From: ronnie@cisco.com (Ronnie B. Kon)
Newsgroups: sci.astro
Subject: Purchasing Amateur Telescopes FAQ (part 1/2)
Date: 2 Sep 92 04:00:36 GMT
Organization: cisco Systems, Menlo Park, California, USA
X-Disclaimer: The views expressed in this message are those of an
	      individual at SRI International, and do not necessarily
	      reflect those of the company.








                  Purchasing Amateur Telescopes FAQ

                                Part I


                           Ronnie B. Kon
                           ronnie@cisco.com



                    Last Updated: August 27, 1992


Questions in this FAQ:

        In Part I:

        1.  What is the single most important thing I should know before
            buying a telescope?

        2.  What is the single best piece of advice to give to someone
            thinking about buying a first telescope?

        3.  What Does All the Jargon Mean?

        4.  What Are Some Good Introductions To Amateur Astronomy?

        5.  What Will I Be Able To See?



        In Part II:

        6.  Buying A Telescope
        6.1     What Company Makes the Best Telescopes?
        6.2     How do Meade and Celestron Name Their Telescopes?
        6.3     Comparison of Schmidt-Cassegrains
        6.4     What is the Best Telescope to Buy?

        7.  Where Do I Buy My Telescope?
        7.1     What About Buying Used?
        7.2     What About Building A Telescope?

        8.  What Accessories Will I Need?

        9.  Why Should I Start With Binoculars?
        9.1     How Do I Hold Binoculars

        10. What Books and Star Charts Are Recommended?
        10.1    What About Computer Programs?
        10.1.1     What Programs Can I Get For Free?


Contributors to this posting include:

 Pierre Asselin          Doug Caprette           Mike Collins
 Kevin Deane             Jay Freeman             Chuck Grant
 Christopher Gunn        Dyer Lytle              Doug McDonald
 Andy Michael            Dave Nash               Bill Nelson
 Leigh Palmer            Alan Peterman           Tom Randolph
 David Smith             Geoff Steer             C. Taylor Sutherland
 Mario Wolczko           Paul Zander



1.  What is the single most important thing I should know before  buy-
ing a telescope?

This is the single most important thing you should  get  out  of  this
FAQ: DO NOT BUY YOUR TELESCOPE FROM A DEPARTMENT STORE.  Ignore every-
thing any literature tells you about magnification and such.  Buy from
a telescope store, where you will get a telescope that doesn't make as
big claims, but will give you FAR better performance.

The reason is that as far as telescopes go, how much you  can  magnify
is  a function of the amount of light the telescope receives, which is
almost entirely determined by the telescope's aperture  (the  size  of
the  lens  or mirror that points at the sky).  As far as magnification
goes, you can expect 50x per inch of aperture on a normal night, up to
62.5x  on  an exceptionally clear night (this is the number Meade uses
in calculating their magnifications).

Department stores always show little 2 1/4 inch refractors for  up  to
300+  dollars and say that the refractor can get up to a whopping 600X
or so.  Strictly speaking, this is true.  However,  applying  the  50x
rule,  it  is  easy  to see that 125x would be pushing the optics, and
that's assuming that they were high quality ones.  With the quality of
the  parts they usually give you are lucky to get 100x with reasonable
resolution.

More details will follow.  I put this up at the top  so  even  if  you
read nothing else, you will read this.


2.  What is the single best piece of advice to give to someone  think-
ing about buying a first telescope?

Find a local astronomy club and attend a star  party.   Find  ways  to
look  through  telescopes  of  different quality and prices so you can
determine what you want to buy.  This FAQ can  give  you  information,
but  cannot  possibly  compare  to  actually going out and looking for
yourself.  Besides, it's the last chance you'll get to look at the sky
for free.




3.  What Does All the Jargon Mean?

OK, by popular request, here is a glossary of common  astronomy  terms
encountered  in amateur astronomy.  Words in bold face (or underlined)
can be found defined under their own heading.

altazimuth mount
     This is what you think of when you think of a tripod  mount.   It
     allows movement in two directions: parallel to the ground, and at
     right angles to the ground.  It is very  useful  for  terrestrial
     observations,  as  it  is a very natural way of observing.  It is
     significantly less useful for astronomical use,  where  an  equa-
     torial mount is preferred.

aperture
     The diameter of the objective.

