Finally made a decent dewcap yesterday and tried it out last night. At one stage I was considering wrapping the whole OTA in EVA foam permanently applied with adhesive, but decided that would be a bit of a shame and in addition, once done, if ever I wanted to undo this it will mean complete disassembly and repainting the OTA. So rather than do that I tried this solution to see if it was sufficient to stop the tube current - and indeed it is.
The aims were:
1. Improve the baffling against stray skylight entering the OTA to increase contrast, and
2. Kill the tube current.
3. Prevent dewing of the corrector without needing a heater strap.
1 sheet of 5mm Coreflute, lightweight hollow plastic sheet like corrugated cardboard much beloved by sign makers, this cost $8;
1 piece of black 3mm EVA foam rubber to line it, or you could use flocking or even black paint.
Velcro strap 1.2m long.
1. Aligh the Coreflute so that the flutes run parallel to the OTA optical axis. Cut the length of the coreflute to cover as much of the OTA plus 1.2X the aperture of the scope (for the dewcap).
2. Using a craft knife and a long straightedge, slit the topside only of every second flute so that the sheet will flex easily in one dimension to wrap around the OTA.
3. Trim to fit the OTA and line the part sticking out the front with black EVA foam. Spray contact adhesive worked fine.
4. To fix to the OTA a velcro strap wrapped around the outside is sufficient.
Set up and ready to observe takes me about 30 minutes. From the start there was NO tube current - zip, nada, none.
The contrast was indeed improved, both on DSOs and on the planets.
And best of all, no issue with dew all night.
This little experiment confirmed that its not really necessary to insulate the entire OTA and backend as others have done here on CN - at least not on this scope.
Will try again next week with an observing site that is typically -5 degrees C with the scope starting at a balmy 21 degrees C.
Edited by luxo II, 04 August 2018 - 08:00 PM.