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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1302914" data-attributes="member: 68323">Thanks for the other data but that Astromomics explanation of phase compensation is bogus <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />No mention of polarization components and their phase lag which is what is actually compensated for. The last time this was posted I posted a follow up that corrected it (search the forums!).You summary is much better.My original point in posting this was to emphasize the following (though I didn't say it:<ul> <li data-xf-list-type="ul">Roofs prisms first came into use at the beginning of the century</li> <li data-xf-list-type="ul">The theoretical need for phase compensation was realized i in the early 1940.</li> <li data-xf-list-type="ul">A practical method did not become available until 1966 (but mainly for use in interferometers and other instruments)</li> <li data-xf-list-type="ul">Commercial phase compensated roof prisms bins became available in 1988 as Zeiss' P* coating. Others followed.</li> </ul>The best example of a controlled experiment as to if PC works I've reported here before comparing three models of Zeiss Dialyt with the same optics but built at different times. One with no PC and single AR coating. The next with no PC and multilayer AR coating (T*). The final one was a post-1988 build multilayer AR coating (T*) and phase-compensation (P*). The biggest difference was in the final model. Remember the good old days when a model could stay in the market for a couple of decades! When the Chinese got into this market they didn't have PC roofs. The Chinese roofs of course had other interesting problems making "good" bins: poor component QA, poor general QA, poor design (e.g stray light issues). They didn't have PC either but most people started to think "no PC" == "terrible view" (which some were but that wasn't just because they had no PC) not "softer and lower contrast along the axis perpendicular to the the roof edge" if they'd managed to deal with the other aberrations, distortions and stray light.Out of all of this I find the socilogical aspects of roofs vs porros to be the most interesting. Why did people change?</blockquote>

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