Re: Off axis mask

From: Guy Macon (http://www.guymacon.com)
Date: 10/11/04 

Date: Mon, 11 Oct 2004 11:44:49 -0700

Martin Brown <|||newspam|||@nezumi.demon.co.uk> says...
>
>Guy Macon <http://www.guymacon.com> writes
>
>>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.

This looks like something that will be a lot of fun experimenting with.
A google search on "apodizing mask" give a wealth of information.

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