XMM-Newton Users Handbook

3.5.10.1 Choice of Science User Defined non-default windows

For OM observations more than one science window may be defined. 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.

Figure 95: When the boundaries of OM science windows are defined in detector pixel coordinates, the relative location of the windows with respect to each other does not change. However, different areas on the sky are imaged under different position angles.

Figure 96: Defining the locations of OM science windows in sky coordinates one makes sure that (approximately) the same area of the sky is imaged under different position angles. Now, however, the OM science windows can change their relative locations. Windows 3 and 5 (which used to be in the upper left corner of window 3, see previous figure) are now partially overlapping, which is not allowed and window 4 is now partly outside the OM FOV (which is also not allowed).

• Maximum number of windows

  Up to five science windows in total (of which up to two in the FAST mode) are allowed.

• Allowed science window locations and sizes

  The size of an OM science window must be integer multiples of 16 in-memory ($0''.5$) centroiding pixels, i.e., $2\times2$ 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 (n$\times$16,n$\times$16), 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:

  • full resolution: 652$\times$652 pixels ( 5'.17 $\times$ 5'.17 )
  • 2$\times$2 binning: 488$\times$480 binned pixels ( 7'.6 $\times$ 7'.5)

  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 22$\times$23 pixels (about $10''.5\times10''.5$).

• Science window overlaps

  Different scenarios for OM science window configurations are displayed in Figs. 95 and 96. 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. 96).

  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.

• Fast Mode slice duration constraints

  The minimum Fast mode time slice duration for a Fast mode window is 0.5 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 the user to define OM science windows is available at:
http://www.mssl.ucl.ac.uk/www_astro/xmm/om/om_wintool/om_tool2.html. This interactive tool may help the users to define and visualize their own Science Windows, it also allows to map 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:
http://www.mssl.ucl.ac.uk/www_astro/xmm/om/grism_tool_current.html