Just as an update on my experiments, I did manage to find some thinner mirrors (described in web page listed on first message of this thread). I put these thinner mirrors on my dish, and repeated the experiment. I still got 3 of the 5 mirrors focusing at different places. :-( see below:
I am bit confused that some of the squares gave bright reflections and some were dim, however it does seem that 2 mirrors reflected in the same place and 3 were off center.
I then moved the aime of the dish around, which moves all 5 spots around in different directions (really interesting, and anyone who thinks that you can put a 2nd lnbf on one of these dishes and get anywhere near a good focal point should try this), and to make a long story short, I found one spot where I got all 5 mirrors to focus on the same point! See below:
In the picture this appears to be a wider area, but the picture was a closeup, so it is really pretty good convergence.
I was hoping to do some experiments comparing these two focal points on an actual sat signal, but I ran into some other problems that I'm put in another message. At this point, I made an adapter for the lnbf holder, which holds the lnbf in the area where the off center focal point was found. I installed this, and connected the lnbf to my old lifetime receiver, and started searching for a suitable sat signal to test reception on, but I got messed up by an unrelated problem, and didn't finish the experiment, although I do seem to be getting relatively good reception using BOTH focal points.
Anyway, I'm going to continue with these experiments and try to compare the two focal points when I get time, but I thought it was interesting that it might be possible to improve the performance of these dishes by finding a different focal point for the lnbf.
Edit: Was having problems displaying images this morning. Sorry if you loaded this message while I was editing it.