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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1310601" data-attributes="member: 68323">You don't see that the first image is elongated along the horizontal axis of he image? Very odd.The difference between the star images is that left hand one (the first image) is elongated along the axis perpendicular to the roof edge (vertical in this image).  The second image on the right is a (properly) circular star image is from the corrected prism. The elongated star image means lower resolution along that axis: by comparing the widths at the 4th diffraction peak it's 40/22 or just a little bit worse than a factor two between the resolutions in the two directions. That's a useful number to remember when considering phase-compensation.I'm pretty sure Zeiss (where these images came from) can do a star test in a phase-compensated and non-phase compensated roof prism. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />See also the MTF plots from the 1991 Swaro paper on another thread in this forum for the measured difference in resolution along the two axes.Of course Ingraham's explanation is barely an explanation but that of course doesn't mean it doesn't happen and that these images aren't accurate measurements.Not sure what "star-test images" you mean here: my eyeball tests or Ingraham's real star test images.If you are talking about the Ingraham images that's not what I said. His images are clearly from a non-PC and PC roof prisms with I presume the same quality of knife edge. I'm not linking them to knife edge quality just to phase-correction and it's effect on the star image (diffraction pattern).For my eyeball tests the non-PC and much more visible knife edges show much bigger spikes. The PC and invisible (to me) knife edges show smaller spikes (but not "no spikes" except for the Pentax and Zeiss). I'm trying to get an explanation for these results. It could be PC related or it could be knife edge quality realted.The issue is suggested by Kimmo is whether the diffraction spikes I see in the bin are related to the quality roof prism knife edge or not rather than just PC as I first suggested. I merely note that I can't see the knife edges in the prisms but I do see the spikes so that puts a lower bound on the quality of the prisms. Alexis says he can see the prisms in all roof prism bins (and that includes the very good ones).I'm leaning towards the knife edge quality as causing the spikes we see given the estimate for the reduction in resolution but even there you can see the diffraction pattern is rather wide from non-phase corrected prisms. Perhaps by using the bright light source I see the tails of the diffraction pattern. I can't tell.I propose two hypotheses and the task is to differentiate between the two. With the data I have I can't do that myself. We need more data.But back to the original question too ... there must be people out there with Monarchs they could test!</blockquote>

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