I too have been using insulating foil around a standard dewcap on my C14 for a few years now with great success (see pictures). Previously even with the dewcap and dew heater on full there was a little misting up of the corrector plate on the -4 to -5 deg. C nights when the telescope was doing long exposures at high elevations. But the insulating foil has stopped that. Frost might form on the very front end of the dewcap but it never affects the corrector. I also have a ‘jacket’ around the OTA made from exercise mat foam which helps prevent the mirror end getting too cold and with it I haven’t experienced misting up of the mirror. The telescope is in a shed observatory with a run off roof and so has plenty of draughty gaps and the resulting ventilation helps dry out the interior after a night’s frost collection!
Richard
