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<blockquote data-quote="Kevin Purcell" data-source="post: 1278140" data-attributes="member: 68323"><p>No, I am not sure. I'd love to get this clarified.</p><p></p><p>I did some rough calculations that seemed to indicate that TIR wouldn't work off the roof surfaces but trying to do a 3D calculation on the back on an envelope and guessing at beam widths means I could just be mistaken.</p><p></p><p>In one optics book I saw it pointed out that the "bottom" surface of the first semi-pentaprism is mirrored. Plus in the original Pechan prism the flat top surface of the upper prism (where the roof is in the SP) is mirrored (that's from MIL HBK 141).</p><p></p><p>But if people have pulled an SP prism apart and can give first hand knowledge (or a citation ... I amazed I haven't found one that indicates the mirroring!) I'd love to hear/see more.</p><p></p><p>The pictures of a SP on zbirding are difficult to interpret (as are any pictures of a mirror!).</p><p></p><p><a href="http://www.zbirding.info/zbirders/blogs/sing/archive/2006/08/09/189.aspx" target="_blank">http://www.zbirding.info/zbirders/blogs/sing/archive/2006/08/09/189.aspx</a></p><p></p><p>And Steve Ingraham does say (my <strong>emphasis</strong>):</p><p></p><p></p><p></p><p>I think part of my problem is confusing the informal use of "roofs" to mean roof prism rather than "roof surface".</p><p></p><p>EDIT: I answer my own question ... with the help of the University of Arizona (nice slides of a range of different prisms with tunnel diagrams). As they say "Note only one surface is mirrored". The lower surface of the first prism.</p><p></p><p><a href="http://www.optics.arizona.edu/ot/opti202/Misc/202%20Prisms.pdf" target="_blank">http://www.optics.arizona.edu/ot/opti202/Misc/202 Prisms.pdf</a></p><p></p><p>That blows my explanation of why my 8x42 Diamondbacks are so "dim" in twilight (i.e. less bright) than my smaller 8x32 porros. With three mirrored reflections it made more sense!</p><p></p><p>I'll fix the WP article. Thanks for the push, Henry!</p><p></p><p>EDIT 2:</p><p></p><p>The first prism is apparently also called a 45 degree Bauernfeind prism this is mentioned on page 358 of "Opto-mechanical systems design" By Paul R. Yoder (on Google Books). He also comments about moving the roof from the Schmidt prism to the bottom Bauernfeind prism. He say's "it lends itselft to packaging in a conveniently shaped housing configuration". I guess that's why Swaro do it.</p><p></p><p><a href="http://books.google.com/books?id=ukkZuvSSeUQC&pg=PT390&vq=Schmidt-Pechan&dq=Schmidt+Pechan+roof+prism&source=gbs_search_s&sig=ACfU3U2ggEpONHbBFR0Gpr9AjJiA0cTU1A" target="_blank">http://books.google.com/books?id=ukkZuvSSeUQC&pg=PT390&vq=Schmidt-Pechan&dq=Schmidt+Pechan+roof+prism&source=gbs_search_s&sig=ACfU3U2ggEpONHbBFR0Gpr9AjJiA0cTU1A</a></p><p></p><p>One page 395 he shows how even the Germans mount SP prisms with urethane glue. Anyone recognize the bin? The trick is in where the glue is applied <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink ;)" loading="lazy" data-shortname=";)" /></p><p></p><p><a href="http://books.google.com/books?id=ukkZuvSSeUQC&pg=PT390&lpg=PT390&dq=Schmidt+Pechan+roof+prism&source=web&ots=c1atins3rC&sig=mpgXKL956yTp85_9usj_Is8Z7Yk&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPT427,M1" target="_blank">http://books.google.com/books?id=ukkZuvSSeUQC&pg=PT390&lpg=PT390&dq=Schmidt+Pechan+roof+prism&source=web&ots=c1atins3rC&sig=mpgXKL956yTp85_9usj_Is8Z7Yk&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPT427,M1</a></p></blockquote><p></p>
[QUOTE="Kevin Purcell, post: 1278140, member: 68323"] No, I am not sure. I'd love to get this clarified. I did some rough calculations that seemed to indicate that TIR wouldn't work off the roof surfaces but trying to do a 3D calculation on the back on an envelope and guessing at beam widths means I could just be mistaken. In one optics book I saw it pointed out that the "bottom" surface of the first semi-pentaprism is mirrored. Plus in the original Pechan prism the flat top surface of the upper prism (where the roof is in the SP) is mirrored (that's from MIL HBK 141). But if people have pulled an SP prism apart and can give first hand knowledge (or a citation ... I amazed I haven't found one that indicates the mirroring!) I'd love to hear/see more. The pictures of a SP on zbirding are difficult to interpret (as are any pictures of a mirror!). [url]http://www.zbirding.info/zbirders/blogs/sing/archive/2006/08/09/189.aspx[/url] And Steve Ingraham does say (my [B]emphasis[/B]): I think part of my problem is confusing the informal use of "roofs" to mean roof prism rather than "roof surface". EDIT: I answer my own question ... with the help of the University of Arizona (nice slides of a range of different prisms with tunnel diagrams). As they say "Note only one surface is mirrored". The lower surface of the first prism. [url]http://www.optics.arizona.edu/ot/opti202/Misc/202%20Prisms.pdf[/url] That blows my explanation of why my 8x42 Diamondbacks are so "dim" in twilight (i.e. less bright) than my smaller 8x32 porros. With three mirrored reflections it made more sense! I'll fix the WP article. Thanks for the push, Henry! EDIT 2: The first prism is apparently also called a 45 degree Bauernfeind prism this is mentioned on page 358 of "Opto-mechanical systems design" By Paul R. Yoder (on Google Books). He also comments about moving the roof from the Schmidt prism to the bottom Bauernfeind prism. He say's "it lends itselft to packaging in a conveniently shaped housing configuration". I guess that's why Swaro do it. [url]http://books.google.com/books?id=ukkZuvSSeUQC&pg=PT390&vq=Schmidt-Pechan&dq=Schmidt+Pechan+roof+prism&source=gbs_search_s&sig=ACfU3U2ggEpONHbBFR0Gpr9AjJiA0cTU1A[/url] One page 395 he shows how even the Germans mount SP prisms with urethane glue. Anyone recognize the bin? The trick is in where the glue is applied ;) [url]http://books.google.com/books?id=ukkZuvSSeUQC&pg=PT390&lpg=PT390&dq=Schmidt+Pechan+roof+prism&source=web&ots=c1atins3rC&sig=mpgXKL956yTp85_9usj_Is8Z7Yk&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPT427,M1[/url] [/QUOTE]
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