Importing Header Information

POLPACK needs to know certain items of header information about each of the supplied data files. These include the half-wave plate position, the orientation of the reference direction, etc. Different instruments may store this information in different ways, and for this reason, POLPACK includes an application which finds the required items of information and stores them away in a standard place within the data file from where the other POLPACK applications can extract them when needed. This standard place is known as the ``POLPACK extension''. The process of copying this information from its original, instrument specific location, into the POLPACK extension is known as ``importing'' the data, and is performed by the POLIMP application⁵, or the POLEXT application (if you have no FITS headers available).

You tell POLIMP where to find each required item of information by supplying it with a ``control table''. This is a text file containing a line for each item of information to be imported into the POLPACK extension. Each such item has a name by which other POLPACK applications refer to it (these are described here). As well as the name of a POLPACK extension item, each line of the control table should contain a description of where to obtained the corresponding value. This description is given in terms of FITS keywords. For instance, the line

   WPLATE HWPANG

tells POLIMP to find the value of the FITS keyword HWPANG and store it as item ``WPLATE'' in the POLPACK extension (this is the half-wave plate position angle).

If your data are supplied in the form of a set of FITS files, then the HWPANG value would simply be obtained from the primary header. If your data is supplied in the form of a set of NDF structures, then the value would be read from the FITS extension in the NDF structure (which should have been created by the data acquisition system). For other data formats, the origin of the keyword value depends on how the data conversion is performed. SUN/55 contains details of how FITS keywords are created from foreign data files.

The POLIMP control table also allows POLPACK extension items to be derived from one or more FITS keyword values, combined together to form a Fortran-like mathematical expression. For instance, this may be useful if the FITS keyword uses a different set of units, or has a different sign convention. When used in this way, each FITS keyword must be ``declared'' earlier in the control table. This declaration tells POLIMP what sort of data values the keyword can take. For instance:

   _REAL  ROTA
   ANGROT 57.29578*ROTA

converts the value of the FITS keyword ROTA from radians to degrees, and assigns the result to the item ANGROT. The first line informs POLIMP that the keyword ROTA should be treated as a single precision floating point value. The recognized data types are:

_BYTE

- Single byte signed integer values.

_CHAR

- Character strings.

_DOUBLE

- Double precision floating point values.

_INTEGER

- Single precision integer values.

_REAL

- Single precision floating point values.

_WORD

- Two byte signed integer values.

See the description of the POLIMP application for further details of the syntax of the control table.

There is also a POLEXP application which reverses the importing process. It copies the information stored in the POLPACK extension into named FITS keywords. The primary purpose of POLEXP is to allow the automatic format conversion facilities of the NDF library to convert a POLPACK NDF structure back into a foreign data file. It is generally not necessary for users to call POLEXP explicitly.

The POLEXT application allows the contents of the POLPACK extension to be set to explicit values supplied by the user. This can be useful if your data does not contain any FITS headers. POLEXT will also list the contents of the POLPACK extension. If your data is stored in NDF format, you can also use the hdstrace command to list the contents of the NDF extension. For instance, to check the contents of the POLPACK extension in the NDF partproc.sdf, do:

   % hdstrace partproc.more.polpack