[KIDS] comparison of KIDS to SDSS photometry

[Francesco La Barbera]

Hi Jelte,

thanks for the detailed message. Let me reply to your questions (on behalf
of Mario, I guess).

>It seems this effect is stronger at RA between 183.5 and 185, and is
weaker (or absent) at RA between 185 and 186.5; @Francesco: is this
confirmed by the SDSS vs. >KiDS comparison?

>
> My interpretation on why this is only obvious in the p2w color and not in
> p2s and p2x, is that the 'problem' is strongest in i-band (the offsets in
> Mario+Francesco's plots seems larger in i than in the other filters) and
> p2w is the only principle color that depends on i.
>


As I was discussing with Mario last week, I can confirm you that the effect
seemed to be larger in i band.
Also, the effect is more significant for the following fields (based just
on my eyeball inspection):
185.0    -0.5
184.0    -0.5 (weak)
184.0     0.5
186.0    -0.5 (very weak)
186.0     0.5
185.0     0.5
i.e. all tiles at 184->186deg, with DEC within +/-0.5 !!! Those with
DEC=+-1.5 seem not to be affected.
I really don't know if this is an issue with either SDSS or KIDS
photometry.
We could give QSO's a try, I guess, but I'm not sure this would solve the
issue.
I will further discuss these issues with Mario on Monday morning......

Cheers,
F.



> Also, the fact that the effect is very small in p2w (~1 %) is probably
> because the calibration problem occurs in all SDSS filters, meaning that
> the colors are almost not affected.
>
> Perhaps there are other ways in which to go deeper into this, but right
> now I'm not sure exactly how. Perhaps another independent calibrator can be
> used... QSO's or something?
>
> In any case I have the feeling that the offsets we see might very well be
> due to a calibration issue in SDSS, or at least in the SDSS petroMags for
> stars.
>
> Cheers,
> Jelte
>
>
> On 28/11/12 10:21, Francesco La Barbera wrote:
>
> 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
>
>
>
> _______________________________________________
> KiDS mailing listKiDS at astro-wise.orghttp://listman.astro-wise.org/mailman/listinfo/kids
>
>
> --
> Dr. Jelte T. A. de Jong
> Sterrewacht Leiden
> Leiden University, Leiden, the Netherlands
> E: jelte at strw.leidenuniv.nl
> T: +31-(0)715275818
> W: jelte.jdejong.net
>
>
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