Barlow
     A Barlow lens is a device which has the effect of increasing  the
     magnification.   It  does  this by shortening the effective focal
     length of the eyepiece you are using.  Thus a 2x Barlow will dou-
     ble the magnification, a 3x will triple it.  Barlows used to have
     a bad reputation, stemming largely from rather poor quality  ones
     being  sold.   Modern  Barlows are high quality and a good choice
     for expanding your collection of eyepieces.  You should keep  the
     Barlow  in  mind  when buying eyepieces--buying a 3mm, 6mm, 12mm,
     and a 24mm and a 2x Barlow is a very dumb idea.  The only use you
     get  from  the  Barlow  is  changing the 3mm to a 1.5mm (which is
     probably going to give you higher than usable magnification  any-
     way).  On the other hand, a 6mm, 9mm, 15mm and 24mm would be com-
     plemented very well by a 2x Barlow.

catadioptric
     Any of a number of compromise telescope  designs,  using  both  a
     lens  and  mirrors.   Examples  are  the  Schmidt-Cassegrain  and
     Maksutov-Cassegrain.  These offer  large  apertures  without  the
     coma  problems  of  a Newtonian.  The light path is folded twice,
     which makes the telescope very compact.  These are pretty  expen-
     sive.   Pictures can be seen in the ads in any issue of a popular
     astronomy magazine: the Meade 2080  and  the  Celestron  C-8  are
     examples  of  Schmidt-Cassegrain;  the Celestron C-90 and Questar
     are examples of Maksutov-Cassegrain.

chromatic aberration
     In refractor telescopes, which use lenses to bend the light, dif-
     ferent wavelengths of light bend at different angles.  This means
     that the stars you see  will  usually  have  a  blue/violet  ring
     around  them,  as  this  light  is bent more than the rest of the
     spectrum.  It is not present at all in  reflectors,  nor  to  any
     significant  degree in catadioptrics.  Different glasses and cry-
     stals (notably fluorite) are sometimes used to compensate for the
     aberration.   Such  telescopes  are  termed  "achromat," or "apo-
     chromat" if the correction is is nearly perfect.

collimation
     This refers to how correctly the optics are pointing towards each
     other.  If a telescope is out of collimation, you will not get as
     clear an image as you should.

coma This refers to the blurring of objects at the edge of  the  field
     of  view,  most  common in short focal ratio Newtonian telescopes
     (at f/10 and longer,  Newtonians  are  very  well  corrected  for
     coma).

declination
     All astronomical objects are located via a pair  of  coordinates:
     Right  Ascension and Declination.  These are easily visualized by
     imagining that the Earth is in the center of a  hollow  celestial
     sphere,  which has all the stars, nebulae and galaxies painted on
     the shell, and the sun, moon and planets move around the  shell's
     inner surface.  We can then project the Earth's lines of Latitude
     and Longitude onto the sphere, and have  a  unique  location  for
     each  object (obviously, these will change rapidly for quick mov-
     ing objects, very slowly for slower objects.  See  also  "preces-
     sion").   The  Declination  is  the celestial latitude, the Right
     Ascension is the celestial longitude.

Dobsonian
     Named for John Dobson of The San Francisco Sidewalk  Astronomers,
     this  is  a  design which allows for very large apertures at very
     affordable prices.  The trade-off is that  they  are  mounted  on
     altazimuth  mounts,  instead of equatorial ones, which makes them
     essentially useless  for  astrophotography,  but  an  inexpensive
     alternative  if you only plan to do visual work.  These are light
     buckets.  If you are planning to build your  own  telescope,  you
     might want to consider a Dobsonian.

double star
     A double star is a pair of stars which appear to  be  very  close
     together.   There  are  two  types of double stars: binary stars,
     where the two stars are actually a part of  a  system  and  orbit
     each  other;  and  optical  doubles,  where the two stars are not
     gravitationally bound at all, they just happen to lie on the same
     line of sight from the Earth.

