Hmm, you have set some quite tight parameters and I think some may be almost mutually exclusive, so I think you may need to consider some compromises. So, what compromises are you prepared to live with? Generally:
A
“pure” spotting ‘scope is optimised for terrestrial viewing:
- Easy set up – fits easily to a tripod with the ¼ threaded tripod adapter.
- Tripod head should provide Up/Down and Right/Left motions.
- Gives a correct way up image and when you sweep the ‘scope, L is L and R is R.
- Can usually focus closely as well as at infinity just by using the focuser, without the need to fit any extension tubes.
- May come with a built in 45 degree eyepiece assembly for easy viewing.
- The range of eyepieces that will fit and focus correctly may be restricted.
A
“pure” astronomical refractor is optimised for viewing the skies:
- It will give an upside down image – the fewer the glass elements the better.
- Requires a star diagonal to use to avoid grovelling on the ground and straining your neck when looking at objects over 45 degrees altitude.
- Star diagonal will give you a correct way image, but it will be mirrored so L is R and R is L.
- A GoTo astronomy refractor on an equatorial mount will be difficult to use for day time terrestrial viewing; the up/down and L/R movements are non-intuitive as one axis is tilted to the same angle as your Latitude.
- Flexibility of an extensive range of interchangeable eyepieces.
- If the mount is a GoTo, thus having motors, are there clutches so you can release the ‘scope and swing it manually? If not, you are stuck with using the motors to move the ‘scope which will make it almost impossible and very frustrating to view boats on the ocean, birds wading, etc.
Other issues:- Both will have a limited light gathering capability due to their relatively small aperture, in the range 60mm to 80mm I suspect.
- Good tripods are expensive.
- Good mounts that provide smooth movement with fine controls tend to be expensive.
Magnification is calculated using the formula:
Focal Length of ‘Scope/ Focal Length of Eyepiece.
E.g. 700mm / 20mm = x35 magnification.
For terrestrial use, I have found that for our 77mm spotting ‘scope, around x30 to x45 is an ideal magnification, as it provides sufficient:
- Magnification (more than x50 makes the image dim with low contrast) with a nice,
- Field of view (you don’t feel like you are looking down a narrow tunnel) and a good
- Eye relief (your eyeball is comfortably some 12 to 20mm from the eyepiece, not resting on it!).
Cheers
Dennis