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dew on primary...help

Started by dan94551, 04/09/2010 05:09AM
Posted 04/09/2010 05:09AM Opening Post
I have an 8" f6 metal tube reflector, and on very cold nights I get moisture on the primary. Has anyone successfully used a dew-zapper or like product on a reflector?
Posted 04/09/2010 06:36AM #1
I've gotten dew on my primary only twice when out with a scope. Each time turning on a blower on the primary and moving air around or across the mirror surface solved it.

If you are referring to dew forming on the primary after bringing the scope indoors from a cold night, then that a blow drier will work to fix that kind of dew.
Posted 04/09/2010 09:45AM | Edited 04/09/2010 09:47AM #2
I added a home-made heater to the rear of the
primary in my 11" Dob. I made a couple chains
of resistors in the form of concentric circles,
and then fastened these to the rear of the
primary using a bit of blue painters tape over
each resistor. The resistors were placed to avoid
interfering with the floatation mirror cell.
Worked pretty well, though there
is a very fine line between keeping dew off, and
making tube currents. It does not need much
heat -- like 1 or 2 watts -- you'll want a
variable controller so as to use the absoulte
minimum power that keeps deew off. I had been
taking this particular scope to Winter Star Party
located 50 feet from the ocean, and sometimes
dew could get really bad (though seeing at
these times was often very steady).

A small fan mounted above and to the side of the
primary and aimed at it, will also help minimize dew.
It won't completely avoid dew, but it will
restrict the formation to a light haze, rather than dripping wet.
Posted 04/09/2010 01:16PM #3
Daniel
I am curious about where you live/observe that this is occurring. I have had it happen once at WSP in Florida in a very humid environment. You say it happens when it is very cold. My experience has been that very cold air doesn't hold as much water vapor as very warm air and I would have thought this less likely to happen in observing environments with extreme cold. How long does it take for your primary to dew/frost up? Do you have a shroud on when this is occurring?
Posted 04/10/2010 01:48AM #4
Daniel White said:

I have an 8" f6 metal tube reflector, and on very cold nights I get moisture on the primary. Has anyone successfully used a dew-zapper or like product on a reflector?

This is a problem with metal tubed reflectors. Most paints are good radiators and the metal is a good conductor. The tube cools below ambient temperature and then the primary mirror can also get below ambient and dew over. My wife's 6" f/8 Edmund frequently had the secondary dew up, and on some nights even the primary got dew. I solved the problem by removing the paint (bare aluminum does not radiate well) and lining the tube with cork. The cork alone would probably do the trick.

Clear skies, Alan