Refractors, Newtonians, and Schmidt-Cassegrains are the most popular telescopes for amateur astronomers.  Maksutov-Cassegrains and Ritchey-Chrétiens are also reasonably common, with catadioptric Newtonians (Schmidt-Newtonians and Maksutov-Newtonians) growing in popularity for certain applications.  For most general-purpose observers, these types of telescopes offer everything that is needed.  But there is no shortage of unique and unusual telescope designs for specialized uses.  The most practical of these designs are those intended for photography, so they are listed first.  The unusual visual scopes, many of which are more novelties nowadays but are sometimes still seen, are listed below.  (No offense intended to the old-school amateur telescope makers, of course, but astronomy in the 21st Century has so far been very much about embracing the amazing new imaging technology available to amateur astronomers.)

Other Designs for Astronomical Imaging

Hyperbolic Astrograph

Takahashi manufactures a line of high-speed imaging telescopes called the Epsilon series.  This design looks basically like a standard Newtonian telescope at first glance.  However, in place of the Newtonian's standard parabolic primary mirror, the Epsilon uses a hyperboloidal mirror, similar to that found in the Ritchey-Chrétien design.  The secondary mirror is simply a flat 45-degree mirror designed to produce an external focal plane on the side of the telescope tube, just like a Newtonian.  Then, a multi-element corrector lens is used just ahead of the focal plane to correct remaining aberrations from the single hyperbolic mirror.  This produces a large, flat field at a very fast focal ratio, usually around f/3 to f/4.

Single-Mirror Designs

There are a variety of single-mirror designs which use multi-element correctors.  In fact, the Hyperbolic Astrograph described above is a member of this family.  However, it uses a second mirror to provide a convenient external focal plane.  A focal ratio of around f/3 is a practical lower limit to using this design.  Any faster and the secondary mirror would become too large.  By placing the camera instead at the front of the telescope (where the secondary mirror would normally be), even faster systems are possible.  This position is normally called prime focus.

Most designs stem from Wynne correctors, originally designed to provide a wide-field option for large parabolic mirrors such as the famous Palomar 200-inch telescope.  A true Wynne corrector uses three elements to correct a parabolic primary mirror.  Hyperbolic mirrors can also be corrected in this way.  An advantage to correcting a fast hyperbolic mirror is that this then provides the option of using the same mirror with a convex secondary mirror to produce a normal Ritchey-Chrétien design.  Of course, the same could be done with a parabolic primary, producing a Classical Cassegrain.

Examples of this design include the Astroworks Centurion telescopes and the AstroOptik Hypergraph system (which is convertible from prime focus to Cassegrain focus).  These systems operate at around f/2.8 to f/3 at the prime focus.  The ultimate extension of this idea is Starizona's HyperStar system.  In this system, the secondary mirror is removed from a Schmidt-Cassegrain and replaced with a correcting lens.  Normally a sub-aperture lens could not fully correct for a spherical primary mirror such as the SCT has, but the Schmidt corrector plate on the SCT allows this to be possible.  This means a well-corrected, very fast prime focus optical system (which is convertible to a long-focus Cassegrain) can be obtained quite inexpensively.  Using HyperStar lenses on compatible SCTs produces extremely fast focal ratios of f/1.8 to f/2.

Other Designs for Visual Observing

Most alternatives to standard visual telescopes are attempts to squeeze more contrast and sharpness out of a telescope than is possible with a standard design.  Whether this is worth the trouble is often questionable and a matter of personal preference.  Some of the designs are difficult to build or optically align (collimate), making them less suitable for most observers.  One of the ultimate goals is to achieve views such as a high-quality apochromatic refractor would give, but at a fraction of the cost.  Observers talk about "refractor-like" optics, in reference to apo refractors being the standard for clarity and contrast.

Off-Axis Newtonian

Above:  Optical layout of an off-axis unobstructed Newtonian telescope

One of the designs that attempts to replicate a refractor's high contrast views is the Off-Axis Newtonian.  The basis for this design is the assumption that the central obstruction (caused by the secondary mirror) in a typical reflecting telescope degrades the image quality.  While technically this is true, for fairly small obstructions the actual evidence supporting this claim is pretty dubious.  Some observers are adamant that central obstructions detract from image quality, while others claim that until the obstruction becomes quite large (around 30% of the aperture) it is not noticeable.  Because of the extra difficulty in making such a telescope, an Off-Axis Newtonian of a given size is usually no less expensive than a good quality standard Newtonian of slightly larger aperture.  Most observers would find the view through the larger, obstructed telescope to be better than the views through the smaller, unobstructed scope.  But, an unobstructed reflector will typically give a better view than an obstructed one of the same aperture if the optical quality is otherwise equal and the observer is experienced enough to see the subtle differences.

An Off-Axis Newtonian uses a mirror that is basically a section of the outer part of a larger parabolic Newtonian mirror.  The secondary mirror sits out of the light path of this off-axis primary mirror so as not to obstruct the optics.  Collimating (optically aligning) an Off-Axis Newtonian is a bit trickier than the same process on a standard Newtonian.

Shiefspiegler

Above:  Optical layout of a typical three-mirror Shiefspiegler telescope

The unobstructed reflector, taken to the extreme.  There was a time when getting high power views, especially of the planets, was difficult with short focal length telescopes.  This was mainly due to the fact that high magnification with a short scope requires a short-focal-length eyepiece.  Until recently, short-focal-length eyepieces all had short eye relief, making them difficult to view through (and all but impossible for eyeglass wearers).  The solution was to build very long-focal-length telescopes and use comfortable, long-eye-relief eyepieces.  Nowadays, though, short telescopes can be built with much better quality, and short eyepieces can now be designed with long eye relief, making high power viewing easy, even with shorter telescopes.

The Shiefspiegler is a multiple-reflection telescope which uses tilted mirrors to produce a long, unobstructed light path.  The usual design is a tri-Shiefspiegler, such as the one in the diagram above.  This design uses three mirrors to allow a compact telescope design with a long focal length.  Several variations are possible, and the picture above shows only one particular mirror arrangement.  Tilted-component telescopes give unusual off-axis star images, but they are intended primarily for narrow-field viewing (especially the planets) and the off-axis star images are less critical than they would be in a wide-field instrument.  The awkward-looking design of the Shiefspiegler has made it the unfortunate butt of jokes, including extreme designs such as the dodeca-Shiefspiegler, a twelve-mirror design giving an unobstructed view of the back of the observer's head.

Above:  The optical layout of the 8" f/60 dodeca-Shiefspiegler, because someone here has too much free time...

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