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Hi Nicola,<br>
<br>
thanks for performing this analysis, and good that the photometry
seems nearly not affected.<br>
<br>
Do you also have plots of the delta-mag between the KiDS-CAT
magnitudes before and after IBSA?<br>
Comparing the difference on the KiDS photometry directly might give
us more detailed information than comparing both separately to SDSS.<br>
That would also allow a comparison at fainter magnitudes.<br>
<br>
Cheers,<br>
Jelte<br>
<br>
<div class="moz-cite-prefix">On 18/01/13 12:09, Nicola R. Napolitano
wrote:<br>
</div>
<blockquote cite="mid:50F92D65.4080300@na.astro.it" type="cite">
<br>
Hi Hugo and everybody,
<br>
<br>
Francesco and I finally got the chance to check the photometry on
the post improved background subtraction algorithm (IBSA) from
Hugo. The IBSA is very well improving the object detection as the
more regular background structure prevents spurious source
extraction. It remained to figure out whether the new background
affects object photometry.
<br>
<br>
To assess this we have compared KIDSCAT magnitudes within 5''
diameter apertures against SDSS petrosian magnitudes.
<br>
<br>
We have considered the KIDS_175.0_-0.5i field (see Fig. 1 before
and after IBSA on the left and right panel respectively) since all
others coadds from Hugo were mostly showing less severe horizontal
features.
<br>
<br>
We have first concentrated on the area affected by the CCD82
vignetting where the background correction has to have been
stronger (green area in Fig. 1).
<br>
Here we have considered all flag=0 sources from KIDSCAT matched to
SDSS and computed the delta_mag for images before-IBSA and
after-IBSA. We have selected about 900 sources in the three
catalogs.
<br>
<br>
The delta mags are plotted in Fig. 2 (delta_mag=
mag_SDSS-mag_KCAT). We have selected only sources brighter than 21
(SDSS i-band). Objects have generally consistent magnitudes
(mean~-0.06 both before and after IBSA respectively, mostly due to
an aperture effect) while the scatter of the histogram grows after
IBSA (sig=0.20, 0.26 before and after IBSA, respectively), in
particular because of an excess of sources after IBSA in the
negative tail. We have checked that this tails is not mag
dependent. Thus there seems that the IBSA possibly subtracts some
of the source flux. This likely happens for ~20% of the sources in
the given CCD82-selected area.
<br>
<br>
We have made the same plot for the whole image in order to see
whether this effect is present also in the area with less
problematic background. The delta_mag plot is shown in Fig. 3.
Here we see that the two distributions, although presenting yet a
negative tail, are much more overlapping with each other, which
means that the overall photometry on the whole field is almost
unchanged. Thus the IBSA does introduces a difference in the
photometry, but apparently this is mainly affecting sources in
regions affected by strong horizontal features.
<br>
<br>
Conclusions: IBSA improves source detection without basically
affecting the source photometry in the regions with moderate BKG
variation (e.g. normal CCD gaps), while it can introduce some
photometry variation in the area of strong CCD vignetting. We
propose to add a flag to the catalog for these latter cases in
order to warn about photometric reliability due to CCD failures.
<br>
<br>
Cheers,
<br>
<br>
Nicola and Francesco
<br>
<br>
On 1/10/13 11:30 AM, Hugo Buddelmeijer wrote:
<br>
<blockquote type="cite">Hi Nicola,
<br>
<br>
Thanks for testing that the bar removal indeed improved
KIDS_175.0_-0.5
<br>
i as well, indeed a troublesome observation.
<br>
<br>
We decided in the telecon to check the effect on the photometry
on a
<br>
full tile. KIDS_184.0_-1.5 is a good test case, since it shows
bar
<br>
problems in at least g, r and i. Could you compare the
photometry for
<br>
these coadds:
<br>
<br>
band: old object_id new object_id
<br>
u: ce4fc4615d66ebf2e0407d81e60e07f4
D2DE8960273B568DE0407D81E60E7CAD
<br>
g: ce4cd66f33e9ad69e0407d81e60e6525
D2DE4D015103EF62E0407D81E60E7A67
<br>
r: ce4cfe555662aa8ee0407d81e60e6745
D2ED2945BFE943D9E0407D81E60E02BC
<br>
i: ce4ebb600dd0e946e0407d81e60e7a9d
D2DE896026D9568DE0407D81E60E7CAD
<br>
<br>
These are all three improved by the new algorithm, although i
still
<br>
shows residual bars. The u band image is visually improved as
well,
<br>
although this is a side-effect. Links to the dbviewer are on the
wiki:
<br>
<a class="moz-txt-link-freetext" href="http://wiki.astro-wise.org/projects:kids:tbarvignetting">http://wiki.astro-wise.org/projects:kids:tbarvignetting</a>
<br>
<br>
Greetings,
<br>
Hugo
<br>
<br>
<br>
On 09/01/13 13:52, Nicola R. Napolitano wrote:
<br>
<blockquote type="cite">Hi Hugo and all,
<br>
<br>
the corrected images look very good, nice job.
