To open the FWHM Preferences page,
open the Mira Preferences dialog and select the
FWHM tab.

Max Radius

Specify the maximum radius to use in the FWHM
calculation. This is the radius of the background beyond the point
source, not the radius of the point source.

Auto Centroid

Check this option to automatically compute the
centroid position for the point source. It is recommended that you
check this box.

Fit Gaussian + Constant

Check this box to compute a fit to the profile of
the point source. If the calculation is made, Mira reports the
point spread function parameters of FWHM, Peak Brightness, and
Background value.

Advanced

Opens the Profile Fit Properties dialog
(see below) for use in the Gaussian + Constant fit.

The Advanced button

The Advanced button
opens the Radial Fit Properties dialog where you set
parameters for radial profile fitting used in the FWHM calculation.
Profile fitting is an iterative procedure which requires starting
values. The FWHM starting value should be reasonably close to the
final value, say within 50% or thereabouts.

Use S/N Weighting

Check this box to weigtht the solution by the
Signal to Noise Ratio of the pixel values. Checking the box usually
gives a better fit to the point source and unchecking the box
usually gives a better fit to the background.

Max Iterations

Set the maximum number of iterations for computing
the fit. A value of around 300 to 400 usually works well.

Initial FWHM

Set the initial value for starting the profile
fitting. His should usually be within 50 to 100% of the actual
FWHM, as measured in [pixel] units.