Re: [ATM] New minimalist single-pole 18" ultralight

[Ross Sackett <rsackett00@yahoo.com>]

--- Scott Milligan <starzkey@charter.net> wrote:

> Some really nice work and innovative ideas here, I
> think.  I hope you'll
> bring it to Stellafane this year.  
> 
> BTW, how thick is the primary mirror?  Is the slight
> compound bend in the
> tube strictly there for aesthetics, or does it also
> play a role in getting
> the 'scope to balance properly?  

My Stellafane scope this year will probably be a
carved wooden reinterpretation of the garden
telescope, but if that isn't finished I will bring the
big blue 18".  

The slight compound curve in the pole is to put the
eyepiece in a more ergonomic position: otherwise you
would have to crane your neck at low elevations.  With
a little reengineering a straight pole would be fine,
but that could move the balance point a bit off the
geometrical axis of the scope.  The primary mirror is
1.6" thick and weighs 31 pounds, which is about half
the total weight of the optical tube assembly.  With
the rocker latched on it all weighs 76 pounds, which
is about all I care to lift at once (and comparable in
weight to my old 10" classic "circus cannon" sonotube
dob).

So far I have spent two good nights with it.  It has
about two-Jupiters flexure during normal driving,
comparable to a big hand-driven scope on a German
mounting.  It takes 3-4 seconds to settle down after a
sharp rap, much less (c.1 second) if your hand is on
the scope to dampen the vibrations.  I have it
adjusted to move with very light pressure, which is
fine as long as I have a Nagler loaded in the focuser.
 It holds collimation well from setup to setup.  The
curved spider and light baffle mean there are no sharp
diffraction spikes, but there is a hazy diffraction
cloud around very bright objects like Venus from all
that hardware in the light path.  Saturn and Venus
looked good, but conditions weren't optimal because of
the air currents around my house.  M13 was nicely
resolved with pinpoint star images, making it look
surprisingly like a big open cluster rather than a
hazy globular.

UPS has the scope right now to get it to California. 
I hope to have some more observing time with it in the
high dry air of the RTMC, though the gibbious moon may
be a problem.

Ross


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