XMM RPS Users' Manual

   
OM grism observations

For OM grism exposures no default configuration has been established yet, mainly due to a lack of data on the grism performance. This means that users planning on conducting observations with the OM grisms must enter all details (OM modes , science window location and size, etc.) manually on the OM settings page for each exposure.

BEFORE starting an OM grism exposure, an image should be obtained of the field of view in order to render possible an identification of the dispersed spectra with sources in the field of view. For the imaging exposure a default configuration (as described above in § 5.3.3.5) should be used. Note that different brightness limits apply to observations with the OM filters , the magnifier and the OM grisms , as listed in UHB Table 19 .

There are two grisms on the OM filter wheel, named Grism1 and 2, covering the UV and the optical wavelength range, respectively (see UHB chapter on OM optical elements ). The dispersion direction of both grisms is the detector y-axis. The dispersed first-order spectra of both grisms 1 are centred with their reference wavelength (260 nm and 420 nm, respectivly) on the detector location that the target would have with a non-dispersive filter in place. Each grism has a zero- and a first-order image. The spectra of the two grisms differ in size (in both orders). The positions of the spectra, with respect to the reference point, y0, are summarized in Tab. 6. Note that the exact positions can only be determined after detailed inflight calibration. It is therefore recommended to define windows as large as possible around the dispersed spectra, as described in the following.

   

Table 6: Positioning of the OM grism spectra with respect to the reference position, y0, of the first order image.

	1st order image					0th order image
	Wavelength [nm]			Position1 of		Position1 of
	at Ref. Pos.	$\lambda_{min}$	$\lambda_{max}$	$\lambda_{min}$	$\lambda_{max}$	$\lambda_{min}$	$\lambda_{max}$
Grism1	260	160	290	-260	+78	-811	-709
Grism2	420	290	550	-282	+282	-767	-699

1) Relative to the reference position, y0, of the 1st order image, along the dispersion direction (=detector y-direction), in units of subpixels.

Preparing an OM grism exposure

Acquiring a target spectrum without further timing information

Only one OM science window is defined.

• The centre of the OM window is defined equal to the target position (use RA and DEC). Note that the target position must be within 3.2' of the XMM boresight coordinates.
• The width of the window in x-direction is set to 1'.
• Depending on whether the optical or UV grism is used, the width of the window in y-direction is set to
  • 4.8' for Grism1 (UV)
  • 5.2' for Grism2 (optical)
• DPU binning is optional.

Obtaining additional timing information from the zero order spectrum

Assuming that OM is prime instrument and the target is on-axis, two windows are defined:

• The windows are defined in the detector coordinate system.
  

  Grism1 (UV)						Grism2 (optical)
  Win.#	X0	Y0	DX	DY	Mode	X0	Y0	DX	DY	Mode
  1	960	752	128	368	image	960	736	128	576	image
  2	1022	208	4	112	fast	1021	256	6	80	fast

• This requires that the position of the spectrum be known in the OM detector coordinate system at the time of proposal submission, which is only the case either for position angle-constrained observations or for on-axis observations with OM as prime instrument.
• Window1 provides the coverage of the first-order spectrum, while window2 provides fast mode data. The appropriate entries for an observation with Grism1 are displayed in Fig. 16. The entries for the first window have been made previously and committed to memory; they now show in the upper part of the form. The entries for the second window are still in the input fields, ready to commit.