Re: [ATM] Re: Weird Foucault readings

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Vladimir,

> No one - except you - is talking about 3-zone mask. You can
> go 11-zone mask and still have reading accuracy compromised by the zonal
> defocus, although in a lesser degree. But zones can be narrowed only so
> much before it starts causing reading difficulties of its own.

So the idea is to get a good compromise, where the light distribution is
reasonably even across the mask opening, and it is also closely similar for
both sides. The narrower zone, the closer this is to ideal, but on the other
hand, the less sensitivity to KE travel. A too wide zone will have a
markedly uneven light distribution, and it is markedly different from side
to side. My zonal calculating algorithm tries to mimick Texereau's mask for
his "standard" 8in. mirror - he seems to have chosen the widths very wisely
from long experience, and simulations seem to confirm it.

But this is where the wave nature of light really matters for Foucault
testing.

Gaviola testing is done closer to the zonal focus for each side, and
consequently the zones can be wider and allow potentially more accurate
reading, but the complexity of the setup is a price you may or may not want
to pay.

>Also, I
> was talking about perfect mirror with perfect zonal defocus structure.
> Actual mirrors are certain to be less than perfect. Consequently, their
> zonal defocus will be more disturbed (less symmetrical) which doesn't
> help reading accuracy.

True - but what if you don't get a tremedously accurate estimate of a figure
that is middling to lousy anyway?


Nils Olof

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