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Re: Secondary Mirror Size

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Posted by Rick Crockett on January 26, 2001 02:51:38 UTC

Hi Walt an Marty:"

Way too much noise has been made about the size of secondary and loss of contrast.

Walt is right about the larger secondary except that you will not need a larger focuser. Reason is that your 100% illuminated area will not be vignetted. In fact your 100% area is still too small unless all you are interested in is stellar and planetary work.

Let's say you want to do moderated deep sky work with a 25mm 60 AFOV. This would give you about a 4mm exit pupil (twice as large but four times dimmer than 8mm). Based on Walt's numbers, less than 30% of your field would fully illuminated. You might as well chisel an inch off your mirror's diameter.

Using the smaller mirror would be even worse. Less than 2% of your field would be fully illuminated.

One solution I suggest is set it up so you can swap diagonals. Make your largest diagonal large enough to fully illuminate the apparent field of your longest eyepiece you intend to use or largest film plane for photography.

Have Fun,


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