# How do I determine the zeropoint magnitude for a Coadd tile?

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I have coadd cutouts from Y1A1 in which I need to know the zeropoint . The header suggests that it is 30, however running SExtractor shows that using 30 gives a magnitude that does not match other catalogs for my specific data set (Strip 82, comparison to SDSS). Additionally, I know there are zeropoint magnitudes found in the Y1A1_coadd_src table which might be what I need, but I am unsure of how to match the COADD_IMAGEID and SRC_IMAGEID to the tiles and/or objects for my coadd cutouts.

When I ran SExtractor on the cutouts and compared to the catalog given by DES, there are offsets in magnitude ranging from 6-8 magnitudes (see attached ps file). There has to be some correction or additional offset to apply to these images. Additionally, I normalized the exposure time to 1s for my cutouts.  mag_auto_comp.ps

Thank you!

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Yep, as Robert Gruendl mentioned, Y1A1 is not calibrated directly against SDSS.  It does use SDSS in the calibrations, but the SDSS data have been transformed to an initial estimate of the DES AB magnitude system using these transformation equations:

Transformation equations for stars bluer than (g-r)_sdss = 1.2:

g_des = g_sdss - 0.104*(g-r)_sdss + 0.01

r_des = r_sdss - 0.102*(g-r)_sdss + 0.02

i_des = i_sdss - 0.256*(i-z)_sdss + 0.02

z_des = z_sdss - 0.086*(i-z)_sdss + 0.01

Y_des = Y_ukidss_ab + 0.238*(z_sdss - Y_ukidss_ab) + 0.00

Note that UKIDSS uses the Vega system, not the AB system.
To convert from UKIDSS Y-band from the Vega system to the AB system, add 0.634:
Y_ukidss_ab = Y_ukidss_vega + 0.634

These were not perfect, but did end up providing absolute calibrations to the DES AB magnitude system at the 0.004mag to 0.023mag level for griz and at the 0.05mag level for Y.

1. Johanna-Laina Fischer

Does this hold for galaxies? That is my main focus

2. Douglas Tucker

These transformation equations should hold reasonably well for normal spirals and ellipticals, but less so for emission-line galaxies (strong emission lines cause all sorts of problems for transforming between different photometric systems).  There could also a small dependence on type of magnitude used (MAG_AUTO, MAG_MODEL, ...) -- the above transformation equations were derived for MAG_PSF, which is effectively a "total" mag for stars and other point sources – but they still should be approximately true.

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I suspect that the Y1A1 DES zeropoint is not tied directly to SDSS system so there would be some formulaic offset.  Douglas Tucker or Eli Rykoff can likely comment.