ephemeris
     Plural, ephemerides.  A table of  the  location  of  a  celestial
     object at regular intervals in time.

equatorial
     An equatorial mount is set to the current latitude, and is  polar
     aligned  (pointed at the north pole) and then moves only in Right
     Ascension and in Declination.  This take a while to get used  to,
     but  offers  the wonderful side effect of being able to track the
     astronomical objects you are looking at as they move  across  the
     sky  (which  is very visible motion at telescopic magnifications)
     by moving in only one direction (Right  Ascension).   Most  equa-
     torial  mounts  come with motor drives that take care of this for
     you.

exit pupil
     This refers to how wide the beam of light  exiting  the  eyepiece
     is,  and  is  equal to the aperture divided by the magnification.
     If it is bigger than 7mm (the size of your pupil in the dark) you
     will  not  be taking in all the light available--effectively, you
     will be using a smaller aperture telescope than you have.

eyepiece
     This is the thing you actually look into.  Almost all  telescopes
     separate  the  Optical  Tube (the telescope proper) from the eye-
     piece.  Essentially, the telescope makes a really tiny  image  of
     what it's pointed at.  The eyepiece acts as a magnifying glass to
     allow you to see the image bigger than  it  would  otherwise  be.
     The magnification is the focal length of the telescope divided by
     the focal length of the eyepiece.  Eyepieces are described by the
     diameter  of the barrel, always expressed in inches (.965", 1.25"
     and 2" are the sizes in common use) and the focal  length  always
     expressed  in millimeters (4mm - 40mm is the usual range).  Short
     focal length eyepieces are also termed  high  power,  long  focal
     length are low power.

     Also significant with eyepieces is the  apparent  field  of  view
     (expressed in degrees) and eye relief (expressed in millimeters).
     The apparent field refers to how big the circle of space you  see
     in an eyepiece appears.  Bigger is better.  Eye relief is a meas-
     ure of how far from the eyepiece you can have your eye and  still
     see.   If  you wear glasses to correct astigmatism, you will need
     fairly long eye relief (the focus knob will  correct  for  almost
     all vision problems except astigmatism).

     There are several types of eyepiece designs.   The  most  popular
     are  Kellner  (inexpensive,  most  popular  for cheap telescopes,
     short eye relief and narrow fields of view.  Good to avoid if you
     can  afford better); Orthoscopic (good price/performance comprom-
     ise); Erfle (wide field of view, expensive); Plossl (perhaps  the
     best  all-around  eyepiece.   Some  moderately expensive versions
     available); and Ultra Wide (very  expensive,  almost  double  the
     number  of  lenses  as other designs makes for more light loss in
     the eyepiece, large exit pupils.  Can  cost  more  than  a  small
     telescope.   Not  a  good  place to spend your money when you are
     just starting out).

     You really don't want to buy many .965" eyepieces--they are  gen-
     erally  not  as  well  made  as  the 1.25" ones, and if you get a
     bigger telescope it will probably  not  accept  your  .965"  eye-
     pieces.   You  can  buy  an  adapter to let you use 1.25" in your
     .965" focuser.  This is probably worth the money.

f/10, f/6,3
     See Focal Ratio

finder scope
     The finder scope is a low power telescope attached to  the  tele-
     scope  you  are using.  Because most telescopes show such a small
     portion of the sky, it is virtually impossible to locate anything
     just  by  looking  through  them.  So you look through the finder
     scope to center the object you want (the finder  has  crosshairs)
     and  then  you  can use your real telescope on it.  Note that you
     can ignore all the claims about big finder  scopes.   You  almost
     certainly  don't  care.  All you need is to be able to point your
     main telescope at something in the sky.  Finder scope  size  only
     matters  when you are starhopping through fairly dim stars (where
     the larger aperture allows you to see dimmer stars).   This  will
     not be an issue for you for quite a while (if ever).  Many people
     use a Telrad sight, which is simply a red LED you can sight  on--
     you  get  absolutely  no  more aperture than your naked eye.  The
     finder scopes are usually advertised  as  8x50  (or  such).   The
     eight  refers  to  the  magnification,  the 50 to the aperture in
     millimeters--just like binoculars.

