FWHM Measurements

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.

To make a FWHM measurement, position the Image Cursor near the target object, then click on the toolbar or click FWHM in the Measure menu.

This command is also operated using the 'F' keystroke command when the Image Cursor is placed near the peak of a point source.

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 PSF + Constant model. The PSF (Point Spread Function) may be selected as a Moffat or Gaussian profile. 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.

NOTE

When the Image Cursor is used to measure centroid positions for an Image Set, the cursor position may appear to be inaccurate after being shifted within the individual images. This happens because the Image Cursor is a feature of the Image Window rather than the individual images. However, the centroid coordinates tabulated in the measurement grid are correct. In contrast, Point Measurements are calculated and marked independently on each image, so they indicate the measured coordinates in each image if the centroid option is enabled on the Centroid Properties page.

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 Declination. 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 menu.

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

FWHM Quantities

#	The sequence number of the measurement.
Image	The name of the image that was measured
FWHM	The Full Width at Half Maximum diameter in arcseconds (WCS calibration) or, otherwise, in pixel units.
Peak Value	The intensity of the peak, measured above the background.
Background	The background intensity near the object.
X	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.
Y	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.
Col	The column coordinate of the object.
Row	The row coordinate of the object.

Measurement Results

Related Topics