[bojan]

Fahim, thanks for the link..
Yes, there are a number of models available out there..
The most common problem with all of them it seems to be astigmatism, in most cases caused by mirror too tightly fastened and therefore deformed ("pinched").
Relaxing of glass (by loosening the retaining rings that holds the mirror in place) generally helps a lot to get rid of those "tear-shaped" stars.
Those procedures on cheaper lenses are very popular in Europe (especially Germany.. people there are known to use their money very wisely :-)
But, the field flatness appears to be excellent in almost all models.
Sometimes, the mirror is simply not good enough... so no help here.

Well, if final image is resampled, those little flaws are not visible anyway.
In my opinion, what we really have to consider here is the price/performance ratio, and what we want to do with the lens.
Personally, I am not prepared to pay 10-20x more for only 1.5-2x better image (visible at sub-pixel level). Some of those "flaws" can be taken care of by post-processing (de-convolution) relatively easily and at no additional cost.
Quite a wide variety of very "serious" work can be done with those, medium-quality lenses (that means photometry etc, not just pretty pictures).

BTW, I still haven't done mirror relaxing on this lens (Rubinar 8/500), the retaining ring thread seems to be glued with brown stuff (shelack?).. so I gave up for the time being - I will try again with stronger tool in the next couple od days when I am on my annual leave.
But I have good outcome with MTO_10/1000A, the resolution apparently doubled after procedure (or, the star shape is now half the size compared to what was before the procedure).

And, finally, if the lens is not good for photography, it will be perfect as a guide scope :-)
Or as a visual telescope.. and a very transportable one :-)