Thermal Plumes and truss tube Dobs:  Are we our scope's own worst performance enemy?

I'm still experimenting on this issue, but based on the pronounced impact that a hand resting on the open end of a reflector near the focuser can have on the image, I'm intellectually convinced that the effects of the heat from the observer's body wafting across the path of the incoming light must be pretty significant.  This is mostly an issue for truss tube Dobs, rather than solid tube ones or refractors.  These latter classes still have to worry about the thermal plume rising above the scope into the light path when it is pointed towards the zenith, though.  This pales in comparison to the K&B style truss tube Dobs, though-- most light shrouds have about the density and fabric properties of an overgrown pair of bike shorts, and I can tell you from first hand experience riding in cold weather those have essentially no insulating value! 

The human body at its most quiescent (basal metabolism) puts out as much heat as a 75 watt light bulb; with a moderate amount of walking around, you can double this.  Unless you are fortunate enough to have a breeze that dissipates this heat away from your optical tube, you may in fact be adding considerably to the thermal distortion of your Dobsonian's optics!  With my old standby Frankenscope, I had nearly 3/4" of Sonotube and similar insulating materials between my body and the optical path.  With a truss tube Dob, you have only a Lycra shroud-- at best!  (Lots of folks like to 'go naked' at dark sky sites to better ventilate their mirrors.)  Point your telescope at a bright star or planet and defocus the optics until the disc fills the whole field of view.  Even putting your hand on the edge of the upper cage will cause dramatic currents to form!   Now imagine how much more heat your head and upper torso give off... and how, if the winds are blowing from your back, they can be carrying this plume right across the optical path!  Kinda' defeats the purpose of using fans to cool your mirrors and making certain that your collimation is dead on...

As a fairly regular Mars observer, I will be testing various simple thermal solutions out between now and opposition (end of Oct.).  The first one I put in was to take an old closed cell foam backpackers' sleeping pad and wrap it around the upper two-thirds of the OTA, securing it with bungie cords.   These pads are lightweight, cheap, and thermally insulating enough that you can sleep atop one laid directly on snow and not melt it!  I also made short segments of this same material to put on the upper cage-- since the light baffle is probably little better at thermal insulation than the Lycra shroud.  I have also made an extension to protrude above the upper cage assembly by a foot or so on the side facing the observer to deflect the rising thermal plume, but this can't be left on the scope in storage and hence will only be tested whenever I have time for a longer observing session and remember to fetch it.

Update:  We had a night of exceptionally good seeing, and the first time I was able to see a clear optical difference between Frankenscope (good Meade mirror with good aftermarket secondary) and Natasha (premium grade primary with secondary to match).  I had an image of Mars at 400X through Natasha that was astonishingly bright and clear-- like looking at a web cam composite digital image.  It was still very sharp at 600X (the highest I could go without Barlowing).  Frankenscope, meanwhile, was still stuck at the mid-300X, and its' image wasn't as bright nor did the focus "snap" in the same way. The following night had "good" but not fantastic seeing, but nonetheless I was able to work at 500X with Natasha.  While it is not conclusive, I think part of this superior optical performance may be attributable to the foam and insulating the light path from my body heat.  

Second update:  I again had a night of superlative seeing (used my non-Barlowed peak magnification of ~605X) with the foam shielding in place.  Removing it and looking revealed that Mars continued to be sharp at 600X, so the verdict appears inconclusive-- or requires that I push the magnification into the Barlowed range, and closer to 50X per inch of aperture (800X).

Third update:  This occurred during a Mars observing session in which the elevation of the planet meant that I was observing from a seated position, with my legs beneath the scope where body heat would rise into the optical path.  I was observing a 450X with mixed success.  When I put my foam insulation baffles on the upper 3/4 of the exposed portion of the optical tube assembly, observable detail improved noticeably, and in fact I was able to see some features for the first time in this observing session.  I believe that this was the most rigorous test of the presence or absence of the observer's body heat I've undertaken to date, and in this case thermal baffling had a demonstrable effect.

Has anyone else tried options like this to minimize thermal interference with optical performance, or are we all collectively fixated on dealing with heat "at the mirror's edge"?

UPDATE AFTER THE 2006 WINTER STAR PARTY:  I spoke to a number of serious observers-- mostly from Canada (where understandably, thermal issues would be more pronounced) who routinely use thermal wraps around their truss tube Dobs.  One-- Attilla Danko (of Clear Sky Clock fame) prefers to use reflective material as is used in automobile front windshield sunshades because it does not attract dew as easily as the closed cell foam I've employed.  Guess this shows there's not much new under the ATM sky after all, eh?

Here is the addition I made to "Brutus" to try this less dew-prone thermal baffle.

Simple closed cell foam wrap-around for OTA to block heat plume from observer's body