After sharing my last entry with the mailing list for our local club, several members were kind enough to take a look at the write up I provided. Consensus among those members was that I am not so much looking at a tracking problem as I am a differential flexure problem between the guide scope and the main scope. They suggested that the pattern of the trailing pixels across the frame, largely in a consistent direction, couldn’t really be explained by tracking problems or polar alignment issues, but rather by a slight but steady flexing problem. The wisdom on this, both from club members and from other reading on the interweb, is that “It Doesn’t Take Much!”.
It is not to say that periodic error and mount performance might not be a contributor to my elongated stars. Obviously, periodic error (and especially non-periodic error) improvements are sure to at least make tracking easier and more accurate. But the hypothesis that was suggested, is that poor tracking would not explain the hot pixel pattern shown in the image of my last post.One straightforward solution to reducing differential flexure is to use an off axis guider. In short, this device uses a small prism mirror to pick off a small portion of the incoming light through the main scope, rather than using a guide scope. While this setup can me a bit more difficult to get set up than a separate guiding scope, it inherently removes potential flexure between the two scopes and the connecting mounting hardware. This is especially recommended on larger scopes and scopes where the mirrors are not firmly attached, and so, can shift as the mount moves to differing orientations. I found a used one and will give it a try once I have the correct combination of spacers to use.In the meantime, though, I decided to try an experiment with what I had on hand. Rather than use my ED80 on by ADM side-by-side setup, I setup my 102mm imaging refractor directly on the top rail of my AT8RC. I wasn’t sure, but wanted to see if this arrangement was more rigid. Also my 102 has a more rigid focuser, and the AT8RC sits directly on top of the mount rather than slightly off-set. I really haven’t heard of anyone having issues with the rigidity of the ADM side-by-side setup, but I decided to try this arrangement, anyhow.The image above is the result of that effort. The tracking isn’t perfect, but the trailing hot pixels are much more tightly grouped together. The image itself is hazy and soft because of the conditions, but the star shape is improved somewhat. Note that the image duration for the subs in this image are the same as the subs in the image from my last post (12 minutes). I am encouraged that with an off-axis guider, results will be even better. And further, I located a higher quality stepper motor gear to replace the existing gear, This represents the last identified anomaly from the periodic error investigation and resultant tune-up that I performed.Note that the conditions were terrible the night I ran this experiment. I couldn’t spot Polaris until after 10PM though the haze and clouds. It was so humid that condensation formed on the glass of my cooled CCD, and after it dried, it left behind a number of spots. Barely any of the stars of Sagittarius were visible.So a tip of the hat to folks at AAAP, especially Nate, Fred, and Frank, who took and look and helped me out.
