[ATM] Understanding field of view

[Göran Hosinsky <hosinsky@ya.com>]

Using www.efn.org/~mbartels/tm/diagonal.htm with mirror
525 mm f4, diagonal 89 mm and distance to focal plane
337 mm there is a note after the resulting table:

"max field for visual use with 68 deg eyepiece and 7mm
exit pupil = 16.61 mm radius."
 From the table the illumination is 83% at that radius,
which is rather more than the 70% acceptable.

How do one come to this 16.61 mm radius and not ~25 mm
where the limit for 70% is?

Goran
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