XMM Users' Handbook

   
Operating modes of the RGS

All RGS CCDs are operated in the so-called ``frame store'' mode, in which half of each CCD is exposed to celestial radiation. The contents of this half is transfered to the second (shielded) half, which works as a storage area before readout, while the first half is acquiring the next frame. Thereby, the two-dimensional dispersed photon distribution is stored.

The RGS has two science modes, ``spectroscopy'' and ``high time resolution'' (see Table 11 for a summary of their basic characteristics). Most data will be obtained in the former. Only if high time resolution (down to $\sim 16$ ms) is required, will the latter be used.

 

Table 11: The RGS science data acquisition modes

$\parbox{3cm}{{\bf Mode}}$	$\parbox{11cm}{{\bf Description}}$
$\parbox{3cm}{Spectroscopy}$	$\parbox{11cm}{2-D readout of up to all 9 CCDs over the full energy... ...out takes 0.6 s. $3\times3$\space pixel OCB is applied. See text for details.}$
$\parbox{3cm}{High time resolution (HTR)}$	$\parbox{11cm}{Continuous readout of strips of 74 pixels collapsed ... ... only 1 CCD is read out, higher time resolution (down to 16 ms) is achieved.}$

1.

Spectroscopy mode

The baseline on-chip binning (OCB) factor in this mode is $3\times3$ pixels. A variety of options to reduce the data rates to within the RGS telemetry bandwidth limits are available. Each CCD readout takes 0.6 s. Each individual chip (and any combination of chips) can be read out. This leads to a duty cycle for the normal sequential readout of the full array of 9 CCDs of 5.4 s. The energy range covered depends on which CCDs are read out and on the position of the source within the FOV. Reading out all 9 CCDs, the full energy range and all spectral orders are sampled, as listed in Table 10. A rough estimate of which energies are sampled if a given CCD (or combination of CCDs) is read out is provided in § 3.4.1.

A fraction of the X-rays produces events in more than one readout pixel. With the basic readout configuration (spectroscopy mode) all detected events are transmitted, stored in the ODF, and (re-)combination of the charges of these pixels (by adding them up) must be performed as part of the offline data analysis. This is the preferred configuration, as full flexibility is retained in the data analysis.

In the case of bright celestial sources, however, these additional data may cause the available telemetry bandwidth to be exceeded. The following configurations of the RGS spectroscopy mode are available for reduction of the instrument telemetry:

• High Event Rate (HER): minor filtering of pixels takes place, which practically is only a noise reduction; split pixels remain to be transmitted;

• HER with Split Event Reconstruction (SER): in this configuration the addition of pixels is performed by the on-board processor before transmission;

• HER with Single Event Selection (SES): Only isolated pixels (without energy contained in neighbouring pixels) are selected on-board for transmission.

2.

High Time Resolution (HTR) mode

In the HTR mode those halves of the RGS chips exposed to celestial light are divided into 74-pixel strips parallel to the dispersion direction. Each of these strips is collapsed into one row in the storage section (the half of the CCD chips shielded from celestial light) for readout. In order to obtain good absolute timing, a short observation of the target in spectroscopy mode must be obtained first, before starting the HTR exposure, to determine the exact location of the source's spectrum in the RGS FOV, which is needed for the determination of the absolute timing. Either all nine CCDs or selected sequences of them can be read out in HTR mode. The time resolution is roughly inversely proportional to the number of CCDs that is read out. As in the spectroscopy mode, the energy range covered depends on which CCDs are read out and on the position of the source within the FOV.

Some basic characteristics of the RGS science modes are listed in Table 11. Each RGS CCD is read out via two nodes.