I know a thing or two about optics, by not means and expert, but I consider myself very well informed. Why? because I'm not only a shooter, but I'm also a photographer and more importantly, an astrophotograper as well. I know optics, I had to learn a lot to get the best optics for my purposes, price be dammed. I will post some of my setups and pictures taken with those below.
Now, to the nitty gritty. there is a thing in astronomy called "seeing". That has to do with atmospheric turbulence. You can have the best optics in the world and if the seeing is crap you might as well pack it in. Atmospherics determine in no small measure what you can see - period. If it's bad you will not see the difference between a $500 spotting scope and a $4000 dollar one. It is that simple.
That said, under reasonable conditions, the most important part of this puzzle is objective lens diameter. There is no substitute for that. You see, there is something in optics we call diffraction limit. You can look that up here:
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/diflim.html
In short, the larger the objective lens, the better the resolution. This has nothing to do with magnification, that's another matter. So in a sense, a cheap scope with a large objective in theory beats an expensive scope of smaller diameter. that's just physics.
So, it sounds like: get the scope with the largest diameter objective lens for the cheapest price. This is indeed true, except that glass quality plays a part as well. You can't use terrible glass on a scope and beat a smaller objective lens scope. that said, most optics are reasonably good these days, so I would always go with the larger objective lens first.
That doesn't work all the time though. Why? because it is all "seeing" dependent. I have shot some of my sharpest astro images with a 76mm objective. Sometimes seeing has been good enough to get really good images with a 106mm objective. But with my 280mm objective "seeing" has to be perfect. This is how much atmospheric turbulence has to do with how sharp you see things. In my experience, in daylight, with bad atmospherics (middle of the day, mirage, etc.), you can't see really good detail past 20X magnification - tops.
Sure, you can spend $4k on a spotting scope and you will not ever go wrong. I know, I have some really expensive little telescopes. That doesn't mean that when the "seeing" is bad you're going to see anything better than a $500 dollar scope. That's life. In my humble opinion, 30x magnification at long range is about the best it is going to get unless conditions are optimal. Ok, maybe 40x but I really doubt that.
Look at the images below. In bad atmospherics a smaller objective which has less magnification works better - period. If you live in a place where it is hot all the time or there is a lot of wind a less expensive spotting scope with a large objective at low magnification is not a bad alternative.
Now for some pics.
Here's my 106mm setup. It was designed in the US, made in Germany, with Russian optics (Russians make the best optics I hate to tell you). Completely computer controlled.
Here's an image from my backyard, shot over 2 nights in great "seeing" - the Andromeda Galaxy:
And here is a 76mm setup (little white scope on top of the big one) in really bad "seeing" The swan Nebula:
If you don't need an expensive scope don't buy it for those 1 out of 100 days when it will make a difference, and keep the magnification LOW.
Best,
JAS
