The Photon Instruments 127mm Refractor
I received some very helpful suggestions over in the sci.astro.amateur newsgroup--including some from Roland Christen, one of the world's most acclaimed telescope makers (he owns Astro-Physics), and in the Equipment Talk forum at astromart.
One suggestion was to adjust the tightness of the front retaining ring that holds the front lens of the objective. This seems to have made a real difference. Too loose, and it appears that the undercorrection problem gets worse; too tight, and it puts pressure on the glass, causing other interesting problems.
With a little fiddling, I at least have the telescope producing a so-so image. Saturn at 190x shows more detail than the Televue Ranger. I did not have ideal viewing conditions tonight, but I was able to see Cassini's Division pretty much all the way around the planet, and there was more detail visible in the planet's atmosphere. It was still slightly fuzzy, but that may be the chromatic aberration problem (no easy fix for that), or it might have been the moisture in the atmosphere.
I still don't have even close to equivalent diffraction rings inside and outside focus, but outside focus there is at least something that looks like it could turn into diffraction rings when it grows up.
Another suggestion, from Roland Christen, was to change the airspacing between the two elements of the objective. There is a 2mm spacer ring in there now; I need to find some 1mm and 1.5mm spacers to try instead, and see if this corrects the problem.
A third suggestion, and probably the easiest to try, is to mask the objective down to 100mm, and see if the quality improves. If so, the objective has a turned edge (not too dissimilar from the same problem with parabolic reflecting mirrors in telescopes).
My attempts at getting any assistance, advice, or even trying to purchase a better objective from Photon Instruments has yielded nothing. My email to them has not been answered. I can't imagine buying anything else from them.
UPDATE: I have been told that Photon Instruments may not have responded to my email because of the MyDoom virus--they apparently were so overwhelmed with garbage from this that they had to throw away vast quantities of email in their inbox. I am going to make another try at contacting them.