Re: ATM field of view

["Michael P. Lindner" <mikel@att.net>]

Kreig McBride wrote:
> 
> Looking for the formula for calculating the widest field of view with
> respect to the draw tube size.
> 
> Example:  55mm is the longest focal length that gives the widest true field
> of view in a 2" focuser. (That's what is in the literature.)

and that's not necessarily true, either. The field covered by an
eyepiece is approximately

        2 x FL x tan(AFOV/2)

where FL is the eyepiece's focal length, and AFOV is the apparent field
of view. For instance, for the TeleVue 32mm Plossl, apparent field is
around 50 degrees, so we have

        2 x 32 x tan(50/2) = 30mm

Of course, TeleVue publishes the field sizes of their eyepieces at
http://www.rahul.net/amall2/tvo/eyepiece.htm and the given number for
the same eyepiece is 27mm (which is why I said "approximate" above.

The point is, it depends not only on the eyepiece's focal length, but
also on its apparnet field (as an example, the TeleVue 40mm plossl has
the SAME field size as the 32mm - note that it's apparent field of view
is smaller).

--
Mike Lindner