focal length
     This is the length of the light path, from the objective  to  the
     focal  plane.  The magnification is the focal length of the tele-
     scope divided by the focal length  of  the  eyepiece.   See  also
     focal ratio.

focal plane
     The plane that the telescope (or eyepiece) focuses on.  When  you
     turn the focus knob on the telescope, you are moving the eyepiece
     back and forth until you make the two focal planes coincide.

focal ratio
     Also referred to as the "speed" of the telescope, is the ratio of
     focal  length  to  aperture,  and  is  always  expressed as an f/
     number.  Thus an 8" telescope with a 2000mm focal length is  f/10
     (because 8" is 200mm, and 2000 / 200 = 10).  An f/10 telescope is
     "slower" than an f/4.

     Fast telescopes give wider, brighter images with a given eyepiece
     than  slower  ones  (but  note that at a given magnification, the
     images are--assuming identical optics--exactly the same: what you
     see  through  a f/6.3 telescope with a 12mm eyepiece is identical
     in width and brightness to what you  would  see  through  a  f/10
     telescope with a 19mm eyepiece).

     In general, the slower the telescope the more forgiving it is  of
     optical  errors  in  the  objective and eyepiece.  A telescope of
     f/10 is fairly forgiving, f/6.3 much less so.

focuser
     This is the thing that holds the eyepiece.  It moves in  and  out
     so  you  can focus the telescope.  It is always included with the
     telescope when you buy one.  The size, almost always .965", 1.25"
     or 2" refers to the barrel diameter of the eyepieces it accepts.

fork mount
     A fork mount is a type of mount where the telescope  is  held  by
     two  arms,  and  swings between them.  A fork mount can be either
     alt-azimuth or equatorial (through the use  of  a  wedge).   Fork
     mounts are most commonly used with Schmidt-Cassegrain telescopes,
     and are almost always equatorial).

German Equatorial Mount
     The first equatorial mount devised and still the most common  for
     small  to  moderate  sized reflectors and refractors.  Unlike the
     equatorial fork, the german equatorial is suitable for telescopes
     with  either short or long tubes (although, if poorly designed, a
     long tube may strike the tripod, preventing viewing at  the  zen-
     ith).   They  usually  are  designed with movable counterweights,
     which make them easy to balance, but heavy and bulky.

     The tube of the telescope is joined to a shaft  (the  Declination
     shaft  or axis) which rotates in a housing that in turn is joined
     at right angles to another shaft (The  polar  axis).   The  polar
     axis  is pointed at the celestial pole (just like any other equa-
     torial mount).  A counterweight, which is required  for  balance,
     is placed on the other end of the decination shaft.

     Tracking an object past the zenith requires that the telescope be
     turned  (both Right Ascension and Declination rotated through 180
     degrees), which reverses the field of view.  Not so much a  prob-
     lem for visual astronomy, but a limitation on astrophotography.

light bucket
     A common slang term for a large aperture.  The cure for "Aperture
     Fever."   Well,  not  really.   After a month with the Keck tele-
     scope, any amateur worth her salt would  be  bitching  about  how
     much more she could see if only she could double the aperture.

Maksutov-Cassegrain
     See catadioptric.

meridian
     An imaginary north/south line passing through the zenith.

nebula
     Plural nebulae.  An unfortunate term which basically means "some-
     thing  up there which isn't a star or a planet."  Until the early
     years of this century nobody knew what the diffuse light  sources
     in  the sky were, so they were all termed nebulae, from the Latin
     for mist.  They encompass galaxies, supernova remnants and  space
     dust.   "Emission  nebulae" are nebulae that emit light, and thus
     you can see, "dark nebulae" are things which  don't  emit  light,
     and  hence  can  only  be  seen  as  a  dark foreground against a
     brighter light.  The Horsehead Nebula is probably the most famous
     dark  nebula.   Note that many astronomers will grimace loudly if
     you refer to galaxies as nebulae.

