Hi Oren,
In my limited experience with larger refractors, most of the thermal
issues are in front of the lens in the dewshield. I've sometimes
seen an instant improvement in the view by taking off the
dewshield from some of these larger triplets. Theres probably a
fine line between preventing dew and preventing warm air plumes
from going up the dewshield.
As for the rest of the OTA tube, I think allowing some small gap
around the lens cell for venting the main tube can only help.
You may also want some small opening at the focuser end to let
air in (though of course, that could be just removing the
eyepiece for a time). This question brings to mind an
experience with a 6" Mak-Newt (also closed tube) and watching
mars once. After an hour outside, it still had a bad case
of warm air pooled-up in the tube and astigmatism. Then I
recalled there was a 1" plug at the rear of the scope, so it
took off the plug, and within 5 minutes it was a completely
different scope giving unbelievably good views. My explanation
for what happened, was that opening the rear let the warm air
rise and flow out through a small gap around the focuser at
the top end. That sort of experience would suggest some value
in having a small gap around the lens cell. Whether it helps
in general probably depends on your day / night temperature
swing, etc. Dust, etc., would be a minor concern, but probably
with some care and keeping the front end covered (or gap
taped-up) most of the time, it would not have much adverse effect.
You might need some attention to avoid dew on the rear surface
of the lens when you bring it in from outdoors. Might be useful
to seal-up the gap before bringing the scope inside. But again
depends on your local conditions.
- John B.
Oren Soffer said:
I did not get any help on the refractors forum so I thought I should try here... I am at the final stages of putting together a tube for a 7" refractor lens I just bought. It is a triplet so I know it will take a little while for it to cool down, and to make things worse, I flocked the entire tube and baffles. Does the 0.03" thick velvet have a major impact on the thermal behavior of a sealed aluminum tube? The lens cell is flush with the counter-cell ring so no gap between them. Should I keep the tube sealed or maybe I should use the push/pull screws to create a small gap, say 1/16", to help the lens cool down faster?(lets ignore the dust issue for the moment). Will it make a major (good/bad) difference? More tube currents, less tube currents?
Thanks, oren.
