Baseplate
The baseplate, mounted in the observatory, is tied directly to the building roof, and separated from the observatory structure itself by blue tape. This decouples the scope from wind forces on the observatory sides. The baseplate is an 18" x 24" plate of anodized aluminum 1/2" thick. It is covered in 1/4"-20 threaded holes on 1" centers. Three bolts for tilting the plate are located in a triangle, with the scope weight located approximately in the center of the triangle.
In order to understand the baseplate leveling procedure, it is necessary to know the placement of the mercury mirror. The mercury mirror is mounted on the az table and is carried around as the scope changes its az.
The mercury mirror sits on the az table. If the az table is parallel to the top surface of the mercury (which is guaranteed to be horizontal due to gravity) then the position of the LED image on the detector will not change when the scope is rotated in az. If the az table is tilted a bit, then the image of the LED will describe a circle as the scope rotates in az.
By measuring the size and phasing of this circle, the baseplate correction can be determined.
With multiple passes and adjustments, the tilt circle can be reduced down to the encoder resolution limit. This levels the baseplate to within about one arc minute. The 5 digits appended to the date give the seconds of UT day, so you can see how I progressed through the evening.
