[KIDS] comparison of KIDS to SDSS photometry
Gijs Verdoes Kleijn
verdoes at astro.rug.nl
Fri Nov 30 12:46:57 CET 2012
Dear all,
I have assembled photometric homogeneity table as part of the release note:
http://wiki.astro-wise.org/projects:kids:data_deliveries:internaldelivery2.0#photometric_calibration
I think column 3 can now be replaced with your results.
That is more informative than current content: stdev in offsets of KiDS
vs SDSS for coadd as a whole.
Is MAD(running median) representative (i.e., free of measurement error)
to quote?
Best wishes, Gijs
ps. I think the flatness allows us to proudly zoom in on +/-0.1 in your
plotting.
On 11/30/2012 12:09 PM, Francesco La Barbera wrote:
> On Fri, Nov 30, 2012 at 12:01 PM, Konrad Kuijken
> <kuijken at strw.leidenuniv.nl <mailto:kuijken at strw.leidenuniv.nl>> wrote:
>
> 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.
>
>
> Yup, these are my conclusions too, Jelte !
> In any case, let's wait to hear also from Mario on Monday morning. We
> should stick at using petromags for galaxies and psfmags for stars,
> measuring KIDS mags accordingly. This is also crucial to properly
> characterize the color terms.
>
> Cheers,
> Francesco
>
> 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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>>
>>
>
>
>
>
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--
--------------------------------------------------------------------
|dr Gijs Verdoes Kleijn | astronomer |
|e-mail: verdoes at astro.rug.nl | OmegaCEN / Kapteyn Institute / |
|www: www.astro.rug.nl/~verdoes | Target |
|tel: +31-50-3638326 | University of Groningen |
|mobile: +31-654658050 | postal address: |
| | Kapteyn Astronomical Institute |
| | Postbus 800, 9700 AV, Groningen|
| | The Netherlands |
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