1 Introduction

PHOTOM is a package for performing photometry of digitized images. It has two basic modes of operation: using an interactive display to specify the positions for the measurements, or obtaining those positions from a file. In both modes of operation PHOTOM allows you to perform photometry using either the traditional aperture method or via optimal extraction. When using the traditional aperture extraction method the target aperture can be circular or elliptical and its size and shape can be varied interactively on the display, or by entering values from the keyboard. Both methods allow the background sky level to be either sampled interactively by the manual positioning of an aperture, or automatically from an annulus surrounding the target object.

PHOTOM is also used by the Graphical Astronomy and Image Analysis tool (GAIA – SUN/214) which integrates the tasks of photometry with an image display tool. This allows the detailed inspection of objects and their environments and provides a highly interactive environment for placing, rotating and resizing apertures.