Animated PSFs

We have generated animated GIF images and MPEG-1 streams showing the evolution of the point spread function for turbulence limited seeing for telescopes of various diameters. These show graphically the evolution of speckles in the PSF, and how the PSF for small telescopes tends to be dominated by a single bright speckle.

These images were constructed by generating a large realisation of a phase screen with the Kolmogorov spectrum with Fried length of 40cm. A circular pupil was then stepped across the screen and PSFs computed using diffraction theory (in the usual near-field limit). The step size was 0.125m, corresponding to a sampling rate of 100 Hz (for a wind speed of 12.5m/s) and the box size is 1.25 arcsec (assuming a wavelength of 0.8 micron). Each realisation shows a sequence of 256 frames.

Animated GIF images should be viewable with commonly used browsers. Each image is about 6MB in size. Images are available for the following image sizes. 1m, 1.5m, 2m, 3m, 4m, 8m,

Also available are MPEG-1 streams, which will require a player such as mpeg_play (for UNIX). These are each about 1MB in size: 1m, 1.5m, 2m, 3m, 4m. 8m.

In the single layer above a single phase screen was dragged across the pupil. In reality one expects there to be multiple layers, and moving with differing velocities. This is expected to dramatically decrease the time-scale for the evolution of the speckles. To explore this we have made simulations with four phase screens of equal strength (whose total seeing strength is the same as in the realisations shown above) moving in the N,S,E,W directions with the same speed (12.5m/s).

1.5m MPEG, 3.0m MPEG, 1.5m GIF, 3.0m GIF

Some increase in evolution rate can be seen for the larger aperture, but for the 1.5m aperture the evolution rate seems little affected.