There is an old lens maker's saying that there is a fixed amount of information content that can be put into any optical system (and this limit is only partly determined by the theoretical diffraction limit of the optics). What this means is that you can either have high resolution in a small field or lower resolution in a larger field, such that there is a practical limit based upon the product of resolution and field size -- an inverse relationship whereas as the field size increases the median resolution tends to decrease. Thus, it's either very difficult or near on impossible to have a high-resolution result over a large field.There is no theory or optical law that states this but given limits on cost, complexity, and manufacturability it's generally true for any mass-marketed optical system.
That said, YMMV. So, now and then it might be possible to find a single sample (or a few samples) of a scope that exceeds the more typical result that is delivered by a particular design or brand (but consider yourself lucky if you happen to be "blessed" with one of these samples).
Okay, so with the above in mind, I've been using a Tele Vue NP127is for several years and I've spent a lot of time in image evaluation and reading on the NP scopes and my opinion is that the Tele Vue NP127is can NOT cover an APS-C sized field without some level of compromise. So, while Tele Vue does make a so-called large field corrector (which I do own) for the NP127is that option is really designed to image at something around three arc seconds per pixel and anything finer will likely result in some disappointment.
What these large field scopes offer is the ability to be used with large sensors (obviously), but with some compromises related to image scale and overall resolution. Thus, you can pair the NP127is (or better yet the NP127fli) with a large sensor like the FLI ProLine 16803 CCD (9um pixels, 52mm diagonal) and with some luck you can get pretty good, large-field results, but at an image scale of about 2.8 arc seconds per pixel.
So, the OP says:
I would like a longer focal length [...and "full frame"...] to reduce the image scale to a better match with great seeing.
And, what I'm saying above is that those requirements may be difficult to obtain with any scope that you are likely to find in the marketplace. In particular (and in my opinion) the NP127is with its large field corrector will NOT give you the kind of results with a full-frame camera that could make full use of a reduced image scale to "better match with great seeing."
In any case, what camera does the OP plan on using that can give both a wide field and high resolution? Furthermore, does the OP really expect to get seeing-limited results with such a combination?
Edited by james7ca, 02 March 2020 - 11:35 AM.