KPG1_ELGAU

Calculates the axis ratio, inclination and minor-axis width of a star image, given the Gaussian widths of marginal profiles at 45-degree intervals

Description:

The reciprocal of the square of the width varies approximately like the length of an ellipse diameter as the angle of projection varies. The routine calculates the required parameters assuming this relationship holds, then iterates, calculating the expected deviation from this law and subtracting it from the data before calculating a new estimate. The solution of the ellipse equations is analogous to using the Stokes parameters of linear polarization to find the ellipse parameters.

Invocation

CALL KPG1_ELGAU( SIG, SIG0, AXISR, THETA, STATUS )

Arguments

SIG( 4 ) = REAL (Given)
The Gaussian widths of the marginal profiles of the star in directions at 0, 45, 90 and 135 degrees to the x axis.
SIG0 = REAL (Returned)
The width of the minor axis of the ellipse.
AXISR = REAL (Returned)
The axis ratio of the ellipse.
THETA = REAL (Returned)
The inclination of the major axis to the x axis in radians (x through y positive).
STATUS = INTEGER (Returned)
The global status.

Notes:

The routine assumes a Gaussian profile for the star.