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Telescope Optics Design

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Posted by John Szabo on May 24, 2002 18:11:22 UTC

Letter to Amateur Astronomers and Telescope Makers:
About a great Telescope that you can't buy anywhere!

I would like to compare notes with others who are interested in improving the quality of optical performance in Newtonian style telescopes.

While examining the optical performance in various telescope designs, I have discovered a little known optical system, with all spherical surfaces designed by; J. L. Houghton in 1944. It is an excellent Rich-field photo-visual instrument in the fast focal ratios and also a superb high resolution general purpose telescope in the medium focal ratio range, rivaling the Apochromatic Refractors, at much lower cost.

The Houghton corrector is a pair of relatively thin, positive and negative lenses made of the same common kind of glass in pairs. The relatively long matching radii of curvature of the spherical surfaces facilitates ease of making and testing the optics.

One version in which a diagonal secondary is mounted on the corrector, shows superior optical qualities compared to two other well known designs.

I have compared the optical performance of three different one mirror catadioptric systems; SCHMIDT, MAKSUTOV and HOUGHTON derived Newtonian designs using the Skew Raytrace program and printed the results at the off axis distances of 2.5, 5.0 and 10 mm, for comparison. All three designs were free from spherical and chromatic aberrations. The Schmidt design showed about half of the coma compared to the classic Newtonian and the Maksutov design had 60% less coma than the Schmidt design.

The Houghton design was coma free and is the only one of the three, with spot sizes that
fit within the Airy disc, right to the edge of a 20mm field. Also it's radius of field curvature is about three times longer then either the Maksutov or the Schmidt, which is great for photography and easier on the eye accommodation.

I'm attaching a chart of spot diagrams comparing the three telescopes in the 6 inch F5
versions. Of course the superior performance of the Houghton is evident in many other
apertures and focal ratios that I have examined.

I have not seen the Houghton Newtonian design on the market. Perhaps someone can help me find a source for the optics or maybe, there is an ATM out there who would be able and willing to make the corrector lenses for a 6 inch F5 at a reasonable price.

For questions or comments, please don't hesitate to contact me.

John Szabo, E-mail;

ps could not attach chart have saved as jpg please email me and I will reply with attachment

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