# NAME

> seeing -- Create a simulated ground-based star image based on a model of atmospheric turbulence

# USAGE

> seeing output xdim ydim gamma fwhmx fwhmy

# PARAMETERS

> output = "" [image]
>   The name for the output image to be created.
> xdim = 512 [integer]
>   The X dimension for the output image.
> ydim = 512 [integer]
>   The Y dimension dor the output image.
> gamma = 1.6666 [real]
>   A parameter which control the shape of the PSF produced. For a discussion of the significance of this please see the discussion below and the references given.
> fwhmx = 5.0 [real]
>   The full width at half maximum of the PSF produced in the X direction.
> fwhmy = 5.0 [real]
>   The full width at half maximum of the PSF produced in the Y direction.

# DESCRIPTION

> This task is for creating simulated groung-based telescope point-spread function (PSF) images. It is based on a model proposed by Saglia et al. 1993, MNRAS, 264, 961 which uses a simple atmospheric turbulence description. The radial profile is derived from the Fourier transform of an exponential power-spectrum model. Apart from the FWHM (which is just a scaling and which can be different in X and Y) there is one free parameter called gamma. The default value of 5/3 comes from the theory but the authors find that slightly different values may give better simulated PSFs, depending on the telescope, site and conditions.

> It is strongly suggested that this task only be used after careful reading of the Saglia paper. It has had little use and there may be problems.

> It is important that the images used are large enough to avoid artifacts which come from truncating the image before the Fourier transform is taken. This may be checked by looking at the values displayed by the program for the extreme values in the images. In rough terms the minimum in the "input image" should be small (eg, 10-8) and so should the minimum in the output real part, relative to the peak (eg, a factor of 10-6 lower). The output imaginary part should be very small (eg, 10-10).

# EXAMPLES

>     st> seeing
>     + SEEING Version 1.3 (April 2000)
>     Output image file name (qqq): test
>     X dimension of output (512):
>     Y dimension of output (512):
>     Full Width half max (in X) of resultant PSF (5.0):
>     Full Width half max (in Y) of resultant PSF (5.0):
>     Gamma value for PSF (1.66666):
>     -Extreme values in input array:   0.1224E-10   1.000
>     -Extreme values in output imaginary part:  -0.2135E-11  0.3030E-10
>     -Extreme values in output real part:   0.1542E-03   9476.
>     st>

# TIMINGS

> The time spent is mostly taken up by the Fourier Transform. Array sizes which are powers of two are most efficient.