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Telescope Buyers' FAQ - Part Two

Telescope Optics

The most important thing in determining the optical performance of a telescope is the diameter of the beam of light that goes into it, its "clear aperture". Obviously, the more light, the fainter the things you can see, but less obviously, image detail is limited by clear aperture, via physical optics. Bigger telescopes produce sharper images, just because they are bigger.

Optical Performance
There are important qualifiers. Firstly, bad craftsmanship can make any telescope perform poorly. Cheesy optics won't work. Fortunately, it is not too hard to make optics of the sizes and types common in amateur telescopes: most manufacturers routinely turn out units that are okay. Bad ones turn up, but major manufacturers will often fix or replace a real lemon, if you have wit to recognize that you have one, and will to complain. (Most of us have neither; that's how some manufacturers make money!)

Secondly, different optical designs perform differently. Schmidt-Cassegrains, Newtonian reflectors, and refractors all have good and bad points. People who love telescopes, or sell them, will be eager to debate the matter. However, variations are relatively minor. It is usually adequate to assume all telescopes of given clear aperture and given quality of optical craftsmanship have the same optical performance: Real differences will correspond to changes in aperture of usually no more than 10 to 20 percent. Shabby optical work will increase that percentage enormously.

Thirdly, atmospheric turbulence ("seeing") limits the ability of a telescope to show detail, and sky brightness limits its ability to show faint objects. Poor seeing usually hits large telescopes harder than small ones. When seeing is poor, there may be no reason to take out and set up a big telescope. If you always observe from such conditions, you may have no reason to buy a big telescope. Yet, even in bright sky, a large-aperture telescope will show fainter stuff better than a small one. Many of us have found dark-sky stable-seeing sites within a reasonable drive of home - from sites near San Francisco Bay, sometimes I have to stare through the eyepiece of my Celestron 14 for several minutes before I can tell that there is any air between me and what I am looking at.

Notwithstanding these caveats, aperture wins, and wins big. If you buy the finest 90 mm fluorite refractor in the world, do not be chagrined if a junior high school student shows up with a home-made 6-inch Newtonian that blows it clean out of the water: The 6-inch I made at 13 puts my world-class 90 mm fluorite to shame. There is no contest, and it's not because I was a master optician at 13, it is because six inches is bigger than 90 mm, hence intrinsically better.

Binoculars as an alternative

Hundreds of deep-sky objects are big and bright enough to show well through apertures of two inches or so, at low magnifications. Thus, medium sized binoculars - 7x50 or 10x50 ('7x50' means '7 power, 50-mm aperture'), make inexpensive, highly portable, easily operated beginner instruments. Perhaps you have one already. To use them well, you must be willing to learn the sky enough to find things with a hand-held instrument. And don't get one that gets too heavy to hold steady before you are done observing.

General Telescope Truths

The most optical performance per unit of clear aperture comes from modern, high-quality refractors - but they are outrageously expensive compared to other designs of the same aperture. Also, in sizes much above four-inch aperture, the tubes are generally long enough to make the whole instrument cumbersome and heavy.

The most optical performance per unit of portability comes from Schmidt-Cassegrain and Maksutov designs - but they are still pretty expensive. There's a qualifier here: What makes them portable are short, stubby tubes, but for small apertures - say, four inches or less -- portability of all types is dominated by clumsiness of the tripod, so the portability advantage of Schmidt-Cassegrains and Maksutovs diminishes.

The most optical performance per unit of cost comes from Newtonians - particularly those with Dobson mountings. Compared to other telescopes of the same aperture, they are clumsier than Schmidt-Cassegrains and Maksutovs, but not nearly as clumsy as refractors.

Let me regroup that information into three questions telescope buyers often ask:

What gives most optical performance for a given aperture?
Usually, a high-quality refractor.

What gives most optical performance for a given car to carry it?
Usually, a Schmidt-Cassegrain.

What gives most optical performance for a given budget?
Usually, a big Dobson.

Refractors

Though costly and cumbersome, small refractors are durable and difficult to get out of whack. Good ones make respectable beginner instruments, particularly for beginners with extra thumbs. And a good small refractor provides a wonderful way for an experienced observer to embarrass folks with humongous Newtonians who lack observing skills to exploit them. But beware of mass-marketed junk refractors, advertised as high-power and sold in department stores.

Altazimuth mounts

Altazimuth mountings tend to be cheaper, lighter, less clumsy, and more quickly set up than equatorial ones, but to use one you must be willing to learn the sky well enough to find things without dialling in celestial coordinates. (Computer-controlled altazimuth mounts allow use of celestial coordinates to find things, or perhaps will look up the coordinates for you, in an internal data base, but they are not cheap.)

What does your telescope say about you?

