It is better to leave a slight gap. The mirror mount and the mirror will expand and contract with temp at different rates. What is no pressure in your living room may be a lot of pressure on a cold winter night.
> On a similar note, a very experienced atm friend uses a trick which may
> be old hat but I have not seen reference to before...he "tunes" out a
> bit of spherical abberation in his primary by heating the back side at
> a certain radius with a ring of resistors with a series reostat to control
> the power. I witnessed a before/after star test at various power levels
> and walked away impressed if not totally convinced.
> I am guessing (if this really works) that he achieves some sort of
>differential
> heating across the mirror. Anyone have any experience with this sort of
> thing?
>
Wow. Just when you thought you've seen it all... I am truely impressed with the cleverness of this scheme. I would imagine it is kind of hard to control without a lot of sensors and a bunch of hardware, but I'd give it 10 out of 10 points for creativity.
Chuck