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Comment: warp images do not use asinh scaling


Another issue with the image astrometry is that the full skycell FITS images do not have a RADESYS keyword.  That leads some software to incorrectly interpret the 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.  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.

The stack image pixel values have been non-linearly scaled using an arc hyperbolic sine (asinh) transformation that converts them to a pseudo-magnitude scale related to the asinh magnitudes (aka "luptitudes") that are used in the Sloan Digital Sky Survey. The scaling is determined by the BSOFTEN/BOFFSET keywords in the FITS header.  Here are the relevant lines from the header of the skycell 1725.051 g-band image:


The asinh flux scaling also applies only to the full skycell FITS images – FITS cutout images are converted to standard linear fluxes. Also note that the asinh transformation is applied only to the stacked images. The single-epoch warp images do not use asinh scaling.  Check for the presence of the BSOFTEN and BOFFSET keywords in the header to determine whether or not the sinh correction is required.