I stumbled on this method of keeping the mirrors free from condensation during imaging sessions largely by accident. The problem that I was trying to solve was how to prevent the build-up of moisture film on the mirrors when the scope is left standing between sessions. The solution was to fill a muslin bag with a few hundred grams of self-indicating silica gel and place it in the recess behind the primary mirror, sealing it with a home made cap. This works perfectly, but what I hadn't expected was that if it's left in place it keeps both mirrors free from condensation during and imaging session as well as during storage. The gel needs to be warmed-up in a microwave every few days, depending on the amount of moisture absorbed, but that's the only inconvenience and at least it's a saving on heater tapes and a controller. I do suspend a second bag from the upper end of the tube if the scope is being left unused for several days, which is obviously removed before imaging!