OGIP Memo OGIP/94-001
Specification of Detector Operating & Observing Modes
and Observation ID in
OGIP FITS files
Code 668,
NASA/GSFC,
Greenbelt,
MD 20771
Version: 1995 May 09
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We list the strings used for the specification of the Detector Operating &
Observing modes of the various instruments within the OGIP data holding.
The definition of the 'observation ID' is also given.
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| Release |
Sections Changed |
Brief Notes |
| Date |
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| 1994 May 11 |
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First (internal) Draft |
| 1994 Nov 30 |
All |
Made compatible with LaTeX2HTML software |
| 1995 Mar 10 |
All |
Revised & Updated |
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In order to facilitate the use of FITS datasets by downstream s/w and
users, the OGIP is adopting a set of standard keywords containing mission
dependent character strings to specify the detector operating,
the detector observing mode and observation identification, used
during the collection of a given dataset.
This memo lists those currently approved for use and should be
strictly adhered to within the OGIP.
Any comments or suggestions (especially if you have requirements not
covered by this memo) should be e-mailed to the
authors (LHEAVX::ANGELINI and/or LHEAVX::GEORGE).
Sequence numbers are commonly
used by missions to identify 'uniquely' a particular
dataset. The definition of the sequence number is not unique across mission
and typically depend upon the different way a dataset is identified
within that mission. Although sequence number suggests a numerical
value only, past mission had given the sequence number as a mixture
of numerical and character values.
To avoid a proliferation of keywords to store the sequence number value,
the OGIP recommend to use OBS_ID. The keywords value is a string,
to allow back compatability with the already archived mission.
The OBS_ID keyword value can be used by database software.
For many instruments, observations could/can be carried out under a
number of different observing modes. Within a FITS file, data from
different observing modes are usually stored in different extensions.
The OGIP has therefore adopted the FITS keyword OBS_MODE to denote
the observing mode of the data within a given extension.
The standard (multi-mission) values of this keyword are described in
Section 3.1, whilst instruments which have
adopted a different standard are individually described in
Section 3.2.
Standard Values of the OBS_MODE keyword
The standard, multi-mission character string values of
the OBS_MODE keyword are as follows:
OBS_MODE='POINTING'
for data when a three-axes stabilized satellite is pointing in a
particular direction of the sky, and a 'mean' pointing position
can be given.
OBS_MODE='RASTER'
for data collected by a three-axes stabilized satellite which is
making a 'raster scan' over a region of the sky.
OBS_MODE='SLEW'
for data obtained whilst a (typically three-axes stabilized)
is moving between one observation and another.
OBS_MODE='SCAN'
for data collected by a scanning (spinning) satellite.
It should be noted that in the case of OBS_MODE='SLEW' and 'SCAN'
observations, a mean pointing position can not be given. In this respect
these two types of observing mode are very similar. However a SLEW mode is
usually reserved for satellites which are capable of a stable pointing
(i.e. capable of OBS_MODE='POINTING') whilst the satellite is moving
between two different pointing positions.
Instrument-specific specification of Observing Modes
For various reasons, the observing mode under which a given dataset
were collected for the following instruments are either not described
using the OBS_MODE keyword, or the allowed values of
OBS_MODE do not conform to the list given in
Section 3.1.
ASCA mission was planning to use the keyword OBS_MODE to store
value related to the on-board computer set-up or to identify manoeuvre
used mainly in the PV phase.
The possible values of the OBS_MODE keyword are:
OBS_MODE='OBSERVATION'
Observation mode
OBS_MODE='LAUNCH'
Launch Mode
OBS_MODE='ASC'
Attitude Control System mode
OBS_MODE='STM'
Star Tracker Memory mode
OBS_MODE='TCU'
Telemetry Command Unit mode
OBS_MODE='BFM'
Bubble File Memory mode
OBS_MODE='GISM'
GIS Memory mode
OBS_MODE='DPM'
DP Memory mode
OBS_MODE='INITIALIZE'
Initialize mode
OBS_MODE='ELD'
Error Log Dump mode
OBS_MODE='UTILITY'
Utility mode
OBS_MODE='UNKNOWN'
any value of obsmode in telemetry that isn't one of the above
Early on in the mission files were produced with the OBS_MODE
keyword set to OBSERVATION. The string value was changed to the value
proposed in this memo, namely POINTING, starting with the
processing software frfread version 3.01.
Note that file created with frfread
software older than that contain
instead the string OBSERVATION.
