[Wavytone]

Hi Nigel,

The usual question is what is the longest eyepiece focal length to suit a given telescope. The answer is Fe = p x f, where p is your eye pupil (normally assumed to be 5-6mm) and f is the focal ratio of the scope.

Alternatively, given an eyepiece focal length Fe = 28mm, and eye pupil 5mm, this suggests the telescope needs to be no faster than about f/5.5.

As for the widest field of view, that's determined by the diameter of the field stop in the eyepiece, and the telescope focal length. If your field stop diameter was d and the scope focal length is F then the angular field of view is (to a very good approximation):
A = arctan(d/F)

You can go as small as you like for the objective. For example, if you cannibalised a 50mm finder - typically a cheap achromatic doublet f/4 to f/6, it will have a focal length around 200 - 300 mm, giving a magnification around 10x with your eyepiece. If the eyepiece field stop was 28mm across (just a guess, you can measure it for yourself with calipers) you'll have a field of view over 6 degrees wide.

To go wider, you could use an even smaller objective - say a 30mm binocular objective (often around f/5) and an ultrawide eyepiece with a larger field stop - a field of 10 degrees or more is feasible, though the image quality might be pretty awful near the edge.

To deal with that you will need to find a better wide-field objective and, for low magnification, the old-school telephoto lenses from film SLR's from the 1970s or 1980s work well enough, and you can still find a few in the junk bins in camera shops. For many years I used a finderscope consisting of a 135mm f/2.8 telephoto coupled to a 25mm eyepiece, giving about 6X magnification and a nice bright, wide field. Camera lenses in the range 135-200 mm work quite well as wide field low power scopes. To put an eyepiece on the back the simplest way is to cannibalise a rear lens cap.