Re: Off axis mask

From: Martin Brown (|||newspam|||_at_nezumi.demon.co.uk)
Date: 10/11/04 

Date: Mon, 11 Oct 2004 13:57:08 +0100

In message <10mir95goo8sn3f@news.supernews.com>, Guy Macon
<http@?.guymacon.com.invalid> writes
>Martin Brown <|||newspam|||@nezumi.demon.co.uk> says...
>
>>You should see an image which looks like the characteristic image
>>through a scope of aperture equal to the size of the holes used crossed
>>by finer dark fringes determined by the separation of the centres of the
>>two holes.
>
>Assuming that I already have a big newtonian reflector set up, I can
>see a use for a single-hole off-axis mask as an essentially zero-cost
>alternative to setting up a smaller OA reflector next to the big scope.
>My question is, is there any point at all to making a multi-hole mask?
>Does it do anything for me that the single hole or the no-mask setups
>don't already cover?

Yes. But it will not give you cosmetically pleasing images. There are
three things you can do with odd shaped masks in front of a telescope:

Learn more about how shape of clear aperture influences the point spread
function. A narrow slit and regular N-gons like triangle, square and
hexagon are worth a try. A hexagon mask can even sometimes be useful to
manage the PA of the diffraction pattern for difficult double star
hunting.

Modify the point spread function by making an apodising mask. Some
people swear by them and others swear at them. In poor seeing it may be
a useful trick sometimes (ditto for hiding any turned down edge on the
mirror).

Detecting double stars with separations very close to the theoretical
limit of your scope in less than ideal conditions. Here you are using
the two sub apertures as a true interferometer and looking for the
fringes caused by interference of light down the two independent paths.

There are plenty of restrictions on this technique - it requires:
        bright stars
        nearly equal in brightness
        knowing their separation to select the right baseline mask

Subject to these restrictions and a few other minor practical ones like
waiting long enough for the air to be stable across both sub apertures
you should see strong fringes (intermittently) when the baseline is
perpendicular to the PA of the double star and none at all when it is
parallel. In the second case the interference peaks from one star sit on
the troughs from the other making the whole pattern vanish if the stars
are exactly equal in brightness. Try using holes about 1/10th diameter
of the baseline as a starting point. It is an interesting thing to try.

Michelson & Pease used a variant of this method in the 1920's to measure
the sizes of Betelgeuse and a handful of other red giants.

Regards, 

-- 
Martin Brown

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