Next: 5.4.3 Additional Keywords
Up: 5.4 Keywords
Previous: 5.4.1 Mandatory Keywords
Subsections
These keywords are optional but
may be used only as defined in this standard.
They apply to any FITS
structure
with the meanings and restrictions defined below.
Any FITS structure may further restrict the use of
these keywords.
Starting January 1, 2000, the following format shall be used.
FITS writers should commence writing the value of
the DATE keyword
in this format starting January 1, 1999 and before January 1, 2000.
The value field shall contain a character
string giving
the date on which the HDU was created,
in the form YYYY-MM-DD, or the date and time when the HDU was
created, in the form YYYY-MM-DDThh:mm:ss[.sss...],
where YYYY shall be the four-digit calendar year number,
MM the two-digit month number
with January given by 01 and December by 12,
and DD the two-digit day of the month. When both date and time
are given, the literal T shall separate the date and time,
hh shall be the two-digit hour in the day,
mm the two-digit number of minutes after the
hour, and ss[.sss...] the number of
seconds (two digits followed by an optional fraction) after the
minute. No fields may be defaulted and no leading zeroes
omitted. The decimal part of the seconds field is optional and may be
arbitrarily long, so long as it is consistent with the rules for
value formats of §5.2.
The value of the DATE keyword shall always
be expressed in UTC when in this format, for all data sets created on earth.
The following format may appear on files written before
January 1, 2000. The value field contains a character
string giving the date
on which the HDU was created, in the form DD/MM/YY,
where DD is the day of the month,
MM the month number
with January given by 01 and December by 12, and YY
the last two digits of the year, the first two digits being
understood to be 19. Specification of the date
using Universal Time is
recommended but not assumed.
Copying of a FITS file does not require changing
any of the keyword values in the file's HDUs.
The value field shall contain a character
string
identifying the organization or institution responsible for
creating the FITS file.
This keyword may be used only in the primary header.
It shall appear within the
first 36 card images of the FITS file.
(Note: This keyword thus cannot appear if NAXIS is
greater than 31, or if NAXIS is greater than 30 and the
EXTEND keyword is present.)
Its presence with the required logical value of T advises that
the physical block size of the FITS file on which it
appears may be an integral multiple of the logical record length,
and not necessarily equal to it.
Physical block size and logical record length may be
equal even if this keyword is present or unequal if it
is absent. It is reserved primarily to prevent its use
with other meanings. Since the issuance of version 1 of this standard,
the BLOCKED keyword has been
deprecated .
The format of the value field for DATE-OBS keywords
shall follow the prescriptions for the DATE
keyword (§5.4.2.1). Either the 4-digit year format or
the 2-digit year format may be used for observation dates from 1900
through 1999 although the 4-digit format is preferred.
When the format with a four-digit year is used,
the default interpretations for time shall be
UTC for dates beginning 1972-01-01 and UT
before. Other date and time scales are permissible. The value of
the DATE-OBS keyword shall be expressed in the principal time
system or time scale of the HDU
to which it belongs; if there is any
chance of ambiguity, the choice shall be clarified in comments.
The value of DATE-OBS shall be
assumed to refer to the start of an observation, unless another
interpretation is clearly explained in the comment field.
Explicit specification of the time scale is recommended.
By default, times for TAI and times that run simultaneously with TAI,
e.,g., UTC and TT, will be assumed to be as measured at the detector
(or, in practical cases, at the observatory). For coordinate times
such as TCG, TCB, and TDB which are tied to an unambiguous coordinate
system, the default shall be time as if the observation had taken place
at the origin of the coordinate time system. Conventions
may be developed that use other time systems. Appendix D
of this document contains the appendix to the agreement
on a four digit year, which discusses time systems in some detail.
When the value of DATE-OBS is expressed in the two-digit year
form, allowed for files
written before January 1, 2000 with a year in the range 1900-1999,
there is no default assumption as to whether it refers to the start,
middle or end of an observation.
The value fields for all keywords beginning with the string DATE
whose value contains date, and optionally time, information
shall follow the prescriptions for the DATE-OBS
keyword.
The value field shall contain a character
string
identifying the telescope used to acquire the data
associated with the header.
The value field shall contain a character
string
identifying the instrument used to acquire the data
associated with the header.
The value field shall contain a character string
identifying who acquired the data associated with the header.
The value field shall contain a character
string
giving a name for the object observed.
The value field shall contain a floating point number
giving the equinox in years for the celestial
coordinate system
in which positions are expressed.
The value field shall contain a floating point number
giving the equinox in years for the celestial
coordinate system
in which positions are expressed.
Starting with Version 1, this standard has
deprecated the use of the EPOCH
keyword and thus it shall not be used in FITS files
created after the adoption of this standard; rather, the
EQUINOX keyword shall be used.
