. I don't know anything at all about astronomy or telescopes. Could you suggest a telescope I should buy? I'd like to spend no more than $200.00. Thank you in advance for your help.

Sincerely,

Santa

Here's what most of these inquirers receive in response.

Dear Santa,

     DON'T buy a telescope for someone as a gift. Especially if you and they have no experience with telescopes and/or astronomy. Rather, look into getting them a few books on the subject. A subscription to Sky & Telescope or Astronomy magazine make ideal gifts for wannabe amateur Astronomers. I know of a really great book for beginners. It's in book stores everywhere. It's called The Lawnchair Astronomer, and it makes a great stocking stuffer too!

Clear Skies!

STRMN

     From there, I often send them via hyperlink to an online FAQ (Frequently Asked Questions) file dealing with all the specifics of telescope purchasing, owning and operating. As much as I could, I've discouraged folks from blindly buying telescopes as gifts or even for themselves, until they learn a little about astronomy and what telescopes are all about. For those who may not have gotten this advice, and/or for those who've ended up owning that shiny brand new telescope, here are a few things that you should know.

Size Matters:  The main factor determining how well a telescope will perform is its size. Specifically, the size of an instrument's objective lens or mirror will determine what kinds of objects the instrument will be able to resolve. An astronomical telescope's main priority is to detect light. How dim an object that a telescope can detect is directly related to its "light gathering" ability. The larger the telescope's primary objective is, the dimmer an object it will be able to resolve.

     Celestial objects are classified in several ways. One of these is related to its apparent brightness. The average brightest stars in the sky are classified as magnitude 1. The dimmest stars, those just at the limit of naked-eye visibility, are classified as magnitude 6, with those remaining stars inbetween the brightest and dimmest as magnitudes 2, 3, 4 and 5. All these are the stars that you can see with your eyes alone. Dimmer stars, magnitudes 7 and dimmer, require some type of optical aid to resolve. Here is where binoculars or telescopes come into play. The larger the instrument, the dimmer the magnitude of object a telescope will be able to detect. Developed by an ancient Greek Astronomer by the name of Hipparchus, this magnitude system works in the opposite direction as well. For objects brighter than magnitude 1, a negative integer is used - e.g. magnitude -1, -2, etc. The sun, moon, the brighter planets and some of the very brightest stars are classified in this way.

Quality and Types:  Telescopes come in two basic designs: Refractors and Reflectors. (See Graphic - Figure 1) Reflectors also come in numerous configurations. The most common small telescopes are the refractors. These gather and focus light via a primary lens which directs the light path down its tube to a focussing secondary lens on the other end of the tube where the image may be magnified. This is the basic ship captain's spyglass design. Telescopes of this type are best suited to observations of the moon, bright planets, stars and the very brightest deep-sky targets.

     Reflectors utilize mirrors in their design to accommodate larger objectives. In a reflector, the light path enters the top/front of the telescope and travels down the tube where it encounters the primary objective mirror. Here it is reflected back up the tube where it is deflected by a smaller secondary mirror which directs the light image out through the side of the tube to a focussing lens where it is magnified and viewed. The advantage of this design is the ability to incorporate a larger main or primary objective without having to build a 30 foot long tube and the subsequent engineering problems that are encountered in large refractors. Also, the quality of either of these design's optics, (mirrors and lenses) play a major role in the instruments performance. For example, poorly ground and/or configured lenses and mirrors will not focus an image correctly. Owners of cheaper instruments can attest to this.

    Additionally, there are some modern designs in reflecting telescopes, those of the Cassegrain types. These bounce the light path from the bottom mirror back up to an opposing secondary mirror which redirects and focusses the light back down the tube again, this time to a focussing lens in the center of the primary mirror. This allows for even longer focal lengths within a smaller tube, making these ideal for mobility, space limitations and other reasons. The cost of these types of instruments are substantially increased over that of the basic designs.

Lenses:  In a refracting telescope, two lenses are used. One is the primary objective lens that is specifically shaped to alter the light path in such a way that it combines the sum of the light/image at a focal point where a second lens is used to focus the image. The secondary lens can be any of several designs and styles. They are used in both refractors and reflectors. Some are best suited to wide angle views while others are utilized for close-up purposes. Some types are very hard on the eyes making eye-relief a factor when considering the choice of lens. Again, the quality of materials and workmanship is important in determining how well any lens, primary or secondary, performs.

Filters:  There are a few situations where a filter can and should be used in operating a telescope. Most people are familiar with or have heard about Solar Filters used for studying or observing an eclipse of the sun. These filters cover the instrument's primary (not secondary) lens or objective, for the purpose of safety. You WILL go blind if you look at the sun with a telescope not equipped with a solar filter. The most common solar filters are made of aluminized Mylar, a material that sufficiently reduces the amount of solar radiation reaching the telescope. The reason a solar filter must go on a telescope's primary objective may not be readily obvious.

