[ATM] spider rotation

[David Weinshenker]

Dale Eason wrote:
> The placement of the secondary is not that critical
> assuming it is collimated.  All that happens is that
> the secondary will not catch the full image of the
> mirror and some vignetting will happen.  For visual
> that is not important the edge of the image will be
> darker but you will not notice it until it is about
> 1/2 as bright.  For imaging it is more important so
> what is usually done is that the secondary is sized
> larger to catch everything.

In any case, some opticians recommend slightly oversizing the
secondary (relative to the calculated size for the desired
fully illuminated field) in order to minimize the effect of
potential edge errors (either departure from flatness in polishing
the glass, or distortions from stresses induced by multilayer
reflection coatings).

The loss of image quality associated with the slightly larger
central obstruction is considered to be minimal compared to
the benefit of positioning the potentially distorted edge of
the mirror (and especially the narrow tip of an elliptical
mirror) beyond the margin of the useful beam so that it cannot
affect the image.

-dave w