Telescopes, a Primer
Stewart Rorer
NYSkies Astronomy Inc
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 CCD’s 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.
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 you’ll 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.