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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
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> <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."