I decided to make a dew heater for the 80mm refractor and after checking the parts box didn't have enough resistors to make the one on the IIS projects. So tried another one that was suggested elsewhere. The former is a section of 90mm PVC pipe and the heater wire was cut off a jug element that was handy. The wire is long enough to give a resistance of 12ohms so at 12 volts it draws 1 amp (12watts heat). A bit too warm but with a controller or a reduced voltage would be OK. The "sock" was bought from the local sewing shop for $1/mtr. A bit of gaffer tape on the top for added insulation and waterproofing and its done.
All being well I should get some extended viewing tonight without the darn dew spoiling things
Bill