Matt St. Helens wrote:

What kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells
you this?

Well, if it uses an 1.25" eyepiece focuser (eyepiece outside diameter is
1.25"), the maximum field stop (the opening at the front of the field lens of
the eyepiece) diameter for such eyepieces is around 27mm or so. The field
stop formula will yield the true field:

TFOV = (180/Pi)*(EFSD/Fl), where EFSD is the eyepiece field stop diameter, Fl
is the telescope's focal length, and 180/Pi is the number of degrees in a
radian (about 57.296 degrees). Thus, and 80mm f/8.75 refractor (700mm Fl)
will yield a maximum true field of view on the sky of about 2.2 degrees. A
24mm Tele Vue Panoptic eyepiece would give you 29x and that maximum 2.2 degree
field, which would be good for examining large objects at low power. If you
replaced the focuser with one for 2" OD eyepieces, the maximum you might hope
for would be about 3.8 degrees (47mm field stop). Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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