Big Iron
This is the giant Dobson-mounted Newtonian, or humongous Schmidt-Cassegrain, that fills your garage. To transport it requires a small trailer, pickup truck, or panel van, and setting it up calls for the concerted efforts of three used fullbacks and a circus elephant. The ladder to climb to the eyepiece is so tall you need supplemental oxygen to deter altitude sickness. This telescope is your galaxy-gazer and cluster-buster supreme, and if it is well made, then when the seeing is good it will show detail that those condescending high-tech dweebs with their confounded itty-bitty seven-inch apochromatic refractors can only dream about.

My 'Big Iron' is a Celestron 14, with a little tiny single-axle cargo trailer to haul it.

Largest Conveniently Portable Telescope
This is the most telescope that will fit easily in your regular vehicle without hiring a bulldozer to clean it out. What it is, depends on what your vehicle is - with a ten-speed, or a subway train, you have a problem. An eight to eleven-inch Schmidt-Cassegrain is the right size for many people; that is one reason these telescopes are popular.

I have had several Largest Conveniently Portable Telescopes over the last few cars. Once I built an eight-inch Dobson whose key design parameter was that the tube just barely fit crosswise across my back seat. I used it a lot till I bought a smaller car. For a while, my Largest Conveniently Portable Telescope was a Vixen 90 mm f/9 fluorite refractor on an altazimuth fork or a Great Polaris German equatorial (I have hardware to fit both), but at present I use a six-inch f/10 Intes Maksutov on the Great Polaris. A somewhat faster Dobson than my 8-inch f/5 would work equally well, and would have more performance for most purposes.

Public Star Party 'Scope
You'll want something pretty portable, with the added provisos that it's nice to have a sidereal drive so you won't have to keep re-pointing it between viewers, and that it shouldn't be so expensive you worry about kids and idiots. Your SCT will do nicely.

I put the Intes or the Vixen fluorite on the Great Polaris, but I set the tripod legs to maximum length, so the expensive optics are out of reach. So far, no one has slam-dunked a rock.

Quick Look Scope
The idea here is to leave something all set up in your entrance hall, or hidden under a stack of old Sky & Telescopes in the back of your car, so you will have a telescope on two minutes notice if a truly close comet comes whizzing by, or if you are too lazy to assemble one of your real telescopes. Such an instrument can also double for nature watching or spying on the neighbors, which may be the same thing - just don't tell your fellow amateur astronomers, or you will lose observer points. Many people have a spotting 'scope on a light tripod, or perhaps a 90 mm Maksutov on one a bit heavier.

Lately, my Quick Look 'Scope has been a 102 mm f/9.8 Vixen refractor with a conventional achromat, on a Vixen bent-fork altazimuth mount that has clutches and slow motions on both axes. I have a couple of smaller refractors that I sometimes use similarly, but since I have room to leave the 102 mm set up in my living room, I benefit from the extra aperture.

Binocular
A good binocular is very useful, and can do much of the work of a 'Quick Look Scope. I have too many; ones I use for astronomy include the 7x35 Tasco ($29.95 at Sears) that I keep in my car for bird-watching (oops, lost observer points), an old Swift Commodore Mark II 7x50 (long out of production), which was one of the first binoculars I saw with BAK-4 prisms, and an Orion 10x50 and 10x70 with BAK-4 prisms and fully multicoated everything, up to but not including the case. At star parties I tend to wander around with one dangling from my neck. I tried two, but lacked sufficient eyes.

High-Tech Conversation-Stopper
This is how you put to shame those grass-chewing hillbilly clodstompers who have giant cardboard Dobsons with tubes so big that they echo. Odds are the seeing will never get good enough for them to demonstrate that a half-metre shaving mirror will blow eighteen centimetres of optical perfection clean out of the water, and if they start talking about faint galaxies you can always change the subject to diffraction rings and modulation transfer functions, and ask them to compare internal baffles and background sky brightness. Besides, your telescope has more knobs than all theirs put together, and it cost more than all theirs put together, too.

The default choice for the High-Tech Conversation-Stopper these days is typically an apochromatic refractor, or some close approximation ("apochromat" is a precise technical term; not all superb refractors are apochromats, and vice-versa), which if well made and well baffled will deliver outstanding performance for its size.

The apertures available suffice for many amateurs who have either recovered from aperture fever or have not yet succumbed, or who have exhausted their supply of fullbacks and circus elephants to set up the Big Iron. Few other kinds of telescopes qualify - you're not allowed to have a Schiefspiegler unless you can remember how to spell it, and nobody wants a Yolo because people expect you to walk the doggie. Some folks like Questars, but not me.

My present High-Tech Conversation-Stopper is the 90 mm Vixen fluorite refractor I mentioned earlier. It is not big enough to be as impressive as I might want, and is rather short on knobs, but I can talk fast enough to make up the difference.

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Author: Dennis Bishop

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