Re: spotting scope question-specific

nicebrid wrote:
I am in western NY so I would guess it is a 6-7 driving distance between us.
Thanks for your offer, I just don't think I will be in your neck of the woods
anytime soon. very generous of you.
I am continuing to read (as time allows) and will sure to have more questions.

here is one now, of the big name makers of "conventional birding scopes" which will
interchange eyepieces?
I am going to guess that none will, as they have no incentive to do so.

If my guess is right, that is a point for going with an astro oriented scope.

What are the practical limits for a "conventional birding scope" in terms of
distance?
How much is that limit improved with an astro scope?
I don't know of any birders that carry a range finder so this will be somewhat
subjective,
in that it is difficult to accurately estimate distances over 100 yards for most
people, myself included.

...

Distance by itself is not a usable measure of telescope's worth. After all, standing on the Continental Divide in a national park, I could see Navajo Mountain -- 140 miles away -- without one. Even magnification doesn't tell the whole story.

People with excellent eyesight can see that the image produced with the best of optics leaves something to be desired when the power exceeds about 25 per inch of objective diameter, or 1 power per millimeter. Although higher powers can't reveal more, they can make it easier to see what detail there is, so up to 50 or 60 power per inch can be useful. Claims that a scope with an 80 mm objective will magnify as much as 200x are absurd, but you can readily find such claims even for 50 mm scopes.

In any event, the images produced even at the diffraction limit of 1 power/mm are too dim. You want an exit pupil of at least 3 mm for reasonable daytime observing, and double that at dusk. The exit pupil diameter is equal to the objective diameter divided by the magnification no matter what service the scope is intended for.

Eyepieces enter into into the comfort of using a scope. Different designs permit different useful apparent fields of view. Kellners are moderately good at 40 - 50 degrees, the larger fields performing poorly at apertures greater than about f/8 or so. Orthocsopic eyepieces have similar fields with better control of aberrations and are useful with larger apertures. There are more wide-angle eyepieces around today than I can keep track of. The granddaddy of the class is the Erfle, with fields of view from 60 to 70 degrees, about as much as can be appreciated while keeping the eyeball fixed. (My 80 mm scope with a 1.25" Erfle gives me about 12.5 power and a 5.75 degree real field, about 300' at 1000 yards and more than 6 mm exit pupil. That eyepiece needs a 2" focuser.) Astigmatism near the edges of the field goes largely unnoticed because peripheral vision lacks the acuity to reveal it. Real field is apparent field divided by magnification. Too much magnification (more than 12 or so for most people) makes objects too hard to find. An auxiliary finder scope is useless without a steady mount, but I can hand hold up to 15 power with something to brace against. With a solid mount, up to 50 power is useful on a fixed object such as a nest. I'd happily settle for 30. With my 5" mirror scope at 150 power, the ground vibrations cause by passing cars are evident in the eyepiece.

Jerry
--
Engineering is the art of making what you want from things you can get.
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