This telescope clearly shows a 50% inner area zone on both sides of focus. The white light image gives the impression that the optics are undercorrected with an turned edge of about 1/3~1/4 wavefront p.t.v. and not very smooth. The Ronchi images however show some overcorrection. When used in startesting a greenfilter makes the overcorrection also clear, because at outside focus the outer bright ring is more sharper and brighter than at the insidefocus outer bright ring, which is soft. With a green filter the lens show still an bit of the mentioned turned edge but has an overal spherical correction of 1/5~1/6 wavefront p.t.v. area
Markus Ludes (APM-Telescopes)
Note: ronchi-testing with 254 lines/inch or 100 lines/cm.