Excellent that the secondary is Cervit. Since a cassegrain secondary can have a power from 2x to 5x or even more, you don't want the figure changing with temperature since it can have a large effect in image quality and focal position.
As for the using the Sun I think that is your best bet in finding the focus at the cassegrain focal plane. I was just try to point out that the diameter of the image your going to get isn't got to by a point but a very large disk. So you need to look for that.
Edited by DAVIDG, 30 November 2018 - 09:40 AM.