XMM-Newton Phase II RPS Users Manual

5.2.4.5 Filling in OM exposures settings

The OM offers imaging and fast modes which can be operated in parallel. An image will always have to be defined (by means of its central coordinates and size) while an additional fast mode is optional.

The image mode only produces brightness information for each pixel, from which a count rate/flux/magnitude for each source in the field can be derived. These values are averaged over the total exposure time. Higher time resolution can be obtained for one or two point sources in the field using the additional fast mode. The higher time resolution is achieved by utilising an event counting mode. This mode accumulates more data and is therefore only available for a small region of the detector, corresponding to $\sim 10\times 10$ arcsec. The additional fast mode is therefore only feasible for point sources with well-known coordinates.

As described in more detail below, both readout modes can be used simultaneously for the same positions on the chip, thus an image and a light curve can be obtained at the same time.

An observation usually includes several OM exposures with different filters, or with the same filter. Users must choose one of the OM optical elements , the exposure time for that optical element, exposure offset (normally 0s) and one of the available OM modes . Consult the XMM-Newton Users Handbook for a detailed description of the instrument and the available observing modes. Also take a look at § 3.2 for a description on how to prepare a sequence of different exposures. A screenshot of an OM exposure form sheet can be seen in Fig. 17.

Figure 17: Screen shot of an OM exposure form sheet for an exposure with the U filter (filter wheel position 3), in Science User Defined mode, with 4400s exposure time. Read-out windows can be defined by clicking the button "Edit Details"; see § 5.2.4.6 for more details. In this case, an image and a fast mode are defined, so the exposure time can not be defined longer than 4400s.

''Filter Wheel''

The user can choose one of the OM optical elements listed in Table 5. For maximum scientific yield, the following three filters are recommended:

1. UVM2 (first choice)
2. UVW1 (second choice)
3. U (third choice)

Since the sequence of exposures using different optical elements must always have increasing filter wheel position numbers¹, the three filters mentioned before have to be observed in the following order: U, UVW1 and UVM2.

Table 5: OM optical elements

Filter wheel position number$^1$	Filter name
0	BLOCKED
1	V
3	U
4	B
5	WHITE
6	VISIBLE GRISM
7	UVW1
8	UVM2
9	UVW2
10	UV GRISM

Notes to Table 5:
1) Exposures using different optical elements must always have increasing filter wheel position numbers.

''Exposure Time (secs)''

Due to internal memory capacity and telemetry bandwidth restrictions, OM science exposures have a limited range of exposure times. The allowed input ranges are listed in Table 6.

Table 6: OM exposure time constraints

Science window configuration	Minimum (s)	Maximum (s)
Science User Def.: Up to 5 Image windows$^{1,2}$	1000	5000
Science User Def.: 1 Image + 1 Fast windows$^3$	1200	4400
Science User Def.: 1 Image + 2 Fast windows$^3$	1200	2200
Science User Def.: 1 to 4 Image + 1 Fast windows$^3$	1200	4400
Science User Def.: 1 to 3 Image + 2 Fast windows$^3$	1200	2200
Full Frame Low Resolution$^1$	1000	5000
Full Frame High Resolution$^4$	1000	5000
EPIC/RGS$^{\rm *}$ Image Mode$^1$	1000	5000

Notes to Table 6:
1) Minimum allowed exposure time is 800 seconds, but multiple observations of 800 seconds duration are strongly discouraged
2) The total number of allowed Science User Defined windows (IMAGE and/or FAST) is 5, with a maximum of 2 of them being FAST windows
3) When defining Science User Def., exposure times within the limits for Image mode are allowed. Additional fast modes can only be added if the exposure time is within the fast mode limits.
4) This mode is not recommended for use because of very large overhead and risk of data loss during downlink.
(*) "EPIC/RGS Image Mode" contains a default combination of 5 exposures in predefined windows.

