[APML]: Mirror Lock?

[steves@LAGUNA.EPCC.EDU]

Someone wrote a few weeks ago that they had a way to lock the mirror of 
their SCT scope into place.  I would be very interested in knowing if 
this can be done and of course how to do it.

Provin and Wallis mention in their book that one could place a spring 
behind the primary to help eliminate mirror movement, but they don't 
mention where one could aqcuire such a spring or how to put it in.

As it stands now, I am limited to 20 minute exposures when shooting 
piggyback with my 500 mm lens.  Anything longer and I get trailed images 
that I trace back to movement of the primary.

Does anyone have any other tips or techniques to help with this problem?