Heat Transfer in Solids+Solid Mechanics boundary conditions

Hi,

I am trying to simulate the behaviour of optics components when being cooled down from 293K to 4K.I am using Solid Mechanics and Heat Transfer to Solids, where i am applying they temperature to all boundaries. I managed to get results, but am not quite satisfied yet. I am using a fixed constraint boundary condition on the surface where the part is being mounted - which isn't really correct, as this surface won't undergo any thermal contraction in the simulation. I have tried to work with a points(also edge)-fixed constraint condition, but they solution won't converge.

Thanks in advance,
Andi