giovedì, gennaio 03, 2008

Aperture masks

A recent thread, on a astronomy forum, focused on the never ending story of masking the aperture of telescopes to supposedly improve views in poor seeing. An idea was to stop down the telescope by using a mask with two off centre holes, instead of one.

Here is a first evaluation of the diffraction patterns for (a) a full aperture scope, (b) a scope with a mask with one hole, centered at half the radius and with a diameter 0.4 the original aperture, (c) a mask with two holes, like b, 180° apart.
The analysis here presented does not consider the effect of turbulence, it only shows the diffraction figure in ideal conditions with and without masks. As seen, the two masks severely enlarge the area where light is focused (this before any seeing effect is included). There does not seem to be any advantage in the double hole mask.
The pictures show the star test for the three cases (a, b, c, from top to bottom). For each case there are three frames. The left frame represents the image 10 waves inside focus. The right image is 10 waves extrafocal. The image at the centre is in focus and in magnifyed 5 times compared to the extra/intra focal images.