MOPEX Online Manual
| Contents | ~ | Previous: Basic Concepts: The Spitzer Point Response Function | ~ | Next: Basic Concepts: Uncertainty Estimation |
Basic Concepts: Aperture Photometry
Aperture photometry may also be measured for each source. The photometry is calculated on the background subtracted image. The program integrates the flux encircled by the aperture centered on the point source (see Figure 1). Multi-frame APEX still uses the background subtracted mosaic image for this purpose. Aperture photometry AP is equal to
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 2). Operation is defined by the Annulus_Compute_Type menu, 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 1: 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 2: The geometry of the annulus used for background estimation.
| Contents | ~ | Previous: Basic Concepts: The Spitzer Point Response Function | ~ | Next: Basic Concepts: Uncertainty Estimation |