Many missions use/used an on-board computer to pre-process the data, before
transmitting it to the ground. A typical pre-processor can/could select or
accumulate different data-streams using one or all of the following
basic criteria:
- detector identifier (for composite instruments, e.g. EXOSAT ME,
ASCA SISs)
- time resolution
- energy resolution
- detector specific parameters
Generally other criteria or parameters can be used in the mode set-up,
which are usually intimately related to the specific of the instrument.
Within a FITS file, the data-streams arising from particular pre-processor
datamodes are usually stored in different extensions, but with similar
layouts or FITS structures.
Within the OGIP, the FITS keyword DATAMODE is used to store a character
string which specifies the type of data-stream information contained within
a given FITS extension.
The content of the keyword is by necessity mission specific and
the string value are given below for each mission/instrument where such
specification is required.
The DATAMODE keyword is used in the Ariel-V light curves obtained
using the ASM instrument in order to distinguish the part of the sky
viewed by the 512 individual elements of the ASM detector.
DATAMODE='ALL SKY'
- 512 detector elements observed the whole sky
DATAMODE='OCTANT'
- 512 detector elements observed only 
of the whole sky
The ASCA Observatory payload consists of four instruments two Gas Imaging
Spectrometer (GIS) and two Solid-state Imaging Spectrometer (SIS).
Depending on the type of instrument, SIS or GIS, the pre-processing on
board is handle by different modes, which imply that different criteria
are used across the two instruments. The string used in the
DATAMODE keyword in the ASCA file contain only part of the
mode definition used by the pre-processing on board computer.
An attempt to construct a more complete string for the DATAMODE
keywords which includes all the relevant parameters associated with
a particular mode has been abandoned to avoid the risk of
breaking pre-existing software which
expects a particular value for that keyword.
Listed below are the strings for the DATAMODE keyword as used
in the file with a description. It is also given a list of the other
relevant keywords which all together define a mode.
Four different modes can be used with the GIS : pulse Height (PH), multi-
channel pulse count (MPC), position calibration (PCAL), and memory-check
mode. Here are the documented the string for only two of them PH and MPC.
PH mode The on-board computer calculates event position and
discards background events using rise-time and position information.
Pulse high spectra of up 1024 channels for each detector can be obtained.
31 bits are used to describe the PHA value, X and Y position, rise time,
spread and timing.
The DATAMODE string value is
DATAMODE='PH '
To uniquely define the mode the FITS file also contains the following
keywords. The value range for the different quantity stored below
are given as number bits. The value stored in the keywords is the power
of 2 of the number of bits.
PHA_BINS number of channels in the energy spectra.
The bits value allow are 10, 8
RAWXBIN and RAWYBIN number of pixels for
image. The bits value allow for positional information
are 2, 4, 6, 8
RISEBINS gives the rise time setting. The bits value
allow for the rise time are 0, 5, 6, 8.
SP_BINS gives the spread.The bits value
allow for the spread are 0, 8.
TIMEBIN gives the time resolution of the data.
This value depends also on the BIT_RATE.The bits value allow
are from 0 to 10. If 0 the time resolution is 62.5ms for
HIGH
bit rate, 500 for MEDIUM bit rate, 2 s for LOW bit rate.
If 10 the time resolution is 61  s for HIGH bit rate,
288 s for MEDIUM bit rate, and 1.95 ms for LOW
bit rate.
BIT_RATE gives the string value of the the telemetry rate.
The possible string values are HIGH, MEDIUM or
LOW.
- MPC
The
MPC mode allows only to record pulse-height and temporal information+
The DATAMODE string value is
DATAMODE='MPC '
To uniquely define the mode the FITS file also contains the following
keywords. The value range for the different quantity stored below
are given as number bits. The value stored in the keywords is the power
of 2 of the number of bits.
PHA_BINS number of channels in the energy spectra.
The number of channel is fixed at 256.
TIMEBINS is the time resolution - values allow (0-8).
If 0 the time resolution is 0.5s for HIGH bit rate, 4s for
MEDIUM bit rate, 16 s for LOW bit rate.
If 8 the time resolution
is 1.95 ms for HIGH bit rate, 15.6 ms for MEDIUM
bit rate,
and 62.5 ms for LOW bit rate.
BIT_RATE gives the string value of the the telemetry rate.
The possible string values are
HIGH, MEDIUM or LOW.
For the SIS, two (concurrent) modes are used to describe how the
data are accumulated: the clocking mode and data mode.
These, together with the telemetry rate parameter (LOW, MEDIUM, HIGH),
defines the time resolution of
the data, the number of detector channels per spectrum and the spatial
resolution/distribution.
Each SIS camera has four CCDs, and the time resolution or clocking mode,
is determined by the ordering of the imaging/read-out cycles of the CCDs.
