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Looking at buying a telescope?

Fantastic, you are looking at purchasing your first telescope!

Before you rush out to the nearest shop or dive into the Online options, please consider the information below.

A BRIEF HISTORY

  • There are several types of telescope, and each of these have there pro’s and con’s.
  • Selecting a telescope is always an individual choice that is driven by factors such as cost, viewing requirements, compromise, portability etc.
  • Nobody really knows when telescopes were first introduced. Records from the early 1600’s acknowledge Galileo as being the first to introduce the telescope in astronomical terms.
  • At that time, the telescope started out life in the form of a basic Galilean Refractor. This was a crude and limited device.
  • Not long after Galileo’s efforts, Johannes Kepler introduced a modification to the design. This design still drives the basic principles of Refractor design today.
  • Since these early times, enhancements have been added to the reflector design in the form of achromatic and apochromatic objective lenses.
  • James Gregory then introduced the reflector concept to telescopes in the mid 1600’s. Sir Isaac Newton subsequently simplified this design into today’s more common Newtonian Reflector

 

TELESCOPES

REFRACTOR TELESCOPES

  • Were Man's first telescope
  • This telescope has progressed over time and has provided some of the best amateur views of the cosmos.
  • Today there are two basic forms of Refractor.

Achromatic refractors

  • Achromatic refractors use two lens elements to help minimize chromatic aberration. This is an optical effect which causes differing wavelengths of light to focus at different points.

Apochromatic refractors

  • These (often called "apos") use three or more lens elements, one or more of which have special properties to eliminate chromatic aberration entirely

PROS:

  • Erect image (right-side up) allows terrestrial use
  • No collimation required
  • Apochromatic refractors give best possible image quality
  • Apochromatic refractors are excellent for photography

CONS:

  • Easily available only in relatively small sizes (ie less than 200mm aperture)
  • Physically longer than equal-aperture Schmidt-Cassegrain
  • Most expensive per inch of aperture (especially for apos)
  • Achromatic refractors are not well suited to photography

 

REFLECTOR TELESCOPES

  • Newtonian telescopes use a curved mirror to focus incoming light onto a second, flat mirror which directs the light to a convenient viewing position on the side of the telescope.
  • Newtonian telescopes must periodically have their mirrors aligned to keep the optics performing perfectly.
  • Aligning the optics of a telescope is called collimation. Collimating a scope usually involves adjusting knobs of the back of the primary and secondary mirrors.

PROS:

  • Least expensive for a given aperture
  • No color aberration like an achromatic refractor
  • Available in wide range of sizes (from about 75mm to 1000mm in diameter)

CONS:

  • Inverted image makes terrestrial use awkward
  • Physically longer than equal-aperture Schmidt-Cassegrain
  • Requires occasional collimation
  • Not usually good for photography, unless built specifically for that purpose.

 

SCHMIDT-CASSEGRAIN TELESCOPES (SCT)

  • The biggest advantage when compared to refractors and Newtonians is their size.
  • By folding the light path, a Schmidt-Cassegrain telescope is much more compact than an equal-aperture refractor or Newtonian.
  • All of the professional telescopes built recently, from Hubble to Keck, are based on the folded Cassegrain design.
  • Schmidt-Cassegrains use a spherical primary mirror to focus incoming light onto a convex secondary mirror.  This sends the light back through a hole in the primary mirror to the eyepiece which is located at the rear of the telescope.
  • Spherical mirrors are less expensive to make than parabolic mirrors, but they introduce spherical aberration.
  • By using a corrector plate at the front of the telescope, spherical aberration is corrected.

 

A variation is the Maksutov-Cassegrain (MCT)

  • Maksutov-Cassegrain telescopes are similar to SCTs, but have a much more highly curved correcting lens on the front of the scope.
  • While they give very good images, Maksutov’s tend to take longer to stabilize from a temperature change such as taking the telescope from inside your house to outside.
  • They have a much narrower field of view and are slower (focal ratio) photographically than SCTs.
  • For these reasons, SCTs tend to be a more popular design

PROS:

  • Less expensive than equal-aperture refractor
  • Most versatile design
  • Erect (right-side up) image allows terrestrial viewing
  • Well suited for photography or CCD imaging

CONS:

