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Polar Alignment Repeatability Pier-Tech 2

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 to perform.

First, I 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 indeed.

Thanks for a great product.

John Smith


 

Polar Alignment Repeatability Pier-Tech 3

John Smith
www.hiddenloft.darkhorizons.org
September 6, 2003


Introduction

I recently upgraded my Pier-Tech 2 to a Pier-Tech 3. At Pier-Tech’s 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.

Methodology

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 Bisque’s TheSky and CCDSoft to create a pointing error model in Software Bisque’s 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 lowered.


Model 1

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 pole.


Model 2

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.


Model 3

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.


Summary

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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