Re: [ATM] Foucault and diffraction

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

vladimir sacek wrote:

>
> Due to diffraction, there is no focal "point", not even a disc,
> rather a cylinder of light extending in the focal zone and widening
> into defocused pattern on both sides. Therefor,
> there is a certain length along which an intercepting edge
> would cause whole of the reflecting surface darken nearly
> instantly. I don't know what exactly determines length of this section,
> but will arbitrarily take 1 wave (~1/3.3 RMS) of defocus error as a
> rough limit at which the darkening begins to be gradual.

Hmmm... I wonder how this relates to Born & Wolf's 20% (Strehl 0.8) measure:

                  +/- 0.5 * (f#)^2 * lambda


_______________________________________________
ATM mailing list http://www.atmlist.net/