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.

As an engineer and self-proclaimed scientist/artist, I am always in search of new ways to express my creativity. I have pursued drawing and painting with marginal success and have since found my way to photography. Composition and processing photographs (particularly nature and landscape scenes) seems to be something that I see myself investing a lot of time in.

Enter astrophotography. Astrophotography combines two of my greatest passions, astronomy and photography. I can express my love for the universe through the capture and processing of nebulae and galaxies hundreds and thousands of lightyears distant. The joy I get from spending nights outside in the dark capturing images to the hours combining and processing the images into one, final, beautiful representation is inextinguishable.

This blog is dedicated to that art. These posts are defined by that science. What follows will be my experience and research into astronomy and astrophotography, what works for me, and what I can find works for others.

Comments are always welcome, as long as they are pertinent and tasteful.