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Re: [ATM] refractive index of air
At 2007-08-12 19:19 +0200, Dominic-Luc Webb wrote:
>On Fri, 10 Aug 2007, Bob May wrote:
>
> > BTW, which ROC is the correct one on a secondary, the one at the
> > edge or the one at the center of the surface for the speciffied
> > ROC of the mirror? I've always assumed that it is the center of
> > the surface.
>
>I do not think there is any ambiguity here, Bob. The equation is
>essentially a conic (alternatively I guess the eccentricity could
>be specified). As near as I can tell, the ROC is for the center,
>and never for any other position.
At 2007-08-13 15:21 -0700, Bob May wrote:
>The ambiguity came in because a previous poster in this thread
>mentioned that working on the edge causes the ROC to change. I
>had always assumed that the center radius was the radius of the
>surface as you have affirmed but you never know and it is
>better to sound dumb a bit rather than to accept the
>misunderstanding and make the secondary improperly.
It is true that the "ROC" (Radius of Curvature) as we use it is the
radius of curvature at the vertex of a conic. At other points, the
local radius of curvature will be different (if the conic isn't a
circle), but we don't care, ROC is being used as one of the
parameters to describe a conic, the other important one being b, the
deformation constant.
Finally, when we ATMs have tested a mirror, found the surface height
x(y) at each zone radius y, and calculated the conic which minimizes
the RMS, it's common practice to say the mirror itself has the
conic's ROC and b. That's what the "previous poster" (probably me)
meant when he said working the edge changes the ROC. This is no
problem figuring a parabola, unless you've already made an non-adjustable OTA.
-- Jim Burrows
-- http://home.earthlink.net/~burrjaw
-- mailto:burrjaw@earthlink.net
-- Seattle N47.4723 W122.3662 (WGS84)
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