Because of gravity, there is always flexure. It's just a matter of
how much.

"Piggy-backed" guide scopes, even if they are light, can be subject
to flexure exerted on the rings by the primary scope.

or, if the primary scope rings are lined with felt or other soft material, the primary scope ITSELF could be moving relative to the guide scope.

I replaced the felt in my 130mm APO rings with solid styrene, as seen here: <http://astronomy.mdodd.com/flexure.html#RingLiner>

That page shows seven things I did to vanquish flexure in my system. You can never pay too much attention to tracking down flexure!

--- Mike
http:// astronomy.mdodd.com