XMM-Newton
Users Handbook
Next: 3.4.4.6 The RGS Background
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Several parameters determine the RGS effective area per dispersion
order: the properties of all optical components in the light path, the
CCD quantum efficiency and the source and order filtering criteria.
The following optical components contribute:
- The effective area of the mirror modules (see § 3.2.2.1).
- The Reflection Grating Arrays: how much of the light from the
mirrors is diffracted to the different spectral orders and then
absorbed on the RFCs. This is determined by the amount of collimated
light that is intercepted by the grating arrays, the reflectivity of
the gratings into the dispersion order and the effective surface of
the gratings that provides useful dispersed light. Since the
gratings are mounted relatively close together, about 1 cm spacing,
light with large dispersion angles is vignetted by the back of the
neighbouring grating, thus reducing the surface area that provides
useful intensity to the camera at these dispersion angles.
- The quantum efficiency of the RGS focal camera CCDs, which
varies from 70% to 95% over the RGS passband from 0.35 to 2.5
keV.
These three components are functions of wavelength and source position
angle. Finally, data selections in the RFC, used to separate orders
and to suppress scattering, will determine the effective area.
To assess the total efficiency of the RGS instrument per spectral
order, the efficiency with which the background is rejected and the
different spectral orders can be selected must also be taken into
account. This is performed by filtering in the imaging domain
(cross-dispersion versus dispersion angles) and in the domain CCD-PI
versus dispersion angle. Standard data selections are indicated by the
white curves in Fig. 62. In the
cross-dispersion direction a filter is applied which includes
typically 90-97% of the total intensity. Default masks for the
spatial extraction of photons from the different orders using PHA or
PI-energy channel vs. dispersion coordinate plots, as in
Fig. 62, are created by the offline analysis
SAS tasks for each RGS . The user can also modify the extraction
masks as described in the SAS User's Guide. In addition the
exposure time per wavelength bin is calculated taking into account the
data selections, rejected columns and pixels and the dead space
between CCDs. The expected efficiency in the post-observation RGS
order selection with the SAS is about 90% (0.95x0.95). This is a
function of wavelength and is less at shorter wavelengths, due to
additional loss of scatter outside of the extracted (dispersion-PI)
region.
Fig. 68 displays the calculated effective area
of both RGS units together, and Fig. 69 the
individual RGS 1 and 2 overlayed one on top of the other. The
calculations have been performed taking into account all the factors
listed above.
Figure 68:
The effective area of both RGS units combined as
a function of energy and wavelength (top and bottom horizontal
scales, respectively). See text for detailed explanations.
![\begin{figure}\begin{center}
\epsfig{file=figs/rgs_specarea2.eps,width=0.8\hsize,angle=0}\end{center}\end{figure}](https://webarchive.library.unt.edu/eot2008/20090830075037im_/http://xmm.esac.esa.int/external/xmm_user_support/documentation/uhb/img182.png) |
Figure 69:
The effective areas of both RGS units separately as a
function of energy and wavelength (top and bottom horizontal
scales, respectively). See text for detailed explanations.
![\begin{figure}\begin{center}
\epsfig{file=figs/rgs_specarea1.eps,width=0.8\hsize,angle=0}\end{center}\end{figure}](https://webarchive.library.unt.edu/eot2008/20090830075037im_/http://xmm.esac.esa.int/external/xmm_user_support/documentation/uhb/img183.png) |
Some clear features can be identified from these figures:
- the effective area has broad gaps due to the inoperative CCDs.
- the efficiency around chip boundaries does not drop sharply nor
completely decreases to zero. This is due to the scattering causing
photons with wavelengths corresponding to the passive areas being
redistributed into non-passive areas. The amplitude of this
sensitivity is strongly dependent on the selection mask in CCD PI
versus dispersion coordinate space.
- the total effective area of both RGS added in
Fig. 68 shows several of these chip
boundaries, as the alignment of the RGS cameras is such that the
inter-chip gaps do not coincide in wavelength.
- a similar effect of lower area as with the inter-chip gaps can
be seen at wavelengths equivalent to bad columns, but there it is
less pronounced.
- absorption edges are due to the existence of passive layers in
the X-ray path consisting of the following atoms: Si (6.74 Å), Al
(7.95 Å), Mg (9.51 Å), F (17.80 Å) and O (22.83 Å)
- apparent oscillations with small amplitude of the effective area
are due to data selections by regions that are defined as polygons
across the surface of binned two dimensional data (cross dispersion
versus dispersion angle and CCD energy versus dispersion angle). The
fact that pixels of these binned images are selected discretely,
gives rise to small scale variations of the selected area within the
region. This is seen as oscillations of the detection efficiency.
This is not a problem, nor is it an additional uncertainty of the
response, as these data selection effects are properly included in
the response generation.
Fig. 70 shows the effective area as a function of
the off-axis cross dispersion angle. The ratio of the median off-axis
effective area to the on-axis values decreases from 0.994 at
0
.5 to 0.83 and 0.60 at -2
.2 and +2
.2,
respectively.
Figure 70:
The RGS 1 effective area as a function of cross dispersion
off-axis angle.
![\begin{figure}\begin{center}
\epsfig{file=figs/rgs_offaxis.ps,width=0.8\hsize,angle=0}\end{center}\end{figure}](https://webarchive.library.unt.edu/eot2008/20090830075037im_/http://xmm.esac.esa.int/external/xmm_user_support/documentation/uhb/img184.png) |
Next: 3.4.4.6 The RGS Background
Up: 3.4.4 In-Flight Performance
Previous: 3.4.4.4 Wavelength Scale Accuracy
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