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Hi all,<br>
<br>
indeed amazingly flat plots, Francesco!<br>
I suppose the conclusion from this is that we shouldn't use
petroMags for comparing stellar magnitudes, and that our coadd
photometry is pretty flat now.<br>
<br>
Cheers,<br>
Jelte<br>
<br>
<div class="moz-cite-prefix">On 30/11/12 11:35, Francesco La Barbera
wrote:<br>
</div>
<blockquote
cite="mid:CAOnmKXeavdpN4ycQ7qhnw0GXWbAdvc51EdEtcYvbftkRts8MHQ@mail.gmail.com"
type="cite">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 moz-do-not-send="true"
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
moz-do-not-send="true"
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 moz-do-not-send="true"
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 moz-do-not-send="true" href="mailto:jelte@strw.leidenuniv.nl" target="_blank">jelte@strw.leidenuniv.nl</a>
T: <a moz-do-not-send="true" href="tel:%2B31-%280%29715275818" value="+31715275818" target="_blank">+31-(0)715275818</a>
W: <a moz-do-not-send="true" href="http://jelte.jdejong.net" target="_blank">jelte.jdejong.net</a>
</pre>
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</blockquote>
</div>
</div>
<blockquote type="cite">
<div><DR8_p2w.eps></div>
</blockquote>
<div class="im">
<blockquote type="cite">
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