Newtonian
     See reflector.
nutation
     Like precession, this is a term you really  don't  want  to  know
     about.   It  turns  out that not only does the sun make the Earth
     wobble in her orbit (see precession),  the  moon  does  as  well.
     This  wobble  (about  10  arc  seconds  with a period of about 18
     years) is termed nutation.  You will never have  to  worry  about
     it.

objective
     This is the thing that gathers light from the sky and  folds  the
     light into a cone.  In a refractor it is the big lens that points
     at the sky, in a reflector it is the big mirror at the bottom  of
     the  tube.   The  job  of the objective is to create a light cone
     which comes into tight focus at a single focal point.

optical tube
     This is the telescope proper.  It is the  tube  which  holds  the
     objective.   The  rest  of  the stuff is accessories, such as the
     mount, tripod, eyepieces, etc.  When reading ads, note that some-
     times  optical tubes are sold by themselves.  You will need to go
     out and buy (or build) a mount for them before you can use them.

precession
     This is a motion which you don't need to think about.  It is  the
     drifting  of  the north pole, and all other celestial coordinates
     with it, in a rough circle.  This occurs over so long a  timespan
     (it  is a 26,000 year cycle) that it will not affect you.  It is,
     however, one of the main reasons (proper motion being the  other)
     that  star atlases are all prepared as of a specific date (Jan 1,
     2000 is the current standard,  the  previous  was  Jan  1,  1950.
     Except  they  are really Dec 31 of the year before--which makes a
     huge difference, as you can well imagine).

proper motion
     Everything in the Universe is moving, relative to the Earth.   It
     is  convenient to divide this motion into two vectors, the motion
     directly toward the Earth (termed radial motion), and the  motion
     perpendicular  to  the  former,  termed  proper  motion.   Motion
     directly towards or away from the Earth  has  no  effect  on  the
     body's position as seen from Earth. Proper motion, however, does.
     This is one reason (along with precession) that star  charts  are
     prepared as of a specific date.

reflector
     A reflector is any telescope which uses a mirror  as  its  objec-
     tive.  The most common type is the Newtonian reflector, which has
     a mirror at the bottom of a tube, which focuses the light into  a
     cone  which  is  deflected by a flat "secondary" mirror (which is
     mounted near the top of the tube in something called a  "spider")
     out a hole in the side.  This is where you put the eyepiece.  The
     advantages of the Newtonian design are numerous:  there  is  only
     one  optical surface on a mirror, as opposed to two on a lens, so
     it is cheaper to make; part of the light path is at right  angles
     to  the  length of the tube, so it can be somewhat shorter than a
     similar refractor; you can get it in much larger apertures than a
     refractor,  and there is no chromatic aberration .  The disadvan-
     tages are that you might get coma away from  the  center  of  the
     image  (in  short  focal  ratio telescopes), and you don't get as
     good resolution as with a refractor of equal aperture (all  other
     factors being similar).

refractor
     This is what you usually think of as a telescope--it has  a  lens
     at  one  end,  and  you look straight through the other.  This is
     sometimes referred to as a "Galilean" telescope, as it is of  the
     same  design  that  Galileo  used  (although strictly speaking, a
     Galilean telescope is a specific kind of  refractor--one  with  a
     simple  double-convex  objective lens and a simple double-concave
     eye lens.  This will not be on the quiz, so you needn't  memorize
     it).  See reflector for a comparison of the two designs.

right ascension
     See declination.

Schmidt-Cassegrain
     See catadioptric.

spherical aberration
     A problem where a spherical lens or mirror in a telescope is  not
     shaped  correctly,  so  the light from the center is focused at a
     different location than the light from  the  edges.   You  should
     never  have  to  worry  about this.  This only shows up in really
     cheap telescopes, like the Hubble Space Paperweight.

spotting scope
     A small telescope, always a refractor or catadioptric,  generally
     used  for terrestrial viewing.  Of limited utility for astronomy,
     though many are marketed as  such.   Probably  the  wrong  choice
     unless you want to use it also for birdwatching, or as a powerful
     telephoto lens on a SLR camera.

