Precise polar alignment of the azimuth axis


Roland Christen
 

Hi Astronuts,

This first post will describe a precise way to polar align the mount's azimuth. Once that is done, it will not require any further adjustment.The idea is to use the natural geometry of the sky with respect to an earth bound mount to produce a perfectly aligned azimuth. That's the axis that rotates the bottom of the RA axis east and West.

Concept #1) If the scope is pointed straight at the zenith on either the west or east side of the mount, that point will remain stationary as you rotate the azimuth axis. So, if a star is in the eyepiece at that point and you bring it to the center of a crosshair, you can rotate the azimuth axis and the star will stay on the crosshair - at least for some minutes if you are tracking it. However, if you then slew to a star down south along the meridian line, it will deviate either east or west depending if the azimuth axis is too far west or east. The star at the zenith is then a pivot point, and the star down toward the south is your calibration star (or azimuth adjustment star). Bring that one to the crosshair by turning the azimuth adjusters and you are theoretically perfectly polar aligned.

So, the simple routine goes something like this:

1) Using your planetarium program, pick a star near the zenith on one side of the meridian. I like to use a star in the west with scope on the east side. That way it won't migrate across the meridian while doing this alignment step.
2) Bring the star to the center of a crosshair and do a Recal or Sync. MaximDL has a nice crosshair if you want to use an imaging camera for this alignment.
3) Now pick a star down toward the south on the SAME SIDE of the meridian (or north if you are in an upside down hemisphere), and slew to it. The star will appear either east or west of the crosshair, so now just turn the azimuth adjuster until the star is on the centerline of your crosshair. It may or may not be aligned N-S but that is of no consequence. No Recal or Sync is necessary because you did not move it via the motors.

Basically you are done with azimuth, but you can re-check by slewing back to the first star and do these 3 steps again for more precise alignment.

Next post - How to align the altitude axis.

Rolando




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