Feeling the heat

It is well known in high humidity and very cold areas that to prevent dew and frost you need to keep the eyepieces warm. There are a number of strategies for keeping them warm; I do not need to go into them here.

My question is: We always want to COOL a primary mirror or refractor objective to get optimal performance. But we prefer under many conditions a WARM eyepiece. Is there any reason to think that a warm eyepiece delivers a compromised image vs. a cold one? My guess is, that with the lens groups sealed and the short distances involved, it wouldn't matter, but I've never seen it discussed.

I suppose at another level it's academic. Even if there is a compromised performance, it's better to have a warm eyepiece that isn't frosted or covered with dew than one that is unusable.

regards
Greg N

My feeling is that you are right about the short
distances. The problem at the objective end of
the scope is not so much the temperature, but
that fact that temperature differences will cause
convection -- rising less dense warm air and falling
more dense cold air -- its a perfect
recipe for seeing problems. But inside an eyepiece
there is no room for air to convect -- temperature
differences cannot drive much of a current. For the
same reason multi-pane windows give good insulation
-- there is not much convection across small distances
between the panes.

Also the optical pupil is small inside the eyepiece
-- only a few mm wide, and the variation in air density
across the small pupil cannot be very large. Unlike
the objective end where the pupil is hundreds of
mm across. For critical viewing like planets, the
pupil is typically less than 1 mm inside the eyepiece.

I have sometimes noticed introducing a warm eyepiece
(from indoors) into a cold telescope will cause
problems, too. But again the problem is not
inside the eyepiece, but in driving air currents
in the larger OTA tube. Of course the scope end
of the eyepiece cools, so it is only a temporary
problem (even if the eye end is slightly heated).

Greg Nowell said:


My question is: We always want to COOL a primary mirror or refractor objective to get optimal performance. But we prefer under many conditions a WARM eyepiece. Is there any reason to think that a warm eyepiece delivers a compromised image vs. a cold one? My guess is, that with the lens groups sealed and the short distances involved, it wouldn't matter, but I've never seen it discussed.

I suppose at another level it's academic. Even if there is a compromised performance, it's better to have a warm eyepiece that isn't frosted or covered with dew than one that is unusable.

regards
Greg N

There are no optical issues involved with warming an eyepiece. "Cooling" one would have one result--fogging and dew. As for primary mirrors, we don't want to cool them either...they'll dew up too. We want them close to ambient temperature...that's the ticket.

8)