[KIDS] KIDSCAT mag lim implementation
labarber at na.astro.it
labarber at na.astro.it
Thu Nov 22 14:20:22 CET 2012
Dear Jelte,
many thanks for the table with expected mag limits. They match what we've
been using
so far.
For what concerns the offsets, as you can see from Nicola's plot, they
amount to ~-0.3/-0.4
mag for r- and i-bands, being as large as -0.6 for u band. In i band, the
situation
is not trivial at all, as we get a wide range of values from -1 to 0!!
I agree that obs epoch might play a crucial role here. In fact, for the 10
images we processed
before ESO meeting, the r-band median offset was ~-0.2 mag, all those
frame being taken
before the end of Feb-2012. On the contrary, all the r-band 52 frames
we're considering now have
been observed after the end of Feb 2012. For i band, I verified that the
two coadds
with offset~0 are the ones observed at the end of 2011!!
BTW, we should look at the offsets as a function of obs epoch. A larger
statistics would be
very useful, I guess .....
I think we should also double-check our estimate of mag errors (once
again!!). We can compute
the errors from variance (i.e. weight) maps, and then compare to those we
get from S-Ex with
KIDSCAT. This is an important check as well, I think.
Cheers,
Francesco
On Thu, 22 Nov 2012 12:00:47 +0100, Jelte de Jong
<jelte at strw.leidenuniv.nl> wrote:
> Dear Francesco,
>
> the limiting mags, as written in the original proposal, are the
following:
> u: 24.8
> g: 25.4
> r: 25.2
> i: 24.2
> (see also for example on the KiDS website:
> http://kids.strw.leidenuniv.nl/techspecs.php).
>
> Seems to me that we're off by about 0.4 mag, of which 0.3 is probably
> attributable to the decrease of throughput of the system due to mirror
> reflectivity?
> Btw, we should be able to see this by looking at the limiting mags as
> function of observation date. Would be interesting to verify this.
>
> Cheers,
> Jelte
>
> On 22/11/12 09:53, Francesco La Barbera wrote:
>> Dear Gijs, et al.,
>>
>> On Thu, Nov 22, 2012 at 6:44 AM, Gijs Verdoes Kleijn
>> <verdoes at astro.rug.nl <mailto:verdoes at astro.rug.nl>> wrote:
>>
>> Dear Francesco et al,
>>
>> Many thanks for the input Francesco. I bring this conversation now
>> into the mailinglist space to ease following the discussion.
>>
>>
>> OK!
>>
>> I ran the procedure for all 52 fields. The procedure produces
>> a single plot, with SNR as a function
>> of 2'' diameter mag. One example is attached here, for the
>> field KIDS_185.0_1.5.r.
>> Black points are all good (flag==0) objects in the field. In
>> red you can see the median trend, while
>> the blue curve is the analytic fit
>> p_1*flux/(p_2*flux+p_3)
>> where p_1, p_2, and p_3 are free fitting parameters.
>>
>> Looks good! Could you share the script you are using? Then Hugo
>> and I will make a python version of it as part of the QC script
>> for sourcelists.
>>
>>
>> I'm attaching here the fortran program, which is part of KIDSCAT now.
>> In order to compile it, you
>> need the port library+pgplot. Binary versions of these libraries were
>> already included in the first KIDSCAT internal release. You should
>> have them already, otherwise, pls, just lemme know.
>>
>> The series of 4 (5?) curves are due to discrete steps in
>> integration times due to the dither pattern, right?
>> The displacement in magnitude seem in rough agreement with that
idea:
>> >>> -2.5*math.log10(4/5.)
>> 0.24227503252014099
>> >>> -2.5*math.log10(3/5.)
>> 0.55462187404089092
>> >>> -2.5*math.log10(1/5.)
>> 1.7474250108400469
>> >>> -2.5*math.log10(2/5.)
>> 0.99485002168009395
>>
>>
>> Yup, different patterns correspond to objects in regions with
>> different texp (because of dithering).
>> Your figures further confirm this.
>>
>>
>> The analytic fit is used to compute the typical mag
>> corresponding to SNR_LIMIT=5 (see dashed and
>> solid black lines in the plot). This is our limiting mag.
>> The procedure also computes the same kind of limit but for
>> different SNR_LIMIT(i.e. 10 and 15). These
>> values are stored in an ascii file (MLIM.dat), into the
>> subdirectory OUTPUT_RES.
>>
>> Good. Please store the analytic fit parameters as well. Those will
>> come in handy.
>>
>> Done. In the ascii file MLIM.dat, we also have now a third column with
>> p_i.
>>
>>
>> I made some tests according to what we mentioned some time ago:
>> 1) I can confirm that computing SNR from either FLUXERR or
>> MAGERR doesn't make any difference at all (just a sanity
check);
>>
>> Good.
>>
>> 2) using different detection tresholds does not change
>> significantly the mag lim estimates; the second attached plot
>> shows the effect of decreasing DETECT_TRESH from 2.5 to 2.0
>> for the same fit as in the example. The mag lim goes
>> from 24.77 to 24.78!!
>>
>> Good to know MAG_APER at 2" diameter is very insensitive to this
>> change in DETECT_THRESH. That might be different for e..g,
>> MAG_ISO. For now I think it is enough to restrict the lim. mag.
>> analysis to MAG_APER with 2". For those we have predictions we can
>> compare to (in KiDS proposal). KiDS team meeting is a good
>> opportunity if/how we need to extend the lim. mag. analysis.
>>
>>
>> Could you kindly provide us the predictions (the ones in hte porposal)
>> for different wavebands ? We (Nicola+me) are performing a systematic
>> comparison to predicted mag_lim's, and want to be sure to use
>> correct values.
>> Of course, the meeting will be an excellenet opportunity to discuss
>> possible extensions....
>>
>> Cheers!
>> F.
>>
>> Best wishes, Gijs
>>
>>
>> Pls, lemme know whatever comment/suggestion you have. Also, do
>> we want/need compute other limiting mags
>> (e.g. using MAGAUTO/different apertures)???
>>
>> Cheers,
>> Francesco
>>
>>
>> --
>>
--------------------------------------------------------------------
>> |dr Gijs Verdoes Kleijn | astronomer |
>> |e-mail:verdoes at astro.rug.nl
>> <mailto:e-mail%3Averdoes at astro.rug.nl> | OmegaCEN / Kapteyn
>> Institute / |
>> |www:www.astro.rug.nl/~verdoes
>> <http://www.astro.rug.nl/%7Everdoes> | Target
>> |
>> |tel: +31-50-3638326 <tel:%2B31-50-3638326> |
>> University of Groningen |
>> |mobile: +31-654658050 <tel:%2B31-654658050> | postal
>> address: |
>> | | Kapteyn Astronomical Institute
|
>> | | Postbus 800, 9700 AV,
Groningen|
>> | | The Netherlands |
>>
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