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. Thus all CCDs are gathering data at all times, independent of the choice of operating mode. The pn camera CCDs can be operated in common modes in all quadrants for full frame, extended full frame and large window mode, or just with one single CCD (CCD number 4 in Fig. 22) for small window, timing and burst mode.
In this mode, all pixels of all CCDs are read out and thus the full FOV is covered.
In a partial window mode the central CCD of both MOS cameras can be operated in a different mode of science data acquisition, reading out only part of the CCD chip.
In large window mode only half of the area in all 12 CCDs is read out, whereas in small window mode only a part of CCD number 4 is used to collect data.
a) MOS + pn
In the timing mode, imaging is made only in one dimension, along the column (RAWX) axis. Along the row direction (RAWY axis), data from a predefined area on one CCD chip are collapsed into a one-dimensional row to be read out at high speed. Since the 2 MOS cameras orientation differ by 90 degrees, the ``imaging'' directions in the 2 MOS are perpendicular to each other.
b) pn only
A special flavour of the timing mode of the EPIC pn camera is the ``burst'' mode, which offers very high time resolution, but has a very low duty cycle of 3%.
The most important characteristics of the EPIC science modes (time resolution and count rate capability) are tabulated in Table 3. Fig. 23 and Fig. 24 show the active CCD areas for the different pn and MOS readout modes, respectively.
|MOS (central CCD; pixels) [1 pixel = 1.1"]||Time resolution||Live time [%]||Max. count rate diffuse (total) [s]||Max. count rate (flux) point source [s] ([mCrab])|
|Full frame (600600)||2.6 s||100.0||150||0.70 (0.24)|
|Large window (300300)||0.9 s||99.5||110||1.8 (0.6)|
|Small window (100100)||0.3 s||97.5||37||5 (1.7)|
|Timing uncompressed (100600)||1.75 ms||100.0||N/A||100 (35)|
|pn (array or 1 CCD; pixels) [1 pixel = 4.1"]||Time resolution||Live time [%]||Max. count rate diffuse (total) [s]||Max. count rate (flux) point source [s] ([mCrab])|
|Full frame (376384)||73.4 ms||99.9||1000(total)||6 (0.7)|
|Extended full frame (376384)||199.1 ms||100.0||370||1 (0.12)|
|Large window (198384)||47.7 ms||94.9||1500||6 (0.7)|
|Small window (6364)||5.7 ms||71.0||12000||50 (6.5)|
|Timing (64200)||0.03 ms||99.5||N/A||800 (85)|
|Burst (64180)||7 s||3.0||N/A||60000 (6300)|
The count rate limitations are defined for a 1% pile-up case (see § 3.3.9 for details on pile-up), which occurs at about 2 photons per MOS CCD frame, and 0.5 photons per pn CCD frame in case of an on-axis source. 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 Pile-up can be alleviated by excising the PSF core at the penalty of losing overall flux, but retaining spectral fitting integrity.
For sources with very soft spectra, a factor of 2-3 lower maximum incident flux limits are recommended, while maximum count rates remain unchanged, see § 3.3.9. For the pn camera also for point sources with hard spectra (power law photon index ) lower count rate limits should be applied, in order to avoid X-ray loading (see XMM-SOC-CAL-TN-0050, available from http://xmm2.esac.esa.int/docs/documents/). For =1.5 and 1.0 the maximum count rate limits given in Table 3 should be reduced by factors 2 and 4, respectively.
One of the major differences between the two types of camera is the high time resolution of the pn. With this camera high-speed photometry of rapidly variable targets can be conducted, down to a minimum integration time of 30 (7) s in the timing (burst) mode.
The SAS task epatplot allows users to have a qualitative estimate of the level of pile-up affecting an input event list by comparing the observed and expected distributions of event PATTERNs. Users are referred to the description of this task in the SAS documentation (see also § 3.3.9 for details on pile-up).
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 115 counts/s for full window and partial window imaging modes and 230 counts/s for timing mode. The pn telemetry limit is approx. 600 counts/s for the imaging modes and approx. 450 counts/s for the timing mode. If the rate is higher, then the so-called counting mode is triggered and for some time the science data are lost. See also § 4.3.1 on the EPIC bright source avoidance.