I asked the same thing a while ago an was directed to this article:
http://www.iceinspace.com.au/63-400-0-0-1-0.html
This gets the air flowing across the primary mirror surface and prevents dew without having to use heaters to bring the mirror temp up above the ambient temp and thus introduce unwanted thermal currents.
I tried it and haven't had a single drop of dew since. Although Im using a solid-tube, not sure if it's effective with a collapsible.
Here's my original thread:
http://www.iceinspace.com.au/forum/s...ad.php?t=76593
Hope it helps
Davin