Wednesday, March 12, 2014

Types of Telescopes

When I first started in astronomy, I thought there were only two types of telescopes: refractors and reflectors. Turns out, I was right! However, I didn't understand the diversity within each classification of telescope. Following is a list of details about the individual telescope classifications with sub-types.

If clarification is needed (or information is incorrect), please let me know!
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Refractor
  1. Achromatic - Two lens design that brings red and blue wavelengths of light to a common focus
  2. Apochromatic - Two or three lens design that brings red, green and blue wavelengths of light to a common focus
  3. Doublet - Two lens objective design – Usually achromatic. Can be apochromatic with ED glass.
    1. ED - Extra-low dispersion. ED glass is used to constrain the dispersion of light when passing through a lens, thus reducing the correcting a secondary lens must perform to reduce or eliminate chromatic aberration.
  4. Triplet - Three lens objective design – High quality with low or no chromatic aberration without ED glass


Reflector
  1. Newtonian - Aperture is open (no lens/corrector plate). A large (primary) parabolic mirror is placed opposite the aperture. The light is reflected off the primary back to a 45° secondary that redirects the light through an opening in the side of the telescope tube. The secondary is held in place by a support structure, called a spider-vane.
  2.  Dobsonian - Refers to a Newtonian design made popular by John Dobson. Large aperture reflector that is supported by an altitude-azimuth base. Most large aperture reflectors are Dobsonian design, due to the physical constraints on an equatorial mount.
  3. Astrograph - A Newtonian reflector made specifically for astrophotography. The design of the telescope is tailored to imaging with a CCD or CMOS sensor and is usually not ideal for visual observing. Low focal ratios (f/3 – f/5) are characteristic of Newtonian astrographs.
  4. Cassegrain – Telescope design that utilizes a concave primary mirror and convex secondary mirror. Longer focal lengths (>1000mm)
    1. Schmidt – Uses a Schmidt corrector lens plate and two mirrors (a parabolic primary and hyperbolic secondary) to “fold” the light and direct it out the back of the telescope, like a refractor. The corrector plate helps correct for spherical aberration.
    2. Maksutov – Uses a spherical section corrector plate with spherical primary. The secondary is spherical and is typically a mirrored section on the corrector plate.
    3. Dall-Kirkham – No corrector plate. Elliptical primary and spherical secondary. Due to the geometry of the primary, Dall-Kirkhams suffer from off-axis coma and field curvature. Modified Dall-Kirkhams utilize an integrated lens within the eyepiece tube to correct for both aberrations.
    4. Harmer-Wynne – Similar to a Dall-Kirkham. Uses a parabolic primary and spherical secondary with a doublet corrector lens after the secondary. First discussed by Harmer and Wynne in a journal in 1976. Usually low focal ratios (<f/5).
    5. Ritchey-Chretien – No corrector plate. Hyperbolic primary and secondary mirrors. Due to the mirror geometry, Ritchey-Chretien telescopes are virtually free of all aberrations (except chromatic), making them the most popular astrophotography telescopes available. Usually very expensive.

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