> The ronchi test, especially with a
> double pass of the light under autocollimation is extremely sensitive to
> zonal defects and in most cases there is no real advantage to the star test.
Astigmatisms that make otherwise great optics useless ( ie 1/2 a wave or more ) are literally impossible to detect with a ronchi screen, where the star test shows it up easily. These tests should always be used together, I think, to give you the whole picture.
Mark