T3D Re: Telescope eyepieces

[john bercovitz <bercov@xxxxxxxxxxx>]

The better telescope eyepieces have eye relief up near one 
focal length.  John O's fov calc looks right to me.  Telescope 
eyepieces typically have enormous distortion (positive or 
pincushion).  The Erfle is one of the better ones in this
respect and the Erfle is what Mike Watters' father-in-law
used in his wide field MF viewer.  (Eye relief of Erfle ~= 
0.8 fl?)  Not sure if it could be massaged to work perfectly 
for us and what we could relax to get more of what we want.  
For really non-critical eye placement, we want more of a 
magnifier type of optic.  The eyepiece of a riflescope is 
what comes to mind but I think the fov on those is probably 
inadequate.  I'll have to take one apart and see.

John B

PS: Regarding loupes, you have to be careful on the magnification.
I seem to recall from when I checked it out years ago that they
all used m = 250/f + 1 even Schneider.

PPS: I checked out a 50 mm Celestron eyepiece in a 2" mount once 
and it had no pincushion to speak of.  Remarkable.  I checked it 
against a grid of lines one mm on center in a 24 x36 rectangle.


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