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The Webfooted Astronomer - February 2002

 

Little Bear,s Corner - In Praise of Small Telescopes

By Tim McKechnie

THIS is the first of what I hope to be a regular (or semi-regular) column dedicated to the users of small aperture telescopes. We,ll talk about what can be seen with them, how to see more, and how to improve their performance. I say "we because I hope that the reader will feel free to communicate back to me regarding thoughts and ideas they might have for future articles. Just send me an e-mail at Docstogie50@aol.com.

About 12 or 13 years ago, the SAS had a group called The Small Telescope Users Group. The group tried valiantly to meet regularly, but dissipated after about one year. Not that there wasn,t interest. Quite the contrary. It was too difficult to arrange a meeting time and place that was convenient. And yet, the increasing availability of good and affordable small telescopes has made the resurrection of a small telescope discussion platform necessary. Perhaps this column is the answer. The name of the column, by the way, comes from the fact that it will be dealing with small aperture instruments. In addition, Ursa Minor, the lesser or little bear, is one of my favorite constellations.

So what is a small telescope? This is a good question, and one we hashed out in the old small scope group. Like everything else from morality to obeying the law, the term is relative and means different things to different people. If you were to talk to a professional astronomer, a small telescope is usually classified as anything one meter or less in aperture-that,s 40 inches and not a small telescope to most amateurs. To many amateurs, especially the deep sky observers, anything under 300 mm. (12 inches or so) would be called small. The average person who is not familiar with telescopes would probably think any scope larger than a hand-held spyglass is big! Nevertheless, in the small scope group, we settled for the old standard that most of us grew up with: 5 inches (127mm) or less for a reflector or catadioptric and 4 inches (102mm) or less for a refractor. This is the size of telescopes we will be discussing in this column.

I,m contemporary enough to appreciate large scopes, yet I,m conservative enough that I am not drawn to wanting one-paying for it, lugging it around, setting it up. I will mention that I do own several 8-inch instruments and use them frequently.

While there is no denying that a larger objective will let you see more stars and usually will show greater detail on the Moon and planets, a small telescope will let you see an amazing amount of the Universe. As an example, let,s take the ubiquitous 4-inch (114mm) Newtonian reflector. This scope has been around for 40 years-virtually unchanged. In quality it has run the gamut from the now defunct Celestron C4.5 (which did just about everything right) to the department store grade instruments (which have done almost everything wrong). The primary mirrors are almost always f/8 spheriods and can range from quite good to very mediocre even within scopes of the same model from the same company. Most individual 4-inch reflectors fall somewhere in between these extremes of optical quality. Remarkably few are really bad. I once found a Tasco 4-inch scope that had a spherical mirror so overcorrected that it was nearly paraboloidal. It gave phenomenal images. I,ve inspected more of these scopes than I care to remember, so this had to be a fluke.

In theory, what can a 4-inch reflector show you? With modern overcoated aluminum on the mirrors and optics that deliver a wavefront error or better at the focus, a 4-inch reflector should be able to show stars down to +12.4 magnitude or so (depending on the visual acuity of the observer) and resolve to 1.1 arc/sec by Dawes and 1.5 arc/sec by Rayleigh,s criteria. In good observing conditions, such an instrument would be able to show five or six belts on Jupiter,s globe as well as the shadows of satellites in transit, on Saturn it would easily reveal the Cassini division, the "C ring and perhaps the color difference in the "B ring, equatorial belts and polar hood darkening as well as five of the moons. Linear detail approaching one mile should be visible on the Moon at higher powers. The larger and closer globular clusters such as M13 and M22 should begin to show resolution into individual stars around their periphery and some of the fainter galactic (open) clusters such as NGC 2158 (right beside M35 in Gemini) should be just visible as a smudge to someone who has keen eyesight and is observing from a dark location.

That,s some mighty fancy shootin, for such a humble instrument! Remember, though, that this would be for a first class example of a 4-inch scope. Not all of them are the same. As we continue with these little excursions into the friendly and I hope enjoyable realms of the various small telescopes in future episodes, we,ll get to know more about how each size and type will do it,s job and how to improve them in order to perhaps squeeze a bit more performance out of them. Stay tuned!

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