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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







SUMMARY

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.

LOG OF SIGNIFICANT CHANGES




Release Sections Changed Brief Notes
Date    
     
1994 May 11   First (internal) Draft
1994 Nov 30 All Made compatible with LaTeX2HTML software
1995 Mar 10 All Revised & Updated
     


Contents

INTRODUCTION

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).

OBSERVATION ID

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.

OBSERVING MODE

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

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.

DETECTOR OPERATING MODES

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.

ARIEL-V

ASM

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 $1/16$ of the whole sky

ASCA

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.

GIS

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 $\mu$s for HIGH bit rate, 288 $\mu$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.

SIS

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 Einstein Observatory

SSS & MPC

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

XTE - Section INCOMPLETE

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) ; $\mu$ 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); $\mu$ 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 [$\mu$ 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

ACKNOWLEDGMENTS

We thank the numerous people both inside and outside the OGIP who have contributed suggestions & comments to this document.

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