Re: ATM Cooling Fans...need more details!

["Alan Garcia" <ngarcia@columbus.rr.com>]

Greetings,

I have a 15" truss tube telescope with a conventional mirror box.  I have
2 - 26cfm fans blowing on the front of the mirror.  They are positioned
across the two lower corners of the mirror box and are aimed at the center
of the primary.  I used two pieces of 1/2" rubber fuel line stretched
tightly across each corner and fastened securely with small wood screws.  I
punched two holes in them and attached the fans to them with nylon screws
and washers.  I then covered the hose and the areas they connect to the
mirror box with flocked paper to give it a nice finished look.  I found the
rubber fuel line was rigid enough to hold the fan in position while being
assured it would deaden any vibrations.   In addition, I have an 85cfm fan
mounted behind the primary.  Actually it is suspended(hangs) between the two
center rungs of my open tail gait,  with 1/8" diameter bungie cords I picked
up at the hardware store.  This seems to have prevented any vibrations from
this rather large fan.  Other than the experiments Bryan Greer and Adler are
doing in reference to this issue, I think most assertions are rather
anecdotal.  All I have is my experience with my particular telescope.  For
what it's worth, I believe you have to have alot of air circulating in the
light path and behind the mirror.  Without adequate air flow you cannot hope
to achieve a reasonable degree of thermal homogeneity in the light path,
which appears to be a important assertion of Greer and Adler.  The added
benefit of course is perhaps a better chance you'll cool  your optic to a
point near the ambient air temperature.  Finally, I run my fans during the
entire observing session.  Adding these fans to my telescope has finally
allowed my telescope to consistently yield images that correspond with the
excellent foucault test data I have for the optic.  In the past those images
were often times "locked" in a thermal mess.

Submitted for your consideration,

Alan Garcia

----- Original Message -----
From: "Richard Ozer" <rozer@ois-online.com>
To: "Dhaval Thaker V" <dthaker@unity.ncsu.edu>
Cc: <atm-digest@shore.net>
Sent: Wednesday, June 20, 2001 10:39 AM
Subject: Re: ATM Cooling Fans...need more details!


>
> Vibration is going to differ depending on scope size and construction.  I
> use velcro to attach the fan to the back of the mirror cell.  The play in
> the velcro serves to dampen the vibration.  I don't notice any vibration
> when I'm observing.
>
> I've read and have been told that positioning the fan such that it blows
air
> across the surface of the mirror is best.  I have not done this for two
> reasons.  First, I don't want to cut another hole in my tube, second, I
dont
> want dust to blow into the tube... I often observe in remote dirt lots.
>
> I always have a pair of sealed lead acid batteries with me wired in
> parallel.  That gives me enough power to run a fan, digital setting
circles
> and a laptop for a whole evening or more.
>
> If you mount the fan in the rear of the mirror cell, the question you are
> not allowed to ask is whether the fan should suck or blow :-)
>
> ----- Original Message -----
> From: "Dhaval Thaker V" <dthaker@unity.ncsu.edu>
> To: <atm@shore.net>
> Cc: <atm-digest@shore.net>
> Sent: Wednesday, June 20, 2001 6:46 AM
> Subject: ATM Cooling Fans...need more details!
>
>
> >
> > Fellow ATMers,
> > I read couple of posts about using PC fans to cool down the scope.
> > can you shed some light on...
> > 1. vibration-dampening mechanism
> > 2. Power supply( battery? )
> > 3. positioning of fans
> >
> > Excuse my ignorance, I have got plenty to share :)
> > Regards,
> > Dhaval.
> >
> >
>
>
>