The value field shall contain a character
string
identifying who compiled the information
in the data associated with the header. This keyword
is appropriate when the data originate in a published
paper or are compiled from many sources.
The value field shall contain a character
string
citing a reference where the data associated with the
header are published.
This
keyword shall have no associated value ;
columns 9-80 may contain any ASCII text.
Any number of COMMENT card images may appear in a header.
This keyword shall have no associated value ;
columns 9-80 may contain any ASCII text.
The text should
contain a history of steps and procedures associated
with the processing of the associated data.
Any number of HISTORY card images may appear in a header.
Columns 1-8 contain ASCII blanks. Columns 9-80 may contain any
ASCII text.
Any number of card images with blank keyword fields
may appear in a header.
These keywords are used to describe the contents of an
array, either alone or in a series of
random groups (§7).
They are optional, but if they appear in the header describing
an array or groups, they must be used as defined in this
section of this standard. They shall not be used in headers
describing other structures unless the meaning is the same
as that for a primary or groups array.
This keyword shall be used, along with the BZERO
keyword, when the array pixel values are not the true
physical values, to transform the
primary data array values to
the true physical values they represent, using Eq. 5.3.
The value field shall contain a floating point number representing the
coefficient of the linear term in the scaling equation, the
ratio of physical value to array value
at zero offset. The default value for this keyword is 1.0.
This keyword shall be used, along with the BSCALE
keyword, when the array pixel values are not the true
physical values, to transform the primary data array values to
the true values. The value field shall contain a floating
point number representing the physical value
corresponding to an array value of zero. The default value for this
keyword is 0.0.
The transformation equation is as follows:
| physical_values = BZERO + BSCALE × array_value | (5.3) |
The value field shall contain a character
string ,
describing the physical units in which the quantities
in the array, after application of BSCALE
and BZERO,
are expressed. These units must follow the
prescriptions of §5.3.
This keyword shall be used only in headers with
positive values of BITPIX
(i.e., in arrays with integer data).
Columns 1-8 contain the string, ``BLANK
'' (ASCII blanks in
columns 6-8).
The value field shall contain an integer that
specifies the representation of array values
whose physical values are undefined.
The value field shall contain a character
string, giving
the name of the coordinate
represented by axis n.
The value field shall contain a floating point number ,
identifying the location of a reference point
along axis n,
in units of the axis index.
This value is based upon a counter that runs
from 1 to NAXISn with an increment of 1 per pixel.
The reference point value need not be that for the center of a pixel
nor lie within the actual data array. Use comments to indicate
the location of the index point relative to the pixel.
The value field shall contain a floating point number ,
giving the value of the coordinate
specified by the CTYPEn
keyword at the reference
point CRPIXn. Units must follow the
prescriptions of §5.3.
The value field shall contain a floating point number
giving the partial derivative of the coordinate specified
by the CTYPEn keywords with respect to the pixel
index, evaluated at the reference
point CRPIXn, in units of the
coordinate specified by the CTYPEn keyword.
These units must follow the
prescriptions of §5.3.
This keyword is used to indicate a rotation from a
standard coordinate system
described by the CTYPEn to
a different coordinate system in which the values in the
array are actually expressed. Rules for such rotations are
not further specified in this standard; the rotation should
be explained in comments. The value field shall contain
a floating point number giving the rotation angle
in degrees between
axis n and the direction implied by the coordinate system
defined by CTYPEn.
The value field shall always contain a floating point
number, regardless of the value of BITPIX. This
number
shall give the maximum valid physical value
represented by the array, exclusive of any
special values.
The value field shall always contain a floating point
number, regardless of the value of BITPIX. This
number shall give the minimum valid physical value represented
by the array, exclusive of any special values.
These keywords are used to describe an extension
and should not appear in the primary header.
The value field shall contain a character
string, to be
used to distinguish
among different extensions of the same
type, i.e., with the same value of XTENSION,
in a FITS file.
The value field shall contain an integer, to be used to
distinguish among different extensions in a FITS file
with the same type and name, i.e., the same values for
XTENSION and EXTNAME. The
values need not start with 1 for the first extension with
a particular value of EXTNAME and need not be in
sequence for subsequent values. If the EXTVER keyword
is absent, the file should be treated as if the value
were 1.
The value field shall contain an integer, specifying the
level in a hierarchy of extension levels of the extension
header containing it. The value shall be 1 for the highest
level; levels with a higher value of this keyword shall be
subordinate to levels with a lower value. If the EXTLEVEL
keyword is absent, the file should be treated as if the
value were 1.
Next: 5.4.3 Additional Keywords
Up: 5.4 Keywords
Previous: 5.4.1 Mandatory Keywords
5/13/1999