This model uses the same underlying mathematical functions as the FixedExp and FixedGaussian galaxies, but rather than drawing the function directly onto the image, it is used as a probability map. A discrete number of photons are placed onto the image, using the probability map to decide where they will go. The result is an image of the galaxy that has photon counting stastics (Poisson).

The parameters accept values in terms of normal astronomical units.

The number of photons may either be specified directly, or given in terms of the magnitude of the object. If calculated, the following formula is used:

photonsPerSec = lam_hc*10.0^(-magnitude/2.5)

where lam_hc=4.2e9, but can be adjusted with the "FluxMultiplier" paramater. The total number of observer photons is then

numPhotons = numExposures*timePerExposure*photonsPerSecond

This calculation is know not be correct, and should be fixed soon with a more accurate model.

Both underlying models have a number of random and non-random transformations applied to their probability functions before photons are dropped. The function is scaled to achieve the desired ellipticity, then randomly rotated. The random rotation can be disabled by setting the VaryRotation parameter to 0.

The transformation may be applied either by drawing the function and applying a transformation to the image, or by transforming the coordinates and then drawing the images. The first method is the default behaviour. The second method is faster in all cases, and has no effect on accuracy. It is turned on with the UseCoordXForm parameter.

This model assumes a pixel width of 0.1 arcseconds.

Parameters

Name	Description
ModelFunction	If 1, use a fixed-ratio Gaussian model. If 2, use a fixed-ratio exponential model.
NumberPhotons	Number of photons to use to create the image. Not used if calculating number of photons based on magnitude.
Magnitude	Astronomical magnitude of the galaxy.
FluxMultiplier	The constant multiple lam_hc use to convert astronomical magnitude to photon counts. The default value is 4.2e9.
NumberExposures	Number of exposures over which to sum the time of exposure. This model does not actually draw more than one image -- this parameter is just used to calculate time.
ExposureTime	Length of each exposure. Used to calculate total exposure time.
MajorAxis	Sigma length of the major axis of the model in pixels. Not valid if Rh set.
Rh	Half-light radius of the model in arcseconds. Not valid if MajorAxis set.
Rh_ratio	Conversion factor between half-light radius and sigma of major axis. Depends on the AxisRatio.
AxisRatio	Ellipticity of objects -- ratio of the major to the minor axis.
SelectGal	If set to 0, do not automatically change random parameters before drawing.
VaryRotation	If set to 0, the rotation value will be fixed to the parameter Theta (default 0).
Theta	Constant value of rotation, in radians from the x-axis, to use if not Varying Rotation.
Verbose	If non-zero, print debugging information to cout.