I've been insulating my Cats for a few years now, both SCT and Mak.
The dual purpose insulation/dew shield does two things:
1, stops internal heat plumes from forming from a warm primary & baffle tube as the insulation stops the tube from getting cold, so the temperature differential and gradient between tube and internals is much reduced to close to nil, so you can start using the scope straight away without needing to wait for the scope to cool - you can use high magnification from the very start.
2, keeps the corrector dew free for longer.
See here:
Thread about insulating a Cat
My Mak, naked:
My Mak with the insulative dew sheild:
This does not mean the corrector does not dew up. If conditions are right dew will form, but much later. I live across the road from a lush park in Sydney, and not far from the coast, dew is ever present even with the weather apps say the dew point is too low for dew to be forming, everything is soaked here... the microclimate at my place is a bugger. I'm finding the corrector of my 7" Mak holds out well for between three and four hours before the corrector begins to mist up (and I've been using 300X and higher from the start) if at all, and the SCT for a couple of hours. A hair dryer is all I use to gently evaporate the dew, but I am not imaging.
Problem with dew heaters is they introduce a heat source that is an issue for a few reasons, 1, Majority of the heat goes into the metal tube and out to the atmosphere, very little into the glass, 2, the perimeter of the glass is heating (expanding), while the glass deeper in the glass away from the heat source remains cooler, and you end up with developing strain in the glass which distorts it, the last thing we want to do!
...
A better solution would be to insulate
and use a dew strap together if you plan to be out for an extended period. The heater could be switched on to a very low setting from the very start. The insulation retains heat for longer inside the OTA, and the dew heater works with the insulation to make for a much more efficient thermal system that vastly reduces any strain inside the corrector due to heat differential. Refractors would also benefit from the same insulation method for the same thermal dynamic reasons as the corrector of a Mak and SCT.
Best of all would be to use a small fan inside the dew shield assembly instead of a heating strap - no heat-induced distortion in the optics or OTA.
Alex.