Monday, August 12, 2013

More on Polar Alignment

Interesting discoveries regarding polar alignment:

First:
The bubbles in the G11 mount are offset.  Centering the bubble is not level.  Instead, it's best to have the edge of the bubble touch one of the lines.  Maybe this is the intended function.  Taking the step of measuring the tripod using a mason's level shows the evidence.

Look at the bubbles:
The mount's level indicators

Altitude level

Level across Azimuth.  It's actually not level here, only slightly off.
Surprisingly, I checked later in the day to find that the offset seen here
is due to uneven heating of the legs by sunlight.

What has this meant?  It means that polar alignment is a much easier task now since the Azimuth and Altitude adjustments don't throw each other off as much.

Second:
I was able to identify that the difference in drift when viewing east and west was mainly due to viewing stars that were too low in the sky and not equal in altitude to each other.  In a follow up effort, I found that using stars that were at 40 degrees elevation -- nearly the same as the pole -- the V shape matched.

By following these steps, I was able to get a Gemini model that was pretty good!



Third:
By reading the notes at http://canburytech.net/DriftAlign/DriftAlign_2.html I was able to confirm that I will not be able to get better than 1 arc minute accuracy in ALT due to refraction.  The refracted pole (which is what is measured in drift alignment) will always appear 1 arc minute too high.

This is not a problem for most of the sky.  So long as I image between 40 and 80 degrees altitude on the meridian (which is most of the interesting targets), it will be a non-issue.  Outside of these thresholds and the tracking will suffer.  I should turn on King rate at those points.

From: http://canburytech.net/DriftAlign/DriftAlign_3.html