Reserved Keywords

Many of the following reserved keywords were originally suggested by Wells, Greisen, and Harten (1981). If a reserved keyword is used, the meaning and structure must be as described here. Keywords other than the reserved keywords should not be used in their place to express the same concepts. Reserved keywords may appear in any order between the required keywords and the END keyword.

Some of these keywords describe the data array.

• BUNIT (character) represents the physical units of the quantity stored in the array, e.g., Janskys, magnitudes/pixel. The name comes from ``brightness units.'' Section 3.1.1.4 discusses recommendations for choice of units.

• BSCALE (floating) is a scale factor used in converting array elements stored on the FITS data set to physical values:

  
  

  $$\mbox{physical\_value} = (\mbox{FITS\_value}) \times \mbox{BSCALE} + \mbox{BZERO}.$$ (3.2)

  
  

  If this keyword is not present, the scale factor is assumed to be 1.

• BZERO (floating) is the offset, the physical value corresponding to a stored array value of zero, in equation 3.2. If this keyword is not present, the offset is assumed to be zero.

  Be careful about possible overflows when using BSCALE and BZERO if the array elements are floating point (value of BITPIX $<$ 0). Pay attention to the likely order of magnitude of the resulting values and be prepared to trap overflows if necessary.

  While the values of BZERO and BSCALE are floating point, the rules of FITS do not specify the data type of the result of scaling, as this result is not part of the FITS file. Whether the result is to be considered an integer or real number depends on the physical significance of the number.

• BLANK (integer) - If the text ``BLANK   '' appears in columns 1-8, then the value will be stored in those elements of an integer array that have an undefined physical value. The value of BLANK appears in the actual data array, without scaling. It should be regarded as a code, not a number; a scaling transformation with BSCALE and BZERO should not be applied to array members that have the value given by the BLANK keyword. Do not use this keyword if the FITS data array is IEEE floating point, becuse this function is performed by the IEEE floating point NaN. The BLANK keyword does not have the same meaning as filling columns 1-8 with ASCII blanks.

The reserved keywords permit complete specification of a linear coordinate system for any axis. Other coordinate systems can be identified through the name given by CTYPE$n$, comments, and user-specified keywords.

• CTYPE$n$ (character) is the name of the physical coordinate for axis $n$ (e.g., frequency, RA, Dec).

• CRPIX$n$ (floating) is a location along axis $n$ called the reference pixel, or reference point, used in defining the range of values for the physical coordinate of axis $n$. It is given in units of the counting index. The counting index for axis $n$ runs from 1 to the value of NAXIS$n$, incrementing by one for each pixel or array position. The value of CRPIX$n$ may be a fractional index number (e.g., 2.5) and/or be outside the limits of the array; if the array runs over index values 1-10, the reference point may still be $-5$. The term ``reference pixel'' originated in the days when the data array was assumed to represent a digital image. The location of the index number relative to an image pixel, i.e., center or corner, is not at present specified in FITS, but the World Coordinates proposal currently under consideration places it at the center, following the common usage in astronomy. A full discussion of this issue appears in section 4.1.

• CRVAL$n$ (floating) is the value of the physical coordinate identified by CTYPE$n$ at the reference point on axis $n$.

• CDELT$n$ (floating) is the rate of change of the physical coordinate along axis $n$ per unit change in the counting index, evaluated at the reference point.

• CROTA$n$ (floating) is the rotation angle, in degrees, of actual axis $n$ of the array from the coordinate type given by CTYPE$n$. As there is no prescribed rule for describing such rotations, the nature of the rotation should be explained in detail using comments.

Default values have not been defined for any of these keywords.

These reserved keywords, from FITS Paper I, allow the definition of simple rectangular coordinate systems, but they do not prescribe the relation between the plane rectangular coordinate system of the FITS array and the spherical coordinate region of the sky that it represents. This question, along with the current comprehensive proposal under consideration by the FITS community, is discussed in section 4. Part of the proposal replaces the simple (CROTA$n$, CDELT$n$) method with a more comprehensive method of defining coordinate transformations in three dimensions.

