Alan Adler's Fan Modification (Revisited)

Okay, so I made this modification some time back and decided that it wasn't
worth the effort considering that my Dob was stored outside in a shed for
most of its life. However, having recently moved into a new home (with no
shed available), and since the observatory isn't even close to being
started, the scopes are now stored in the garage. The temperature
differential is hence greater than it used to be (garage is
attached/insulated), and the results are now more obvious.

The fan modification is definitely worth the effort for scopes that have to
transition even a mere 5 degrees F. Previously, I also had thought that fan
vibration was a problem, but I no longer believe that to be the case. What
might be the problem is the velocity at which the air flow is sweeping the
mirror as I am still running at the full fan speed at 12VDC. Velocity of air
flow is quantifiably visible in the out of focus image during star test
collimation. I think I was interpreting that as vibration in earlier tests
(and may even have stated this theory at that time). (I really need to get
speed control. Can someone remind me how to do this with a 12VDC fan? (And
don't say 9VDC battery)).

I think the main difference with the fan is the controlled "sweeping" of air
across the mirror, in lieu of the turbulence of natural convection. The
sweeping of air is of even greater benefit than simply cooling the glass, as
it gives an immediate result. I was very nicely surprised at last night's
results.

Adler rocks!

--
-Stephen Paul

In article , "Stephen Paul"
wrote:

Previously, I also had thought that fan vibration was a problem, but
I no longer believe that to be the case.


I never had vibration problems either. The key would seem to be having a
sanwhich approach. I used a layer of GE silicone glue to attach window
insulation foam to the fan base. Then I used another thick bead of the
silicone to attach the foam to the mirror box of my scope.

My first iteration used urethane adhesive for the fan-foam connection,
it seemed equally effective. Could run both of the 3-1/2 fans full out
with no detectable vibration.

--
Jeff Morgan
email: eliminate the number and substitute mindspring for nospam

"Stephen Paul" wrote in message
...
Okay, so I made this modification some time back and decided that it
wasn't
worth the effort considering that my Dob was stored outside in a shed for
most of its life. However, having recently moved into a new home (with no
shed available), and since the observatory isn't even close to being
started, the scopes are now stored in the garage. The temperature
differential is hence greater than it used to be (garage is
attached/insulated), and the results are now more obvious.

The fan modification is definitely worth the effort for scopes that have
to
transition even a mere 5 degrees F. Previously, I also had thought that
fan
vibration was a problem, but I no longer believe that to be the case. What
might be the problem is the velocity at which the air flow is sweeping the
mirror as I am still running at the full fan speed at 12VDC. Velocity of
air
flow is quantifiably visible in the out of focus image during star test
collimation. I think I was interpreting that as vibration in earlier tests
(and may even have stated this theory at that time). (I really need to get
speed control. Can someone remind me how to do this with a 12VDC fan? (And
don't say 9VDC battery)).

I think the main difference with the fan is the controlled "sweeping" of
air
across the mirror, in lieu of the turbulence of natural convection. The
sweeping of air is of even greater benefit than simply cooling the glass,
as
it gives an immediate result. I was very nicely surprised at last night's
results.

Adler rocks!

--
-Stephen Paul



A 250 ohm pot in series will get you good speed control.

For another fan install that works well, this might be worth a read:

http://tinyurl.com/qhte

Active cooling works.

Clear skies,

Tom

"Tom Hole earthlink.net" tomhole@NOSPAM wrote in message
...

A 250 ohm pot in series will get you good speed control.

Thanks Tom.

Know where I can get one with a mini foot print that can handle the current
(and how much current that is)?

-Stephen

(I really need to get speed control. Can someone remind me how to do this
with a 12VDC fan? (And don't say 9VDC battery)).

A simple variable resistor placed in the circuit should do it. Try
your local Radio Shack.

Ritesh

"Stephen Paul" wrote in message
...
"Tom Hole earthlink.net" tomhole@NOSPAM wrote in message
...

A 250 ohm pot in series will get you good speed control.

Thanks Tom.

Know where I can get one with a mini foot print that can handle the
current
(and how much current that is)?

-Stephen


Stephen,

Radio Shack has one. One fan should draw from 90-140 milliamps depending on
your fan. Every pot Rat Shack sells should handle that. I got one with an
on off switch built in to keep from having to unhook the battery.

Clear skies,

Tom