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Comment: added FAQ in RADESYS

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This is explained in PS1 Stack images, the "Photometric Calibration" section.

Why are the coordinates incorrect when I display a skycell image in DS9?

If the coordinates in your image appear to be far off (by arcminutes or more) from what you expect, you probably have encountered the RADESYS problem using recent versions of DS9.  The full skycell FITS images do not have a RADESYS keyword.  That leads some software to incorrectly interpret the PS1 image coordinates as being equinox 1950 rather than equinox 2000.  At the moment the only known software with this issue is DS9 v8, but it could happen with other software as well.  (Older versions of DS9 correctly interpret the PS1 FITS headers as J2000 coordinates.) 

The fix is to insert the keyword RADESYS = 'FK5' in the header.  FITS cutout images have a correct RADESYS keyword (as of 2019 March 13), but full skycell FITS images do not.

We hope eventually to fix this issue (and some other problems described in the "FITS image format quirks" section of the PS1 DR2 caveats page), but since the PS1 archive includes 1.5 petabytes of PS1 images, the task of updating all the image files is not simple.