Quote:
Originally Posted by bigjoe
An old Japanese 12.5 Kellner I had from Unitron had the sharpest and clearest image Ive ever seen on axis !
bigjoe
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Joe there's even better - take the Kellner apart and use the doublet alone, plano side towards the telescope. The clarity and contrast will astound you, though the useful field of view is typically around 30 degrees.
An achromatic doublet used this way is actually a Dollond eyepiece and just goes to show that 18th century observers really did not lack good eyepieces. You want a really, really, fast doublet - say f/2 - and 12mm and 6mm focal lengths could be a bit hard to find.
I can safely say that a doublet used this way is better than the original Steinheil monocentric, and the contrast is almost as good as you would get from a spherical eyeiece (single element being a glass ball, with extremely limited field of view and virtually impossible to apply multilayer coatings). The bonus is the 30 degree FoV which, in its era, was first class. You can see why the RAS types swore by this eyepiece at the time, if they had one from a competent optician. No wonder the Steinheil was controversial in its day.