If it is important to ensure that any particular part of the extended
object does not fall on a CCD gap, users must choose the pointing
direction appropriately (see UHB sections on EPIC pn chip array
and
EPIC MOS chip array
).
For each of MOS and pn, it must be decided if any part of the source is
bright enough to give rise to local photon pile-up that would degrade
the calibration beyond the science goals when using the full frame
imaging mode (see UHB section on EPIC pile-up
). If not, the full
frame mode should be used.
RGS data of faint extended sources are compromised by low total count
numbers and a reduced energy resolution. It should be left on in the
default SPECTROSCOPY mode for serendipitous background information
(see UHB section on RGS modes
).
For a faint extended source the OM should be best operated in its
imaging mode. It must be decided if a specific field of view coverage
is needed or if the default OM sequence of 5 exposures per filter is
adequate. If yes, a default configuration should be chosen (see
§ 3.3 and also the UHB sections on OM modes
and
OM default configurations
).
If the observation is long compared with the visibility window (as
reported by the online XMM Target Visibility Tool), the user must consider how best to
split it up, e.g., into multiple observations that fit into continuous
visibility periods. Other criteria for the split of an observation
could be optimal filling of gaps between CCDs, or a calculation to
fit in a certain sequence of OM filters
observed with one of the
OM default configurations
(i.e., with exposure multiples of five per filter).
Users must check the visible magnitude of in-field or nearby optical
targets. If the soft X-ray response is important, one should choose
the thinnest filter compatible with the brightest visible objects, as
described in the UHB section on EPIC filters
.
For standard SPECTROSCOPY mode observations with RGS no inputs are
necessary to choose a non-default CCD readout sequence (see UHB section on RGS modes
).
For the OM it must be decided if specific filter coverage is necessary
for the science or if the recommended filter sequence is adequate (see
UHB section on OM default configurations
and OM optical elements
).
While no limits are expected on the length of exposures for the X-ray instruments while observing faint sources (except for visibility constraints and the length of the observation), OM exposures are further constrained by telemetry and memory capacity limits (§ 5.3.3.5).