Re: [ATM] New 16" telescope design

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: "John Sherman" <atm@mail.johnspics.com>

>>Also, why use a shroud?

>1) To protect my mirrors from dust, pollen, fingerprints, and bugs.

I'll agree with the fingerprints issue...especially at public star parties
at schools.

I don't think shrouds will help much with bugs/dust/etc.  My best results
have been to use a "lid" mirror cover that contains a very small volume of
air between it and the mirror's face....but that only helps when the scope
is in storage.  During use?  I don't know how to prevent dust/etc. buildup
very well except by putting down astroturf around the scope so people don't
kick up much dust.

>2) To keep the wind from getting to my spider and vibrating it.

And now the entire scope vibrates (at a lower frequency, but probably larger
amplitude, and for a longer time).  That appears to be trading one problem
for another.  I don't see this as a solution.

>4) To keep light from shining on the inside of the UTA where
>   the eyepiece can see it.

If you have a low profile focuser, yes, your eyepiece probably sees lots of
UTA.  With my 'not low profile' setup...and a small aperture baffle below
the focuser...very little is seen.

With the setup pictured at:
http://overton2.tamu.edu/aset/krajci/dobsonian/focuser-baffle-67pct.JPG
(and no shroud)  I have observed Mercury, Venus, Mars, Jupiter, and Saturn
when the sun was above the horizon...broad daylight.  Sometimes when
observing Mercury...sunlight is falling on the main mirror's face...and the
mirror's clearly dusty with a seven year old coating...and contrast is still
pretty good.  I don't see other folks at star parties observing Mercury
while the sun is up, only me.  I can't quote a numeric value of the scope's
contrast...but the baffling seems adequate without a shroud, and wind
profile is low.

Tom Krajci
Cloudcroft, New Mexico



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