APER will sum the intensities of pixels falling within circular apertures centered on an object in the 'source' image. Using apertures of differing radii will enable you to characterize the radial intensity distribution of an object.
Up to 10 aperture radii in arc seconds can be specified with the RAD keyword. Alternatively, a linearly increasing sequence of apertures can be specified with the STEP keyword, where 'incr' is the radius increment in arc seconds, and 'n' is the number of apertures. A geometric sequence of apertures can be set up with the GEO= keyword, with the first value being the first aperture size and the second the multiplier for subsequent apertures; as many apertures as will fit in the image will be used. The pixel scale in arcsec/pixel is specified with the SCALE keyword. The centers of the aperture must be calculated ahead of time with the AXES centroiding command, or loaded with the C keyword. Lastly, the OLD keyword tells the routine to use any unspecified parameters from the last time.
The routine will produce a list of the apertures, the sum of pixels interior to them, the average interior surface brightness, and the same quantities for the rings defined by sequential apertures. The summations are done to the nearest pixel at the edges of the aperture, that is, no fractional pixel interpolation is done at the edges. For this reason, results obtained with very small apertures may be inaccurate. The results are stored in a common block, and can be examined with the PRINT command, and stored or retrieved with the GET and SAVE commands. With the PLOT keyword, the apertures will be displayed on the image display (for most video devices).
APER does NOT do sky subtraction. The background light is included in the calculation of the brightnesses. Use SKY to compute the background of the image, and SUBTRACT to remove that background.
Specifying a list with the MAG word will give observed magnitudes in the listed apertures. (What does this mean?? good question! Inspection of code seems that MAG= keyword allows user to specify magnitude values which just get output right back.)
The STORE keyword saves some results into VISTA variables. The total intensity in the first aperture is loaded into variable TOTAL, the area of the first aperture will be loaded into APAREA, and the average intensity into variable AVERAGE.
To extract aperture photometry in elliptical apertures, see the EMAG command.
See Also: EMAG, ELLMAG, PROFILE, SKY