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.
I was out last night mostly to work on focusing the Takumar 200mm lens, hoping to reduce chromatic aberration but I decided to image the polaris region to see if i could spot this beauty. In the middle of the highlighted circle above you can see for sure Gaia DR2 1152635154843863808, a magnitude 12 star and just up and to the right is NGC 3172. I can see it with the eye of faith but it’s tiny at 1X.7 arc minutes and low surface brightness at 13.7 mag/arcmin2
The plate solve below from astrometry.net is the only way I knew it was in frame then i tracked it down in stellarium.
I had a bit of a debacle the first time i tried the ioptron skytracker – i had no luck at all with the polar scope and i just roughly aligned it with the bore hole. I had the brilliant idea to roll up a sheet of paper to stick in the sighting hole which let me find and center polaris and then i was able to graduate to their polar scope. iOptron makes an app that gives you these little pictures of where polaris should be in the reticle – it seemed loony to me but i was actually able to line it up pretty much spot on.
I was working in a cramped spot with no good view of the sky so my reward was this nondescript image of part of ursa minor and draco with zero trailing that i can see. These are fairly close to the pole but I think they would have shown some trailing in 30 second images. These were taken with the Canon t3i, f/4 ISO 400.
I’ll get out into the country and try my luck on M31 tonight. That’s much more likely to smear but i’m optimistic. I need to get much better at pointing the camera once it’s on the mount and i need to remember to focus both the camera and the reticle before i start. it’s interesting to note that it makes very little difference in sighting whether i have my glasses on or not!