XMM Users' Handbook

   
Science modes of the EPIC cameras

The EPIC cameras allow several modes of data acquisition. Note that in the case of MOS the outer ring of 6 CCDs remain in standard imaging mode while the central MOS CCD can be operated separately. The pn camera CCDs can be operated in common modes in all quadrants, or a single CCD in one quadrant can be operated independently. Thus all CCDs are gathering data at all times, independent of the choice of operating mode.

 

Table 3: The science data acquisition modes of EPIC

$\parbox{4cm}{{\bf \htmladdnormallink{EPIC}{http://astro.estec.esa.nl/XMM/user/build/buildxmm_top1.html\char93 epic}\ MOS Mode}}$	$\parbox{11cm}{{\bf Description}}$
$\parbox{4cm}{Full window}$	$\parbox{11cm}{Full field of view on all seven chips; low time resolution (ca. 3 s).}$
$\parbox{4cm}{Partial window}$	$\parbox{11cm}{Only part of the central chip is read out as a 2-D image; several partial window sizes are possible.}$
$\parbox{4cm}{Timing}$	$\parbox{11cm}{The central chip is not read out in a 2-D mode, but ... ...1-D row over a user-defined pixel area for fast photometry (down to 1.5 ms).}$
$\parbox{4cm}{{\bf \htmladdnormallink{EPIC}{http://astro.estec.esa.nl/XMM/user/build/buildxmm_top1.html\char93 epic}\ pn Mode}}$	$\parbox{11cm}{{\bf Description}}$
$\parbox{4cm}{Full window}$	$\parbox{11cm}{Full field of view on all chips; relatively low time resolution (70 to 250 ms).}$
$\parbox{4cm}{Partial window}$	$\parbox{11cm}{2-D imaging readout over part of the array with increased time resolution. Several sizes of partial windows are allowed.}$
$\parbox{4cm}{Timing}$	$\parbox{11cm}{The information from one chip is collapsed into a 1-D row before readout. Max. time resolution of 30 $\mu$ s.}$
$\parbox{4cm}{Burst}$	$\parbox{11cm}{A special flavour of the timing mode, with extremely... ...n to 7 $\mu$ s), but also a very low duty cycle with only ca. 3\% live time.}$

 

Table 4: Basic numbers for the science modes of EPIC

$\parbox{6cm}{{\bf MOS (central CCD; pixels)}}$	$\parbox{2cm}{{\bf Time resolution}}$	$\parbox{2.5cm}{{\bf Max. count rate$^1$\space diffuse (total) [s$^{-1}$ ]}}$	$\parbox{3.5cm}{{\bf Max. count rate$^1$ (flux) point source [s$^{-1}$ ] ([mCrab])}}$
Full window (600$\times$600)	2.5 s	700	0.70 (0.24)
Full window 2 nodes(600$\times$600)	1.4 s	1250	1.30 (0.45)
Partial window (100$\times$100)2	0.4 s	125	5 (1.7)
Partial window (300$\times$300)	0.9 s	500	1.8 (0.6)
Partial window (600$\times$600)	0.2 s	9000	10 (3.5)
Refreshed frame store4	1 ms - 2.8 s
Partial Window (100$\times$100)3	0.2 s	250	10 (3.5)
Partial Window (300$\times$300)3	0.7 s	600	2.5 (0.9)
Timing	1.5 ms	N/A	100 (35)
$\parbox{6cm}{{\bf pn (array or 1 CCD; pixels)}}$	$\parbox{2cm}{{\bf Time resolution}}$	$\parbox{2.5cm}{{\bf Max. count rate diffuse (total) [s$^{-1}$ ]}}$	$\parbox{3.5cm}{{\bf Max. count rate (flux) point source [s$^{-1}$ ] ([mCrab])}}$
Full window (400$\times$384)	73.4 ms	1000(total)	8 (0.9)
Full window extended5	0.28 s	300	2 (0.26)
Large partial window (200$\times$384)	48 ms	1500	20 (2.1)
Small partial window (64$\times$64)	6 ms	1500	130 (14)
Timing	0.03 ms	N/A	1500 (200)
Burst	7 $\mu$s	N/A	60000 (6300)

Notes to Table 4:
1) ``Maximum'' to avoid deteriorated response due to photon pile-up, see § 3.3.10. Note that telemetry limitations are in some cases more stringent than the pile-up constraints. For the MOS cameras the maximum count rates are about 150 counts/s for full window imaging and 300 counts/s for partial window imaging and timing mode. The pn telemetry limit is approximately 1150 counts/s in all modes.
2) 1 node readout.
3) Free running readout.
4) Variable live time for full field imaging of bright extended sources. Usage of this mode in AO-1 is discouraged.
5) ``Extended'' means that the image collection time is extended by a programmable parameter (not user-selectable).

