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Re: Is Dall-Kirkham Coma Visually Less Perceptible Than In A Comparable Newtonian?

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Posted by Robert F. Royce/">Robert F. Royce on October 23, 1998 12:42:21 UTC

I have designed a 12.5" f/35 Dall-Kirkham as a dedicated high resolution CCD lunar/planetary scope. The primary is to f/6. Spot diagrams indicate that coma is not a problem (exceeds to Airy disk) at a distance of 10mm off axis at the image plane. But look what I did, I did not use a fast primary. Too many people use very fast primaries and coma increases when fast primaries (or small diameter secondaries) are used. The great advantage of the Dall-Kirkham is it's spherical secondary, which can me fabricated at high accuracy as a smaller central section of a larger piece. This a very important consideration for high resolution work inasmuch as the hyperbaloidal secondary is very difficult to figure and test; at the least you'll need a Hindle sphere or full size flat - and even the the surface may be rough and zoney. But still the coma in a Dall-Kirkham can be a problem unless the design use is correctly understood. It's going to be a small field, lunar/planetary/double star scope.

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