     It is imperative that the filter be placed inbetween the sun and the telescope's primary lens or objective. Placing a solar filter between the observer and the focussing lens of the telescope can be dangerous. The reason is that if it is placed over the focussing lens, the sun's radiation will have already heated the telescope's critical optics, and in the case of mirrors and lenses, can cause them to crack and/or melt. This includes the solar filter itself, potentially being damaged while covering an exploding lens. Should your eye be nearby during such an event,... well, you can "see" how that might not be a pleasant surprise.

     Other filters are used to add contrast to the view of an object. Colored glasses and other materials are used in these types of filters, and can be a great help in seeing more of an object. For instance looking at a full or nearly full moon with a clear lens can be quite irritating. By adding a red, blue, yellow or even green filter, you can get contrasting views and a better look at some of the features that may be washed out in an unfiltered observation. As with everything else, quality is a critical factor in how well a filter will work. There are both good and bad products on the market.

Mounts:  Most telescopes come with a mount of some sort. Tripods similar to a camera stand are the most common in the lower price range instruments. A good sturdy mount, however, will hold the instrument perfectly steady, even in a brisk wind. This is important for several reasons. Looking in the eyepiece, the last thing you want to have to deal with is seeing a target object bouncing around the lens. Also, as you train a telescope on a celestial object, you need to realize that as you increase magnification you are, in essence, looking at a smaller and smaller piece of sky. At extreme magnification, the slightest movement of the instrument translates into major motion of the object in the eyepiece. There are several types of mounts. From the basic tripod and the more advanced equatorial mounts, to the very basic and simple Dobsonian swivel box for larger reflectors, mounts can be found in dozens of variations and styles. Stability is the primary factor for any design.

Automation:  So, you think you might want to take pictures of some of those dim fuzzies? Well, you'll need to be able to track the sky to take anything other than those star-trail pictures you may have seen. The Earth rotates. Therefore, pointing a telescope at an object and taking its photo means that you need to follow that object. Just looking at an object requires moving the telescope regularly. Most amateurs do this manually by moving the telescope slightly in concert with the object being observed. In wide angle lens views, looking at a larger piece of sky, this can mean moving the scope, a little, every few minutes or so. In a close-up view, this could mean gingerly moving the scope every several seconds. It's not hard to do this. However, it won't suffice with regard to astrophotography, where the need is for tracking an object perfectly in order to take long-exposure photographs. Here is where you can spend some real money. Automated systems and mounts which use battery and AC powered, worm geared, clock drives come in both expensive and ridiculously expensive price ranges. Some of these systems coupled to special cameras are even designed to be hooked up to your computer so that you can "watch" while sitting in the comfort of your living room or wherever your PC lives.

Price:  As I've alluded throughout, price is a major consideration when considering a telescope purchase. Quality is directly proportional to performance and therefore, you'll get what you pay for. That $100 or $200 instrument in the slick ads or on display at the local department store looks really expensive, but be forewarned, you're dealing at the bottom end of the scale. Size, quality optics, a sturdy mount, additional lenses and filters are not cheap. So before you make that decision to open up the wallet or zip out the credit card, do some homework and shop around. There are both superior and inferior products available through all sorts of dealers, mail-order houses and retailers. As with everything else, buyer beware.

Views:  No matter what type of telescope you use, DO NOT expect to see glossy, magazine quality astrophotographs in the eyepiece. DO NOT expect to see a lot of color. Human eyes, unlike photographic plates, cannot detect the colors, sometimes colors enhanced via computers, that you may have seen in the aforementioned pictures. Only in the very largest instruments will you get any hint of color. You will see the red in Mars, Antares and Betelguese, the blue and gold of Alberio's double stars. Even a hint of green in the Great Nebula in Orion. But these are just hues at most. Don't expect fire engine red or cobalt blue. Besides the lack of color, don't expect to see any fine detail in planetary views. You will be able to see the four largest moons and cloud belts of Jupiter. They'll look like pencil lines crossing the small white disk. And perhaps even the Great Red Spot if you happen to be looking at the right time when it makes its every two and a half hour pass. You'll also be able to see Saturn's rings. If your instrument is of sufficient size and quality, you may even be able to see the major division in the ring system. Deep-sky objects are also targets for amateur instruments. The brightest are easiest, though again, don't be disappointed when you don't see the fine interior galactic structures clearly visible in those telescope advertisements. Everything will be substantially dimmer, smaller and much less dramatic than what you may be expecting.

     Now I don't mean to discourage anyone from becoming involved in amateur astronomy. Quite the opposite in fact. My aim is to prevent potential enthusiasts from being turned-off of Astronomy due to an erroneously purchased and inferior instrument. One that will, more often than not, end up in a starring role in next summer's garage sale, or worse yet, receive a life sentence buried in a back closet. This is a great hobby and absolutely anyone can enjoy it. Becoming an amateur astronomer requires nothing more than a desire to learn about the sky. So, as you set off to explore the universe, my hope is that by providing you with some insight and some essential information, you'll have an even more enjoyable experience. Best wishes for the coming New Year!

The Lawnchair Astronomer


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