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Posted by Robert May on July 3, 2003 19:48:13 UTC

The problem that you are running into with the secondary is that the scope is such a short focal length. You will note that the cone of light from the primary to the secondary is reflected to the side by the secondary. This is the basic problem as the cone is so fat where the secondary needs to go.
You can shrink the secondary requirements slightly by making a sled type focuser where the focuser and the secondary slide up and down the scope tube which allows for the mounting plane for the eyepiece to be right at the edge of the tube and thus more of the cone of light will be going up the tube rather than going towards the side to the focuser. Doing this will allow the smaller secondary to be used instead of the larger one.
I'll also note that the .21 diameter size of the secondary isn't that bad for that scope due to the needs of getting the light to the side of the tube. Many professional scopes use secondaries that are on the order of 25% of the area (not the diameter as above) on their scopes and get good results.
If I did the regular focuser, I'd use the 2" secondary without any fears and if I wanted minimal zize for the secondary, I'd do the sliding sled focuser. BTW, old floppy disk drives have an excellent linear slide in them that you can use and a spring to hold the traveling part against the adjusting screw (1/4-20 allthread) makes things easy. Use a nut on the allthread which moves the carriage for best results.

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