XMM Users' Handbook

   
Choice of non-default OM science windows

For OM observations more than one science window may be defined. However, it is not possible to select the entire OM FOV for science observations. There are several boundary conditions that limit the user's choice and which must be taken in account properly if a non-default instrument configuration is to be used. Note that the use of non-default OM instrument configurations is discouraged by the SOC for AO-1.

  

Figure 72: 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 73: 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 Fig. 72) 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 (Fig. 72).

• Allowed detector 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.

  The minimum width of a detector window is the equivalent of 4 CCD pixels, the maximum width in one direction is limited to 64 CCD pixels (equivalent to 512 in-memory pixels).

  CCD windows must start at an even pixel number in both the horizontal (x-) and the vertical (y-) direction. OM science windows can therefore start, e.g., at the in-memory pixel coordinates (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.

  A science window may be larger than $512\times512$ in-memory pixels, but if so it requires more than one detector window (as defined, e.g., in the imaging mode default configurations).

• Science window overlaps

  Different scenarios for OM science window configurations are displayed in Figs. 72 and 73. 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. 73).

• Limited onboard memory capacities

  OM tracking frames are stored in the so-called ``small word memory'' (SWM). From one tracking frame to another, the pointing direction can change due to spacecraft drift. Using one tracking frame as reference, the attitude solution of the other is determined, a mean shift calculated and applied, before they are added in-memory. Because of the possible shifts, the allocated SWM area must include a $\pm16''$ margin around the specified science window. Including this margin, the maximum SWM capacity is 1,007,616 in-memory pixels.

  The final image(s) is (are) then accumulated in the ``big word memory'' (BWM). The BWM will also contain tracking history information, reducing the size of the available memory space for science data. The sum of all image mode pixels for each exposure must not exceed 432,698 pixels.

  Fast mode data are stored in a special memory partition. The maximum length of a fast mode exposure is 4.4 ks with one fast mode area in use and 2.2 ksec with two. These numbers assume that there is only one bright source in each fast mode window.

• Limited onboard processing capabilities

  The tracking frame time for image mode observations must be long enough to calculate the attitude solution and to complete the shift & add of the previous tracking frame. For $2\times2$ pixel binning of a $700\times700$ pixel size image mode window, this takes ca. 4.2 s.

  The minimum fast mode time slice duration (in ms) for a fast mode window must be longer than the total number of fast mode pixels in that area divided by 64. An additional 50% safety margin will be added to the time slice duration.

For a more detailed explanation see the technical note on Constraints on the OM Window Configuration and Definition of the OM Default Configurations (§ 8).