EMAG performs aperture photometry within concentric elliptical apertures of increasing semimajor axis radius but fixed ellipticity and position angle. The image is assumed not to have been sky subtracted, but the background level of the image is assumed to be contained in the VISTA variable SKY. Each pixel then gives a contribution (pixel value - SKY). The aperture radii (i.e. semimajor axes) are incremented by one pixel at a time. The resulting aperture total intensities are recorded in the PRF common block, replacing the values of the "elliptical totals" computed by PROFILE.
EMAG attempts to account for the presence of perturbing stars (or any other glitches) on the image of the galaxy as follows: AEDIT is first used to mark those pixels which are contaminated; these pixels are given the flag value -7777. EMAG will not use any pixel with a value less than zero (note therefore the importance of not performing sky subtraction!); instead, it will look for that pixel's "symmetric partner", i.e. the one reflected about the galaxy center. If the partner is not also flagged, it will use its intensity level as an approximation for the original, flagged pixel; if it is also flagged, however, neither pixel is used by EMAG.
The user can explicitly specify the center, position angle, and ellipticity of the apertures, if desired, with the CENTER=(row,col), PA=theta, and ELL=eps keywords on the command line. It is more common, however, to use EMAG following the use of PROFILE. In this case, the ellipse parameters may be determined by accessing the PRF common block. (The ellipse centers, however, may also be learned from the AX common block, following use of AXES.) The radius at which the ellipse parameters are taken from the PRF common block is set either directly, via the PROF=p keyword on the command line, or by the ERAD keyword, which then gets the radius from the VISTA variable ERAD, which has been set by the interactive routine EMARK.
The number of apertures for which the computation is performed (or, equivalently, the maximum radius for photometry), may be set in one of several ways. The command line keyword N=n sets it explicitly. Alternatively, the keyword RMAX instructs EMAG to obtain the maximum radius from the VISTA variable RMAX, which can be set interactively with the routines RMARK or EMARK. If neither N=n nor RMAX are given on the command line, EMAG will use the existing maximum radius in the PRF common block (provided the latter is loaded.)
The command line keyword APPEND instructs EMAG to tack on to the PRF common block not only the elliptical totals, but also the PA and ellipticity EMAG works with, and surface brightnesses computed along the way. This will only work if the radius for photometry determined by the N=n or RMAX keywords is greater than the previous radius in the PRF common block.
To compute the magnitudes within elliptical apertures with varying ellipticity and position angle as computed by PROFILE, see the ELLMAG command.
At present, the results of an EMAG calculation can be viewed by writing the contents of the PROFILE common block using PRINT PROF.
See Also: EMARK, RMARK, ELLMAG