Q: Why do most targets have two Obs(ervation)ID's?
A: A standard NuSTAR observation of a celestial source requires two
slews and so results in two ObservationID's (obsIDs).
The first ObsID starts with the slew from the previous target to the
new target. This slew is performed in STELLAR ACS mode and can take
up to an hour for the observatory to reach the new target. However
this mode is unsuitable for science observations as it will include
a roll maneuver of about 1 deg/day to maintain the solar array oriented
to the Sun. So a second, usually short, slew maneuver is performed in
INERTIAL ACS mode which freezes the observatory attitude, pointing the
observatory at the celestial target for extended periods. The length of an
observation is limited by how long the orientation of the solar panels in
INERTIAL mode can remain within operational limits, usually about 1 week.
To maximize efficiency the STELLAR slew is timed to arrive at the new
target when it is occulted by the Earth. The following INERTIAL attitude
slew maneuver is timed to complete within the same occultation period.
An additional period of observatory settling is also allowed to complete
before the celestial target exits Earth occultation and the science
observation can begin. As an example, here are the entries in the as flown
timeline around the observation of Mkn 421 on 2013 January 2nd:
observationID Name Start End ACS
60061256002 NGC5728 2013-01-02 04:20:05 2013-01-02 18:10:00 I
60002023001 Mkn421 2013-01-02 18:34:29 2013-01-02 18:40:00 S
60002023002 Mkn421 2013-01-02 18:40:02 2013-01-02 23:00:00 I
60021009001 COSMOS_MOS009 2013-01-02 23:16:40 2013-01-02 23:25:00 S
(ACS I = INERTIAL mode, S = STELLAR mode)
The slew in STELLAR mode from NGC 5728 to Mkn 421 began at 18:10:00
and completed at 18:34:29. The slew in INERTIAL mode on Mkn 421 began
at 18:40:00 and completed at 18:40:02. The observatory in held in
INERTIAL mode from 18:40:00 to 23:00:00. The position of Mkn 421 was
occulted by the Earth from 18:22:24 until 19:00:44 and so the observatory
will have completed all slews and attitude settling by the time the
target emerged from behind the Earth. The observation ended when the
observatory slew to the next target (COSMOS_MOS009) at 23:00:00.
Observations of the same target at later dates will continue the visit
numbering scheme. For example the next observation of Mkn 421 was on
2013 January 10th and contained obsID's 60002023003 & 60002023004.
Note that some observations may have an additional obsID if it was
deemed necessary to refine the observatory pointing. The small maneuver
is usually performed within the first 10 orbits of an observation.
Some targets may only have a single obsID associated with the observation
if they are part of a survey program. These programs are usually planned
as a mosaic of positions where the distance between the tiles in the
mosaic are less than 10 degrees. No STELLAR mode slew is required to
set the orientation of the solar array when observing these tiles
Q: How are the NuSTAR file names constructed?
A: The filename format for the NuSTAR science files uses the following
convention: "nuObservationID[M][xx]_[ll].ss" where:
'nu' is the prefix indicating the mission name (NuSTAR);
'ObservationID' is an 11-digit number identifying the observation (ObsID);
'M' is a one-character string that identifies the Focal Plane Module (A or B);
'xx' is a code to identify the observing mode as defined below:
01 (SCIENCE): normal observing scientific mode;
02 (OCCULTATION): Earth in the field of view;
03 (SLEW): data taken during a spacecraft slew;
04 (SAA): South Atlantic Anomaly passages;
05 (CALIBRATION): on-board calibration radioactive source in the field of
06 (SCIENCE_SC): attitude reconstruction from the spacecraft bus star
'll' indicates for event files the processing level ('uf' for Level 1 and
Level 1a files, 'cl' for Level 2 files). For other data files it describes
their content (e.g. 'met' for raw metrology data, 'mast' for mast aspect
solution file, 'ex' for exposure maps);
'ss' is the file extension and indicates the data type (e.g. 'evt' for event
files, 'img' for sky images, 'hk' for housekeeping files, 'lc' for
light-curves, 'pha' for energy spectra).
The quantities in square brackets may not always be present, for example,
(i) 'M' (A or B) is not used for data files from the metrology laser system
(ii) the observing mode code 'xx' is not used for Level 1/1a event files since
the data splitting is carried out during Stage 2 of NuSTAR data processing
(see Chapter 4 of the NuSTAR Data Analysis Software
Users Guide for more details).