STARTEST IMAGES GALLERY

 Refractor 120_2

Type: Achromatic Refractor  Doublet
Aperture: 120 mm
Focal length:600 mm(F6)

startest Images copyright
Markus Ludes (c) 2001

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Inside Focus

Inside Focus

Outside Focus

Outside Focus

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This is a new model doublet airspaced achromats 120F/5 or D=120 mm f=600 mm. The optical quality on this very fast is very good and impressive. The Ronchi images show an better quality than the real startest with white light or greenfilter. The lens is global very good polished with no zones, but somewhat overcorrected. If  I would not have seen the Ronchimages I would say this lens has only one deffect, a turned edge.  On both sides of focus the inner fresnell rings are getting dimmer and fainter towards the center, a sign of excellent spherical correction. Only the outer bright ring looks rather diffrent, a typical sign for turned edge. However the Ronchiimages confuse me a little bit, because the show no sign of turned edge but a global overcorrection. Comparing only the inner fresnell rings, i would say this lens is nearly perfect, but comparing the outer bright rings, my rating is about 1/4 wavefront p.t.v. Its nearly impossible for me to give a good estimation of the strehl, because so far we have not tested yet these simple cheap chinese achromats in interferometers, so I have nothing to compare. Using 200x magnification in startesting the lens shows several diffraction rings , maybe due the worse color-correction pushing much energy away from the ideal focus, maybe also/and from the overcorrection. In any case this fast achromat can be called an excellent telescope for low power deep Sky observing , I would use it myself, because very seldom you find better optics in such fast achromats

Markus Ludes (APM-Telescopes)