Re: [ATM] Clock drives and Tracking systems [Long and rambling]

[Woodchuck <djv@bedford.net>]

On Sun, 29 Aug 2004 artbianconi@blast.net wrote:

> The current Ball & Socket design is an attempt to build a truly
> compact portable truss which has the ease of a "point and shoot"
> aiming of a Port-a-Ball, without the huge sphere.
>
> It's not anything ground shaking as ideas go or at least should
> not be: it's simply a ball mounted solidly underneath the primary
> cell which is then fitted inside an adjustable clamp in much the
> same way your leg is fitted to your hip. I've tested it with
> temporary fixtures and it aims and stay put easily within any
> spot in given hemisphere. Presumably it will continue to do so
> after an OTA is fitted.

This sounds similar to one done by Ike Newton around 1670.

(See
http://www.egglescliffe.org.uk/physics/astronomy/telescope/newtontele.html)

Modern versions of this sometimes use a bowling ball resting in a flower
pot.

> The problem is this: how does one aim it electronically with any
> degree of certainty? Worse yet, how does one aim it with any
> degree of repeatable precision?

Therein lies the problem.  As you think about it, you'll probably
discover why this mount is nice for visual instruments, particularly
for fixed telescopes, such as spotting scopes at a rifle range.

Problems:  You don't have an "obvious" position of the OTA relative
to the mount.  That ball is too symmetrical.  So there is a zeroing
problem, somewhat akin to setting up a conventional amount and
aligning it.  This is part of a larger problem: your mount is
constrained in only the three translational degrees of freedom, and
is free to rotate in all three rotational degrees.  This is one
more rotational degree than a conventional mount.  In the practical
sense, the ball is free to rotate about infinitely many axes,
although this can be resolved to three, of course, mathematically.
The problem is that the telescope will not have a unique position
in which it points at a unique target.  There are infinitely many
ways that it can put Sirius in its crosshairs; conventional mounts
limit this to one or two (GEQ) (The extra degree is something like
rotating the OTA about the optical axis, but in a ball/socket set
up it involves translations of the OTA in addition to rotations).
This extra degree of freedom is why it is easy to point visually;
compare to a camera on a ball-socket tripod vs one with a alt-az
"pan head".  When pointing, ball-socket is nice, when tracking,
it's nice if the only degree of freedom is the same as the motion
of the object tracked.

As Mel suggested, the "straightforward" approach is to use two
orthogonal shaft encoders and stepper motors in a classical 2-axis
closed loop servo system.  This would be pretty interesting to see
in practice.  I'd also suggest looking at track balls for design
possibilities.

An objection I see is that this system cannot be balanced, unless
there's some funny business with the ball (it is non-homogeneous,
such as being hollow with counterweights inside it).  This means
that the servo motors will have to overcome (or be themselves the
source of) clamping forces.  This increases the demands on the
servo motors, adding to the $-factor.  Having to drive through
a clamp that is restraining significant torque may prove unsatisfying.

I think you may be trying to do the impossible or the implausible.
The B&S mount is like a Dob alt-az, but better, it's like an
alt-az with the addition of a friction-controlled eyepiece positioner.
Grab a handle on the OTA and point it where you want it, with the
eyepiece where you want it.

My tempation is to move the complexity of driving the scope into
a separate mechanism, i.e. a polar platform of some kind.

> Port-a-ball acknowledges that its clock drive system is only
> accurate for a limited period which may eliminate it for serious
> astro-photography. Such accuracy is fine for a small aperture
> (6")  telescope whose primary design intent is portability and
> ease of use.
>
> Because there is no surface for conventional optical readers to
> use, I must come up with another way of tracking. Anyone have any
> ideas?

The surface of the ball in my computer's Logitech trackball is speckled.
It's positioning system would work for your application, it depends
on your demand for precision.  There might be some sort of way to
use speckles besides just counting them, some sort of system where
the image of the speckles are projected on a CCD.

Also, you might consider mounting the sensors in the ball, and sensing
something in the socket or elsewhere in the earth-fixed environment.

A single vertical gyro inside the ball might be useful.  (What's
your budget?  You can probably tell I used to do "aerospace" long
ago in another life.)  I wonder if you could stick a big permanent
magnet or two inside the ball and  measure the field? (sounds a little
preposterous, but in theory...  maybe embed two fluxgates inside
the ball and measure the earth's field, or a local (powerful) one
in the socket or environs?)

My ideas are probably good only for visual tracking, if that.

Dave

_______________________________________________
ATM mailing list http://www.atmlist.net/