Mira Product Brief

Radial Profile Plots and FWHM Measurements

Mira provides two ways to measure the Point Spread Function ("PSF") of the image:

Radial Profile Plot, and
Direct FWHM measurement.

The PSF is an important tool for quantifying image sharpness as measured by the width of the image of a point source. The width is characterized using the Full Width at Half Maximum, or "FWHM" of the radial intensity profile of the point source. Mira provides 2 methods: The Radial Profile Plot provides a visual confirmation of the measurement, whereas the direct FWHM measurement uses the same computational code in Mira but lists its results in a table. In both methods, Mira first measures the centroid position of the source. Then all pixels with a specified radius are collected into a set of (radius, value) pairs. Finally, a Gaussian + Constant function is fit to these data.

	Sample Image At left is shown a star-field image displayed in Mira version 7. The contrast is boosted to show faint point sources to be plotted and measured. The PSF is measured using either the FWHM measurement or a radial profile plot. Both methods use the image cursor to mark the measurement location. The image cursor is the large red rectangle with a central crosshair. If moves independently of the "live" mouse pointer. To measure the PSF, first switch to image cursor mode (the black, pressed button at the top left), then position the image cursor near the reference point. Click the plot or FWHM command to make the measurement. The image cursor automatically moves to the centroid coordinate and makes the PSF measurement there.

	Bright Source The plot window at left shows the radial profile of a bright point source in the image above. Notice how well the model (smooth curve) fits the data (squares). The FWHM is measured from the parameters of the fit, which you can see listed above the plot outline.

	Faint Source This plot shows the radial profile of a much fainter source in the same field of view. The model again fits the data very well despite the significantly lower Signal to Noise Ratio. Do you trust the quality of Mira's FWHM measurement? The radial profile gives you proof.