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A recent endeavor has been to try my hand at auto guiding the LX using a Quickcam. Auto guiding is strange. It isn't as easy as it looks, but then it is easier than it looks. I guess the problems I had were just not the problems I expected.
I collaborated with Marty Niemi on a port of his great autoguider program, Guider, to C. In the process I learned quite a bit about the things Guider does, and came to appreciate that none of the things it does were created in a vacuum.
First and foremost among my hard learned lessons is that what you and I think of as solid in the way we measure solidity, and what the camera thinks of as solid, with its 10 micron ruler, are not even in the same universe. It can very accurately follow the flex in something you didnīt think was flexible.
The ported version of Guider will maintain the lock on a guidestar to within about 0.5 pixels. That works out to be about 0.8 arcseconds with the ETX. That is a very respectable job for a retired webcam and an ETX. The problem is that the guidescope being locked that close is no guarantee that the main scope is doing the same thing. In fact, the main scope can be drifting at a very high rate while the guidescope remains locked on a star.
It might be better to think of it from a new perspective. The LX mount now points an ETX, and tracks flawlessly. We just added a 10" piggybacked camera to the ETX. The problems seen in the system are with the piggybacked camera - not the ETX. It happens that the solutions from each perspective are mostly the same. The alignment between the guidescope and the main scope cannot change during the cource of an exposure. Not even by a few very tiny arcseconds.
Assuming the link between the two scopes is non-flexible from the guiderīs viewpoint, it remains that the camera on the main scope is still essentially outside the feedback loop created by the guider. Any wobble in the optical axis of the main scope will not be detected by the guider, and will not be corrected out. The LX series of scopes is defined by the inability of the OTA to hold the primary mirror an the same plane as the scope changes angle relative to the horizon. This shortcoming is called mirror flop in the extreme case, and is caused by the required clearances between the mirror mechanism and the tube which carries it forward and backward inside the scope. |
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