I you mount a bare fan on the back of the mirror cell, you can leave the fan running to help keep the mirror closer to ambient temperature as the temperatures falls. The bad thing about fans mounted to the mirror cell is that they *will* cause the scope to vibrate - especially when running full throttle! Once the mirror is cooled, it's best to run the fan at a lower speed.
Better performance can be gained if the fan is mounted on a baffle so that the back of the telescope is sealed. Air moving around the mirror and up the tube helps clear any turbulence from the tube. Even with a baffled fan, you need to turn the fan speed down once the mirror is cooled.
Pat Dotson
Newtonian Cooling Fans
www.scopefans.com