Re: ATM fan capacity

["Bill T." <twentiethwave@hotmail.com>]

Jan van Gastel wrote:

> I am going to buy a fans for my 20"f/5: one to blow across the top of the
> primary and one blowing against the bottom (or the last one sucking, I
> haven't decided yet). Capacities of the fans in my catalog go are 11, 38,
> 71, 82 and 114 m^3 per hour. They are all about the same price. Is it best
> to choose the one with the highest air output?

Jan

I've used both high and low volume fans on mirrors large and small.  I can't
say that I've seen any difference in cooling times.   Both seem to work
equally well, but I haven't done measured experiments.  I went from high
volume to low volume mainly for noise considerations -- fans I can't even
hear in the shop sound awful out in the quiet night.

I also have a clip-on fan which I direct at the front, top of the mirror for
a while after set up, with the telescope tilted down.  It's in the optical
path.  But even so views while the mirror is still hot are usually better
with this fan than without it.  When the mirror is equilibrated, it doesn't
seem to help at all.  I use it only during cool down.  It's low flow,
blowing from a distance of about 1 mirror diameter.  It definitely does
reduce cool down times.

Please tell us about your "push-pull" fan experiences.

Bill T.