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

[Ross Sackett <rsackett00@yahoo.com>]

--- Michael Lindner <mikell@optonline.net> wrote:

> Ross,
> 
> It looks really cool, and very innovative. Great
> job!
> 
> I am skeptical about a few points on its performance
> that perhaps we can 
> discuss. The 3 point altitude bearings are an idea I
> toyed with but 
> abandoned because of stability issues. If you
> consider the area between 
> the altitude support points, this has 1/2 the area
> of a traditional dob, 
> and the center of mass of the scope is actually
> towards the narrow end 
> of the triangle. In my prototype this made the scope
> a bit tippy and 
> wobbly. 

Hi Michael,

Interesting questions.  I am not sure I understand
exactly which support triangle you mean.  If you mean
the ground support triangle between the three feet on
the ground board then yes, it is important to account
for that in locating the azimuth axis.  I shift the
azimuth axis a little towards the arm side (away from
the trunnion side) to keep it under the center of
gravity of the scope as a whole. Tippiness in that
sense isn't an issue--it still just depends on the
distance between the feet on your groundboard.  For
insurance, I made my feet a little long to act as
outriggers, but it turns out in the 18" that I didn't
need to.

There is another kind of rocking you might mean,
though--on the altitude axis itself.  In my four
scopes using this design I've never experienced
anything like that, and frankly don't think it is a
practical problem since the three points of the
altitude bearing (two pads and a pivot) would always
be in contact, barring major torsion that caused it to
jump from the dob pads (in which case, just move the
pads farther apart).  A conventional 4-pad dob design
would have an even bigger problem, both from that kind
of torque and rocking from a slightly misplaced pad.

> Also, the single pole looks like it would be
> a source of flexure 
> and be bouncy when the scope is moved.

Of course: any superstructure, whether solid tube,
truss, or a single pole, is bouncy to a degree.  The
only important issue is whether it is sufficiently
stiff for its intended use.  This was built as a
grab-and-go backyard scope for visual use.  I find
that I can tolerate about 2 apparent Jupiter diameters
of flexure in a scope before the springback  gets too
annoying for most visual observing.  This is
comparable to the flexure during focusing and hand
guiding of a biggish Newtonian on a hefty equatorial
mount.  (And, I might add, similar to some underbuilt
truss dobs you see at star parties.) I sized the pole
to give no more than 2 Jupiters of total flex during
guiding and focusing, based on my experience with 8"
and 12" versions of the design, and a little beam
theory.  With the 2" pole the scope actually flexes
only about 1 Jupiter, probably because I've adjusted
the knobs on the pivots to allow the scope to move
with a light touch.

To optimize the scope for high power, though, I would
probably increase the diameter of the pole to 2.25-2.5
inches.  In a cantilevered design like this stiffness
increases with the cube of the pole diameter, so a
small increase in size has a big effect.

Vibration is another matter: with the long
cantilevered arm it takes 3-4 seconds to settle down
after a rap with the knuckles, which could be too much
for some people.  The easy fix is to just leave your
hand on the pole in usual guiding position: then
vibrations damp out in 1 second.  I've tried packing
the pole with foam and suspending a rope inside the
tube, but neither works nearly as well as briefly
touching the tube.

Best,
Ross
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