The K-20 Project

The K-20 Project

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6/2/2007 - Squaring the Focuser

Being the upper end of the telescope, the Secondary Cage holds the focuser and secondary mirror and attaches to the truss tube.  This requires that the focuser be square to the optical axis of the cage and centered on the secondary mirror to allow for accurate collimation.  The process involves first adding a center dot to the secondary mirror and then using a Cheshire eyepiece, a long bolt and a ruler to square the focuser to the cage.

ENTRY PICTURES


Dave Kriege provides a nice page in his book that when copied 1:1 allows you to cut out an ellipse template that matches the minor axis of your secondary mirror to locate its center.	The center is located with a Sharpie through the pinhole.

A dot of black paint is added.	To square the focuser I first centered the spider horizontally in the secondary cage. Next I removed the secondary mirror and dropped a bolt of the same diameter as the mounting post down through the spider. After inserting a Cheshire eyepiece into the focuser I then centered the vertical crosshair of the eyepiece on the bolt by shimming the focuser left to right. After removing the bolt I drew a line on the Kydex opposite the focuser that line up with the vertical crosshair in the Cheshire. I then removed the Cheshire and put it in the focuser from the INSIDE of the cage and measured the distance from the top of the upper cage ring to the center hole in the eyepiece. I measured this same distance on the opposite side of the cage along the vertical line just created. I put the Cheshire back in the right way and centered on this point by shimming the focuser up and down. Squaring complete. Lastly I marked this location with a dot of silver ink and is my reference point for checking the squareness of the focuser once a year.