Thanks Barry
Paul - I only went to -5 and no further. At -5 it was already at 54% or thereabouts, so ambient was pretty high. I'm running a little 50w 240-120vAC step-down at the moment, and that little thing gets pretty warm so I don't want to push the cooling cycle too hard just yet. Frames I took either side of the Tarantula look clear, so I don't know what the cloudiness is - extra nebulosity?
With the shutter closed, how would you know if you were in fact frosting? Take an exposure, shine a light down the scope and take a look?