FITS Header Rules

All data acquired from science instruments of the Subaru Telescope are output saved in a FITS format.

These sets of data from the telescope are archived at the Hilo Base Facility in Hilo, the island of Hawai‘i, and is released from there. As a general rule, all search items of the archive must be contained in a header in a FITS primary HDU. Also, for the purpose of being able to search data across multiple science instruments simultaneously, common FITS keywords must be used for the instruments to the extent possible. From a perspective of data analysis, since parameters needed for analysis are mostly extracted from FITS headers, the use of common FITS keywords is desirable to achieve efficient software development through standardization of analysis software. For these ends, the Subaru Telescope stipulates the FITS header rules below.

• The organization that supervises the Subaru Telescope’s FITS headers is the Subaru Telescope FITS Committee. FITS files and contents of headers generated from all science instruments of the Subaru Telescope are subject to the committee’s review and approval.

• In principle, FITS files generated by the Subaru Telescope’s instruments are in conformance with the FITS Standard developed by the IAU FITS Working Group.

• The keywords defined in the Header Dictionary shall be used as defined, and not be used for any other meaning. The keywords classified as Common in the Header Dictionary shall be always included. In addition, some of the keywords, including the terms classified as Imaging and Spectroscopy, are essential to be contained for data acquired in designated observation modes.

• Science instrument groups shall develop and publish a dictionary for the List of the Instrument-specific Headers, similarly to the common header keywords. Each of the instrument groups has discretion to revise its dictionary, but shall release revision and a revision history on the web. The group shall notify the person in charge at Subaru Telescope (fits@naoj.org) of such revision if any. A Support Astronomer (SA) for each instrument shall release such information on its instrument webpage as for the current facility instruments. For PI instruments and decommissioned instruments, the information shall be published on this document by Subaru Telescope. All the information shall be put together in this document.

• After data start to be generated from any instruments, keywords used in the past shall not be used for any other meaning. Units used in the past shall not be changed, either.

• When a dictionary for an instrument is revised after it starts to generate data, some change shall be added to its header keyword INS-VER.

• When a new value is required to be set for a keyword whose allowable value is predetermined, it is subject to inquiry to the committee (fits@naoj.org) and its approval before being used.

• A science instrument-specific keyword shall be used in either of the following two ways:

  (a) Put the keyword into a block behind a group of basic keywords. In this method, a comment line, such as COMMENT Subaru Device Dependent Header Block for FOCAS, shall be inserted, followed by the keyword.

  (b) Put keywords of similar nature as well as basic keywords together into a block, while any necessary comment is inserted for better readability.

• The first two characters of instrument-specific header keywords shall be provided as the instrument’s ID. The remaining six characters can be used at the discretion of a developer of the instrument, which should be in conformity with the Abbreviation List as much as possible. An Instrument ID shall consist of such forms as A_, B_, and C_, with no overlapping allowed. The instrument IDs that are currently reserved are as follows:

  Instrument ID	Instrument
  A_	AO36
  B_	FMOS
  C_	CIAO
  D_	AO188
  E_	MEC
  F_	FOCAS
  H_	HDS
  I_	IRCS
  K_	MOIRCS
  L_	LGS
  M_	MIRTOS
  O_	OHS
  P_	HiCIAO
  Q_	COMICS
  R_	MIMIZUKU
  S_	Suprime-Cam
  T_	Hyper Suprime-Cam
  U_	VAMPIRES
  V_	VTOS
  W_	PFS
  X_	SCExAO
  Y_	CHARIS
  Z_	IRD
  2_	SWIMS
  3_	Kyoto3D-II

• In reference to the keyword categories, the order of abbreviations to create a keyword shall be as follows: Image, Instrument, Telescope / Time / Environment / Statistics, Unit / Action (see the Abbreviation List, and these abbreviations can be further shortened as necessary). For example, a keyword of a slit position angle at the start of exposure is created in combination of SLT (slit), P/PA (a position angle), and STR (start of exposure), in the order of SLT (Category=Instrument), P/PA (Statistics/Unit) and STR (Action); the keyword can be SLT_PSTR.

• WCS shall be described for imagining observation.

• The ASCII Table Extension, the Binary Table Extension, and the Image Extension can be only used for extension.

• The names of astronomical objects certified by IAU shall be used as much as possible.

• The units of values shall be in conformity with those described in the dictionary. In principle, the SI unit system shall be applied.

• When a value consists of numerals, its format shall be defined with consideration given to significant figures of each parameter.

• In-line comments shall specify meanings of keywords and units of values.

• In a pixel coordinate system with the center of the pixels as a reference point, the pixel number shall start from 1.

• Keyword OBS-MOD

  • The value patterns of the keyword OBS-MOD below shall be consistently followed so that a type of data is indicated at a glance. Any of the following shall be used for the first 4 characters with all in upper case.

    Category	OBS-MOD
    Imaging	IMAG
    Spectroscopy	SPEC
    Imaging polarimetry	IPOL
    Spectropolarimetry	SPOL

  • With one underscore (_) added, any subsequent description is permitted; provided, however, that the same string of characters shall not be used for different meanings at any time. For coordination between observation control and analysis, it is recommended that OBS-MOD is obtained as a status value from the summit observation control system (when an abstract command is used for observation).

• Keyword DATA-TYP

  • The keywords below are currently recommended for use. In case any other keywords are needed to be used, the committee (fits@naoj.org) shall be contacted first for permission before its use.

  DATA-TYP
  ACQUISITION
  BIAS
  COMPARISON
  DARK
  DOMEFLAT
  DOMEFLAT_ON
  DOMEFLAT_OFF
  FLAT
  FOCUSING
  INSTFLAT
  OBJECT
  SKYFLAT
  STANDARD
  STANDARD_STAR
  TEST

• Filter and grism

  • When a science instrument is equipped with multiple sets of filters and grisms, a keyword shall be assigned a name or a sequence of numbers that ensures uniqueness of each of the filters and grisms. When a new filter or grism is replaced with the current one, the name shall be always changed.

• Compression of FITS data

  • An entire FITS file can be compressed (e.g. Rice, and Gzip), or, data stored in a FITS file can be compressed by means of Tiled image compression which is stipulated in the IAU FITS Standard; provided, however, that these two compression methods shall not be applied to the same data.

  • Lossless compression must be applied when data are compressed.

  • When data are compressed, such format shall be compatible to widely-used tools to read, write and view FITS files.