Reducing Cone Error(Centering Polaris)

Cone error is the divergence between where the mount is pointing and where the attached camera or telescope is pointing. I decided to tackle this with the mount and camera in the polar home position – i.e. polaris should be close to the center of the image. Horizontal error is just a matter of rotating the camera around the declination axis – orthogonal to the polar axis, but vertical error is trickier. A telescope would have a means for adjusting it but with the camera on a dovetail bar attached to the mount there’s no obvious approach. When I first measured it in the left image above, Polaris was at 2810 vertically instead of the 1728 which would be the midpoint. Since the FOV was about 4.3 degrees, the error is about 1.34 degrees. After shimming the attachment plate Polaris last night was at about 1860, so still about .16 degree low.

In the picture below you can see the shims in place. I’ll try moving them halfway to the mounting bolt. I’ve tried doing calculations but they never work out. I’m a bit worried about making the attachment unstable but i might as well try.

In terms of reducing Chromatic Aberration I don’t think I’ve done much. The two images of Polaris below were before refocusing and stopping down on the right and after on the left. The left image is a bit better but not specifically the CA and there’s a lot of other stuff going on that affects the image.

UPDATE -here’s my cone error 20-05-18 – still a bit below center. I make it .2 degrees below center. I’m not sure I can do any better than that.


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