Telescope Optics

The first step in choosing a telescope is to decide on the optical configuration. This has many repercussions, affecting the amount of sky that is imaged, the size of the Optical Tube Assembly (OTA) and hence the size of the dome, and the type of telescope mount.

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We start with a basic review of the most common optical arrangements.

Refractor

In its simplest form the refractor consists of two or more lens elements which are either air spaced or cemented together to form the "objective". Light passes through the glass and is bent (refracted) to reach a focus where an eyepiece or image detector is placed. Refractors require almost no maintenance unlike reflectors which must be recoated every few years. This feature made the refractor very popular in the 20th century for astrometric measurements where long-term stability is important.

Refractors have considerable drawbacks:

  1. They can only be supported around the edge of the lens. This limits the size of the optic as it sags under its own weight.
  2. As the light passes through the lens it suffers from chromatic aberration. By combining different glasses this can be effectively corrected. However, there is usually still a significant residual color term which makes the refractor unsuitable for UBVRI or other standard photometry programs.
  3. Light is absorbed by the lens, especially in the blue / ultra-violet region.
  4. The tube length is very long.

The refractor is rarely used as the main optical component in a modern research telescope. However, it is often found employed as a small auxiliary telescope, mounted piggy-back on the main OTA, and is used for solar observations, as an off-axis guider, or as a finder telescope.

Newtonian

The simplest type of reflecting telescope employing a concave parabolic primary mirror. The light reflects off the mirror and comes to a focus on-axis. This is an impractical position as the observer would block the incoming starlight. Therefore a small flat mirror, called the Newtonian Diagonal, is placed before the focus to direct the beam to the side where it can be readily examined. The size of the diagonal mirror increases as the focal ratio of the optic becomes faster. Newtonians are very popular with first-time amateur telescope makers.

The Newtonian has has the following drawbacks:

  1. The tube length is very long compared to a Cassegrain telescope of the same effective focal length.
  2. The field is very poorly corrected, especially for fast focal ratios. For an f/5 system the coma becomes very objectionable to most observers at fields greater than 0.3 degrees. Field coma can be partially compensated with a Ross corrector, a lens placed close to the focal plane.
  3. For visual observations the observer must climb tall ladders to reach the eyepiece.
  4. For electronic detectors the equipment must be placed far from the telescope axes so that the system is difficult to balance and longer cable lengths are required.
  5. Heat from the equipment or observer is likely to degrade the image quality.

For these reasons the Newtonian is a poor choice for a modern research instrument or public outreach facility.

Cassegrain

The Cassegrain is the most common system for the modern observatory. It is available in several optical variations. The flat Newtonian diagonal is replaced with a secondary mirror with a convex surface. Light is reflected back through a hole in the primary mirror.

A Cassegrain telescope has the following advantages:

  1. The tube length is compact.
  2. The focal plane and hence instrumentation is readily accessible.

There are several basic optical variations.

Classical Cassegrain

The original design invented by French sculptor Sieur Cassegrain employed a parabolic primary and a hyperbolic secondary mirror. The field of view of the classical Cassegrain is rather small. The diameter of the secondary mirror is also small. Typical focal ratios are f/12 to f/15. For systems faster than about f/10 the coma in produced by a classical Cassegrain will be several times worse than a Newtonian of the same focal ratio.

Ritchey-Chretien Cassegrain

A wide-field Cassegrain which can easily be identified by the rather large secondary mirror, typically 40% of the primary diameter. This is the best configuration for CCD imaging as the optics produce round star images at relatively large distances off-axis. A suitably designed field aplanatic field corrector will yield diffraction-limited images over the entire field. The full capability of the RC system is lost in many telescopes that cannot keep the optics collimated and that suffer from tube currents. ACE provides Ritchey-Chretien optics with or without the field corrector depending on the client's scientific requirements.

Dall-Kirkham

This variation on the Cassegrain telescope has a primary mirror that is ellipsoidal and a secondary that is spherical. The chief advantage is that the primary and the spherical secondary are relatively easy to manufacture. This is at the expense of the main aberrations which are coma and field curvature. Not an effective solution for a professional telescope.

Corrected Dall-Kirkham

Employs a spherical secondary and a primary with a conic constant around 0.8 together with a flat field corrector. Produces a beautiful wide-field (2°). Can be used from very fast (f/3.5) to moderate (f/9) systems. Has the distinct advantage of a spherical secondary which is very easy to collimate and stay in collimation. Many R-C systems, although in theory produce diffraction limited images, are incapable of doing so due to misalignment of the primary-secondary with tube flexure or due to tube currents in closed- tube OTA designs. All ACE telescopes use open-tube truss designs.

Schmidt

A wide-field telescope that has a spherical primary mirror and a "Schmidt corrector" lens at its radius of curvature. The focal plane is located half way between the two elements and any equipment located there acts as an obscuration. Although the Schmidt is a very fast system, typically around f/2, the tube length is very long. This is not a general purpose telescope. Like the refractor, the Schmidt is limited in size because the corrector cannot support its own weight.

Schmidt-Cassegrain

A secondary mirror, placed at or integral with the Schmidt corrector plate, send the light through a hole in the primary to the Cassegrain focus. Often found as a mass-produced telescope because the spherical primary is easy to manufacture. Not a viable solution for a large-aperture research grade telescope.