Inside Focus

Inside Focus

Outside Focus

Outside Focus

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

This is a second sample of this chinese made airspaced doublet achromat. It has excellent optics, similar to the one I got shipped to Aries for selecting Chromacorrs. Looking to the images of the ronchigratings experienced people can see the little under and overcorrection, looking to the Ronchigrating by star makes it more difficult and only by let moving the bands to the sides you can find an very small bending. Using an whuite light star and greenfilter it is visible that this lens is  slightly undercorrected, but how much ? Many customers would maybe say 1/8 to 1/10 wavefront, but using the lens with higher power indicates that it is about 1/6 wavefront p.t.v. because it become clearer visible the  inside and outside image diffrences. The lens is smooth but not ultrasmooth, a strehl of about 96~97 % is quite possible.This lens is an perfect match for an deluxe chromacorr, some people would call this lens maybe null corrected, I do not.From my personal experience I would say about 15% to 20% of these telescopes have such good quality.