Quote:
Originally Posted by clive milne
Guide star availability for a given area of imaging sensor is more or less proportional to the square of the inverse of the focal ratio.
Aperture has little to do with it..
fwiw
~c
|
Perhaps that's true for conventional guiding, but star brightness is important for AO guiding since it's often the limiting factor in determining the maximum tracking rate. All else being equal, the same star with a larger aperture scope will give either a higher ADU count, or a shorter exposure for the same ADU count (therefore higher tracking rate).
If a rotator is used and the position angle of the main camera isn't important (e.g. square chip), then guide star availability will be significantly affected both by the angle of the guide chip relative to the axis of rotation (i.e. whether the long vs short axis is tangential to the axis for non-square guide chips), and by the distance of the guide chip from the axis of rotation (greater distances sweep a larger area during a full rotation).