Centroid Radius

Specifies the maximum radius, in pixels units, for pixel value used in computing the centroid position. The optimal radius is usually about 2 times the actual FWHM. For example, if the FWHM of a point source is critically sampled near 2 pixels, a sample radius of about 4 pixels gives the most accurate centroid position.

Auto Centroid

Check this box to compute the centroid position as part of the FWHM computation. If this box is unchecked, the existing estimate of the position is used as the centroid for the FWHM computation. The existing position may be an existing centroid position or just the "raw", but fractional, array position in the image.

Max Radius

Specifies the maximum radius, in pixel units, for pixel values used to compute the FWHM. The FWHM is determined from the fit to a Gaussian + Constant model for the source and background. If the radius is too small, the background pixels will be under represented in the fit. Conversely, if the radius is too large the pixels in the source profile will be under represented in the fit. The optimal radius is usually about 4 times the actual FWHM.