Simulation of an NGS Speckle Observation with a 30 meter Telescope

Here is a movie showing simulations of wavefront propagation through turbulence for a 30 meter telescope.

30 meter simulation using a single layer atmosphere

On the left side of the movie is the wavefront phase in the pupil plane of a 30 meter aperture. On the right is the phasor amplitude of the electric field in the image plane (i.e. the square root of the intensity that would be detected by a camera). This simulation was performed by creating a turbulent layer at the ground, placing an aperture at the ground, applying these "optics" to a plane wavefront to get the pupil plane phase, and then propagating the resulting wavefront to the far field to get the image plane amplitudes. In this movie you can see that the motion of individual speckles is correlated with the wind velocity, which for this simulation was due north.

The parameters for this simulation were:
30 meter aperture
1 micron radiation
r0 of .3 meters at 1 micron
3 cm pixel scale in the wavefront
3 cm pixel scale in the turbulence layer
time step of duration 20 milliseconds
10 seconds (500 20 ms frames) of simulation time

This simulation was generated using the program simple_simulation, which is provided as part of the arroyo distribution.

Here is another simulation in which a six layer atmosphere has been used, and wavefronts have been propagated through this atmosphere using diffractive rather than geometric propagation.

30 meter simulation using a 6 layer atmosphere

The simulation was run with parameters that differed slightly compared to those above. First, the time step is 10 milliseconds rather than 20, and the duration of the simulation is 5 seconds instead of 10. Second, the six layer atmospheric model that I used has an equivalent r0 of .37 meters at 1 micron.

On the left appears the wavefront phase in the pupil plane of the 30 meter telescope. In the middle appears the wavefront amplitude in the pupil plane of the thirty meter telescope. The wavefront amplitude shows the effects of scintillation, which arises from diffractive propagation effects. Briefly, phase errors introduced by high altitude turbulence are mixed into amplitude fluctuations as the wavefront propagates through free space. Finally on the right appears the resulting PSF. I put the PSF into a separate movie to permit closer inspection.

PSF only

There are a couple of interesting differences between the single layer 30 meter simulation above and this simulation. First, the multilayer atmosphere permitted simulation of diffractive propagation, which allows us to qualitatively inspect scintillation in the pupil plane. The wavefront amplitude fluctuations from scintillation are of order 10 percent at 1 micron, and consequently the intensity fluctuations would be of order 20 percent.

A second point of interest is that the apparent velocity of the pupil plane phase is up and to the right, while that of the pupil plane amplitude is up and to the left. This effect arises from the fact that the velocities of the low altitude layers and the high altitude layers were different in the simulation. The turbulence profile is exponentially weighted towards the ground, so that low altitude layers - which happen to move up and to the right in this particular simulation - dominate the wavefront phase. Scintillation is dominated by high altitude layers, which happened to be moving up and to the left in this particular simulation.

A final point of interest is that the lifetime of speckles in the psf appears much shorter in the six layer simulation than in the single layer simulation. And while the speckles in the single layer simulation appeared to translate in a direction correlated with the wind vector of the layer, speckles in the six layer simulation appear to wander around a short time before fading away.