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Light Cone Theory

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Posted by Steven A. Crosby on September 20, 2001 22:13:43 UTC

I am working on a 10" f6 reflector design using NEWT. I adjusted the parameters such that the 75% illumination diameter equals about 40mm, the largest ocular I expect to use (2" focuser). The 100% illumination diameter is just over 20mm. The diameter of the tube was adjusted so it just clears the 75% light cone without vignetting at the front of the tube.

So, if I understand this correctly, 20mm and shorter focal length oculars will be fully (100%) illuminated by the primary mirror. And, oculars with longer focal lengths will have the center 20mm fully illuminated, with the amount of light transferred to the eyepiece tapering off towards the edge of the field of view. Am I on track so far?

Then, since the 75% light cone is illuminating my largest ocular, I won't see any benefit in increasing the diameter of the tube - allowing more light in to the primary - since any of that light will arrive outside of the ocular anyway.

Is my thinking on track here, or am I missing something? Thanks for any help or advice.

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