Re: ATM Eyepiece Selection

["Bill T." <twentiethwave@hotmail.com>]

> I just completed my 12.5" F5 scope and am in the market for a nice wide
FOV
> eyepiece. Here's what I've narrowed it down to:


If you multiply the f-ratio by your personal maximum iris dialation, you get
the focal length of the eyepiece that will give you the widest possible
field while still delivering 100% of the available loptical path light to
your eye.  This is a little simplistic, but gives a good ballpark figure,
and matches my empirical experience pretty well.

Most adults have about a 5mm wide dark adapted pupil.  So for an f5 scope, a
25mm eyepiece will be optimal.

There are still good arguments for oversized exit pupils for very wide
objects like M31 and M33.  But with the 35mm Panoptic, you would technically
"throw away" 2/7 of the available optical path brightness, and 3/8 of the
brightness with the 40mm.

I have built several scopes to fit a particular eyepiece: a 14.5" f5.3 to
fit a 27mm Nagler (5.3x5=26.5), a 20" f6 to fit a nice 30mm (6x5=30).

Bill T.