''Exposure Offset (secs)''

The exposure offset should be left at its default value of 0 sec to ensure continuous operation of the instrument. Only under very special circumstances may one want the instrument not to operate for a certain amount of time. In that case an offset with respect to the previous instrument exposure has to be defined here. The valid range is between 0 and 1000 sec.

''Instrument Mode''

The instrument mode is chosen depending on the target characteristics (size, variability, etc.) and on the available total observation time. Table 7 shows an overview of possible OM modes. It should be noted that a FAST mode window in addition to an image mode window does not affect the integrity of the image.

Table 7: OM mode names

Mode Name	Prime Instrument
Science User Defined	Any
Full Frame Low Resolution	Any
Full Frame High Resolution$^1$	Any
EPIC Image	EPIC
RGS Image	RGS
GO OFF$^2$	Any

Note to Table 7:
1) This mode is not recommended for use because of very large overhead and risk of data loss during downlink.
2) This mode is related to Mosaic observations and should not be set for any other observation. Any change to exposures already created for this mode must first be agreed with the SOC.

All the possibilities are described in detail in the XMM-Newton Users Handbook, where also some examples are given in Sect. 3.5.9.6 .

The user can choose between a user-defined setting and predefined defaults. The user-defined mode is strongly encouraged over the default modes and requires only one extra step to define the central coordinates and size of the read out windows. This is accessed via the "Add Details" button, and described in more detail in the next section § 5.2.4.6.

The default EPIC/RGS Image mode consists of five exposures in which two science windows are read out. A central 2' x 2' window with the boresight coordinates in the centre has a higher spatial resolution (0.5 x 0.5 arcsec). An additional lateral window is read out at 1 x 1 arcsec resolution. In all five exposures, the central window will always be around the target while the lateral windows will ultimately cover the entire chip. This is illustrated in Sect. 3.5.9.2 of the XMM-Newton Users Handbook.

Two full frame configurations are available, where the high-resolution mode is not recommended because of excessively large overhead and risk of data loss during downlink.

Important considerations for the choice of mode are, e.g., the source extent and expected variability of the source. If the target is not variable it could be of interest to obtain spectral information performing exposures in different filters. Variability on time scales shorter than 800 seconds, down to 0.5 seconds, can be followed with the FAST mode centred on the source. The FAST mode is offered by selecting the Science User Defined mode. In this mode, two FAST windows can be selected in addition to at least one IMAGE window. Data from IMAGE and FAST windows are acquired simultaneously without extra time consumption or telemetry expenses, and the source will be in the IMAGE and in the FAST window.

An important consideration is the required overhead times before each exposure. A smaller number of longer exposures requires less overhead than a large number of short exposures. A series of short exposures will not deliver a continuous stream of data because of the presence of overhead between exposures.

When selecting to observe with a Grism in Science User Defined Mode the IMAGE window is automatically defined for an optimal extraction of the spectrum of a target.

If V Grism or UV Grism observation of several targets in a field is desired, the "Full Frame Low Resolution" Mode has to be selected. Note: if the field is crowded it may not be possible to separate the spectra of the different objects.

XRPS will read the choice of prime instrument made by the user and will offer in the OM modes pull-down menu only those that are compatible with that prime instrument. Thus, the user will see only part of the configurations/modes from Table 7 listed in the pull-down menu on the screen. When reconsidering the choice of prime instrument AFTER having specified OM exposure details, one must go back and change these entries in compliance with the newly defined prime instrument. Otherwise the exposures are invalid (XRPS will flag an error condition). The use of non compatible filters and modes would give an error message, for instance V GRISM and UV GRISM filters are not compatible with "EPIC/RGS Image" modes.

''Order (ins. based)''

In this field you can define the order of multiple exposures using the same instrument and filter within an observation. This number is calculated automatically if left blank.

Note: If the user does not include OM exposures in the observation, then the SOC will consider one of the following approaches: i) to use the available time for OM calibrations, ii) to include OM exposures suited for the particular target, iii) to introduce OM exposures in filters ''UVM2'', ''UVW1'' and ''U'' in ''Full Frame Low Resolution'' mode.