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Re: Dall-Kirkham Primary Mirror Testing

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Posted by Robert May on May 17, 1999 20:41:58 UTC

: Do I still use two blade edges in front of the LED or not? Is there something on the WWW you know of that describes this method in more detail?

If you do the tracing of the rays, you will notice that the image comes back inverted so there is no need for a second blade as the only light that you see when you get to the shading is the light that comes from the edge of the blade. In other words it creates it's own slit and that slit is as perfectly aligned as the blade is straight. F15 is a good ratio. The plate can be used if you want. You may want to insure that the mounting of the secondary on the plate is balanced so that there isn't any flexing of the glass and thus miscollimation due to the flexure. Making a flat surface can be hard on single thickness glass. Celestron uses a plaster type support tool to make thier corrector plates. The plaster is for holding the glass to the right shape under the vacuum pan method. You might like to consider casting a plaster to the back of the plate and get one side nice and flat - note that it is more important to get a smooth figure than worry about the curvature. If you end up with a few waves of curvature, you can always polish in the matching curvature (like a Maksutov corrector) which will provide very little change in the wavefront of the light entering the scope. Another thing that you probably haven't thought of is making a set of lenses to go near the EP and correct for any residual coma that may exist. These lenses are typically called Ross Corrector lenses although particular versions have other names.

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