FWHM Measurement

The FWHM Measurement tool is used to compute the Full Width at Half Maximum, or "FWHM" of a point source or other small object.

This command uses the Image Cursor to locate the approximate center of the object, then it computes the centroid and then the fit of a Gaussian + Constant model. This command works with the Image Cursor so that the cursor is left in a position for making a Column or Row plot or computing statistics at the point without having to re-position.

If an image set is open in the Image Window, you can choose whether to measure only the active member of the image set or to compute the FWHM for the entire image set. If measuring the entire set, Mira likewise centroids each position as it processes the set.

Measurement Results

Measurements are tabulated in the Fwhm Measurements window as shown below. This is a Report Window with built-in functionality for sorting, rearranging, copying, and saving to a file.

The example above shows results for images having a World Coordinate Calibration ("WCS") with FWHM results in arcseconds and the X,Y values in terms of Right Ascension and Decliination. If the images do not have a WCS calibration, the results are listed in units of pixels.

Measurements for all objects in all images and all Image Windows go into a single Report window. If you want to clear the Report window of prior measurements, make sure the window has focus, then click Clear in the View pull-down menu.

The items in the Report window are described in the table below.

FWHM Quantities


The sequence number of the measurement.


The name of the image that was measured


The Full Width at Half Maximum diameter in arcseconds (WCS calibration) or, otherwise, in pixel units.

Peak Value

The luminance of the peak, measured above the background.


The background luminance near the object.


The X coordinate of the center of the object. If the image has a WCS calibration, this is right ascension, otherwise this is the column coordinate.


The Y coordinate of the center of the object. If the image has a WCS calibration, this is declination, otherwise this is the row coordinate.


The column coordinate of the object.


The row coordinate of the object.

Related Topics

Measuring Images, Report Windows, Image Set, Image Cursor, FWHM Preferences page