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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1278049" data-attributes="member: 68323">For those that are having a problem visualizing this (like me) I refer to the diagram in Wikipedia<a href="http://en.wikipedia.org/wiki/Roof_prism" target="_blank">http://en.wikipedia.org/wiki/Roof_prism</a> shows the roof idea clearly in a pentaprism for those in need of a refresher.And this article shows the "standard" layout with the Schmidt prism on top (BTW, I wrote most of the text in this article so let me know of any errors).<a href="http://en.wikipedia.org/wiki/Schmidt-Pechan_prism" target="_blank">http://en.wikipedia.org/wiki/Schmidt-Pechan_prism</a>So Henry is saying in the alternative Sawro design the roof is moved from the top of the top prism to the bottom of the bottom prism (and the flat surface moves from the bottom of the bottom to the top of the top prism) and the whole prism assembly is mounted "backwards" in the light path (i.e. the light comes in on the right and exits to the left in the Wikipedia diagram. Is that right?Of course the next question is: why? What's the win in this design? The only thing I can see is it points the "wide" end of the prism at the objective. Does it make for a better fit in, or perhaps a smaller, tube?And to nitpick (I shouldn't do this with Henry as I'm sure he's forgotten more than I know about optics) the Pechan prism is actually a two prism composite (the top prism has a planar mirrored surface not a roof). From the MIL HBK 141 on optics design.</blockquote>

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