The Starry Messenger
     The Starry Messenger (or TSM) is a classified advertising monthly
     for astronomy stuff.  Lots of ads, so a must-have for anyone con-
     sidering buying used.  It has started charging the sellers  a  4%
     commission,  though,  which has prompted a new paper, "The Cosmic
     Exchange," to begin publishing without charging  the  commission.
     I  expect  only  one of them to last.  Personally, I'm betting on
     TSM, but it's hard to say.

wedge
     This is the thing that a  fork-mounted  Schmidt-Cassegrain  tele-
     scope  will  attach to, to connect it to the tripod.  You want it
     to be sturdy.

worm drive
     This is the sort of drive most telescopes come with, if they come
     with  a drive.  It is a very accurate and smooth drive.  However,
     due to imperfections in the manufacturing process, there will  be
     periodic  errors that occur at the same point in every worm cycle
     (usually about 8 minutes).  To deal with this, higher  end  tele-
     scopes  come  with  drives  which  compensate  for the mechanical
     defects.  Celestron's is the  PEC  (Periodic  Error  Correction),
     Meade's is the Smart Drive.

zenith
     The sky directly overhead.  An object "transits" when its line of
     right ascension crosses the zenith.


4.  What Are Some Good Introductions To Amateur Astronomy?

In the United States, there are two popular astronomy  magazines:  Sky
and  Telescope  (S&T), and Astronomy.  Of the two, S&T is more techni-
cal, while Astronomy has more  things  like  "artist's  conception  of
Jupiter-rise  on  Ganymede"  which  are very pretty.  I consider S&T a
necessity, but getting both is not a bad idea.

In the U.K., there is a magazine called Astronomy Now which you  prob-
ably  want to subscribe to.  It is rather like Astronomy in style, but
slightly less bulky.  Also, fewer pretty pictures.  There  is  also  a
magazine called Popular Astronomy (which is not sold on newstands--you
have to join the Junior Astronomical Society).

There are many good introductory books.  I can  recommend  The  Light-
Hearted  Astronomer  by  Ken Fulton as being an excellent introduction
for the complete neophyte.  The writing style is a little  irritating,
but  it  is full of practical information.  It is more about observing
than astronomy, though.  It has advice like "if you are in bear  coun-
try, make a lot of noise so the bears don't bother you."

P. Clay Sherrod's A Complete Guide  to  Amateur  Astronomy,  available
through Sky Publishing Company, is a more technical introduction.

Sidgewick's books are absolutely excellent books,  probably  the  very
best ever written on amateur astronomy.  They are also probably over a
beginner's head.  Holding off on these for a while would not be a  bad
idea.

Also see below, the section on Books and Starcharts.


5.  What Will I Be Able To See?

The best way to find out is to go observing with someone.  Look for  a
local  astronomy  club  (S&T lists them periodically).  This is also a
very good way to get a good price on a used telescope of proven  qual-
ity.

In general, you will be able to see all planets except Pluto as disks.
You  will  be  able  to  see the bands on Jupiter and the rings around
Saturn.  You may be able to see the ice caps on Mars (although Mars is
probably  the  most  disappointing object in the Solar System).  Venus
and Mercury will show phases but not much else.

You will be able to see four of  Jupiter's  moons  as  points.   Ditto
Saturn's moon Titan.  You will be able to see comets.

Do not expect your images to be anywhere as nice as the ones  you  see
from  the  Voyager  spacecraft.  If a $2000 telescope could get these,
nobody would ever have spent the billions to  send  a  spacecraft  out
there.

As far as "deep sky" objects, you will be able to see all the  Messier
objects in most any modern telescope.  Galaxies will tend to look like
bright blobs.  Look a while longer and you may find some  spiral  arms
or  dust  lanes  (assuming  it  has them).  Galaxies look nothing like
their pictures--you will not see the arms anywhere near as clearly.

You will also find that the colors you see are considerably more muted
than the pictures you see.  This is because our retinas work by having
two different types of light sensitive organs, rods and  cones.   Rods
are  very  sensitive  to  dim  light, but relatively useless for color
vision.  Cones are the opposite.  Thus when looking  through  a  tele-
scope you are using your rods, and you aren't seeing a lot of color.
-- 
-------------------------------------------------------------------------------
Ronnie B. Kon           | "Boy, they were big on crematoriums, weren't they?"
ronnie@cisco.com        |
(415) 688-4592          |           -- George Bush at Auschwitz, Sept 1989
Newsgroups: sci.astro