Precision Centroid

Check this box to compute a precision centroid measurement. If this is not checked, the centroid is computed from the pixels accrued by the Detection phase, which must be above the threshold setting. The precision centroid gives the same results as using the interactive centroid measurement.

Click [Setup...] to open the Centroid Parameters dialog.

Precision FWHM

Check this box to compute a precision measurement of the Full Width at Half Maximum (FWHM) on all sources found by the Detection phase. This measurement gives the same result as using the interactive FWHM measurement. Note that the precision FWHM is extremely CPU intensive and will dominate the time spent in the entire extraction process.

Click [Setup...] to open the FWHM Parameters dialog.

Source Photometry

Check this box to make a precision photometric measurement for each source. This measurement is made like the aperture photometry measurement used in astronomy, which gives the integrated signal above the local background within an elliptical aperture centered on the object. The result is labeled as the "Total Lum", or Total Luminance, as measured above the locally estimated background which is more precise than the background surface value estimated during the Background phase. Also returned are the measurement uncertainty, signal to noise ratio, and local aperture background for each source. See Source Extraction Definitions.

Click [Setup...] to open the Source Photometry Parameters dialog for setting the aperture radii, ellipticity, orientation, and other parameters.

Detect Variability

Check this box to detect sources that vary in brightness among images in the image set. The threshold variability is specified in the Variability Parameters dialog opened using the [Setup...] button.

Note

Source Photometry gives the best background estimates when sources are separated by empty background. This is because the local background used by the measurement samples pixels lying inside an annulus surrounding the source. The Modal Brightness Median Brightness background methods do rejection of non-background pixels but still can give a biased result when non background pixels comprise a significant portion (e.g., 10% or more) of the local background sample.