I assure you, Jon, quality is never an abstract characteristic. Never. No matter which metric you may choose to measure it; or benchmark used to define it.
I am a little surprised we're even having this argument as we seem to be in somewhat violent agreement in that its all about priorities.
You are reading into this a bit. Never said a single characteristic defines quality. Rather, what I said/implied is this: its the sum of the priorities driven by a subjective value system. Yours are different then mine - that does not make it inferior/superior. It just makes it different. This is a straw man debate.
In my opinion, far off-axis sharpness is over-rated unless one is choosing to purchase an instrument for photography. I have never once heard anyone say that "...wow, the center is mush; but my God the off axis image is impeccably sharp and contrasty." What is not opinion is this: if the lens is not high quality it won't matter what the off axis performance is to the visual observer.
To state the obvious, refractors have limited aperture. To this end, I want the best optics possible so the performance of the lens is only limited by its aperture.
I've observed with, and tested a couple of NP127's, TEC140ED's and TSA120's. So, at least I come by my conclusions honestly. And, based on this, I made my recommendation as it pertains to the $5k budget.
Regarding the Don Bruns' experiment - I read the thread when it was active. But, really, he had no choice but to use an NP101 (or similar) regardless of its overall optical quality. His primary needs in a telescope were FOV and focal ratio for this experiment, not optical quality. Though I am sure he wanted a diffraction limited OTA and the NP101 certainly provided that.
Anyway, that's how I see it.
We clearly have different attitudes towards what quality means.
- Regarding field flatness: The center being mush, while the edge is sharp, that's not a flat field, that's what you can get with a short focal length doublet or triplet viewing at low powers wide fields.
- I don't think in terms of what 4 inch (or whatever) telescope provides the best views of a particular object, I think in terms of what telescope provides the best view, aperture is one of the many variables.
- Optical quality is suitability of a particular telescope system for a particular task. Don Bruns used an NP-101 because it had the particular set of optical characteristics/qualities that fit his needs.
- Quality depends on the job at hand, in that sense it's not abstract, it's not subjective. It's difficult to argue that the NP-127 will not provide the most perfect 3.65 degree star field of any of the 5 inch scopes under discussion.
On the other hand, if one is looking for the sharpest, most contrasty view of Jupiter, there are other 5 inch refractors that would very likely be a better choice.
But in my world, a 5 inch refractor is not the best choice for achieving the sharpest, most contrasty views of Jupiter.