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Old 24-01-2009, 04:37 PM
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gary is offline
Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,497
Telescope Pointing Analysis System - G-11

Originally Posted by mick pinner View Post
would there be a noticable advantage in upgrading from 8192 to 10,000 step encoders on a G11? based on accurate polar alignment.
Hi Mick,

Thanks for the post.

When going from 8192 steps on each axis to 10,000 steps on each axis,
there is a potential 49% increase in pointing resolution.

However, because the geometric, eccentric bearing and gravitational flexure
errors within the mount/OTA might be causing pointing error residuals significantly
more dominant than those attributed to encoder resolution alone, the approach
we would recommend would be to perform a TPAS analysis.

TPAS stands for Telescope Pointing Analysis System and it is built-into the
Argo Navis firmware. It is described in the User Manual in the section on

After one has performed a star pointing test, TPAS can analyse and potentially
compensate for many of the most common mount errors and can even report
and refine any residual polar-misalignment.

With your G-11, what we would specifically recommend is to initially polar the
scope in the way you prefer, either using a polar alignment scope or drift test.
Then set the SETUP MOUNT menu to the GEM EXACT ALIGN setting.
To assist with the information TPAS can report on polar-misalignment,
check that the time is reasonably accurately set in SETUP DATE/TIME
and that your location has been set in SETUP LOCATION.

Then devote one evening, perhaps during full moon, to do an extended
star pointing test. This might entail sampling the positions of say 50 to 100
stars across the entire sky from zenith to within about 10 degrees of the
horizon. Whilst doing the sampling, one can go into the SETUP MOUNT ERR menu
from time to time and perform an analysis to get some feel as to how things
are progressing. You can even start to put an initial pointing model in place
which then starts improving your pointing performance straight away, easing
the identification of stars during subsequent sampling.

Once you have sampled enough data, one can determine which types
of errors within the mount/OTA are likely to be persistent from session to
session and then save these terms into the unit's non-volatile memory.

The real magic of TPAS is that once you have a model in place, on
a subsequent observing session you can re-synchronize the model on as
few as perhaps two to five stars and your pointing performance will be
as good as it was after the extended star sampling run.

A classic problem with GEM's is non-orthogonality between the Dec axis and
the optical axis. This is because the human operator is responsible
for mounting the OTA onto a dovetail or into the rings and if the OTA is
not 'square' with respect the mount head, the telescope will tend to 'look'
to one side compared to where the mount and the associated encoder
system 'thinks' it is pointing.

You might find this case study of a TPAS analysis done on a G-11
interesting. See

The above case study includes links to graphs which are in Scalable
Vector Graphics (SVG) format. These are the most intersting part
of the document. Some new browsers, such as Internet Explorer, support
SVG format natively. If your browser does not, to view SVG content, you will need
to load a free Adobe SVG viewer plugin, which is available for
Windows, Linux and Mac OS-X from

If one passes the mouse cursor over the sampled data on any of the graphs,
the graphic actually reports the error residual for that data point. So the graphics
are interactive.

The 'before' and 'after' cases are most dramatic. Whereas the raw pointing
performance of this particular telescope was about 13.6 arc minutes Root
Mean Square (RMS), the after case dropped it down to 1.1 arc minutes RMS
with the largest error residual being only 1.9 arc minutes. Given the 8192 step
encoder resolution is about 2.6 arc minutes a step, this is an excellent result.

RMS is a statistical metric and popular in engineering applications.
When we say a telescope has a raw pointing performance of RMS of
13.6 arc minutes, we are saying, on average, approximately 68% of objects
fell within a radius of 13.6 arc minutes from the center of the eyepiece
(i.e. a diameter of 27.2 arc minutes) and nearly all of them fell within a radius
of three times that, i.e 40.8 arc minutes.

So you can appreciate to get the mount errors down to the point where the
pointing residual effectively becomes encoder resolution limited can be
highly desirable for some users.

So, in a nutshell, rather than investing in the 10,000 step encoder solution,
I recommend you stick for now with the 8192 step encoders and steep yourself
in TPAS. There is a bit to absorb at first, but using the system in practice
is relatively easy once one is familiar with the concepts.

By the way, the techniques employed here are essentially identical to that which
professional practitioners perform on all of the world's largest and most
expensive telescopes and the lexicon - terms like RMS, Non-Perpendicular
Axis Error, etc. - is identical as well.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
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