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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.

PS1 Timing

Why

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do the observation times in the image header and catalog

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disagree?

The times in the warp image headers and in the catalog (e.g., the obsTime column in the Detection table) are defined using international atomic time (TAI) rather than UTC time.  Those times differ by the addition of leap seconds, which leads to header times that differ by 34 or 35 seconds from the UTC times. (See Rots et al. 2015 for more details.) If you are concerned with timing at this level, you may need to convert the times to UTC.  For the warp images, the fix for this is to insert the keyword TIMESYS = 'TAI' in the header.  FITS cutout images have a correct TIMESYS keyword (as of 2022 January 20), but full skycell FITS images do not have a TIMESYS keyword. For the database times, the fix is to add subtract 34 or 35 seconds (depending on the date) to from the TAI time to get the UTC time.

Also note that the epoch given in the FITS header for the warp images is the start time of the observation, while the epoch in the database for Detection entries is the mean time of the observation, which is the start time plus 15 seconds (since the exposure time for PS1 images is 30 seconds).   So you should add 15 seconds to the warp header's MJD-OBS keyword value to get the equivalent value from the Detection.obsTime column in the PS1 database.

Thanks to Peter Van Wylen for discovering this issue and identifying the TIMESYS issue and helping identify the fix. And thanks to Jules Halpern for pointing out an error in the original description of the fix.