[KIDS] comparison of KIDS to SDSS photometry

[Francesco La Barbera]

Dear All,

I  summarize  here  the  comparison  of KIDS  to  SDSS  photometry  we
(Mario+me) have performed so far for  the 52 coadds of KIDS INTR2. All
plots are included in the attached archive.

Mario has  also performed a  comparison to CFHTLS  as well as  a first
attempt to characterize of color terms using SDSS and CFHTLS data, but
I'm not going to include it in  here (we didn't have much time to look
at it in detail, so far).

We have matched the KIDSCAT catalogs to SDSS DR8, separating stars and
galaxies according to SDSS classification.

The plots

sdss_${SOURCE}_${BAND}_${FIELD}_full_DR8.png

show differences  in magnitude as a  function of RA  (upper panel) and
DEC (lower panel), where
SOURCES -> gal/st refer to galaxies and stars, respectively
BAND -> ugri
FIELD -> f135 plots the three fields with RA~131, 135, and 139
         f185 plots the ten fields with RA~184, 185, and 186

We  are using  petromags  from  SDSS (for  both  stars and  galaxies).
KIDSCAT  mags have  been  interpotaled to  match  the SDSS  4xPetroRad
(diameter) aperture  for each object.   Lines are running  medians for
different bands/pointings (the u-band comparison for galaxies is quite
meaningless,  considering the  low S/N  of SDSS  u-band  data). Median
KIDS-SDSS offsets have been subtracted off for each field.

On  average, the  agreement is  good, with  no  significant variations
(larger than a few cenths of  mags) across the fields. This is further
shown in the attached plots

cmp_${BAND}band_phot_ra_dec_${SOURCES}_errmag0.05.jpg

where

BAND->ugri
SOURCES-> stars/galaxies

plotting magntiude  differences as a function  of RA and  DEC with all
fields overplotted (after subtracting off the RA and DEC of each coadd
center).   Median magnitude  offsets have  been removed  (as  done for
previous  plots),  and  only   objects  with  better  SDSS  photometry
(petroerr_mag<0.05mag) have been  selected. Circles plot all available
sources (for  all 13  coadds in each  band). Curves  are median-binned
trends (each bin including the same number of objects), with different
colors corresponding to different  fields. Notice that the u-band plot
for galaxies is not included, for the reasons mentioned above.

Finally, the plots

off_${BAND}band_phot_ra_dec_${SOURCES}_errmag0.05.jpg

show  the distribution  of median  magnitude  offsets for  all the  13
fields available in  each band.  In general, the  distributions have a
width  of a  few cenths  of mags,  but in  one  case (KIDS_185.0_-0.5,
g-band) there is a large offset of ~0.1mag. We think it's important to
investigate why this happened.

I'm also  attaching two tables  (produced by Mario, hopefully  in wiki
format)  summarizing the  the median  offsets, mad,  and peak  to peak
amplitudes  in RA  and DEC  (to have  an upper  value to  the internal
photometric accuracy):

ov_sdss_gals.tab
ov_sdss_stars.tab

If  necessary, I can  provide further  details/info on  the comparison
during today's teleconf. We'll try to put all this material on the wiki as
soon as possible.

Cheers,
Francesco & Mario
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