[glend (Glen)]

I have been using Argon purging on my cold finger modified Canon DSLR for about a year now and it has proven a very effective way to prevent internal condensation on the sensor and camera internals (the T-Adaptor is capped with a Baader filter so the camera is a closed aperture). Argon can not transport water vapour in the way that plain air does, and it is heavier than air so will not percolate away as fast as some other gases, and it will displace air in a closed vessel. It has occurred to me that for any closed tube scope, Argon filling may assist with the prevention of internal optic fogging. My Skywatcher MN190 is a closed tube scope and could easily be filled with Argon, and I am going to try this out at some point. Of course external glass surfaces like the Corrector would still need a heat strap but the internal mirrors should not be at risk of fogging on even the most humid cold nights - provided the Argon can be held in the tube without leaking out. As stated above,being heavier than air Argon tends to stay in vessels, simply sealing the rear of the scope would prevent the loss of the gas provided you don't 'tip' it out the focuser. Argon gas is readily available in any Bunnings that sells welding equipment, and the 1 litre compressed disposable bottles are cost effective. Of course over the course of some days it would probably leak out of the tube but if you had a long winter imaging session planned it could be very useful for extension of the session without worry about internal mirror surface fogging. I am not sure of how it would go in SCTs, but would welcome the discussion re other closed tube designs.