For additional information, select: Note #4 by Bruce Johnston
Figure 19 - Shims used to raise fork arm
After either adjustment has been made tighten the bolts, and recheck the DEC axis along the vertical line as described in Step 8. Repeat this procedure until the beam does not stray from the vertical line.
Recheck the horizontal line with the RA one final time to make sure that nothing has moved during the adjustment procedure. If the beam sweeps along the horizontal and the vertical lines without straying then the DEC axis is now perpendicular to the RA axis and you are ready to check the OTA LOS axis against the RA axis.
Aligning The OTA LOS Axis To Coincide With The RA Axis
The theory behind this procedure is that with the scope DEC locked at
90º a laser beam reflected off the surface of the corrector place
onto a flat surface a distance away from the corrector plate will not
move when the scope is rotated around its RA axis. That is if the OTA
LOS axis coincides to the RA axis. If the two axes do not coincide then
the reflection will sweep in a circular motion as the scope is rotated
around its RA axis. The curve in the corrector plate does not affect the
reflected beam because as the scope is rotated around its RA axis the
beam will remain on the same portion of the curved surface on a perfectly
aligned scope. Editor's Note: Any method that relies on bouncing a laser off the corrector to test the LOS (line of sight)
axis will only test the axis of the corrector plate. Since an SCT has three optical elements, only one of which is adjustable
in collimation, the corrector axis is not necessarily exactly identical to the LOS axis.
Step 1. First find a location that will allow you to reflect the laser beam several feet from the corrector plate with the scope DEC set to 90º to a flat surface that can be observe closely. I used my basement. This allowed me to reflect the beam approximately 22' onto the basement wall from the corrector plate. I used a piece of 3'x3'x_" plywood to mount the equatorial wedge to hold the scope. I was then able to set the entire unit on the floor, set the wedge latitude to 25º and tilt the whole unit slightly so that the beam reflected to the basement wall instead of the basement ceiling. Make sure the OTA is balanced with its weight system for this portion of the procedure.
Figure 20 - Temporary mount used
to align LOS axis with RA axis