“This is a doublet airspaced ED Apochromat. We think it is some kind of SD or Fluorite design. From a set of four of these Apos this is the second best. The best was really outstanding and did show during a a Zygo test (632nm) an error of 1/11 wavefront p.t.v. and 99.1 % strehlratio. This lens show in my CCD images some additional colour, which are for human eye hardly visible, except when you are using very high magnifications on bright stars. And even than you will have to look twice to find any traces of violet. The images are looking quite good and are slightly misleading on the real quality of this instrument. The lenses are polished very smooth but high power Startesting shows an clear undercorrection with a bit of a turned edge. My impression on this lens is about 1/4~1/5 wavefront p.t.v. only , which can be seen only at higher power. The longer focallength of this scope is quite forgiving on such errors and makes them for most users invisible” Markus Ludes (APM-Telescopes)
|
