E PHOTOPT - examining PHOTOM’s performance

PHOTOPT is an auxiliary package to examine the performance of the various sky estimators used by PHOTOM. As indicated in section B the best choice of estimator depends on the circumstances. The idea behind PHOTOPT is to put down an aperture on a random piece of sky, estimate the background from a concentric aperture using one of the sky estimators offered by PHOTOM, and subtract this estimated sky from the flux in the central aperture. If the sky estimator correctly estimates the sky then the difference of the two will be zero. Since an automatic procedure cannot be certain of selecting a representative piece of sky, the procedure is repeated a number of times to increase the statistics.

PHOTOPT selects regularly spaced points within an image, puts down an aperture at each of the points and calculates the difference between the central aperture and the sky estimate from the surrounding annulus. Each of the three sky estimators offered by PHOTOM are tried in turn on the same set of points so that a comparison can be made between the three methods. The output is in the form of two graphs for each estimator. The first shows the difference between the central aperture and the sky estimator for each of the samples. The sample number forms the x-axis and the difference (object - sky) given in photons per pixel forms the y-axis. The second plot shows a histogram of the differences for the samples. The differences are binned into suitable intervals and form the x-axis of the histogram, with the y-axis giving the number of samples in each difference bin.

PHOTOPT has a number of parameters in common with PHOTOM: the aperture shape parameters, SEMIM, ECCEN and ANGLE, the scaling factors PADU and SATURE and the background annulus size INNER and OUTER. The two parameters unique to PHOTOPT are NP, the number of points to sample, up to a maximum of 100, and the RANGE, which defines the bounds of the data value used in the plot. The number of points sampled may not exactly equal the number requested as the program automatically positions the points to be on a rectangular grid which evenly covers the whole data array. The program also ensures that the density of points does not result in the central apertures overlapping.

PHOTOPT can be run from ICL or the C-shell. From ICL use :

  ICL> photopt

From the C-shell use (assuming photomstart has been executed) :

  % photopt

The full description of PHOTOPT follows.