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Introduction

Large, optically fast telephoto lenses are an ideal tool for astrophotography and wildlife photography. But large telephoto lenses are very expensive. Another problem with large refractive lenses is aberrations, particularly chromatic aberration. Chromatic aberration is the failure of different colors to come to the same focal point. Manufacturers typically use multiple optical elements in order to control and minimize aberrations. But there is another way!

Astronomers need large optics that are optically fast too. Optically fast mean low f/ratio (focal length / aperture). But instead of lenses, the vast majority of large telescopes use mirrors.

L In the photo world, one can buy mirror telephoto lenses, but they do not generally have a good reputation, and they are often optically slow (like f/11).

However, it is my opinion that certtain telephotos using mirror optics can outperform lenses. My background is both professional and amateur astronomy, Some of my earliest publications were on optical design, and I've ground a couple of mirrors for telescopes. I've also designed and built optical systems for spectrometers.

It is my opinion that simple mirror optics can deliver superb optical performance for use as a telephoto lens. However, simple mirror systems require mirrors in the center of the optical path to bend the light to deliver it to the camera. This is called the secondary mirror, and an example is shown in Figure 1.

Figure 1. Optical diagram for a Newtonian Telescope,

The requirement of a secondary mirror, considering the size of the film plane in a 35mm camera, means the secondary mirror must be large enough to intercept and deliver the light to the camera. In a simple telescope design, like the Newtonian, the secondary must be slightly larger than the film plane. For 35mm film, this means the secondary must e at least about 2-inches across.

The secondary size means the main mirror must be considerably larger or all the light will be blocked. Thus the primary mirror must be in the 4 to 6-inch diameter range, with 6-inch performing better.

draft... still writing.

Figure 2. Results of comparing resolution of the 760 mm and a Canon 100-400 mm lens (at 400mm). The test target was at 40 feet for the 760 mm and at 21 feet for the 400 mm so the target recorded about the same size on the film

Views of the 760 mm f/5 on a Dobsonian mount. This is a wood construction with little regard for weight. The tube assembly weighs about 21 pounds. Using weight-saving materials the weight could probably be under 12 pounds.

This link shows the optical performance of Newtonian, Maksutov-Newtonian, Schmidt-Newtonian, and Wright-Newtonian telescopes. Note the improvement woth the wright design. http://silverstar.pccenter.ru/fov.htm Newton, Mak-Newton, Schmidt-Newton, Wright-Newton: useful field of view.

Other Wright-Newtonian, Schmidt-Newtonian links:
http://www.mindspring.com/~woharris/astro1.htm

http://www.meade.com
http://www.meade.com/catalog/lxd55/lxd55_series.html

http://www.celestron.com/products.htm

R. N. Clark Email contact (is encoded to prevent spam):
has the following form: username@qwest.net where
username is rnclark

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Last updated May 2, 2002