Bulk Data Frames

The Descriptors NAXIS, NPIX, START, STEP, CUNIT, IDENT and LHCUTS must be present, in order to define the data as an image frame.

[IDENT] Character Descriptor of up to 72 characters, to contain the identifier of the frame or object.

[NAXIS] Integer Descriptor containing the number of dimensions (or axes) of the data.

[NPIX] Integer Descriptor of NAXIS elements containing the size (i.e. number of pixels) of each axis.

[START] Double precision Descriptor of NAXIS elements containing the world coordinate value of the first pixel of each axis. Note: the value refers to the center of the pixel (e.g. sensitivity weighted mean).

[STEP] Double precision Descriptor of NAXIS elements containing the distance or step between pixels on each axis expressed in units of the world coordinate system corresponding to that axis.

[CUNIT] Character Descriptor of 16*(NAXIS+1) characters containing the unit of the data itself and the units of the world coordinates of each axis. Here is specified, in what units the world coordinates and data are given. It is recommended to use units defined in the IAU Style Manual.

[LHCUTS] Real Descriptor of 4 elements containing the user cuts and physical minimum and maximum of the data frame. Note: some applications may use elements 5 and 6 for storage of special cuts values.

The descriptors described above are also called the standard descriptors of an image (cf. the SCDCOP and STDCOP interfaces).

A number of other descriptor names are reserved and may be present. These descriptors may not be used for any other purpose than the one defined here. They are:

[REFPIX] Double precision Descriptor of NAXIS elements containing location of the frame reference point in pixel coordinates where the value 1.0 corresponds to the center of the first pixel. This Descriptor is created by the FITS reader.

[ROTA] Double precision Descriptor of NAXIS elements describes the rotation of the coordinate system in degrees and corresponds to the FITS 'CROTAn' keyword. This descriptor is disregarded if the CTMATRIX descriptor exits. This Descriptor is created by the FITS reader.

[CTYPE] Character Descriptor of 16*NAXIS characters defines the type of coordinate system used for the axes and corresponds to the FITS 'CTYPEn' keyword. This Descriptor may be created by the FITS reader.

[CTMATRIX] Double precision Descriptor of NAXIS*NAXIS elements defines the coordinate transformation matrix for the axes. The matrix elements are stored column by column. This Descriptor may be created by the FITS reader.

[O_TIME] Double precision Descriptor with information on time, duration etc. of an exposure.

1.

epoch at start of exposure (years) (note: the epoch 1992.0 corresponds to 1992-01-01 00:00 UT),

2.

month of the epoch (January:1) (note: must be zero if element 1 is a fraction of year),

3.

day of the epoch (note: must be zero if element 1 is a fraction of year),

4.

Modified Julian Date (MJD) at start of exposure (days),

5.

Universal Time (UT) at start of exposure (hours),

6.

Local mean Sidereal Time (LST) at start of exposure (hours),

7.

exposure time in seconds.

This Descriptor may be created by the FITS reader.

[O_POS] Double precision Descriptor of 3 elements with information on the position of the object observed.

1.

right ascension in degrees!,

2.

declination in degrees, and

3.

equinox of coordinates.

This Descriptor may be created by the FITS reader.

[RADECSYS] Character Descriptor of 16 characters defining the type of celestial coordinate system used e.g. FK4 or FK5. This Descriptor may be created by the FITS reader.

[O_AIRM] Single precision Descriptor with mean airmass of exposure. This Descriptor may be created by the FITS reader.

[OBSERVER] Character Descriptor with the name of the observer. This Descriptor may be created by the FITS reader.

[TELESCOP] Character Descriptor with the name of the telescope used for acquisition. This Descriptor may be created by the FITS reader.

[INSTRUME] Character Descriptor with the name of the instrument used for acquisition. This Descriptor may be created by the FITS reader.

[FILENAME] Character Descriptor with the original file name. This Descriptor may be created by the FITS reader.

[HISTORY] Character Descriptor containing the history of the data. This Descriptor may be created by the FITS reader.

[HISTORY_UPDA] Integer Descriptor containing the update flag for descriptor HISTORY of the same frame. If set to 1, descriptor HISTORY is updated. If set to 0, descriptor HISTORY is not updated.

[COMMENT] Character Descriptor with comments relating to the data. This Descriptor may be created by the FITS reader.

[ESO_LOG] Character Descriptor with the observing log. This Descriptor may be created by the FITS reader.

[HIERARCH] Character Descriptor containing hierarchial keyword information for the frame. This Descriptor may be created by the FITS reader.

[NULLVALUE] Integer Descriptor of 8 elements defining the bit-patterns of the NULL values used. Its value must not be changed. This Descriptor may be created by the FITS reader.

[DISPLAY_DATA] Integer descriptor of 9 elements holding information about the way the frame was loaded into the display. First 2 elements contain scaling values, negative values indicate expansion, positive values indicate shrinking of the image (note, that for historic reasons the sign is the opposite of what you use in the LOAD/IMAGE command). The next two values are the center pixels in x,y. DISPLAY_DATA(5) = 1 or 0 depending upon if the frame was averaged or not when loaded with scaling factors < 0. The values of DISPLAY_DATA(6,7,8,9) are the frame and screen pixels (x,y) for a given fixpoint.

Descriptors starting with underscore '_' are used for special storage of acquisition data. Such descriptors should not be created by users. ESO has defined a number of hierarchical keywords which are used by the Archive for storage of information relating to the data acquisition e.g. instrument and telescope parameters. This information is stored in descriptors starting with '_E' followed by characters defining the logical hierarchical level. The level field is separated from the lowest level with a '_'