INTRODUCTION

A spectral file can be created by accumulating all X-rays detected within some region on an imaging detector. This spectral file then retains no information about the distribution of the X-rays accumulated or of the size and shape of the region used. However, the instrumental response may depend on this information. For instance, the vignetting correction might vary across the region selected so that a proper calculation of the effective area requires a knowledge of the spatial distribution of the selected events. Another example is where the region used does not collect all the events from a point source. In this case a calculation of the effective area requires knowledge of the shape of the region used and the position of the point source.

One way of encoding the required information is to place an image of the selected region as the primary image in the spectral file. We refer to this as the WMAP and describe the required keywords below. To minimize the file size the image should be of the rectangle which bounds the selected region.