[KIDS] KIDSCAT completeness

[Francesco La Barbera]

Dear All,

I apologize for leaving yesterday before the end of teleconf .....
here it's an update about completeness and its implementation within
KIDSCAT.

The completeness algorithm is based on the idea of Garilli+99, A&A, 342,
408,
where one determines the magnitude at  which objects start to be lost in
the catalog because they are
below the brightness threshold in the detection cell. You can see an
application of this algorithm
to SDSS and UKIDSS data in our SPIDER-I paper, La Barbera et al.
2010MNRAS.408.1313L (e.g. Fig.~22).

Completeness is computed using the MAG_AUTO vs. DETECTION_MAG diagram, as
illustrated, for one field,
in the attached jpg
file, COMPL_Sci-DBOXHOORN-OMEGACAM-------OCAM_g_SDSS---Coadd---Sci-56240.5315550-6.jpg.
The bz2 archive also includes the same kind of plots (automatically
produced when running KIDSCAT) for all the 52 images.

Symbols in the plot(s) are:
blue dots -> stars, according to KIDSCAT classification
grey dots -> objects whose MAG_AUTO and DETECTION_MAG have low S/N (<5 and
4, respectively). These are excluded
in the computation of the limiting mag to make the computation of detection
limit more stable (see below).
black dots -> all remaining sources in the catalog (considering only
objects with S-Ex and IMA flags=0)

Notice that since we do not have the detection brightness for the sources,
I'm using the 2pixels diam. mag from KIDSCAT
as a proxy for it (hereafter, and in the plots, I refer to this as
DETECTION_MAG). This approximation should be OK for most purposes.
The distiibution of points in the diagram is approximately limited, by
construction (S-Ex detects objects if they are above a given S/N
in the detection cell), wrt DETECTION_MAG. I determine the detection limit
(vertical dashed line) by binning the data wrt
MAG_AUTO, and computing in each bin the 98-th percentile of the
DETECTION_MAG distribution. This gives us the red, solid, curve
in the diagram. A fixed part of the red curve, *below* the detection limit,
is fitted by a least-squares line (cyan line).
The intersection of the cyan line with the detection limit (red, dashed
line), gives you the completeness magnitude of the catalog
(horizontal dashed line). By construction, the catalog should be more than
98% complete below this completeness value.

Notice that one might use a second-order polynomial fit, rather than a
line, to model the solid, red, curve. After several
experiments, I decided to keep the linear fit. It gives more stable results
(in any case, the completeness estimates do not change
by more than 0.1-0.2mags). Notice that for u-band, because of the low
number of galaxies with good S/N, the completeness
estimates are somewhat uncertain, but still reasonable, in my view.

Of course, using external, deeper photometric catalogs, would help us to
obtain independent estimates of completeness,
to compare with our internal ones.

As usual, any comment/suggestion is more than welcome !!

Cheers,
Francesco
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://listman.astro-wise.org/pipermail/kids/attachments/20121129/0d38b038/attachment-0001.html 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: PLOTS_COMPLETENESS_KIDSCAT.tar.bz2
Type: application/x-bzip2
Size: 7118170 bytes
Desc: not available
Url : http://listman.astro-wise.org/pipermail/kids/attachments/20121129/0d38b038/attachment-0001.bz2 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: COMPL_Sci-DBOXHOORN-OMEGACAM-------OCAM_g_SDSS---Coadd---Sci-56240.5315550-6.jpg
Type: image/jpeg
Size: 168605 bytes
Desc: not available
Url : http://listman.astro-wise.org/pipermail/kids/attachments/20121129/0d38b038/attachment-0001.jpg