The Equatorial Imitation Game by Mel Bartels
Why does a telescope need to track?
Because the Earth rotates once every day, the stars at night appear to
move across the sky in arcs. The telescope needs to follow the stars.
We call this sidereal tracking or equatorial motion.
Equatorial mounts were invented to easily track the stars by rotating a polar axis at a slow constant rate (atmospheric
refraction causes the tracking rate to slow ever so slightly near the
But the tipped polar axis in an equatorial mount adds bulk to the
telescope and the popular German equatorial design requires counterweights,
making transport and setup in the dark oftentimes a real chore.
Consequently much thought and innovation has gone into alternatives
that imitate equatorial motion.
The altazimuth mount is often a more transportable and stable design
but suffers from continuously varying tracking rates and field rotation
at the eyepiece or camera. The Dobsonian, depending on your point of
view, is either free from the complications that come with tracking devices,
or is handicapped because the object drifts out of the field of view
too quickly at higher magnifications.
The Poncet Platform and the Equatorial Table
A significant advance occurred in the 1976 with the invention by Adrien
Poncet of the tracking platform, originally called a Poncet Table, but
now more often called an equatorial table. He reasoned that if the
astronomer was willing to accept limiting sidereal tracking time to say
an hour before having to start over, then a flat platform could be
devised that imparts equatorial motion to any telescope placed on top.
Subsequently the Poncet Table evolved into what we call equatorial
platforms that better support heavy telescopes and track more
All tracking tables suffer from the restriction that they are custom
built to specific latitudes. I’ve worked to overcome this restriction
with my omni-latitude table. Continue reading...
Designers invariably think of equatorial tables in terms of limited
equatorial motion. The English Yoke mounting becomes the Alan Gee
design with a pivot point and cylindrical bearing sector. The Horseshoe
equatorial mount (think Palomar 200 inch telescope) becomes the modern
equatorial table with dual cylindrical bearing surfaces.
But can a table that imitates equatorial motion such that it supports a
telescope aimed in any desired direction be built from different
motions, different placement of axes? Let’s investigate.
The Bartels Turret Table
Can an equatorial platform be devised using the well understood
altazimuth Dobsonian design where the primary axis, aimed straight
upward, spins like a platter and the secondary axis pivots at right
angles much like a crane going up and down? We should only need a
snippet of limited motion in both directions, potentially an easy to
build design. We know conceptually that a three axis mount can
perfectly imitate sidereal tracking motion and that the three axes can
be arbitrarily aimed anywhere and in any order of preference. Is there
a solution where a limited tracking time and a vertically oriented axis
can be found such that the third axis can be foregone such that motion
in only two axes is necessary? It is said that constraints help problem
solving. I’m about to find out if that is true here.
I’ll use Sketchup, a free CAD program, to help picture the platform as
it goes through its three dimensional motions. I’ll snapshot the
platform as it begins tracking, and then take a second snapshot after
one hour of sidereal tracking motion. I’ll then experiment with moving
the snapshots around in order to find two axes of limited motion, one
axis aimed upward and the other aimed sideways, just like a Dobsonian
Any flat table, as long as it pivots around the polar axis, will allow
a telescope placed on top to be aimed anywhere in the sky and continue
to track equatorially.
Here's the flat table oriented flat to the ground for a latitude of 45 degrees.
Now I'll rotate it through a half hour of sidereal tracking each way,
snapshoting it. The three snapshots like this.
As long as their orientation is not changed, we are free to reposition
the tables as desired, so I’ll place them so that a common point in their
centers that represents a vertical axis of rotation. The tables
intersect like this.
Here are the three tables from their south end: the first set the
starting orientation, the second set a half hour into the tracking (the
table is flat to the ground) and the final set an hour into the
tracking, showing equal and opposite tip to the starting orientation.
The azimuth axis rotates +- 5 degrees as does the altitude axis. There
is a slight tip to the azimuth axis of 0.6 degrees over the hour of
tracking. That means a +- error in the eyepiece of 1/3 degree. For this
to be an issue, the table will have to be polar aligned to 1/3 degree
For latitudes closer to the pole the azimuth rotates more and the
altitude roates less; the reverse occurs for shallower latitudes.
For a half hour of tracking, these numbers are halved.
The drive rates should vary by +- 2% causing a fraction of a degree
positional error, but can be ignored for visual high magnification use.
The pluses of this design is that simple Dobsonian styled bearings can
be used. The disadvantage is that the scope will tip a bit off-balance.
The top plate will need springs to counteract the tendency to tipping.
There are other placments of the axes that can be studied also, but
they are similar in nature.
So there's the result of my deisgn process. Onto other ideas, looking for constraints that lead to good solutions.