Understanding Telescopes: Types, Advantages, and Disadvantages

Telescopes Overview MAS April 20, 2017

Three Basic Types

Refractors Typical 3 to 6 Disadvantages - Generally have small apertures, typically 3 to 6 inches. Advantages -Easy to use and consistent due to the simplicity of design. - Images are very sharp - Smaller apertures mean poorer viewing of distant galaxies and nebulae. - Excellent for lunar, planetary and binary stargazing especially with larger apertures. - Heavier, longer and bulkier than reflector and cassegrain telescopes of equal aperture. - Sealed tube protects optics and reduces image degrading air currents. -Great for wide field deep sky astrophotography. - Good-quality refractors cost more per inch of aperture than any other kind of telescope. - Rugged, needs little or no maintenance and its sealed tube protects optics from image degrading.

Refractors Chromatic Aberration filters CA Killer or Fringe Filter <-Single Lens <- Achromatic or Doublet <- ED or well corrected doublet or poor triplet <- Apochromatic or Triplet/Quad

Types of Refractor Glass Abbe is a measure of the material's transparency or dispersion, higher is better. Here are the Abbe numbers (Vd) of some commonly used ED glasses: Crown & Flint (Schott): 60 FPL-51 (Ohara): 81.54 H-FK61 (CDGM): 81.61 FCD1 (Hoya): 81.61 OK-4 (LZOS): 92.1 FCD100 (Hoya): 94.66 FPL-53 (Ohara): 94.93 CaF2: 94.99 (Fluorite)

Refractor Quality There is more to quality than the type of glass. There are different quality levels for each of the types of glass. It is basically how much imperfection is allowed. You can get very high quality FPL51 glass that is better than low quality FPL53. So the types of glass is only one thing to consider. Add to his how well the glass is ground or figured. On top of this there are coatings to prevent reflections and enhance transparency.

Reflectors Typical 4 to 36 Disadvantages - Images are a little softer than a refractor - Generally, not suited for terrestrial applications Advantages - Usually have larger apertures and long focal lengths which mean excellent viewing of faint deep sky objects (remote galaxies, nebulae and star clusters). - The tube is open to the air, which means dust on the optics even if the tube is kept under wraps - Low in optical irregularities and deliver very bright images. - Reflectors may require a little more care and maintenance (collimation) - A reflector costs the least per inch of aperture compared to refractors and catadioptrics since mirrors can be produced at less cost than lenses - May require a Coma Corrector - They are large and possibly heavy and storage space and transportation need to be considered.

Reflectors Long focal length Longer focal length

Compound Typical 3 to 16 Disadvantages - Images are a little softer than a refractor - More expensive than reflectors of equal aperture - Required DEW prevention - Mirror flop on some designs, focus is achived by moving the primary mirror - Heavy weight on some designs, especially with fork mounts - Its appearance may not be suited to everybody's taste Advantages - Generally considered the most versatile type of telescope with excellent lunar, planetary and deep space observing plus terrestrial viewing and photography. - Best near focus capability of any type telescope - Very Good for deep sky observing - Ideal for planetary astrophotography - Closed tube design reduces image degrading air currents - Compact and durable - Very long focal lengths

Compound

Variations Catadioptric telescopes (Catadioptrics) Argunov Cassegrain telescope Catadioptric dialytes Klevtsov Cassegrain telescope Lurie Houghton telescope Maksutov telescope Maksutov camera Maksutov Cassegrain telescope Gregory (Spot) Maksutov-Cassegrain telescope Rutten Maksutov-Cassegrain telescope Sub-aperture corrector Maksutov- Cassegrain telescope Maksutov Newtonian telescope Modified Dall Kirkham telescope Schmidt camera Baker-Nunn camera Baker-Schmidt camera Lensless Schmidt telescope Mersenne-Schmidt camera Schmidt Cassegrain telescope ACF Schmidt Cassegrain telescope (Meade Instruments) Schmidt Newton telescope Schmidt-V is l camera Refracting telescopes (Dioptrics) Achromatic telescope dialytic refractor Reflecting telescopes (Catoptrics) Cassegrain telescope Dall Kirkham telescope Nasmyth telescope Ritchey Chr tien telescope Gregorian telescope Herrig telescope Herschelian telescope Large liquid mirror telescope Newtonian telescope Dobsonian telescope Pfund telescope Schiefspiegler telescope Stevick Paul telescope Toroidal reflector / Yolo telescope Apochromatic Binoculars Opera glasses Copyscope Galileoscope Monocular Non-achromatic Galilean telescope Keplerian Telescope Aerial telescope Superachromat Varifocal gas-lens telescope Baden-Powell's Unilens