Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site amdahl.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!whuxlm!harpo!decvax!decwrl!sun!amdahl!canopus From: canopus@amdahl.UUCP (Alpha Carinae) Newsgroups: net.astro Subject: Re: To buy a telescope (LONG) Message-ID: <1715@amdahl.UUCP> Date: Wed, 26-Jun-85 13:12:51 EDT Article-I.D.: amdahl.1715 Posted: Wed Jun 26 13:12:51 1985 Date-Received: Sat, 29-Jun-85 01:27:57 EDT References: <894@sdcsla.UUCP> Distribution: net Organization: RA: 6h 22m 30s; Dec: -52d 36m Lines: 98 > <894@sdcsla.UUCP>: > I am in the market for a telescope. Nothing fancy. > However I know next to nothing about them. I would like to get > a reflective unit (i think). > I plan on looking at the moon, saturn, animals, the people across > the canyon. > > My questions are: > > 1. What does the diameter have to do with power? > Or are they two different things? There are two kinds of "power" here: light-gathering ability and magnification. In the case of a reflector, the larger the diameter, the more surface area for collecting light, hence a more powerful telescope. Magnification is a relationship between the focal length of the mirror and the focal length of a given eyepiece. Example: a 6 inch mirror with a focal length of 48 inches would yield 48 power with an eyepiece of 1 inch focal length (a 25mm Orthoscopic, for example). > 2. If power and diameter are not directly related can I get a 4" > scope and change the power by changing the eye piece? Yes and no. Changing eyepieces will allow you to increase/ decrease the magnification; the actual light-gathering power of your scope would be fixed by the diameter. NOTE: While you could theoretically increase the magnification to whatever you wanted, there IS a relationship between objective diameter and maximum *useful* magnification. The rule of thumb is that one can get, as a maximum useful value, approximately 50 power per inch of aperture. Therefore, I wouldn't recommend exceeding 200 power with a 4" scope. > 3. What size should I get for general viewing? This is a difficult question to answer; different people will give different answers based on their biases (myself included). You mention the Moon, planets, animals, and people. You mention nothing about portability. Telescope mounting is important, especially if you are interested in using "high" magnifications - as is a method of following the celestial object which you are observing (motor-driven mounts). As a general rule, short-focus reflectors don't perform well on the Moon and planets because they are designed for use with low magnifications. My guess is that a small refractor would suit your needs quite well (2.4 to 3"). Also, since all telescope images are inverted, you will need to buy a Porro Prism (unless viewing people and animals upside-down is OK). > 4. How much should I pay? Again, this is based on telescope type. A large (10") reflector on an altazimuth mount can be purchased for 300-400 dollars. A good 3" refractor on a decent equatorial mount may run 650 dollars. Check out publications like "Sky & Telescope", and ASTRONOMY magazines. Investigate used equipment, as well. > 5. What should I look for in a reflective scope. Primary mirror: is it over-coated (Silicon Monoxide)? What is the wave-front error (i.e. 1/4 wave or less?) Type of cell it is mounted in, and ease of adjustment (reflectors need to be collimated fairly regularly). Focal ratio - you would need to decide what is suitable for you. Secondary mirror/support: My preference is for 4-vane spiders for secondary support. You may wish to solicit other opinions. Eyepiece holder/focuser: Look for a smoothly operating rack and pinion eyepiece focuser. Most handle 1 1/4" eyepieces. If you buy a "big" reflector with a short focal length, you may wish to have a 2" eyepiece holder. Mounting: There are several types of mounts: altazimuth, fork, German equatorial to name a few. The important thing is, is it steady, and free (as possible) from flexing, and does it damp out vibrations well? This is rather long and windy - and I didn't address a lot of other issues. If you really are a budding amateur astronomer, my personal recommendation is to invest in a good Star Atlas, like Will Tirion's SKY ATLAS 2000, and a good pair of 7x50 binoculars. Then spend a year or so just learning the night sky. Join a local Astronomy Club, and see what their members have in the way of scopes. By then you will have a good idea of what you really want, and you will have saved yourself a lot of pain and money! -- Frank Dibbell (408-746-6493) ...!{ihnp4,cbosgd,sun}!amdahl!canopus Amdahl Corporation, Sunnyvale CA [This is the obligatory disclaimer..] - - - - - - - - - - - - - - - - - - - - - - - "I call it 'tranya'. I hope you relish it as much as I."