Introduction
Exact format specifications for exchange of data within the VO are still under development (status March 2004). Those specifications are going to be based on data models and will be developed in different threads dealing with images, tabular information, theory data etc.
Nonetheless, many VO enabled features are available to the community today once the data are loaded into a tool like the AVO prototype. Here's a summary of supported FITS keywords that the AVO prototype V1.0 is aware of.
FITS Images
General documentation on FITS is available from the FITS Support Office.
Mandatory FITS image keywords
The minimal list of FITS keywords for displaying a 2d image is:
SIMPLE, BITPIX, NAXIS, NAXIS1, NAXIS2
Supported astrometric reductions
WCS astrometry reduction
CRPIX1, CRVAL1, CTYPE1, CD1_1, CD1_2
CRPIX2, CRVAL2, CTYPE2, CD2_1, CD2_2
Another supported WCS format
CRPIX1, CRVAL1, CTYPE1, CDELT1, PC001001, PC001002
CRPIX2, CRVAL2, CTYPE2, CDELT2, PC002001, PC002002
Some other old dialects for astrometry
CRPIX1, CRVAL1, CDELT1
CRPIX2, CRVAL2, CDELT2
- without CROTA (defaulted to zero)
- alternatively with either CROTA1 and/or CROTA2
The old DSS1 format
PLTRAS, PLTRAM, PLTRAH, PLTDECSN, PLTDECS, PLTDECM, PLTDECD, PLTSCALE
[AMDX1,AMDY1 .. AMDX12, AMDY12]
XPIXELSZ, YPIXELSZ, CNPIX1, CNPIX2
Projection
Projections are encoded in the CTYPEn keyword. Supported are:
SIN, TAN, ARC, AIT, ZEA, STG, CAR, NCP
Optional keywords:
| Keyword | Remarks |
|---|
| BLANK | integer value used for undefined pixels |
| EPOCH | a float value which deals for EQUINOX in the old RADIO images, and for EPOCH of observation in others |
| EQUINOX | a float value defaulted to 2000.0 |
| MJDOBS | Modified Julian Date given as a floating point number |
| RADECSYS | coordinate system; supported are: FK4, FK5, ICRS, GLON, SLON, ELON;
GLON for Galactic coordiantes, SLON for Supergalactic and ELON for ecliptic |
Sample Header
The FITS header of an image with defined WCS and equinox looks like this.
SIMPLE = T
BITPIX = 8 / Bits per pixel
NAXIS = 2 / Number of dimensions
NAXIS1 = 768
NAXIS2 = 768
CRPIX1 = 384.0
CRPIX2 = 384.0
EQUINOX = 2000.0
CRVAL1 = 189.20660782916502
CRVAL2 = 62.21594557221128
CTYPE1 = 'RA---TAN'
CTYPE2 = 'DEC--TAN'
CD1_1 = -2.798812690843424E-4
CD1_2 = -5.5335921184083845E-6
CD2_1 = -5.5335921184083845E-6
CD2_2 = 2.798812690843424E-4
END
|
Edit menu
