The task will only handle images which are a tangent plane projection of the sky, for example those created from the X and Y columns of EPIC event lists using evselect. Images from the optical monitor, which in their raw form contain a significant amount of distortion, could in principle be mosaiced after reprojection to the tangent plane projection.
The output image is the additive mosaic of the input images, in other words no average is performed. If the input image list is matched with a corresponding list of exposure maps, these exposure maps are additively mosaiced in parallel, then the output image is divided by the mosaiced exposure map. This approach is preferable to mosaicing exposure corrected images, as it helps avoid problems of poor counting statistics in areas of low exposure. Binary masks can be simulated by providing exposure images which have a pixel values of zero or one.
The output pixel size (in degrees) and world-coordinate system (WCS) reference coordinate for the output image are set to those of the first input image.
The task can read images in any of the dal-supported numeric data types. If all the input images have the same data type (and if –withexposure is not set), the output image also has that type; otherwise the output is written in real64. If –withexposure is set, this task also looks up the input exposuresets as well, then the datatype with a larger bytesize one (e.g., FLOAT rather than INTEGER) between input images and exposuresets is used for the type for the output image, providing that all the input sets have the same data type; otherwise the output is written in real64.
The task support FITS images which have their image array in the PRIMARY extension of the file. It also, exceptionally, supports XMM-Newton mask files, which hold the mask array in an extension called MASK. In the latter case, the output file also contains an extension MASK which holds the mosaiced mask array.
XMM-Newton SOC -- 2021-11-30