Re: ATM Nothing but heat....

[Chris Westland <westland@uxmail.ust.hk>]

Taking this one step further, I had thought of sealing the scope except
for the UTA, and using a blower, with dessicant on the intake to force a
higher pressure inside the tube.  You get complete control of thermal
cells and dew with this arrangement.  

The drawback is that blowers are notoriously inefficient for this type
of application. They are very good at producing high pressure with low
volumes of air ... with a telescope, you want exactly the opposite. 

There are two solutions.  First, you could use a muffin fan on the end
of a large (6") clothes dryer exhaust pipe as the "blower" ... this is
clumsey because of the large tube ... but it does isolate the vibration
and heat of the fan from the scope, allowing higher fan speeds. 

The second approach is the elegant one.  Vortex Industries makes a
device for around US$100 which takes the high pressure, low volume input
from a blower, and converts it to low pressure high volume throughput
... by a factor of about 20:1 on both statistics.  The device is about
8" round by 5" thick, with no moving parts.  So you could mount this on
the mirror cell, and run a thin compressor hose to the scope, attached
to a portable compressor (12VDC ones cost perhaps US$25 at discounters). 

I'd be interested in hearing the experience of anyone who tries this.

-c


> off.
> 
> I was wondering if you could use a fan to force air over a desiccant, and then
> flow this into the tube assembly (with appropriate filtration to prevent the
> desiccant from getting onto the mirror). The air could be at ambient, but
> should have
> a lower dewpoint. It might not suck moisture off an already dewed mirror, but
> perhaps it could inhibit dewing. Nothing but heat?  Anyone willing to test?
>                         ....RICH

-- 
Chris Westland
Associate Professor, Hong Kong University of Science & Technology
Clearwater Bay, Kowloon, HONG KONG  
Tel: ++(852) 2358-7643  Fax: ++(852) 2358-2421
EM:westland@uxmail.ust.hk  WWW:http://home.ust.hk/~westland/westland.htm