For OM observations more than one science window may be defined. The different science windows may overlap (as long as they overlap fully) without loss of information in any of these windows. Specifically, a FAST mode WINDOW inside an IMAGE mode WINDOW does not punch a hole into the image, thus, the source will be in the image and in the fast window.
There are several boundary conditions that limit the user's choice and which must be taken into account properly if a non-default window configuration is going to be used.
Up to five science windows in total (of which up to two in the FAST mode) are allowed.
The size of an OM science window must be integer multiples of 16 in-memory (0''.5) centroiding pixels, i.e., 22 CCD pixels.
Science windows must start at an even pixel number in both the horizontal (x-) and the vertical (y-) direction [e.g.: (16,16), (32,32)], and end at (n16,n16), where n is an integer number. Starting at pixel coordinate (0,0) is not possible because of the necessity to allocate a 16 pixel wide margin to accommodate spacecraft drift.
The maximum size for a single window in the Sci.User Defined Image Mode is:
or any rectangular size with a total equivalent number of pixels. If more than one image window is defined, the sum of the number of pixels (binned or not) of all windows should not exceed 432698.
The maximum size of a Fast Mode window is 512 pixels in full resolution, equivalent to 2223 pixels (about 10''.510''.5).
Different scenarios for OM science window configurations are displayed in Figs. 112 and 113. OM science windows must overlap either entirely or not at all. This must be true for all satellite position angles allowed by the specifications made during proposal submission, in particular if the windows are specified in celestial coordinates (Fig. 113).
Particular care must be employed if windows are defined partly in pixels and partly in sky coordinates within the same exposure. It is responsibility of the PI to ensure that these windows do not overlap for any position angle. Violation of this constraint can lead to unrecoverable loss of science exposures.
The minimum Fast mode time slice duration for a Fast mode window is 0.5 s while the maximum duration is the tracking frame duration (20 s).
For a more detailed explanation the reader may refer to the technical note on Constraints on the OM Window Configuration and Definition of the OM Default Configurations (§ 7).
An interactive tool for planning OM observations of point sources is available at the MSSL URL: http://www.mssl.ucl.ac.uk/www_astro/xmm/om/om_tool_current.html
Another tool to help defining OM science windows is available
This interactive tool may help defining and visualising OM Science Windows and mapping out the dispersion direction of the grisms for different position angles to check for a possible superposition with a bright star.
An interactive grism tool for planning OM grism observations is available at: