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Focus Problems.

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Posted by Robert May on September 16, 2001 19:51:05 UTC

Are we discussing a 60mm F11.6 refractor here?
Please remember that the primary optics (the lens or mirror of the telescope) is going to focus the light at a particular point away from the primary lens or mirror. Since the scope has a 700mm FL then that focal point is going to be 700mm away from the primary lens or mirror. The design of that scope is probably such that the EyePiece is going to be located properly in it's original configuration. Adding a right angle mirror to that scope will mean that the light has to travel that much further to get to the EP and, if the travel of the focuser isn't long enough, it will be impossible to get the EP to the same distance from the primary lens or mirror. The barlow allows for the focusing because it changes the effective FL of the scope by 2x or whatever the barlow is designated as and as such, may bring the focal point out to a point at which you may be able to focus the scope. It might be noted that the barlow is going to do it's rated magnification of the image only when it's applied to the EP and not to the right angle mirror. When put between the scope and the right angle mirror, the magnification will be larger and thus, the focal point even further away than the basic focal point of the scope.

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