Quote:
Originally Posted by gregbradley
They seem to need a flattener though which is odd as they display coma something an RC should not be displaying.
Greg.
|
The elongated star images at the edge is defocussed astigmatism. RC's are coma free but suffer strongly from astigmatism without a field flattener with which they can produce very sharp round image at the edge. I've always thought , at least for the shorter focal length RC's at 12" and under , that the " coma -free " moniker was a bit of a furfy because of the off - axis astigmatism. the big observatories don't mind that for astrometry purpose as its still easy to find the centroid of an ellipse.
Mark