A Survey of Refractive Systems
for Astronomical Telescopes
by
Roger Ceragioli
This Website is Dedicated to Wolfgang Busch,
Who made the First Practical Oil-Immersion
Triplet Apochromat for Amateur Astronomy in 1974.
Introduction.
This web site is meant to describe and illustrate for
amateur astronomers and telescope makers some types of refractive systems in
use for visual observing with astronomical telescopes--primarily doublet
achromats and triplet apochromats--in long and short focal ratios. It
assumes that the reader already has a fundamental knowledge of how telescopes
work, and will not discuss the basics. That information is easily
available both from books and many currently existing websites. Moreover,
books and the web also contain extensive information about reflective
telescopes, especially the Newtonian reflector. But what has been lacking
is equivalent in-depth information about the design and function of refractive
telescopes. This website aims to remedy that lack.
The site does not try to be comprehensive, although it does strive for depth.
Thus, the term "refractive" does not include catadioptric
systems such as Maksutovs, Schmidts, or Houghtons which use mirrors to collect
light and refractive elements to correct image errors. Rather, it
concentrates on systems which employ either lenses alone to collect light and
correct images, or lenses to collect and mirrors to help correct images.
Therefore, it includes not just achromats and apochromats, but also Schupmann's
Medial and Brachymedial telescopes. The reader should also note that the
site is not an equipment review. Names of living people and current
products are avoided, except where minimal reference is necessary.
Reviews of current products can be found on other websites and in books and
magazines.
An excellent place from which to begin examining telescopic systems--for both
newcomers and those already somewhat familiar with them--is the book, Telescope
Optics--a Comprehensive Manual for Amateur Astronomers, by H. Rutten and M.
van Venrooij (Willmann-Bell, 1988). This
text covers all the basics of telescope function and design, as well as the
principles of optical ray-tracing, which will be used extensively on this web
site to illustrate the error residuals ("aberrations") of the various
designs. In fact, since the subject of refractor design is already an
advanced topic, I will assume that the reader has a good working knowledge of
this book or something similar. A more advanced treatment can be found in
Uwe Laux's Astrooptik, 2nd ed. (Sterne und Weltraum, 1999), which
gives an extensive description and analysis of optical systems currently in use
for amateur and professional telescopes, as well as a copious
bibliography. Another excellent text is G.H. Smith's, Practical
Computer-Aided Lens Design (Willmann-Bell, 1998), which clearly discusses
the basics of geometrical and physical optics and shows in-depth how to go
about designing optical systems using current ray tracing software for PCs.
One of the best and most widely used of these software packages is ZEMAX,
marketed by ZEMAX Development Corporation. All the graphics on this
website which show image spots, ray fans, lens layouts and other data regarding
refractive systems were derived using ZEMAX. In Smith's book, mentioned
above, he discusses how ZEMAX functions and how it represents optical
performance through its many different graphical outputs [cf. Smith, pp.
39-97]. Anyone wishing more information than I give regarding my ZEMAX
graphics will do well to consult Smith. My goal is essentially to extend
the discussion of refractive systems found in Rutten and van Venrooij to more
types than they cover.
N.B. Special thanks go to Wolfgang Busch, and to
his friends Walter Stephani and Hans-Christian Schröder for their valuable
help in understanding classic German lenses. Special thanks also go to
James R. Lynch III, an optical engineer of 30 years experieince, who has given
useful criticism of my work and made it possible to extract the maximum amount
of information from the early Schott glass catalogs. Without the labor of
these gentlemen, my work would be much poorer. Of course, it goes without
saying that the remaining errors on the site are my own.
Table of Contents:
Chapter
1. Optical Conventions.
Chapter 2. Primer of Image Aberrations and their Graphical Representation.
Chapter 3a. Achromats (General
Considerations & Cemented Doublets).
Chapter 3b. Achromats (Contact &
Non-Contact Types).
Chapter 4a. Apochromats (General
Considerations & Doublets).
Chapter 4b. Apochromats (Triplet
Systems).
Chapter 5. The Petzval
Telescope & Sub-Aperture Color Correctors.
Chapter 6. Schupmann's
Medial Telescopes.
Last Updated: June 14, 2005.
Contact: rogerc@as.arizona.edu