Your ultimate theoretical endpoint is 135 degrees at 1x -- looking up at the night sky. Anything else will be approaching that theoretical limit (14x, 10x, 8x, 7x, 6x....1x). Wide field is very much linked to magnification or the lack of it. Actually I suppose you could use negative magnification and get the entire 360 degrees projected in front of you, but the effect is weird.
The engineering constraint in terms of cost very much hinges on whether one wants color correction and the fast focal ratio. The Orion 80mm ED f/7.5 is $500 their 80 mm f/6 is $950. Yes it's carbon fiber but it pretty much illustrates how maintaining color correction shoots cost up as you reduce focal ratio.
Below f/6 tends to be Petzval (televeu) or similar (Pentax 125, four lenses) territory though I think Astro-physics pushed the boundaries with the original f/5 Stowaway, but they never went back. Some people like Jon Issacs thinks the f/5 Petzval wide field is the cat's meow, a view I respect, and that those of us who get by with f/6.3 or f/6.5 (let alone f/8) are mere pikers in wide field enjoyment. This is truly an aesthetic judgment. As far as I'm concerned anything at 20x or lower in commercially available scopes with 2" eyepieces is wide field but some people say it's not wide field till you're down around 10x.
You can shrink focal ratio in achromatic designs and decreasing magnification and aperture helps, and cost is not prohibitive--which is why a pair of non-ED binoculars can be pretty good. Not as good as apochromatic swaros but pretty good. And so the achromatic designs are available from $50 to $500, within that, some are much better than others.
Much hinges on how much you think you *won't* be having wide field views with your theoretical wide field scope. The best wide field views are in the Milky Way and the Milky Way is not up all night when it is up and it is not up every season of the year. There are some nice views to be had outside the Milky Way but you're thinning the herd of potential objects.
One thing for sure, the more you shrink your focal ratio the harder it is to maintain "reach" into the upper magnifications and the more expensive it is to do so and maintain apochromatic correction. An f/8.1 FS128 gets to 300x with a .43 exit pupil. An Astrophysics GT130 requires a .33 exit pupil to get into the same territory. That may not sound a like a lot but a 25% reduction at these tiny radii has pretty noticeable effects in terms of pushing the organic limits of your eye. So if you like razor sharp 300x views in a 5 inch apo you need to keep in mind that you are "buying" the difference between 25x (the FS128) and 20x (the GT130) at the low end with a noticeable shrinkage of exit pupils to get equivalent high magnifications in the zone of 200x to 300x.
So the "limit" as far as I'm concerned is how much you want to take a telescope out that will be handicapped when it comes time to look at objects that require some magnification. It's not just planets. There are planetary nebulae and galaxies which only begin to show to advantage, and in some case even become visible at all, at 50x, 80x, etc., in 3 to 5 inch apertures. Even more for teasing out double star separations.
I have two personal views. One is that wide field views of 2.5, 3.5 degrees on up are pretty d*mned good and that it is worth sacrificing some wide field at the low magnification end to have more options at the high magnification end. For the very low magnifications I reach into my trunk and pull out--binoculars. I've pretty much decided I don't want five and six degree fields in my telescopes.
The other personal view is that the development of highly corrected refractor optics using specialized glass has popularized the idea of "how low, how wide can you go" as a kind of astro-limbo game where we bend as low backwards as we can in terms of native astigmatism and field curvature tolerance in our eyes in order to squeak under an ever diminishing focal ratio. And part of the game is digging ever deeper into our purses for ever increasingly expensive glass and exotic designs to achieve that goal. For myself limbo is a game I leave to the kids and in a sense I feel that the ultra wide views are best left to the younguns who have the curvature accommodation and lack of astigmatism that many of us oldsters lack. But even the younguns, assuming they have the budgets for high end optics, may find that there's a limit to how "low they can go" in terms of having a versatile telescope that can split a close double at high powers and also offer a "wide" field--however defined.
Owning an achromat taught me that I don't really want a cheap scope "just for low power views." I invariably want more performance out in the field.
Edited by gnowellsct, 14 February 2019 - 03:15 PM.