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| RA | Right Ascension, usually set at the observatory and not formally part of WCS It is usually in sexigesimal format (hh:mm:ss.ss), but may be in degrees. WCSTools imwcs writes out a value approximating the center of the image. |
| DEC | Declination, usually set at the observatory and not formally part of WCS It is usually in sexigesimal format (hh:mm:ss.ss), but may be in degrees. WCSTools imwcs writes out a value approximating the center of the image. |
| EPOCH | By the FITS standard, this is the equinox of the observation coordinates in years, but if the EQUINOX keyword is present, it may be the epoch of the observation in years |
| EQUINOX | The equinox of the observation coordinates in years |
| RADECSYS | The system of the coordinates: ICRS, J2000, B1950, GALACTIC, ECLIPTIC |
| SECPIX | Arcseconds per pixel for assumed square pixels. This is an observatory keyword and may be from the instrument model. WCSTools imwcs writes out a value approximating the fit image scale. |
| SECPIX1 | Arcseconds per pixel in the horizontal dimension. This is an observatory keyword and may be from the instrument model. |
| SECPIX2 | Arcseconds per pixel in the vertical dimension. This is an observatory keyword and may be from the instrument model. |
| CTYPE1 | WCS Type of the horizontal coordinate, including coordinate and projection, each allowed four characters, such as RA---TAN. |
| CTYPE2 | WCS Type of the vertical coordinate, including coordinate and projection, each allowed four characters, such as DEC--TAN. |
| CRPIX1 | Reference pixel on the horizonal axis |
| CRPIX2 | Reference pixel on the vertical axis |
| CRVAL1 | Horizonal axis WCS value at the reference pixel |
| CRVAL2 | Vertical axis WCS value at the reference pixel |
| CDELT1 | Change per pixel in horizonal axis WCS value at the reference pixel |
| CDELT2 | Change per pixel in vertical axis WCS value at the reference pixel |
| CROTA1 | Rotation angle of image around the reference pixel in degrees (same as CROTA2 and unnecessary) |
| CROTA2 | Rotation angle of image around the reference pixel in degrees (same as CROTA1, if it is present) |
| CD1_1 CD1_2
CD2_1 CD2_2 |
Rotation and scaling matrix |
| PC1_1 PC1_2
PC2_1 PC2_2 |
Rotation matrix, used with CDELT1 and CDELT2 in the final version of the FITS-WCS standard as an alternative to the CD_j matrix. |
| PC001001 PC001002
PC002001 PC002002 |
Rotation matrix, used with CDELT1 and CDELT2 in draft versions of the FITS-WCS standard. Some data files were written using these keywords before they were dropped from the standard in favor of PCi_j. |
| LATPOLE | Latitude of the pole of the coordinate system. In the final FITS-WCS standard, this may also be set by PVi_4. |
| LONPOLE | Longitude of the pole of the coordinate system. In the final FITS-WCS standard, this may also be set by PVi_3. |
| IMWCS | If the WCS is set by imwcs, the value of this keyword is a string giving the version of the program which was used. |
| CO1_1 CO1_2 CO1_3
CO1_4 CO1_5 CO1_6 CO1_7 CO1_8 CO1_9 CO1_10 CO1_11 CO1_12 |
These keywords are the horizontal axis coefficients of a set of polynomial expansions of the CD matrix which can be fit by imwcs. |
| CO2_1 CO2_2 CO2_3
CO2_4 CO2_5 CO2_6 CO2_7 CO2_8 CO2_9 CO2_10 CO2_11 CO2_12 |
These keywords are the horizontal axis coefficients of a set of polynomial expansions of the CD matrix which can be fit by imwcs. |
| PROJP0 PROJP1 PROJP2
PROJP3 PROJP4 PROJP5 PROJP6 PROJP7 PROJP8 PROJP9 |
These are projection parameters used by draft versions of the FITS WCS standard, but dropped in favor of PVi_j in the final standard. |
| PV1_0 PV1_1 PV1_2
PV1_3 PV1_4 PV1_5 PV1_6 PV1_7 PV1_8 PV1_9 |
Projection parameters if axis 1 is latitute/declination |
| PV2_0 PV2_1 PV2_2
PV2_3 PV2_4 PV2_5 PV2_6 PV2_7 PV2_8 PV2_9 |
Projection parameters if axis 1 is latitute/declination |
