Re: [ATM] Unique New Equatorial Platform Design

[creator <creator@subarcsec.com>]

Greetings Friends!

Here's a report on "prior art" for you...

Jay Kirkland very kindly sent me a scan of an article by Bob
Barclay from Telescope Making #17. It is a great article, with
many good insights, and based on Mel Bartels' descriptions I
am pretty sure this must be at least one of the articles that
Mel had mentioned to me.

I must acknowledge that Bob Barclay's platform could be easily
mistaken for a "dead ringer" for mine, and our platforms do in
fact incorporate some of the same principles. Reading Bob's
article was a delight because I can see that he went through
a number of the same thought processes that I have. I hope one
day if possible to communicate with Bob directly and compare notes.

In his article, Bob explicitly acknowledges uncanny parallel
thinking among ATMs at that time, and his article helpfully
references an article by Richard Berry in January 1982 Astronomy,
and also one by Chuck Olson in TM#15, both of which I will try
to get hold of for review.

Bob mentions that the center of gravity is off-center. He recognizes
that a larger diameter cylinder would allow for finer regulation of
flow rate. He also remarks on the somewhat precarious nature of tilting
the system and loosing balance. He contemplates the need for some sort
of declination control mechanism. Finally, he describes his plans for
a future attempt at a smaller platform with a shorter hinge.

Summarizing, there is one major and very significant difference between
Bob's design and mine, namely the design of the polar hinge. There are
also subtle and important implications of the overall design that Bob
did not contemplate, at least in this one article that I've now read,
for example the significant benefits of having the weight of the system
deliberately out of balance.

My polar axis is better than the "hinged box" design for several reasons.

First, going back to fundamental "theory of operation", it comes as close
as I could conceive of to being a physical mechanical representation of
the basic ideal specification for a geometric line, namely "two points".

If you can define your polar axis with two points, and can finely adjust
the position of those points independently of one another, you can very
precisely adjust and define a line parallel to earth's rotational axis.

In the McCreary Mount, the two points are the centers of two spheres. The
spheres are chrome steel bearing balls, commodity items available off
the shelf at a very low cost but having a very high precision. These two
bearing balls define the polar axis, completely eliminating the shafts,
pins, roller bearings, sleeves, etc. of other designs. Every moving and
rolling or sliding contact mechanical element that you can eliminate from
the system is a multiple design win, because it is one less element that
needs to be precisely machined and one less element to cause tolerance
stack-up problems.

If you look at the "Photo" page at http://subarcsec.com you will see the
north and south bearing ball support adjustments. The south adjustment
moves north/south to adjust the altitude angle of the axis. The north
adjustment moves east/west to adjust the azimuth angle of the axis. Because
of the wide spacing of the two points and the fine-pitch (10-32) threaded
adjustment rods, polar axis pointing can be very precise indeed.

If you stipulate that, once adjusted, the bearing balls can be held in
place firmly and securely without moving or shifting, and that the platform
you build is sufficiently rigid to well support your instrument, then you
can see that ALL of the precision of the path of the platform motion is
governed by the smoothness and spherical regularity of the bearing balls.

The one-inch chrome steel balls that I bought for the prototype are spec'd
to be spherical to within 24 one-millionths of an inch (!). At that precision
and with the geometry of my platform, the path smoothness of travel of the
platform is theoretically less than one-sixth of an arc-second throughout
the entire range of platform motion! This could not be said of a box-hinge
design. For chrome steel balls of this precision, I paid the princely sum
of $2 each! Try to get anything "custom machined" for that price... :)

That Bob Barclay was NOT headed in this direction is clear by the fact that
he concludes his article by discussing SHORTER and unitary hinges to be used
in his future designs, i.e. he was targeting "compactness" rather than
"precision".

And so, I see once again I've no doubt "run on" too long, so I'll sign
off for now with my special thanks to Jay Kirkland for sending that
article, and again to Mel for having mentioned it to begin with. :)

As always, please keep those cards and letters coming! :)

Best regards,

-Dann
================================================================
Dann McCreary http://flyinghouse.com    creator-at-subarcsec.com
SubArcSecond Tracking Accuracy!  --   Visit http://subarcsec.com
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  is declaring the work of His hands"  -     Psalm 19:1    (NASB)
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