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28. Adjusting for wobble in declination axis
Notes: The main source of tracking error appears to be wobble in the declination axis. Wobble in DEC appears in the images of stars as a bulge on the horizontal axis. Evan theorized that we needed to imbalance the weight on the DEC axis to give the motor something to work against. With our current polar alignment, the DEC correction in PHD Guiding is predominantly south. We read this under the guiding window during the guiding. M63 is in the northwest. Guiding south should mean that the guide star appears to climb. The scope, which is polar aligned slightly eastward, wants to dive relative to the guide star as both rotate westward, so the scope is guided (and lifted) upward (south) to follow the guide star. For imaging M63 (See 27), we moved the scope forward on the mount about 1/4" from perfect balance to create a small forward imbalance. This did not appear to affect focus. There is no reason why it should so long as the focus stop screw is tight. The result was that the DEC error appeared on the PHD chart as square drops to -1 pixel lasting for a few seconds and returning to near zero with an irregular frequency pattern of roughly one per minute. This was a different pattern than the large up and down variations on 3 minute intervals that we have seen with the scope weighted on the camera end. The frames appeared to be sharper, but we still were able to use only 3 out of 10. The subexposures with the roundest stars came from near the end of the run. Perhaps the tracking was better on the late subexposures because at the lower azimuth (closer to horizontal alignment) the imbalancing on the DEC axis was most effective. This argues for additional imbalancing closer to the meridian. Zoom in/out with ⌘↑(+/-) (Mac), ctrl (+/-) (PC).