giovedì, gennaio 03, 2008

Aperture masks with seeing

Here is an investigation of the effect of aperture masks in bad seeing conditions. The full aperture picture has been computed with 0.25 rms turbulent wavefront, according to Suiter's model (with q=1.3). As seen, the large amount of turbulence disrupts the ideal diffraction figure, which now takes the speckle aspect.

The pictures relative to the masks intercept only a part of the full aperure turbulent wavefront (and thus their relative ptv and rms is less than for the full aperture case).
Unlike the previous post, the holes here are 30% of the original diameter and placed at radius 0.6 (they are tangent to the external original diameter) In the previous post holes were 40% diameter and placed at radius 0.5. The smaller holes are here used in hope to exclude as much turbulence as possible. Furthermore their location clears a possible central obstruction of 40%. The in-focus images are here magnified only twice.

It is easy to see that the 1-hole mask preserves a fair diffraction pattern. It is indeed very little affected by turbulence. However, the size of the diffraction figure is somewhat greater than the speckle of the full aperture case. Thus the mask only makes a bigger diffraction figure which covers the speckle. Moreover, the light in the full aperture case, although distributed over the large speckle area, is still more concentrated in the centre than the masked case. This means that the resolution, or the mtf functions, will be better for the full case, also in the bad seeing conditions.

The two holes mask needs no further comment. It jons the disadvantages of the the two.