• DATAMAX (floating) is the maximum data value in the array, after any scaling transformation has been applied to the stored array value.

• DATAMIN (floating) is the minimum data value in the array, after any scaling transformation has been applied to the stored array value.

Note that DATAMAX and DATAMIN apply to the physical values represented, not to the numbers in the FITS file. In determining the values for DATAMAX and DATAMIN, special values such as the IEEE special values and values derived from integer array members set to the value of the BLANK keyword are not considered.

Some keywords provide information on the observations represented or the production of the data set.

• DATE (character) is the date the file was written ('dd/mm/yy'--note order). UT is recommended. The value may refer to the creation date of the original file rather than that of the current copy. This rule allows files to be copied without changing the value of the DATE keyword. Currently, date, month, and year are always two digits, with a leading zero if necessary. With this format, the century is ambiguous.

• DATE-OBS (character) is the date of data acquisition (UT recommended); it tells when the observations were made ('dd/mm/yy'-- note order). Whether this value (and that of DATE) refers to the start, midpoint, or end of the relevant time interval is not specified. Use comments to say.

  With the turn of the century approaching, there have been extensive discussions as to how to distinguish the century when specifying a date. A proposal based on the ISO 8601 format is now under consideration by the regional FITS committees.

• ORIGIN (character) is the installation where the FITS file is being written.

• TELESCOP (character) is the data acquisition telescope.

• INSTRUME (character) is the data acquisition instrument.

• OBSERVER (character) is the observer name or other identification.

• OBJECT (character) is the object observed.

• AUTHOR (character) identifies who compiled the data associated with the header. Use this keyword when the data come from a published paper or have been compiled from many sources. It refers to the author of the data being carried by the FITS file, not to the creator of the computer-readable form. It should not be used to identify the software that created the FITS file or the writer of that software. The CREATOR keyword (section 5.6.1) has been suggested to identify the software creating a FITS file.

• REFERENC (character) is the bibliographic reference for the data associated with the header.

• EQUINOX (floating) is the equinox of the coordinate system (in years). In early FITS data sets, the keyword EPOCH was used with this meaning. Since the term ``epoch'' has a different meaning, referring to the actual date of observation, the correct term EQUINOX should be used in the future. EPOCH will still be accepted with this meaning in old data sets and should not be otherwise used. Software to read FITS data sets should interpret EPOCH as equivalent to EQUINOX. The epoch or date of observation can be given as the value of the DATE-OBS keyword.

• BLOCKED (logical) - deprecated - when set to the value T advises that a tape may be blocked with more than one logical record per physical block. A value of F has no meaning. Its significance is now primarily historical and is discussed further in section 3.8. However, because the keyword name BLOCKED has been reserved, it cannot be used for any other purpose.

Other keywords signal a card image with comments or other text.

• Columns 1-8 blank, no keyword, means columns 9-80 are a comment.

• COMMENT (none) means columns 9-80 are a comment.

• HISTORY (none) means columns 9-80 are a comment. This keyword is intended for use when the associated text discusses the history of how the data contained in the array were processed.

These keywords are the only ones without values that can have ``='' in column 9. Users may define other keywords to contain comments as well. For these keywords, column 9 may not contain ``=''. The reason this restriction does not apply to the COMMENT, HISTORY and keywords is principally historical; the original FITS paper defined columns 9-80 as being a comment and did not restrict the content. For the COMMENT, HISTORY, and blank field keywords, the reader will be able to tell by the keyword name that there is no value. However, for user-defined keywords, the reader has no other way of telling a priori whether the field has a value or not. To minimize confusion, it is best to avoid ``='' in column 9 even after COMMENT, HISTORY, and blank keyword fields as well.