jsg What is causing this discrepancy?
Short answer: your OTA's focal length is not 550mm.
Whenever you do a plate solve (either manually, or when the ASIAIR does a polar alignment), the plate scale is used to compute the true focal length of the optics ahead of the camera. Since the angular distance between stars are fixed, and the sensor's resolution is fixed, the computation gives a very accurate equivalent focal length.
Temperature changes (or any defocusing) can cause the computed focal length to change, but only very slightly -- usually, by less than a tenth of a millimeter.
Imprecise measurement of the star asterisms and star centroids can also cause the computed focal length to change, but again, usually quite small.
My Takahashi FSQ-85 has a design target focal length of 450mm, and with a 1.01x flattener, comes to 454.5mm. ASIAIR measurements had placed the measured focal length at between 456mm and 457mm depending on the time I measured it. So, even a Takahashi can be off by a little from their design target.
Chen
