FWHM Properties

FWHM Properties

Fitting Radius

Sets the radius over which a PSF (Moffat or Gaussian) function is fit.

 Auto Centroid

Check this box to cause the FWHM to be computed using a centroid position as its center. If not checked, the FWHM value uses the image cursor position as its center.

[Advanced]

Opens the FWHM Fit Properties dialog for setting advanced fitting properties. See below.

Centroid on

Selects the direction of intensity profile used for calculating centroid positions. This is measured from the pixel values and is not affected by the choice of display settings, such as the palette or contrast.

Positive Points

Negative Points

Selects whether to centroid on positive-going values (peaks) or negative-going values (valleys).

Sample Radius

Defines the distance from the target point within which pixel data are used to compute the centroid position. A reasonable radius value is approximately half the diameter of a point source image.

Weight fit by Signal to Noise Ratio

Check this box to weight 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.

Min iterations for fit convergence

Set the minimum number of iterations for computing the fit. A value of around 200 to 400 usually works well.

Initial estimate of FWHM, in Pixels

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

PSF Model

Selects the mathematical model used for fitting the Point Spread Function ("PSF") to the data. The choices are Moffat or Gaussian. For the Moffat PSF, the beta parameter is estimated by the FWHM calculation.

Opens the Moffat Profile Properties dialog for setting the Moffat beta value. The Moffat beta value is the starting point for the calculation as it is a fit parameter. This is an advanced feature.