  • Requires occasional collimation
  • More expensive than equal-aperture Newtonian

 

TELESCOPE MOUNTS

  • German Equatorial Mount
  • Fork Equatorial Mount
  • Alt/Az Mount
  • Dobsonian Mount

 

German Equatorial Mount (GEM)

  • An equatorial mount is a mount that has one rotational axis. This axis is parallel to the Earth's axis of rotation.
  • This axis on a German Equatorial mount is the Right Ascension (R.A.) axis which is aimed toward a celestial pole to polar align the mount.
  • This RA axis allows motion from east to west.
  • This mount uses a counterweight on a long shaft opposite the telescope to counter-balance the weight of the telescope and accessories.
  • The equatorial mounted telescope is able to track the sky aound a polar axis to compensate for Earth's rotation.
  • Once aligned, the telescope can track the sky using slow-motion controls or a clock drive to rotate the Right Ascension axis.
  • The telescope rotates around the mount's declination (dec) axis in order to allow movement north and south.

PROS:

  • Allows automatic tracking with clock drive
  • Very stable
  • Easy to point to most areas of the sky
  • Very good for photography or CCD imaging

CONS:

  • Heaviest type of mount
  • Longer setup time for large scopes

 

Fork Equatorial Mount

  • A Fork Equatorial mount holds the telescope on the end of one or two arms. The term comes from the original two-arm design's resemblance to a tuning fork.
  • Some smaller, lighter model telescopes incorporate a single arm to reduce weight when two arms are not necessary.
  • The fork arms of this type of mount are pointed toward a celestial pole to allow the mount to track the Earth rotation.
  • A Fork Equatorial mount has a Right Ascension (R.A.) axis (the fork arms) which is aimed toward a celestial pole, to polar align the mount.
  • Once aligned, the telescope can track the sky using slow-motion controls or a clock drive to rotate the Right Ascension axis. This axis allows motion from east to west.
  • The telescope rotates around the mount's declination (dec) axis in order to allow movement north and south.

PROS:

  • More compact than German equatorial mount
  • Lighter weight and quicker setup then German equatorial mount
  • Allows tracking using a clock drive
  • Good for photography or CCD imaging

CONS:

  • Eyepiece ends up in awkward position when telescope points north near the celestial pole.

 

Alt-Az Mount

  • An Altitude-Azimuth (or Alt-Az) mount moves parallel with and perpendicular to the horizon.
  • This motion is very intuitive and is especially easy to use for terrestrial viewing
  • Altitude refers to height above the horizon, and azimuth is the angle along the horizon from north.
  • Like a compass bearing (0 is north, 90 is east, 18 0 is south, and 270 is west).
  • Alt-Az mounts are easy to use for terrestrial viewing. For astronomical viewing, they have a great advantage over equatorial mounts in that they keep the eyepiece in a convenient position at all times.
  • However, they cannot automatically track unless the telescope is computer-controlled.
  • Therefore, most GoTo telescopes are Alt-Az mounted.
  • The Dobsonian is also a type of Alt-Az mount.

PROS:

  • Most compact type of mount
  • Eyepiece is always in convenient position
  • Easy to use for terrestrial viewing

CONS:

  • Will only track if computerised
  • Must be mounted equatorially (using a wedge) in order to be used photographically or for CCD imaging

 

Dobsonian Mount

The Dobsonian is very easy to use and so is very popular with beginners.  It is the least expensive type of telescope for a given aperture.  As a result, it is popular with advanced observers who wish to own a very large telescope

PROS:

  • Easy to use
  • Inexpensive
  • Easy to setup

CONS:

  • Does not track (unless a special device is purchased, which may not be available for many models)
  • Difficult to do photography or CCD imaging without being able to track
  • Wide range of eyepiece positions depending on where telescope is pointing

 

See our other articles on Calculating Focal Ratio and Calculating Magnification

Practical Magnification

  • In good seeing, never exceed 60 X magnification for each 25mm of aperture.
  • This quickly reduces to around 40 X magnification when viewing from poor/city skies.
  • To do so will exceed your telescopes resolving capabilities and your image quality will be significantly reduced.
  • In general, viewing at magnitudes greater than 300 X will provide poorer seeing and will lose contrast.
  • This can be exceeded at times in dark skies and excellent seeing conditions