[KIDS] comparison of KIDS to SDSS photometry
Jelte de Jong
jelte at strw.leidenuniv.nl
Thu Nov 29 12:15:05 CET 2012
Hi Francesco and all,
since I was quite intrigued by the strange offsets in the SDSS vs. KiDS
comparison for stars (and not for galaxies), I had a look at the DR8
data in the area of the 10 contiguous tiles in INT-DR2.
The problem here is of course that we need an independent calibrator
that we are certain is constant over the field-of-view, and the obvious
thing to me seemed to be the stellar locus.
Here is what I did:
- downloaded the DR8 petroMag data for stars for (183.5 < RA < 186.5)
and (-2 < DEC < +2)
- applied extinction correction (using values included in DR8)
- calculated the principle colors
(FYI: from Ivezic et al, 2004, AN, 325, 583
p2s = -0.249*u0 + 0.794*g0 - 0.555*r0 + 0.234
p2w = -0.227*g0 + 0.792*r0 - 0.567*i0 + 0.050
p2x = 0.707*g0 - 0.707*r0 -0.988
p1s = 0.910*u0 - 0.495*g0 - 0.415*r0 - 1.28
p1w = 0.928*g0 - 0.556*r0 - 0.372*i0 - 0.425
p1x = 1.0*r0 - 1.0*i0
)
- for the stars with r(petro,SDSS)<18 plotted the principle colors p2s,
p2w and p2x (using only the part of the stellar locus that is vertical
in these colors, e.g. see
http://wiki.astro-wise.org/projects:kids:photometry#sdss-style_principle_colors)
In the p2s and p2x projections I couldn't see anything specific, but in
p2w there does seem to be a variation, corresponding to the same offsets
seen in the plots Francesco sent around.
The attached plot shows the p2w color vs. DEC for three different RA
bins (middle panel is full RA range, top is low-RA half and bottom is
high-RA half of the area), with a running mean plotted in red.
I hope you can see (squinting might help ;) there is a 'modulation' in
the top and middle panel, and the 'phase' of it agrees with the offsets
in the SDSS vs. KiDS comparison!
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.
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 list
> KiDS at astro-wise.org
> http://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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