Telescopes, a Primer -------------------- Stewart Rorer NYSkies Astronomy Inc www.nyskies.org firstname.lastname@example.org 2009 December 3 initial Basic Types 1. Refractor Optical a. Achromatic (2 lenses) i. Cemented (interior curves the same, easier to make) ii. Air Spaced (different curves, better correction) F15 b. Apochromatic (3 or more lenses) i. Oil Spaced leaky, stay away from it. ii. Air Spaced highly corrected to about f6 c. Petzval (4 lenses) i. Flat field. No color. Heavy. Expensive. Best design. 2. Refractor Mechanical a. Folded design to reduce tube length. Adds a mirror. 3. Reflectors 0 to 3 mirrors a. Herschelian tilted mirror, no central obstruction, lots of aberrations unless F20 F30. With long focal ratio, primary can be left spherical. b. Newtonian (Dobsonian uses this design) i. Simple, 2 mirrors, 1 parabolic, 1 flat ii. Open tube, requires spider c. Cassegrain usually requires perforated primary. All designs difficult to baffle to keep out stray light. i. Classical primary parabolic, secondary hyperbolic. Usually f15 ii. Ritchey-Chretien coma free design. Primary hyperbolic, secondary hyperbolic. Strong field curvature, more astigmatism. Off axis star images are round making computer correction fairly easy. F7 to F9. iii. Pressman Camichel primary is spherical, secondary strongly elliptical, strong off-axis aberrations. Easier to manufacture. F20+ iv. Dall Kirkham secondary is spherical, primary elliptical, strong off-axis aberrations. Easier to manufacture. F20+ d. Gregorian has an erect image and inward curving field. Not favored because of greater length than Cassegrain. i. Classical parabolic primary, elliptical secondary. ii. Other variants exist as with the Cassegrain, but are not manufactured. e. Naysmith flat diagonal redirects light cone out of side of tube near primary mirror. Good for nearly fixed eyepiece. 3 mirrors. Type applies to Cassegrain and Gregorian designs. 4. Catadioptric mixed lens and mirror designs. a. Schmidt thin strongly aspheric corrector. Sealed Tube. No Spider. i. Camera has strongly curved but aberration free field. ii. Newtonian Sealed tube, no spider. Reduced but not eliminated off-axis aberrations. F6 iii. Cassegrain usually F10 F11. Very popular, short tube, light weight. b. Maksutov Very thick, heavy, spherical meniscus corrector. Sealed Tube, No Spider. Strongly curved corrector keeps stray light out of the field. i. Newtonian better off-axis than Schmidt Newtonian. Offered at F5.3. ii. Cassegrain Questar, F15. iii. Gregorian design has erect image and had been made as a small terrestrial telescope by Celestron at F10.
Discussions of Technical Advances in Telescope Manufacture 1. Anti-Reflection Coatings. a. Multiple coatings on surface reduce reflectivity of under .5% overall. Be sure what you buy is fully multicoated. 2. Reflective Coatings a. Plain Aluminum, 89%, 2 mirrors 79%. b. Enhanced Aluminum, 96%, 2 mirrors 92%. c. Dialectric, 99%, 2 mirrors 96%. At present, very expensive and only for diameters up to 4. 3. Substrates the lower the coefficient of expansion, the better. a. Plate Glass inexpensive, high expansion b. Pyrex most frequently used. About 1/3 the expansion of plate glass. May no longer be available for telescope mirrors. c. Quartz i. Fused quartz expensive, near zero expansion. ii. Corning ULE expensive, zero expansion, stable at high temperatures. This is quartz doped with a low percentage of titanium dioxide. d. Astro-Sitall zero expansion, but cannot be heated as it will break down. 4. Figuring of Surfaces a. Classical pitch lap, Ό - 1/10 wave surface accuracy. b. Computer controlled Ion Beam Figuring 1/100 wave surface accuracy. 5. Aberrations a. Spherical eliminated in all designs. b. Coma eliminated in a few designs. c. Astigmatism eliminated in a very few designs d. Field or Petzval Curvature very difficult to eliminate. Flat CCDs make it desirable to eliminate. Latest Celestrons and Takahashi astrograph at F2.8 e. Chromatic not an issue with Catadioptrics or Apochromats. Some secondary color in two lens refractors.
Bibliography Telescope Optics, Evaluation and Design Harrie Rutten and Martin Venrooij Comprehensive discussion of telescope optical systems. Very thorough but easy to read.
Star Testing Astronomical Telescopes, a Manual for Optical Evaluation and Adjustment, 2nd Edition Harold Richard Suiter More than youll ever want to know about collimation, secondary obstructions and aberrations effect on images.
Reflecting Telescope Optics, I & II R. N. Wilson If you really are strongly motivated you can tackle these two volumes, which contain enormous amounts of information about reflecting telescopes.