XMM-Newton Science Analysis System: User Guide

6.1.1 OM observing modes and data types

The OM can be operated in two basic science modes: image and fast mode (see the OM dedicated section of the UHB for further details). These modes can be used separately or combined, in different instrument configurations:

• Default modes: Image_only and Image$+$Fast. This is a set of 5 successive exposures with predefined windows configurations covering nearly 92% of the field with/without a Fast mode window centered at the boresight location.

• Science User Defined Image and/or Fast mode windows: the user can define in pixels or in (RA, Dec) up to 5 windows, from which up to 2 can be defined in Fast mode

• In addition, the OM detector can be operated in Full Frame mode to obtain images of the entire FOV, either at high resolution (0.5 arcsec/pix, $2048 \times 2048$ format) or at low resolution (1 arcsec/pix, $1024 \times 1024$ format).

In the windowed modes, the detector windows are re-centered to correct for pointing errors (Field Acquisition, FAQ). Also a shift_and_add mechanism is used to compensate for spacecraft tracking stability. Full frame modes do not have these capabilities.

The observing modes determine the type of data obtained with the OM: 2-D images for image modes and events lists if fast mode is used.

The default modes mentioned above will produce the following data types: since each exposure has two image windows associated with it, there will be two image files per exposure which correspond to a small high resolution window at the centre of the FOV and a large low resolution one placed at different locations in each exposure. If the default mode was Image$+$Fast, there will be an additional event list file corresponding to the Fast window.

In the Science User Defined windows configuration there will be one image file or event list corresponding to each of the windows.

The Full Frame images consist of a single image file if high resolution was used and four adjacent image files in case of low resolution. (It should be noted that the four image files correspond to a unique exposure).

In addition to these image and/or event list files, for each exposure, there will be several auxiliary files containing instrument configuration parameters. There will also be files containing OM house-keeping data for the whole observation. All these files together with the spacecraft attitude and time correlation files, common for all instruments, constitute the ODF for OM data.

Table 10 contains a summary of the possible OM files contained in an ODF (for one exposure).

Table 10: ODF data file identifiers associated with a single OM exposure.

Data Identifier	Contents
00IMI	Image window 1
01IMI	Image window 2
FAE	Event list (if fast mode was used)
THX	Tracking history
WDX	Priority window data
PFX	Priority fast mode data
RFX	Priority reference frame data
PAX	Field acquisition data
PEH	Periodic house-keeping
NPH	Non-periodic house-keeping
E4I	Full frame high resolution image

OM Images files have a file name ending in "IMI.FIT". There is one image file per image mode window per exposure. Raw image files can be displayed with e.g. the ds9 image viewer. It is also possible to combine all processed image windows of an OM default configuration before viewing them using the SAS task ommosaic.

OM full frame high resolution images have a name ending in "E4I". Full frame low resolution follow the general nnIMI rule, with nn=00,01,02,03 (four files per exposure).

OM Fast mode event lists have a file name ending in "FAE.FIT". There is one dataset for each fast window. It is possible to inspect the data files using e.g. the ftool viewer fv.

OM reference frame data have a file name ending in "RFX.FIT". Generally one file is present for each exposure, except if tracking was switched off. This is the case for certain OM engineering modes and certain filter elements (e.g. grism).

OM tracking history file data have a file name ending in "THX.FIT". Generally one file is present for each exposure, except when tracking was explicitly switched off, or when no suitable stars for tracking were found in the reference frame at the beginning of the exposure. It is possible to inspect these files with e.g. the ftool viewer fv. A plot of column DX and DY shows the attitude stability during the exposure. The PPS product file whose name ends in "TSHPLT.PDF" provides a visualisation of the pointing stability during the exposure in PDF format.

OM field acquisition data have a file name ending in "PAX.FIT". There is only one field acquisition dataset present per observation. In order to get a feeling on the absolute pointing accuracy, the user can look at this field acquisition dataset (PAX.FIT). The parameters DX, DY provide the absolute pointing error in units of 0.001 subpixel.

OM priority window data have a file name ending in "WDX.FIT". One priority window dataset is present for each exposure. This file is used, e.g. to compute the exposure dead-time fraction.

OM priority fast mode data have a file name ending in "PFX.FIT". There is one dataset for each exposure containing fast mode window(s).