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Message: <blockquote data-quote="kabsetz" data-source="post: 1348780" data-attributes="member: 10167">Thanks, Jean-Charles, for doing the theoretical part needed to keep faith in the booster method.I would like to add one bit of practical caution, mainly for those who have not used boosters much yet but are going to try - Henry and Ron surely have come across this already. That is, if the small part of the objective of the booster that takes in the light from the optics that is being tested includes the prism roof edge of the booster, the roof edge can cause enough problems to the image of the booster to visibly compromise the evaluation of the star-test. In this special case, the aberrations of the very small area of the booster are actually significantly higher than the compound aberrations of the whole objective. This is more a problem when viewing a tightly focused star-image, and can be overcome by placing the image just slightly off-axis in the booster image. This, of course, is not a problem if one uses a porroprism binocular or a standard, high-quality finderscope as the booster, but it can be a factor when using boosters such as the Zeiss 3x12 or the Leica 8x20 Ultravid, both of which I use. Although both of these have a very well-ground roof prism, its effect can be seen (and ignored, to a large extent, when you know what it is) when the roof edge line crosses the diffraction image being examined.KimmoP.s. Ron, I think that you have had such a steep learning curve recently that I'm beginning to feel a bit uneasy whenever you cite my experience as surpassing yours. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" /></blockquote>

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