FITS File Names for the Chandra Data Archive

Please note: These file names are the ones created by standard processing for everything that goes into the archive, but not necessarily what the user sees; files may be renamed when transferred to the user (e.g., dropping digits in the start time, leaving off processing run versions).

The prime motivations for the convention are:

  1. Unique file names
  2. Uniformity in file names
  3. File names that are sufficiently informative
The first consideration leads to the requirement to carry the processing level as well as the processing version in the file name for at least some products. For the sake of uniformity, we require them on all. Also for reasons of uniformity, it was decided to use TSTART throughout: it becomes difficult to relate products from different levels if some are identified by time, others by obs_id, and yet others by sequence number; time is the universal discriminator ;-) Finally, leading zeroes are required; the gain of one character for the first few years of the mission does not outweigh the bother of having to count characters to decide whether files belong together or are years apart.

File Name Specification


    <i>     instrument: "acis", "hrc", ...
    <s>     data source: "x", "f", "t", "b", "s", "u"
    <t>     TSTART (integer part): "#########"
            For higher level products: ObsId and observation interval
    <v>     processing run (version): "N###"
    <f>     optional discriminator (e.g., FEP Id, TU)
    <c>     contents: "evt", "prf", "hst", "src", "win", "bias", ...
    <l>     processing level: single digit: "0", "1", "2"
    <sl>    sublevel: single lower case letter; e.g., 1.5 is "1a"
Example: acisf123456789N000_evt1.fits

Instrument Values

    acis    AXAF CCD Imaging Spectrometer subsystem
    hrc     High Resolution Camera subsystem
    ephin   Electron-Proton-Helium INstrument subsystem
    pcad    Pointing Control and Attitude Determination subsystem
    tel     Telescope subsystem
    sim     Science Instrument Module subsystem
    obc     On-Board Computer software subsystem
    ccdm    Communications Command and Data Management subsystem
    cpe     CPE hardware and software subsystem
    eps     Electrical Power Subystem
    thm     Thermal control subsystem
    sms     Structure subsystem
    prop    Propulsion and pyro subsystems
    misc    Other AXAF signals
    orbit   Orbit ephemeris
    sun     Solar ephemeris
    moon    Lunar ephemeris
    angles  Viewing angles
    clock   Clock measurements
    axaf    Certain mission-level items

Data Source Values
    x       XRCF
    f       Flight
    t       TRW
    b       Ball Bros
    c       Other lab calibration
    s       Simulated
    u       Unknown

Processing Version

It is assumed that decisions about reprocessing of production data will be made at a kind of CCB level and that the processing version number will hence be provided "manually" to the processing pipeline.

Optional Discriminator

On purpose, I have not defined the format of the optional discriminator. I would urge developers, though, to design them as compact as possible, preferably without underscores. Similarly, it is assumed that common sense will be used in defining the instrument and contents strings. Where necessary, changes to ICDs may be suggested. The value "TU" is reserved for files containing quantities in telemetry units that duplicate other files containing the same data in engineering (calibrated) units.

User-level File Names

We will not finalize this convention, yet, but one might envision shortening the above example to:

This page is written and maintained by Arnold Rots (

This page was last modified on 1999-01-26T18:20:00.