Re: [ATM] Couder Masks and Zone Testing

["Richard Schwartz" <richas@earthlink.net>]

And what is the best size for the mask holes?  If they are too small,
diffraction will cause the cone of light from each hole to be spread out
and it will be hard to tell when you have the knife edge set right.   If
the holes are too large, then you see shading variation inside each hole
and it is hard to judge when the holes appear equally bright...   This
leads me to wonder if there is a better way that does not use holy masks.

. . . Richard


> [Original Message]
> From: Nils Olof Carlin <nilsolof.carlin@telia.com>
> To: Guy Brandenburg <gfbrandenburg@yahoo.com>; Michael Lindner
<mikell@optonline.net>; Byron <byron@astronomygeek.us>; <atm@atmlist.net>
> Date: 4/15/2004 7:59:57 AM
> Subject: Re: [ATM] Couder Masks and Zone Testing
>
>
>
> Guy, Michael and Byron,
>
> > The math is not hard, really. If you have a 6 inch
> > diameter mirror, and you want 4 zones, and you want
> > them to have the same area,
>
> This is not necessarily a good idea - particularly regarding the central
> zone. At
> http://w1.411.telia.com/~u41105032/couder/zcalc.htm
> I have explained why I think so - in brief, a central zone does not behave
> like the others, it is much less useful and better left out.
>
> > It will help a LOT if you have a ruler that is marked
> > in decimal fractions of an inch!
>
> You could also use one marked in decimal fractions of a meter ;-)
>
> > As far as what portion of the zone to cut out, both
> > Texereau and the LeCleires suggest drawing chords that
> > are parallel to the diameter, and cutting between the
> > chords. The problem here is that the outer zones
> > become much, much smaller than the inner zones, and
> > thus are harder to read. A simpler and better solution...
>
> A possibly even slightly better solution is to make them of equal area,
but
> still use a chord (and diameter) to limit the cutouts. The reason is that
> comparison is always along lines perpendicular to the (most commonly
> vertical) knife-edge, and the extra "triangle" between the diameter and
the
> corresponding chord can only confuse. One way is to make the "height" of
> each cutout about 1/3, or even 1/2 of the zonal radius.
>
> Nils Olof
>
>
>
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