Dew on UC primary

[gb_astro]

How much of a problem is dew forming on the primary of a ultra compact trus dob compared to the standard rocker box design?

Thanks,
gb.

[Bassnut (Fred)]

gb, im no expert, but the exposure of the primary on a truss design to the elements would indicate the absolute need for a dew heater.

[gary]

Originally Posted by gb_astro

How much of a problem is dew forming on the primary of a ultra compact trus dob compared to the standard rocker box design?

Anecdotally, owners report the shroud, which comes down quite low, tends
to help keep dew away. Keep in mind to start collecting dew, the primary has
to have reached the dew point and all that glass means a large thermal mass.
That's why most large Dobs are fitted with fans beneath the primary to help them
reach ambient and most large Dob owners are consumed with getting the mirrors to
cool down fast enough.

The dew heaters on large Dobs are typically on the secondary and the eyepiece
rather than the primary. The 18UC also has the option of the dew heaters on the
secondary and eyepiece.

Dave Kriege at Obsession would undoubtedly have some additional advice
to offer with regards how the dew potential of the 18 UC compares to the classic
18.

Best Regards

Gary Kopff
Mt. Kuring-Gai

[gary]

Hi GB,

Dave Kriege of Obsession Telescopes, who designed the entire range, reports as
follows -

Originally Posted by Dave Kriege, Obsession Telescopes

There is no difference on the primary mirror. So much thermal inertia that
dew is never a problem for either. The secondary on the UC is more prone
to dewing tho. A dew heater is highly recommended for the UC secondary.
DAVE K

[Starkler (Geoff)]

Originally Posted by gary

Keep in mind to start collecting dew, the primary has
to have reached the dew point and all that glass means a large thermal mass.
That's why most large Dobs are fitted with fans beneath the primary to help them reach ambient and most large Dob owners are consumed with getting the mirrors to cool down fast enough.

Indeed and im convinced that when I leave the shroud off my scope that the mirror cools quicker being able to partially radiate directly to the sky. Getting rid of the heat is the challenge.

If i owned a scope with an exposed mirror like a UC, I would want to have a shroud though simply for the purpose of protecting the primary from foreign objects especially in the company of others.

[tnott]

Dew on large primaries is rarely a problem as the ambient temperature drops faster than the mirror throughout the night:

http://www.cruxis.com/scope/mirrorcooling.htm

Shedding this heat is much more important than worrying about dew, which why a fan is needed. In fact, data has shown that without some sort of active cooling the mirror will almost never get to within a degree above ambient. My open style 16" X 1.5" thick primary has never dewed in over 2 yrs of use.

Even if it was a problem the UC has stub tube that fits around the primary anyway that would conceivably act as a dew shield. Running a fan would also help stop dew from forming.

The secondary is different - in these open scopes generally a dew heater is used there.

[gb_astro]

Guys, thanks for the info.
(I have only used SCTs and refractors so when I
see exposed glass I think dew.)

gb.