Soon after this thread came out, I tried my ST80 using the half-a-dozen diagonals in my collection: (1) Celestron 1.25", mirror, 90 deg, no CI; (2) Smart Astronomy (GSO?) 1.25", mirror, 90 deg, dielectric, no CI; (3) Orion 1.25", prism, 90 deg, CI, BK-7?; (4) Unknown provenance 1.25", prism, 90 deg, no CI, BAK-4?; (5) Orion 2", mirror, 90 deg, no CI, dielectric; (6) Kson 1.25", 45 deg, CI, BK-7? Verdict: In every case, no discernible improvement. No miracles seen.
On the other hand, since my prism/mirror trials, I have taken my ST80 to a dark site (green/blue zone) and discovered for myself the short tube achromat's true forte: Wide field views of nebulae and star fields. I upgraded to a single-focus Crayford (why would I need a dual-focus for low powers?). I fitted it with a Knight Owl UWA 30mm 2" 80 deg AFOV eyepiece, which yielded 13x, 6 deg TFOV, 6mm exit pupil, and a brightness factor of 1.1. And I screwed on a Celestron 2" OIII filter. The results: Excellent views of the North America, Pelican and Veil Nebulae, in fact the first view of them I'd ever managed to have. I could see all three major parts of the Veil with no difficulty. My wife and daughter also had no problem seeing these nebulae, so it was not a matter of my visual-observing skills. My personal conclusion based on my experience: These short tube achromats are best used for wide field Milky Way vistas and nebulae. Or you could rumage through your own collection of prisms and hope to get lucky. But wouldn't it just be better to use a long focus refractor or a good Newt or Cat for the planets and Luna? Just my two cents. YMMV.

May you have better luck with your prisms,
Mike