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Hi Francesco and all,<br>
<br>
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.<br>
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.<br>
<br>
Here is what I did:<br>
- downloaded the DR8 petroMag data for stars for (183.5 < RA <
186.5) and (-2 < DEC < +2)<br>
- applied extinction correction (using values included in DR8)<br>
- calculated the principle colors <br>
(FYI: from Ivezic et al, 2004, AN, 325, 583<br>
p2s = -0.249*u0 + 0.794*g0 - 0.555*r0 + 0.234<br>
p2w = -0.227*g0 + 0.792*r0 - 0.567*i0 + 0.050<br>
p2x = 0.707*g0 - 0.707*r0 -0.988<br>
p1s = 0.910*u0 - 0.495*g0 - 0.415*r0 - 1.28<br>
p1w = 0.928*g0 - 0.556*r0 - 0.372*i0 - 0.425<br>
p1x = 1.0*r0 - 1.0*i0<br>
)<br>
- 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
<a class="moz-txt-link-freetext" href="http://wiki.astro-wise.org/projects:kids:photometry#sdss-style_principle_colors">http://wiki.astro-wise.org/projects:kids:photometry#sdss-style_principle_colors</a>)<br>
<br>
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.<br>
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.<br>
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!<br>
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?<br>
<br>
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.<br>
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.<br>
<br>
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?<br>
<br>
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.<br>
<br>
Cheers,<br>
Jelte<br>
<br>
<div class="moz-cite-prefix">On 28/11/12 10:21, Francesco La Barbera
wrote:<br>
</div>
<blockquote
cite="mid:CAOnmKXdxsq-8RhL+eMjforXCN8hKJ+KUOHWvLHAnMK2Zb0rs1A@mail.gmail.com"
type="cite">
<div>Dear All,</div>
<div><br>
</div>
<div>I summarize here the comparison of KIDS to SDSS
photometry we</div>
<div>(Mario+me) have performed so far for the 52 coadds of KIDS
INTR2. All</div>
<div>plots are included in the attached archive.</div>
<div><br>
</div>
<div>Mario has also performed a comparison to CFHTLS as well as
a first</div>
<div>attempt to characterize of color terms using SDSS and CFHTLS
data, but</div>
<div>I'm not going to include it in here (we didn't have much
time to look</div>
<div>at it in detail, so far).</div>
<div><br>
</div>
<div>We have matched the KIDSCAT catalogs to SDSS DR8, separating
stars and</div>
<div>galaxies according to SDSS classification.</div>
<div><br>
</div>
<div>The plots</div>
<div>
<br>
</div>
<div>sdss_${SOURCE}_${BAND}_${FIELD}_full_DR8.png </div>
<div><br>
</div>
<div>show differences in magnitude as a function of RA (upper
panel) and</div>
<div>DEC (lower panel), where </div>
<div>SOURCES -> gal/st refer to galaxies and stars,
respectively</div>
<div>BAND -> ugri</div>
<div>FIELD -> f135 plots the three fields with RA~131, 135, and
139</div>
<div> f185 plots the ten fields with RA~184, 185, and 186</div>
<div><br>
</div>
<div>We are using petromags from SDSS (for both stars and
galaxies).</div>
<div>KIDSCAT mags have been interpotaled to match the SDSS
4xPetroRad</div>
<div>(diameter) aperture for each object. Lines are running
medians for</div>
<div>different bands/pointings (the u-band comparison for galaxies
is quite</div>
<div>meaningless, considering the low S/N of SDSS u-band
data). Median</div>
<div>KIDS-SDSS offsets have been subtracted off for each field.</div>
<div><br>
</div>
<div>On average, the agreement is good, with no significant
variations</div>
<div>(larger than a few cenths of mags) across the fields. This
is further</div>
<div>shown in the attached plots</div>
<div><br>
</div>
<div>cmp_${BAND}band_phot_ra_dec_${SOURCES}_errmag0.05.jpg</div>
<div><br>
</div>
<div>where </div>
<div><br>
</div>
<div>BAND->ugri</div>
<div>SOURCES-> stars/galaxies</div>
<div><br>
</div>
<div>plotting magntiude differences as a function of RA and DEC
with all</div>
<div>fields overplotted (after subtracting off the RA and DEC of
each coadd</div>
<div>center). Median magnitude offsets have been removed (as
done for</div>
<div>previous plots), and only objects with better SDSS
photometry</div>
<div>(petroerr_mag<0.05mag) have been selected. Circles plot
all available</div>
<div>sources (for all 13 coadds in each band). Curves are
median-binned</div>
<div>trends (each bin including the same number of objects), with
different</div>
<div>colors corresponding to different fields. Notice that the
u-band plot</div>
<div>for galaxies is not included, for the reasons mentioned
above.</div>
<div><br>
</div>
<div>Finally, the plots </div>
<div><br>
</div>
<div>off_${BAND}band_phot_ra_dec_${SOURCES}_errmag0.05.jpg</div>
<div><br>
</div>
<div>show the distribution of median magnitude offsets for
all the 13</div>
<div>fields available in each band. In general, the
distributions have a</div>
<div>width of a few cenths of mags, but in one case
(KIDS_185.0_-0.5,</div>
<div>g-band) there is a large offset of ~0.1mag. We think it's
important to</div>
<div>investigate why this happened.</div>
<div><br>
</div>
<div>I'm also attaching two tables (produced by Mario, hopefully
in wiki</div>
<div>format) summarizing the the median offsets, mad, and peak
to peak</div>
<div>amplitudes in RA and DEC (to have an upper value to the
internal</div>
<div>photometric accuracy):</div>
<div><br>
</div>
<div>ov_sdss_gals.tab</div>
<div>ov_sdss_stars.tab</div>
<div><br>
</div>
<div>If necessary, I can provide further details/info on the
comparison</div>
<div>during today's teleconf. We'll try to put all this material
on the wiki as</div>
<div>soon as possible.</div>
<div><br>
</div>
<div>Cheers,</div>
<div>Francesco & Mario</div>
<div><br>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
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<a class="moz-txt-link-freetext" href="http://listman.astro-wise.org/mailman/listinfo/kids">http://listman.astro-wise.org/mailman/listinfo/kids</a></pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Dr. Jelte T. A. de Jong
Sterrewacht Leiden
Leiden University, Leiden, the Netherlands
E: <a class="moz-txt-link-abbreviated" href="mailto:jelte@strw.leidenuniv.nl">jelte@strw.leidenuniv.nl</a>
T: +31-(0)715275818
W: jelte.jdejong.net
</pre>
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