Achromatic Refractors

© Geoff Gaherty 2003
Toronto Centre RASC

Back in the first half of the 20th century, there were only two types of telescopes in general use by amateur astronomers: achromatic refractors and Newtonian reflectors. By far the most common types were 3" f/15 refractors and 6" f/8 reflectors. Amateurs at that time were well aware of the optical law governing acceptable chromatic aberration in refractors: the focal ratio had to be five times the aperture in inches (Conrady: Applied Optics and Optical Design (Dover)). Sidgwick (Amateur Astronomy for Amateurs (Faber)) calculated a slightly more liberal standard: three times the aperture in inches. Note that this is the focal ratio, not the focal length. Since the focal length is the aperture times the focal ratio, the require focal length increases as the square of the aperture:
 
D (mm) Sidgwick   Conrady  
  f/ F.l. (mm) f/ F.l. (mm)
50 6 300 10 500
75 9 675 15 1125
100 12 1200 20 2000
125 15 1875 25 3125
150 18 2700 30 4500

As you can see, the focal lengths of the telescopes quickly become enormous, which explains why there were few amateur refractors much larger than 4" aperture. Occasionally refractors were built for special purposes, such as comet hunting, with lower focal ratios; Leslie Peltier’s 6" f/8 refractor was one of these rare birds.

In the late 1960s the mass produced Schmidt-Cassegrain was introduced, and this pretty much wiped out the demand for large refractors among amateurs. Around 1980, new glass types and computer lens design became available, and it became possible to produce apochromatic refractors for the first time. These were and are very expensive compared to other scopes, but offered the color correction of a Newtonian combined with a very wide well corrected field of view, hitherto unavailable.

The existence of short focal ratio apochromatic refractors has caused a resurgence of achromatic refractors attempting to offer similar wide fields of view, but without the color correction of the apochromats. Because portability has become a major selling point, modern achromats are usually made in focal ratios dictated by marketers rather than by opticians. Many people buy these based on the impression that they will get the legendary performance of long focus achromats or short focus apochromats in a compact inexpensive package. This is a physical impossibility.

What you get in a modern mass produced achromat is a compromise. If you want a really wide field, you get a lot of chromatic aberration, and are limited to low powers. If you want high magnification for lunar and planetary observation, you can choose a medium focal length refractor, and still get a moderate amount of chromatic aberration. The only way to get the legendary refractor resolution and lack of chromatic aberration is to try to find a long focal ratio achromat (which hardly anyone makes any more) or pay a lot more money for an apochromatic refractor. None of the compromises out there will deliver the goods, no matter how much hype you hear about "semi-apochromats."

If none of the achromatic refractors currently on the market adequately suppress chromatic aberration, why do people buy them? Partly it is irrational: there is a mystique about refractors being "real" telescopes. Different people vary in their sensitivity to chromatic aberration, and if you observe most of the time with it present, your brain learns to cancel it out. I have personally owned and/or used refractors ranging from f/5 to f/20 in apertures ranging from 2" to 36". From an optical point of view, I strongly prefer those with long focal ratios, f/15 or more. From a practical point of view, taking into account portability and observing comfort, I’m prepared to compromise down as far as f/8, largely because I own a minus violet filter and an Aries Chromacor which can mask or correct much of the chromatic aberration. I won’t go over 5" aperture because of the size and weight of even an f/8 refractor in this size. Finally, my refractor is not my primary instrument; for most of my observing I prefer the larger aperture and perfect color correction of a Newtonian reflector.
 

2003-05-28