[Shiraz (Ray)]

Hi - need help.

often see the assertion that long fl scopes need better tracking for imaging than shorter ones - basic question is why?

Tracking accuracy is an angular term, so if the scope angular resolution is limited by the atmosphere (which is generally a fair assumption for typical conditions and scope sizes), a given tracking error will produce the same degree of angular distortion in star shape regardless of scope fl. As a numerical example, if the resolution of two different scopes is limited by the seeing so that they both produce 3 arc sec star blobs and the tracking introduces a wobble of 2 arc sec in RA, then both scopes will produce stars that are 5x3 arc sec in shape. The stars might have completely different sizes in the final images, depending on the fl and sampling, but they will still have the same underlying 5x3 aspect ratio. As far as I can see, unless the seeing is so good that at least one scope is not seeing-limited, fl does not come into it at all.

However, the idea that focal length determines required tracking performance is widespread and is held by experienced users, so clearly I am missing something - very grateful for an explanation of what it might be.

regards Ray