[mental4astro (Alexander)]

Hi all,

I struggled a bit trying to work out which forum to put this in. Could have been Equipment, DIY, or this one. It deals with contemporary thinking behind Mak and SCT insulation/dewshield, DIY modifications, and a bit of artistry. Not entirely one or another exclusive theme, so I've opted to place it here.

Popular thinking about Catadioptric telescope "cooling" and dew control is to wait until the OTA cools before attempting high magnification viewing or photography. However, new thinking takes a different approach. Instead of waiting for the OTA to cool, or for that matter force its cooling, the new approach is NOT to let the OTA cool. This doesn't mean to apply heat, but instead to insulate the OTA in order to dramatically slow its cooling.

The problem Catadioptic scopes have is the tube cools very quickly, however, the primary mirror, its cell and the baffle tube do not cool anywhere as fast, so the surrounding air inside the OTA experiences a significant temperature differential between the warm primary/cell/baffle and the cold tube. This in turn generates those dreaded tube currents.

Now, if the tube is NOT ALLOWED to cool quickly, but rather the OTA is insulated, then the air inside the OTA does not experience a temperature differential, and those dreaded tube currents are not generated. Best of all, this also means that you can begin to use the telescope immediately as there is no need to wait for "thermal equilibrium" of the OTA as there are no thermal currents to wait to abate.

The insulation method also sees this material be extended beyond the corrector plate doubling as a dewshield. The insulation/dewshield combination also helps to extend the dew-free period of the corrector, if not totally eliminate dew formation.

With my 7" Intes Mak, I made this insulation/dewshield out of white Corflute. Colour selection is a critical part of this combination. Black is the very worst colour for our gear to be painted. It loses its heat the fastest and cools to below ambient temperature, making it attract dew and even frost long before any other section. Black as the colour of any insulative material will actually be counter productive with what we are trying to achieve.

The Corflute was prepared to facilitate its wrapping around the OTA by having every second channel cut on one side. The Corflute will then very easily wrap around the OTA with the cut section facing out. The part of the Corflute that doubles as the dewshield, the inside was lined with black felt. The Corflute also had notches cut out to accommodate various items, like finder scope blocks, handle and anchor points of the dovetail plate. These slits also make rolling up the Corflute to stow very easy.

The result is quite remarkable. This Intes Mak is a heavy item, with a very thick aluminium tube, and the meniscus corrector lens is very thick. The heat differential would happen for a long time if the OTA was allowed to cool unchecked. But with this simple Corflute dewshield, I've been able to commence high magnification viewing straight away on setting up. And so far I'm still to be overwhelmed by dew on the corrector plate.

Alex.