<br>
We had a quick check to the image that looked worse than the
others with
<br>
clear severe features for CCD vignetting, i.e.
KIDS_175.0_-0.5i from the
<br>
wiki page.
<br>
In attachment the catalogs before and after the correction.
Red are all
<br>
sources, green the "good" ones.
<br>
The situation is indeed improved and also if there remains
some slight
<br>
horizontal features in the coadd in the area of CCD82, the
catalogs are
<br>
note significantly affected. Of course we want to look in more
details
<br>
at the extracted sources in that area.
<br>
<br>
All other images you sent in your examples with residual CCD
gaps did
<br>
not actually present spurious detection in catalogs, thus we
expect the
<br>
improvement here is on the photometry measurement uncertainty.
<br>
<br>
Cheers,
<br>
<br>
Nicola
<br>
<br>
<br>
<br>
On 1/9/13 9:56 AM, Hugo Buddelmeijer wrote:
<br>
<blockquote type="cite">Dear KiDS members,
<br>
<br>
An improved background subtraction algorithm for
RegriddedFrames was
<br>
created to remove the vignetting effect at the edges of the
CCDs. We
<br>
(Groningen) plan to use the method for KIDS ESO DR1.
<br>
<br>
* Example *
<br>
See this example CoaddedRegriddedFrame of KIDS_184.0_-0.5 in
r:
<br>
before: <a class="moz-txt-link-freetext" href="http://tinyurl.com/arce9oj">http://tinyurl.com/arce9oj</a>
<br>
after: <a class="moz-txt-link-freetext" href="http://tinyurl.com/aso2q5k">http://tinyurl.com/aso2q5k</a>
<br>
And in the i-band, KIDS_131.0_-1.5:
<br>
before: <a class="moz-txt-link-freetext" href="http://tinyurl.com/b33pu47">http://tinyurl.com/b33pu47</a>
<br>
after: <a class="moz-txt-link-freetext" href="http://tinyurl.com/a3muamz">http://tinyurl.com/a3muamz</a>
<br>
<br>
* Procedure *
<br>
The method is selected by choosing
BACKGROUND_SUBTRACTION_TYPE 4 for a
<br>
RegriddedFrame. (Only in 'current', not yet in 'AWBASE'.)
This performs
<br>
the following:
<br>
1) Create a mask to flag all sources and bad pixels.
<br>
2) Calculate the median value of the background pixels of
each row.
<br>
3) Subtract this median value from the row.
<br>
4) Let Swarp remove the rest of the background (as usual).
<br>
<br>
* KIDS ESO DR1 *
<br>
Based on our experiments the Groningen team suggests to use
the new
<br>
method for the KIDS ESO DR1 release. We plan to start
processing next
<br>
week so there will be time to reprocess bad cases, if they
arise. Do you
<br>
agree?
<br>
<br>
* Caveats *
<br>
There are currently no known cases where the quality of the
<br>
CoaddedRegriddedFrames decreases. Chip-filling galaxies will
probably be
<br>
a problem, but that was already the case with the original
method. The
<br>
vertical bar pattern seen occasionally at the corner CCDs is
not treated.
<br>
<br>
* Future *
<br>
The 'bar pattern' arises because the vignetting of the
background is
<br>
slightly different in the science images than in the
calibration flats.
<br>
It might therefore be possible to improve the flatfielding
to remove the
<br>
bar already during the creation of ReducedScienceFrames.
This requires
<br>
more investigation and will not be achieved in time for ESO
DR1.
<br>
<br>
* Sources *
<br>
The flux of the sources is affected differently by the
vignetting than
<br>
the background. Correcting the source flux requires a
separate solution
<br>
and is therefore beyond the scope of this discussion. (There
can be a
<br>
flux error of about 1-2% in the affected regions of the
individual
<br>
exposures, leading to a potential flux error of 0.2-0.4% in
the coadds.)
<br>
<br>
* More Information *
<br>
The wiki contains more information about the bars in general
and
<br>
examples of the improved background subtraction:
<br>
<a class="moz-txt-link-freetext" href="http://wiki.astro-wise.org/projects:kids:tbarvignetting">http://wiki.astro-wise.org/projects:kids:tbarvignetting</a>
<br>
In particular in the section "Improved background
subtraction".
<br>
<br>
Greetings,
<br>
Hugo Buddelmeijer
<br>
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<br>
</blockquote>
<br>
<br>
<br>
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</blockquote>
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</blockquote>
<br>
<br>
<br>
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<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
KiDS project manager
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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