Oh dear.
I did enjoy the Tamron SP 500mm mirror lens and it did make really nice crisp pictures, but it it really a bit too much faff for the sort of snapping I do. If I want super telephoto reach, I've got scopes that will do the job, and my magic drainpipe 80-200mm lens with a teleconverter gives me 400mm at f5.6 with autofocus.
So I have sold it on and, as you do, went looking at old Takumar lenses. I've just taken delivery of a rather lovely 28mm f3.5 SMC Takumar and initial tests show that it should be great as a general purpose tourist lens and ought also to produce good widefield astro shots. If the clouds go.