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Discussion of Example 1 (VLA Image Header)

This header describes an image of 3C405 (OBJECT keyword) on 27 October 1982 (DATE-OBS keyword) by an observer with the designation ``Perl'' (OBSERVER keyword). The data matrix has four axes: right ascension, declination, frequency, and polarization (NAXISn and CTYPEn keywords). The matrix here contains an image for only one frequency and polarization. The sine projection from the sky sphere to the tangent plane is used. The coordinates are equinox 1950 (note the use of the EPOCH keyword, now discouraged; use EQUINOX instead). Right ascension is in degrees, extending from approximately 299.47779 degrees at point (pixel) 1 to approximately 299.3925 at point 2048, with the value 299.435165226 at point 1024 (CRVAL1, CDELT1, CRPIX1). Declination, in degrees, runs from approximately 40.57486 to approximately 40.61848, with the value 40.5961940065 at pixel 513 (CRVAL2, CDELT2, CRPIX2). The measurement is at 4866.35 MHz (CRVAL3). CDELT3 is not significant here; there is only one point. The values are stored in 16-bit integer format (BITPIX keyword). Values in the matrix are converted to flux in Janskys per beam (BUNIT keyword) by multiplying by 7.04625720812 (BSCALE keyword) and adding 2.18688869476 (BZERO keyword), corresponding to a range of data matrix values of -0.121747084 (physical value) or -32764 (in file) to 4.495524406 (physical value) or 32764 (in file) Janskys per beam. The EXTEND keyword has the value T, indicating that this Basic FITS unit may be followed by extensions; the comment notes that it is followed by tables. The use of the HISTORY keywords may appear to be analogous to the HIERARCH proposal; however, the embedded ``keywords'' have meaning only within AIPS; to the rest of the world they are comments and can be treated as such.


next up previous contents
Next: Example 2:  M87 and Up: Examples of FITS Headers Previous: Example 1:  VLA Image