RE: ATM Spider Design

["Alan Levin" <ailevin@netcom.com>]

> From:          andydtg@alpha1.phoenix.net
> Date:          Mon, 19 Aug 1996 09:55:54 -0500
> To:            atm@shore.net
> Subject:       RE: ATM Spider Design
> Reply-to:      andydtg@alpha1.phoenix.net


> I don't think the thickness of the vanes (or anything else) in the
> optical path has anything to do with the amount of diffracted light in
> the image plane, it's a function of the length of the edges. The thickness
> does affect the amount of obscuration because of the increased area.

It may be merely wording, but the thickness and length both count
since the total area of the spider determines how much light is in
its diffraction pattern.  The shape of the spider determines the
shape of its diffraction pattern. Coherently summing the spider
diffraction pattern with the diffraction pattern of the clear
aperture and the diffraction pattern of any other obstructions gives
you a new diffraction pattern. The square of that pattern then gives
you the observed intensity pattern. 

As with central obstruction, the issue is not the loss in light
gathering, but the loss in contrast because  light that would have
been in the central diffraction spot that is now directed to some
other place.  The shape of the spider or any other obstruction where
the light goes.  The area determines how much light goes there.

Good Observing,
Alan