Hi All,<div><br></div><div>this is a "quick" test, showing the delta(mag) stacked plots (i.e. overplotting all 13 fields),</div><div>when one adopts PSFmags from SDSS and KIDSCAT large-diameter (8'') apertures.</div>
<div>I used the SDSS-KIDS matched catalogs produced by Mario. </div><div>I didn't plot single values but only binned trends. The x- and y-scales are the same as in previous plots.</div><div>Amazing how the 2.5deg patterns disappear!! Although in some cases (e.g. magenta curve for g-band comparison), you can see some trend, we should consider (IMHO) that PSFmags can still be affected by systematics of the order of 0.01-0.02mag, and we are using an arbitrarely(!) large aperture for KIDS data. Measuring mags in the same (large) apertures would possibly solve any residual aperture effect. </div>
<div>BTW, the amplitude of the trends is amazingly small !! </div><div><br></div><div>Cheers,</div><div>Francesco</div><div><br></div><div> <br><br><div class="gmail_quote">On Fri, Nov 30, 2012 at 10:40 AM, Konrad Kuijken <span dir="ltr"><<a href="mailto:kuijken@strw.leidenuniv.nl" target="_blank">kuijken@strw.leidenuniv.nl</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div>Hi all,</div><div>Beautiful work! Jelte and I looked at the plots yesterday and indeed concluded that the step functions correspond to the size of the Sloan chips. The pattern in iband is 2.5 deg wide, exactly the width of a stripe in Sloan. Two interleaved exposures, six chips with large gaps each, similar to a 1d version of the paw print pattern in vista, would give the step pattern you find. </div>
<div><br></div><div>One thing to check: does the effect show up if you use Sloan psf magnitudes? Petrosian mags might be a little seeing dependent, psf mags should be optimal if they have done their analysis right. </div>
<div><br></div><div>Thanks</div><div>Konrad<div><div class="h5"><br><br>On 29 Nov 2012, at 12:15, Jelte de Jong <<a href="mailto:jelte@strw.leidenuniv.nl" target="_blank">jelte@strw.leidenuniv.nl</a>> wrote:<br><br>
</div></div></div><div><div class="h5"><blockquote type="cite"><div>
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 href="http://wiki.astro-wise.org/projects:kids:photometry#sdss-style_principle_colors" target="_blank">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>On 28/11/12 10:21, Francesco La Barbera
wrote:<br>
</div>
<blockquote 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></fieldset>
<br>
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</blockquote>
<br>
<pre cols="72">--
Dr. Jelte T. A. de Jong
Sterrewacht Leiden
Leiden University, Leiden, the Netherlands
E: <a href="mailto:jelte@strw.leidenuniv.nl" target="_blank">jelte@strw.leidenuniv.nl</a>
T: <a href="tel:%2B31-%280%29715275818" value="+31715275818" target="_blank">+31-(0)715275818</a>
W: <a href="http://jelte.jdejong.net" target="_blank">jelte.jdejong.net</a>
</pre>
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