1.

``full window'',

In this mode, all pixels of all CCDs are read out and thus the full FOV is covered.

2.

``partial window''

In a partial window mode one of the CCDs of both types of EPIC cameras can be operated in a different mode of science data acquisition, reading out only part of the CCD chip.

3.

``timing''

In the timing mode no two-dimensional imaging is performed, but data from a predefined area on one CCD chip are collapsed into a one-dimensional row to be read out at high speed.

4.

``burst'' (pn only)

A special flavour of the timing mode of the EPIC pn camera, with very high time resolution, but very low duty cycle.

The most important characteristics of the EPIC science modes and the allowed choice of parameters for astronomical observations are tabulated in Table 3. Table 4, especially for the MOS camera, provides specimen performance data concerning time resolution and count rate capability for selected modes. The actual time resolution performance may change in detail by launch, but as count rate guidelines will scale inversely with time resolution and are not expected to change greatly, the GO is advised to be aware that minor modifications may be announced in the future. The count rate limitations are defined for a 1% pile-up case (see § 3.3.10 for details on pile-up), which occurs at about 2 photons per MOS CCD frame, and 0.5 photons per pn CCD frame. Early estimates of spectral fitting errors without any response matrix corrections show that a doubling of these count rates could lead to systematic errors greater than the nominal calibration accuracies. The SOC Team intends to develop tools to alleviate this pile-up effect, but GOs are advised that this will be a long-term effort.

One of the major differences between the two types of cameras is the high time resolution of the pn chip array. With this camera high-speed photometry of rapidly variable targets can be conducted, down to a minimum integration time of 30 (7) $\mu$s in the timing (burst) mode.

The XRPS forms currently have non-descriptive ``Partial Window n'' choices for the EPIC MOS. This is because at the time of preparation of the XRPS the window modes were not yet well-defined. At the time of writing it is suggested to use the following mapping (but again there might be minor changes in the future).

XRPS Name	Descriptive Name	Window Size (active pixels)	Time Resolution (s)
PRI FULL MOS	FULL WINDOW (1 node)	600$\times$600	2.5
PRI PART RFS	REFRESHED FRAME STORE	600$\times$600	selectable 1 ms - 2.8 s
PRI PART W2	PARTIAL WINDOW (1 node)	100$\times$100	0.4
PRI PART W3	PARTIAL WINDOW (1 node)	300$\times$300	0.9
PRI PART W4	PARTIAL WINDOW (free running)	100$\times$100	0.2
PRI PART W5	PARTIAL WINDOW (free running)	300$\times$300	0.7
PRI PART W6	FULL WINDOW (2 nodes)	600$\times$600	1.4
FAST UNCOMP	TIMING	100 wide	1.5 ms

Explanations:

Refreshed frame store (RFS):
This mode is designed for the case where the user requires to collect data from the whole of the inner CCD (600$\times$600), but knows that the photon flux rate is so high as to lead to significant pile-up. Then the refreshed frame store mode can be invoked, in which the frame integration time is defined, but the data which collects during the remainder of the normal readout of 2.8 seconds must be discarded. Thus, a frame collection of as rapid as 1 ms could be utilised, but in this case the live time is only 0.04%. Only one RFS value will be available, which must be determined in-orbit and is therefore not known yet. There are very few extended objects that can be considered to require this facility, rather than a normal windowed selection, and therefore it is not recommended for normal GO usage.

Window mode with ``free run'':
The SOC instrument team has identified windowing modes with either timed and synchronised readouts, or a free-running readout. The advantage of faster readout with free-running mode should be traded-off against a possible problem of clock interference between readouts of different CCDs.