[KIDS] comparison of KIDS to SDSS photometry
Konrad Kuijken
kuijken at strw.leidenuniv.nl
Fri Nov 30 12:01:37 CET 2012
Hi all,
indeed amazingly flat plots, Francesco!
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.
Cheers,
Jelte
On 30/11/12 11:35, Francesco La Barbera wrote:
> Hi All,
>
> this is a "quick" test, showing the delta(mag) stacked plots (i.e.
> overplotting all 13 fields),
> when one adopts PSFmags from SDSS and KIDSCAT large-diameter (8'')
> apertures.
> I used the SDSS-KIDS matched catalogs produced by Mario.
> I didn't plot single values but only binned trends. The x- and
> y-scales are the same as in previous plots.
> 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.
> BTW, the amplitude of the trends is amazingly small !!
>
> Cheers,
> Francesco
>
>
>
> On Fri, Nov 30, 2012 at 10:40 AM, Konrad Kuijken
> <kuijken at strw.leidenuniv.nl <mailto:kuijken at strw.leidenuniv.nl>> wrote:
>
> Hi all,
> 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.
>
> 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.
>
> Thanks
> Konrad
>
>
> On 29 Nov 2012, at 12:15, Jelte de Jong <jelte at strw.leidenuniv.nl
> <mailto:jelte at strw.leidenuniv.nl>> wrote:
>
>> 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 <mailto: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 <mailto:jelte at strw.leidenuniv.nl>
>> T:+31-(0)715275818 <tel:%2B31-%280%29715275818>
>> W:jelte.jdejong.net <http://jelte.jdejong.net>
>> <DR8_p2w.eps>
>> _______________________________________________
>> KiDS mailing list
>> KiDS at astro-wise.org <mailto:KiDS at astro-wise.org>
>> http://listman.astro-wise.org/mailman/listinfo/kids
>
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