The allow values are 4-CCD, 2-CCD, 1-CCD,
Parallel mode. In 4-CCD all 4-CCD are exposed at the same time.
and the time resolution in high bit rate is 16 seconds.
In 2-CCD only 2 CCDs are exposed at the same time, but the field of
view is half that obtained in 4-CCD mode, and the time resolution is 8
seconds.
In 1-CCD only 1 CCD is exposed, the field of view is a quarter
of that obtained in 4-CCD mode, and the time resolution is 4 seconds.
In Parallel mode only 1 CCD is exposed, but all spatial information
is lost. The time resolution is 15,625 ms and is used in the FAST mode
data mode.
The charge cloud produced by an X-ray photon is not localized to one pixel
but spread out over several pixels. The data mode describes
the different way of transmitting the 'spread' to the ground.
The allow values are FAINT, BRIGHT, and FAST.
The keyword DATAMODE only contains the data mode.
FAINT The SIS spectra have 4096 channels. The pixel
position at the center of the event is given together with
the pulse height recorded in the nine pixels around it.
BRIGHT The SIS spectra have 2048 channels.The channel
compression is obtained. compared with the faint mode,
binning by a factor of two channels 1024-2047 and by a
factor of four channels 2048-4095.
FAST The SIS spectra have 2048 binned as in the bright
mode.This mode, which makes used the parallel sum mode (see
below), sacrifices the positional information for time
resolution. timing information is available with a precision
of 16ms
The original datamode 'name' for the Einstein SSS instrument were
PH mode, DELTA-T (DT) mode, and MS mode.
Within each mode different
data-streams could have been accumulated setting either different time
resolution parameter and/or different number of energy channels. A typical
Einstein SSS FITS raw data file contains ten extensions (eight
dedicated to SSS data and two to the simultaneous MPC data set), all of
them having the same FITS layout/structure
(HDUCLAS1='TEMPORALDATA', HDUCLAS2='COMBINED').
Within each SSS extension, stream of data, obtained
within an observation, from the three different modes with 'similar'
characteristics are combined together. Therefore the original datamode
is not unique within an extension, and the original value is kept within a
column (MODE).
Combining data from all three different modes, the SSS extension can contain
either high time resolution data, or spectral data or event data or time
histogram data. The string for the DATAMODE keyword contains a short
description of the extension content.
Below are listed the string value for
the DATAMODE keyword both for SSS and MPC.
- SSS data
DATAMODE='SPEC_2.56s_128chan'
- The extension contains 128 channel spectrum every
2.56 seconds. The FITS extension layout is such that
each row has one spectrum integrated 2.56 seconds.
DATAMODE='TIME_2ms'
- The extension contains intensity value every 2 ms
over the full energy range. The FITS extension layout
is such that each row contains 1024 bin integrated 2.5 ms.
DATAMODE='TIME_5ms'
- The extension contains intensity value every 5 ms
over the full energy range.The FITS extension layout
is such that each row contains 512 bin integrated 5 ms.
DATAMODE='TIME_10ms'
- The extension contains intensity value every 10 ms
over the full energy range. The FITS extension layout
is such that each row contains 256 bin integrated 10 ms.
DATAMODE='SPEC_1.28s_128chan'
- The extension contains 128 channel spectrum every
1.28 seconds. The FITS extension layout is such that
each row has one spectrum integrated 1.28 seconds.
DATAMODE='EVNT_1.28s_512pha'
- The extension contain PHA channel value for 'first' 512
events detected every 1.28 seconds. The FITS extension
layout is such that each row has an array of 512 PHA
channel value.
DATAMODE='HISTO_1.28s_DT'
- This extension contains an histogram of 128 bins
obtained with the DELTA-T mode in each row.
Each bin contains the number of events arrived at the
detector which were equi-spaced in time (1.28/128) * n.
DATAMODE='SPEC_0.64s_128chan'
- The extension contains 128 channel spectrum every
0.64 seconds. The FITS extension layout is such that
each row has one spectrum integrated 0.64 seconds.
- MPC data
DATAMODE='SPEC_2.56s_8chan'
- The extension contains a 8 channel spectrum
every 2.56 seconds. The FITS extension layout is such that
each row has one spectrum integrated 2.56 seconds.
DATAMODE='EVNT_1.28s_512time'
- This extension contains time values for the 'first' 512
events detected every 1.28 seconds. The FITS extension
layout is such that each row has an array of 512 time
value.
