### PHOTOPT

Perform sampling experiments with different sky estimators

#### Description:

PHOTOPT examines the performance of the three different sky estimators used by PHOTOM on a particular frame. It does this by performing aperture photometry on random parts of the frame, subtracting the estimated sky level from a concentric aperture from the level in the central aperture. If the estimator is good then the expected result is zero, as long as there are no objects in the central aperture. This is repeated a number of times over the frame to ensure that a fair representation of the frames characteristics is obtained. The results are shown in graphical form as a set of difference graphs. The histogram of differences will indicate which is the best suited estimator for the frame.

#### Parameters:

##### ANGLE = _REAL (Read)
The orientation of the ellipse defining the aperture. This is defined in degrees going anti-clockwise from the positive y-axis. This is equivalent to a position angle.
##### DEVICE = DEVICE (Read)
The name of the device to receive the graphical output.
##### ECCEN = _REAL (Read)
The eccentricity of the ellipse defining the aperture. For a circular aperture this should be set to 0.0.
##### IN = IMAGE (Read)
Name of the image on which the sampling test will be performed.
##### INNER = _REAL (Read)
The radius of the inner edge of the annular sky aperture in units of the object aperture size. The actual dimension in pixels is obtained by multiplying this factor by the object aperture semi-major axis in pixels.
##### NP = _INTEGER (Read)
The number of points to sample in the image up to a maximum of 100. This number is factorised so that a regular grid of samples is taken. The actual number of samples is restricted to ensure that the central apertures do not overlap.
##### OUTER = _REAL (Read)
The radius of the outer edge of the annular sky aperture in units of the object aperture size. The actual dimension in pixels is obtained by multiplying this factor by the object aperture semi-major axis in pixels.
The limit of the plot in photon units ( data units $\ast$ PADU ). This can be used to limit the range of the plot to the interesting region.