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Message: <blockquote data-quote="henry link" data-source="post: 2003983" data-attributes="member: 6806">Thanks for the photos, Paul. For me the most interesting bits for evaluating the optics are the defocused glitter points on the water in the center left photo. They tell us a few things we can't tell from the nice sharply focused bittern. For instance, they're close to perfect circles so no significant astigmatism or pinching is present The central point is well centered and the outer ring is evenly illuminated so collimation is good. They remain round to the edge which indicates no vignetting. No visible color fringes, even on out of focus glitter points, indicates excellent correction for longitudinal CA.The outer ring of the diffraction pattern is very bright which gives the out of focus disk a hollowed out look. That pattern on glitter points more distant than the focal plane indicates undercorrection for spherical aberration. I can't guess how much from the photo, but some undercorrection is typical in a lens or scope like this.  It would be interesting to see if using the prism changes this. Prisms add spherical correction so I wouldn't be surprised to see better correction in the visual configuration. I use a prism diagonal instead of a mirror with my Tak SKY90 because by sheer good luck the prism adds about the right amount of overcorrection to compensate for the objective lens' undercorrection.</blockquote>