An idea for a table of WCS information

While it might be nice to place this information into FITS IMAGE HDU keywords, some of the world coordinate system (WCS) keywords themselves are already doubly-indexed. The 8-character limit does not provide enough keyword namespace to permit sensible encoding of two more indices. Such an attempt inevitably produces numeral-jammed keywords whose structure defies trivial parsing. Furthermore, the 8-character constraint creates field widths with uncomfortably small upper limits on the indices.

The archival FITS format for DEIMOS will store data from separate CCDs into separate FITS HDUs. This permits the WCS information to be stored in precisely the manner prescribed by the draft WCS standard document. However, for purposes of the DEIMOS simulators and quick-look reduction software it is desirable to store all WCS information in a single configuration file.

A possible solution comes with a phrase we have found ourselves using over and over in the DEIMOS project.

Don't use keywords to do the job of a table.

Question: Should these tables also contain URIs in the sense used by the proposed FITS Grouping Convention? If they do then the image-based WCS rows could point directly to the images that they describe.

A FITS ASCII table containing WCS information

A FITS Binary table containing WCS information

An editable plaintext representation of the WCS table

Representative Values of the various columns

CCDPOS

An indication of the position within the detector mosaic occupied by the CCD in question. For DEIMOS this will typically be an integer in the range 1 to 8. The value may also be something like 'GUIDE' to indicate that the WCS applies to the TV guide camera. These values require further consideration.

AMPPOS

An indication of the position within the detector mosaic occupied by the amplifer in question. For DEIMOS this will typically be 1 or 2.

CTYPE

RA---TAN, DEC--TAN

Equatorial coordinates as in the WCS draft.

GLON-TAN, GLAT-TAN

Galactic coordinates as in the WCS draft.

ELON-TAN, ELAT-TAN

Ecliptic coordinates as in the WCS draft.

MASK_X, MASK_Y

Coordinates on the slitmask.

DEWAR_X, DEWAR_Y

Coordinates on the dewar focal plane.

The use of the tangent plane (-TAN) projection above is merely illustrative. The projection best suited for DEIMOS images has not yet been determined.

CUNIT

For transformations to the sky as described in the WCS draft this will be degrees ('deg '). For transformations to slitmask or dewar focal plane this will be meters ('m ').

RADECSYS

The values 'FK5 ', 'FK4', 'FK4-NO-E', and 'GAPPT ' are as in the WCS draft. Values which are specific to DEIMOS include

SLITMASK

transformation is to the slitmask coordinate system

DEWAR

transformation is to the dewar focal plane

In addition, for spectral purposes we will need a form of WCS which transforms from a FITS pixel position to a slitletID and wavelength. This will require an additional table not yet specified.

DOMAIN

This column is proposed to permit the transformations to apply from coordinate systems other than FITS pixels. Its implications and possibilities are not yet clear.

The DOMAIN is the coordinate system from which the transformation is being done. Without this mechanism it would be necessary to use FITS pixels as an intermediate step in any coordinate conversion. Possible values are

FITS_PIX

Transformation is from FITS pixels. This is the default.

CCDPIXEL

Transformation is from unbinned CCD pixels.

SLITMASK

Transformation is from slitmask coordinates.

DEWAR

Transformation is from dewar coordinates.

Note that there is currently no way to specify the units of the domain. Thus coordinates from each domain must be given in some established canonical system.