[bojan]

I simply could not resist recent ebay item on sale..
It was Tamron Popular 400mm F6.3, from '70-ies last century... for $30.
When it arrived, I found moisture on the rear glass element, inside.. no idea how the water get in ?? But it was easy to loose retaining ring and clean the element with soft tissue. Assembled again, it looked very good.

The reports on this lens (and most of others) on the web were useless, of course: lots of "poetic" descriptions, lots of images of flowers and birds, but none in terms of MTF or some high-contrast objects images that would indicate the true quality of the lens.

So, tonight I did the usual thing: star field at full aperture, 10sec exposure time, couple of shots only because mosquitoes were unbelievable hungry... So I run inside as soon as I could.

Some bad news: lens suffers from significant astigmatism, visible on the Beta Cruci image (attached, 100% crop from centre of the frame).

Good news: very similar distortion in corners. It seems the glass (front element, it will be the major contributor to the problem) is probably just "pinched". If this is true, relaxing the retaining ring will help here.
Further indication that this may be true is, none of the astigmatism is visible at f11.. so the central part of the front element is not "potatoid", only the edges are deformed.

So, after relaxing the glass, I may have another decent lens for photometry (aperture being 62mm diameter, not bad for the purpose). Or, it will be good as finder or guide scope.
I will certainly post the results of the procedure when I have them.