Re: [ATM] Re: Weird Foucault readings

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

Bob,

> Yep, I know I introduced the 3 zone mask into this as the mask zone count
> does matter as to how accurate you make those measurements.  One guy doing
a
> test with 3 zones can come up with a much different result than another
who
> uses 11 zones for the testing.  I just wanted to make sure that the first
> individual fully understands that his great results really aren't true.

But are they more or less true than those of the second individual? More
zones may be better if the mirror parameters demand more zones, worse if
not.

> Yeppie, you get some error just from the "focus" error (quotes because a
> really short FL mirror will show the shadow from dark to light in the
zone,
> for example) is really not that important as is the slope error across the
> zone.

If any mirror shows a marked progression from dark to light, the reason is
not that the mirror has too short FL, but that the zone is too wide for the
mirror it is used with. This is preventable of course, if you have an idea
of how wide a zone can be and still be readable at the KE.

> I wonder if the real limitations are not the width of the zone as measured
> on the mirror but rather the angle of the zone width as you go across the
> mirror????  It seems to me that the zone count goes up as you shorten the
> focal ratio and only hits a limit as the zones become small due to the
> diameter of the mirror.

The change of contrast between the sides is approximately inversely
proportional to the zonal width, at least for narrow zones where the light
distribution is fairly symmetric across the zone. So I would guess the total
error in estimate will be about proportional to the square root of the
number of zones used, as long as they are clearly narrower than the optimum.
Given an average error in estimation over each zone, of course a large
mirror with many zones will have a larger error of the estimated error.
Nils Olof


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