FWHM Preferences Page

The FWHM Preferences page configures Radial Profile Plots and the FWHM Measurements tool used for computing the point spread function of the image.

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.


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.

Related Topics

Radial Profile Plot, FWHM Measurements, Mira Preferences