[Shiraz (Ray)]

Sam, Ken Rick - thanks guys. So it's a perception issue, not physics.

again taking a numerical example to summarise: assume a 1000mm fl scope is nicely matched to a camera with 5 micron pixels. if the scope is changed to 2000mm fl without changing the camera, the stars will be oversampled and the (dimmer star) tracking distortion will be more noticeable to the eye than with the 1000mm fl scope. Alternatively, a 500mm fl scope will produce an undersampled image and the dimmer stars will appear as irregular blocky points almost regardless of their underlying shape. (the current Polarie craze shows that idea taken to the extreme).

However, if I chose a camera with 10micron pixels for the 2000mm scope and one with 2.5 micron pixels for the 500mm scope, the star images for these setups would be identical to the 1000/5 setup if the tracking is the same - star shape would be independent of focal length.

So the assertion that a longer focal length scope requires better tracking should have the very important caveat "If you use the same camera".

You could just as accurately say "a longer focal length scope does not require better tracking if the angular pixel scale of the camera is maintained"

thanks, I can understand that better now. regards Ray