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The following is an article written by
one of our customers who was able to discriminately test our product and
critique it for polar alignment repeatability.
Telescope Pier Used: Pier-Tech2
Polar Alignment Repeatability Specifications
I ran some polar alignment stability tests last night. It took roughly 3 hours
set and measured polar alignment with the pier fully extended. Then
I fully lowered it and measured again. Finally, I raised the pier
and measured again.
Lowering and rising resulted in an almost imperceptible shift of 0.2
arc minutes in azimuth and 0.1 arc minutes in elevation - very good
repeatability. From rose to fully lower resulted in a 1.1 arc
minutes shift in azimuth and a 9 arc minute change in elevation.
The repeatability is outstanding in the up position. Even as it is
lowered, which is something I was doing for visual observing,
objects always remained within the 30' FOV of my eyepiece.
Needless to say, I am quite pleased. As you know, CCD imaging is my
primary interest and it looks like the Pier-Tech2 is doing fine
Thanks for a great product.
September 6, 2003
I recently upgraded my Pier-Tech 2 to a Pier-Tech 3. At Pier-Techs request, I
did a detailed analysis of the polar alignment stability as the pier is raised
and lowered. This report describes the equipment and test method used.
My system consists of a Software Bisque Paramount ME, an RCOS 12.5
Ritchey-Chrιtien telescope and SBIG ST-10XME camera system. On top
of the RC is a Takahashi FSQ-106 with an SBIG STV camera head. The
entire load on the Pier-Tech 3 is estimated at 230 lbs. The
Pier-Tech elevating pier is a key element in the design of my
observatory since I must lower the telescope in order to fit it
under the roll-off roof on top of my garage. Construction details of
the observatory are on my web site.
Figure 1: Pier retracted and scope parked
Figure 2: Pier extended
I collect modeling data using New Astronomy Press Automapper II
software controlling Software Bisques TheSky and CCDSoft to create
a pointing error model in Software Bisques Tpoint. Each model was
comprised of over 80 individual points covering the entire sky down
to an elevation of 40Ί. Appropriate analysis of the model including
fitting appropriate terms to represent the mount and telescope
system was performed. Once this is complete, a quantitative measure
of polar alignment error is available.
Three models were developed as follows:
1. Pier retracted
2. Pier fully extended
3. Pier retracted again
The differences between 1 and 2 would indicate how much the polar
alignment changes as the pier is raised and lowered. The difference
between 1 and 3 would indicate the repeatability of polar alignment
in returning to its lowered state after the pier is raised and
This model was taken before the pier was raised.
Figure 3: Pier retracted
This is the Tpoint fit data window. The key terms are ME, polar axis
elevation and MA, polar axis azimuth. In this case, my polar
alignment target is the refracted pole. For my latitude and height
above sea level, this corresponds to a polar axis elevation of 72
above the true celestial pole. The ME term of 119.81 indicates I
am 50 arc-sec. high. The MA term indicates I am 12 arc-sec. west of
the pole. This is quite close within 0.9 arc-min. of the refracted
The pier was then elevated to its maximum upward travel of
approximately 20 inches.
Figure 4: Pier fully extended
Note that the ME and MA terms have changed value. The shift in polar
axis is given by
Elevation: ME2 - ME1 = 197.65 - (-119.81) = 317 arc-sec. change.
Azimuth: MA2 - MA1 = 91.91 - (-11.82) = 104 arc-sec. change.
The pier is once again fully retracted.
Figure 5: Pier fully retracted
To assess the repeatability after pier movement, we compare the
model 1 and model 3 terms as follows:
Elevation: ME3 - ME1 = - 106.73 (-119.81) = 13 arc-sec. change.
Azimuth: MA3 - MA1 = -11.01 (-11.82) = 0.8 arc-sec. change.
We can see that through the elevation of the pier, the elevation of
the polar alignment moves from above the celestial pole to below it.
This seems like a reasonable tolerance compromise and if we were to
polar align with the pier partially extended, we might expect a ± 40
arc-sec. accuracy in elevation and azimuth. A reasonable overall
expectation is ± 1 arc-min. polar alignment accuracy based on this
sample. For visual use with a high quality mount, this should put
most objects within the field of view of a medium power eyepiece.
Repeatability of the retracted position seems to be excellent and
within the measurement error of this methodology. The differences of
13 and 8 arc-sec. are essentially dead-on. This meets the
requirements for high resolution CCD imaging.
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