For STIS/CCD spectroscopic data in this archive, x and y describe the spatial position of a particular location in the STIS slit; details are given below. They are offsets in right ascension and declination, respectively, measured in arcsec.
The x=0, y=0 reference point is nominally located at
RA0 = 10h 45m 3.620s = 161.2650833 deg (J2000), dec0 = -59d 41' 4.10" = -59.6844723 deg (J2000),which we take to be the position of Eta Carinae, the central star.
WARNING: This absolute position has not been well determined; RA0 and dec0 are merely the values listed in HST observing plans, and operationally the star was located by a peakup at wavelengths near 7280 AA. For more information about Eta Car's absolute position, see the Astrometry page. The offsets x and y are better, and are probably meaningful to an accuracy of about 0.03 arcsec.
If we express each coordinate in seconds of arc,
x = 0.5047616*(RA - RA0),
y = (dec - dec0).
Following usual practice, x increases eastward and y increases
northward - a left-handed coordinate system.
In every case x and y refer to offsets at the time of observation, i.e., there are no corrections for nebular expansion.
Reference location in the data images:
Coordinates x and y refer to a particular location along the STIS slit, which corresponds to a particular fractional row number in each data image. This row number is specified by keyword CRPIX2 in the FITS header, and is often (but not always) close to 513.
Note that FITS row numbers begin with 1, not 0.