Circular aperture photometry can be done for each source. The photometry can be calculated on the original or background-subtracted image. If done on the original image, the background can be estimated in a circular annulus. The program sums the flux encircled by the aperture centered on the source using fractional pixel areas (see Figure 8.25). APEX Multiframe does aperture photometry on the mosaic, not the individual images. Aperture photometry AP is equal to
Equation 8.26
which is the sum of the products of the pixel values Ii weighted with the exact area overlap ai.
If the image is in surface brightness units (FITS keyword BUNIT = MJy/sr or microJy/square_arcsec), it computes the results in the units of micro-Jy.
Optionally, if Use Annulus is selected, the background B can be estimated and subtracted from the aperture photometry for each point source. It is estimated within an annulus of a user specified size, as
B = Operation (Pixels with R >= Inner Radius and R <= Outer_Radius).
R is the distance from the point source position to the center of the pixel. Fractional pixel computation is not done; each pixel is either in or out (see Figure 8.26). Operation is defined by the Annulus Compute Type selection, and can be either "mode" or "median". In order to proceed with the background computation, there must be at least Min Number Pixels of good (non-NaN) pixels in the annulus. The background subtracted aperture photometry is computed as:
Figure 8.25: A circle of a specified radius centered on the point source (red) is overlaid on the image and aperture photometry is computed by summing the pixel values weighted by the overlap area.
Figure 8.26: The geometry of the annulus used for background estimation.
