A note on SDSS a) magnitudes from fluxes for faint targets and b) zeropoint shifts for all targets

[sndik16]

For fainter targets, the formula in the equation (2) of the manual (v 3.1) is not exactly correct for SDSS magnitudes, because SDSS uses asinh magnitudes with the "softening parameter" term for very faint objects.

Also, SDSS zero points for u and z bands have a slight shift from AB system, for all targets.

Just a note, in case somebody's science question needs a very precise determination.

References:
https://www.sdss.org/dr12/algorithms/magnitudes/
http://www.sdss3.org/dr8/algorithms/fluxcal.php#SDSStoAB

Thank you very much,
Maria