As said above, the SSS FITS file can be a combination of one or all of
three modes available: PH, DT (DELTA-T),or
MS. To have a short description
of which mode was used to create the FITS file, the primary array only
will contain the keyword SSSMODE with specified which mode has been used
to create the file. The string for this keyword are :
PH_DT_MS if data coming from the all three modes are used to
create the FITS file
PH_DT if data coming from the PHA or DELTA-T mode are used to
create the FITS file
PH_MS if data coming from the PHA and MS modes are used to
create the FITS file
DT_MS if data coming from the DELTA-T and MS modes are used to
create the FITS file
PH if data coming from the PHA mode only is used to
create the FITS file
DT if data coming from the DELTA-T mode only is used to
create the FITS file
MS if data coming from the MS mode is used to
create the FITS file
The XTE payload comprises the Proportional Counter Array (PCA), the
High Energy X-ray Timing Experiment (HEXTE) and the All Sky Monitor (ASM).
Data from each experiment are 'pre-organized' by on board computers.
Data from the PCA are packaged and delivery to the telemetry by the
Experiment Data System (EDS) which contains six programmable Event Analyzers
(EAs) assigned to the PCA. The EAs are fully programmable with a variety of
modes, which can be divided into several categories:
- Binned mode; Counts as function of time, energy, PCU,layer.
- Events mode; Single events in up to 30 bits per event.
- Single-bit mode; Timing events with a resolution of 0.1 ms
Two of the EAs will be dedicated to generate PCA data in two different
'standards' modes to have a continuous and consistent set of PCA data.
Within a FITS file for the XTE data, the keyword DATAMODE contains a
string which is a short description of the mode used by the on board
computer to sample the data. The string, for each of the categories
itemized above, contains always a prefix (to identify the categories),
the time resolution, the energy/channel boundaries (the last two can be
missing for specific mode), plus a number of identifier specific to certain
categories.
Events mode: The string value is coded in the following way
DATAMODE='E_ttt_xxxA_lll_rrr' where
E : mode prefix
ttt: time resolution [ms value (1,2) ; s value
(1,2,4,8,16,32,64,128,512)]
xxx: number of energy channel [value (4,8,16,32,64)]
A: selects the set of energy boundaries (table to be
inserted) [value (A,B,C,M)]
lll: lower PH limit for the lowest energy channel. For
this category, the upper PH limit for the highest channel is
always 255. [value (0,14,18,24,36,50)]
rrr: readout time [value s (1,8)]
Note that not all the combination of the listed values for each
item are allow.
- Binned mode:
The string value is coded in the following way
DATAMODE='B_ttt_xxxA_lll_hhh_b_propane' where
B : mode prefix
ttt: time resolution [ms value (1,2,4,8,16,64,250,500);
s value (32,64,128,256,512)]
xxx: number of energy channel [value (2,4,8,10,14,16,19,
32,45,64,128)]
A: selects the set of energy boundaries (need a table)
[value (A,B,C,M)]
lll: lower PH limit for the lowest energy channel
[value (0)]
hhh: the upper PH limit for the highest channel
[value (14,18,24,36,50,255)]
b: how many bits are used for each bin [value (H,Q,F)
where H= 8 bits (Half words), Q=4 bits (Quarter words),
F=16 bits (Full words)]
propane: used in the string only if the propane layer
is added in with the xenon layer.
Note that not all the combination of the listed values for each
item are allow.
Note also that the b and/or
propane components can be absent.
- Single-bit mode: The string value is coded in the following way
DATAMODE='SB_ttt_lll_hhh_rrr' where
SB : mode prefix
ttt: time resolution [ s value (30,61,122,244,488)]
lll: lower PH limit for the lowest energy channel
[value (0, 12,18,24,36)]
hhh: the upper PH limit for the highest channel
[value (12,18,24,36,50,255)]
rrr: readout time [value s (1) value ms (250,500)]
Note that not all the combination of the listed values for each
item are allow.
Note also that the b and/or
propane components can be absent.
The standard modes used in two of the EAs are set up such to give either
high time resolution with low energy resolution (Standard Mode 1) or
low time resolution with high energy resolution (Standard Mode 2).
- Standards Mode 1: The string value is coded in the following way
DATAMODE='Standard1_ttt' where
From this mode nine time series histograms are generated, each spanning a
time period of ttt divided into 1024 bin,therefore the time resolution
for each bin is 1024/ttt [ttt values (8,32,128)].
Six histogram represent broad energy range from the xenon layers; one is
for propane layer; two give counts from anticoincidence information.
All histograms are integrated over all PCUs.
Note that if ttt is 128 the string can be simply Standard1.
- Standards Mode 2 The string value is coded in the following way
DATAMODE=' Standard2_ttt' where
Section Incomplete
We thank the numerous people both inside and outside the OGIP who
have contributed suggestions & comments to this document.
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