[bojan]

Finally I found some time to better document and conclude this astigmatism saga:
MTO-1000A glassware is pretty good but the assembly of the lens is not.
It is obvious on the attached images, how the tight retention ring that holds the mirror in place deforms the glass to such extend that the image becomes unusable for astro-photography.
Now, I have mirror which is actually quite free (not good, I am afraid: it slightly rattles in its cell and I will have to think of some way to keep it in place (and prevent damage from accidental external shock), while at the same time no pressure is applied which may deform the glass (silicon rubber pads, perhaps?).
There is still some residual astigmatism left (perhaps because now mirror "hangs" of axis slightly?), but nothing compared to what it has been before I started to play with it. I will try to remove this later, if it can be done, I will do it.
My biggest problem was in understanding what other people said about how they solved this problem. I thought that they were using too colorful language when writing that "mirror should be relaxed such that it almost rattles".. This had to be taken as it was written.. and so on.
Anyway... this lens is now much better than it ever was....
The images are 30 sec exposures with Canon 400D, ISO1600, 200% crop from central part of image, displayed with DPP. There is no visible difference of star images from corners, which means the focal plane is pretty flat.

EDIT:
To illustrate further the effect of too tight bracket around the tube, here I added a central part of two shots, taken when I was not aware I was squeezing the tube (and mirror with it) too much. They are deliberately taken slightly out of focus to emphasize the astigmatism.