This is one of the chinese airspaced doublet achromats. In this telescope I found a defect which I had not found during the past 2 years :a clear inhomogeneity at one edge. The overal correction of this lens would be quite good, except that we can see in startest and in the Ronchitest an small turned edge. I have had extreme difficulties to rate this model with a Ronchigrating (using 5 lines, 3 lines and only 1 line, all where nearly perfect straight). There is no clear over- or undercorrection, since inner surface is very well corrected. During startesting with white light as well as with a greenfilter the undercorrection however easy visible.The inhomogeneity produced an alike astigmatism effect in defocused image visible and infocuse the airydisc is not clean , more a bit diffuse and triangular shaped. This telescope goes back to the importer for exchange. It is not usefull for higher power or chromacorr. If we forget about the inhomogeneity , i would rate this lens as about 1/4 wavefront, not better, the difference between inside and outside focuse is just too big even with greenfilter.
Markus Ludes (APM-Telescopes)
Note: ronchigram made with 254 lines/inch or 10/mm