| andydj5xp |
| (professor emeritus) |
| 05/18/08 11:16 AM |
|
Re: Strange Results in Eyepiece FOV Comparisons
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Quote:
Good point! I thought of this, but OTOH there is (I would hope at least) likely no one who is not by now aware of the highly non orthoscopic character of the Nagler's and Panoptics, and this certainly has not stigmatized their very well deserved reputations. I therefore rapidly concluded that there is simply no valid stigma associable with being either orthoscopic or not orthoscopic, so I completely discounted this potential.
The term "highly non orthoscopic" sounds a bit negative and is kind of inappropriate for an ultra-widefield eyepiece. They are normally designed for small angular magnification distortion (AMD) which - as the apparent field gets much larger than about 60° - is by definition not orthoscopic. They are simply not ment to be orthoscopic. AMD means constant focal length from axis to the edge which will invariably bend outwards straight lines at the edge of field. For astronomical observations this will be no disadvantage while OTOH other advantages will be achieved. Orthoscopic means increasing focal length from axis to the edge which will keep the lines straight but will squeeze the outer part of the field.
Orthoscopic without distortion will give AFOVs as
AFOV = 2arctan(FS/(2*FL)) called also tan-formula
and AMD without distortion will give
AFOV = (FS/FL)*57.3° called also arcus-formula
with FS = field stop diameter and FL = paraxial focal length of the eyepiece.
Distortion will of course change the AFOVs more or less and since it is not normally known it is better to rely upon the FS which can be determined quite easily (either by star drift or by observing a tape measure).
Andreas