Cluster Cosmology » running stellar locus on VLT

h1. Wiki

This can be divided into 3 steps

h2. Step 1 run modelfitting photometry

you can find the scripts at /big/users/shantanu/VLTmfp/catalogs/runmfp_b.sh (for b band and so on)

note that since initial catalogs will be uncalibrated , use -mag_zeropoint for b,I and z bands.

The zeropoints for HST bands is :

F606W 26.493mag, F814W 25.947mag.

the sextractor param file used in the scripts is 

/big/des_home/stable/etc/sex.param_sersic

you can edit to remove params you don't need.

h2. Step 2 Produce a matched catalog across all 4 bands

This is the most tedious step.  Unfortunately you cannot run sextractor in dual image mode

since coadds are of different size.

The way I did it was to run a modified version of a code 

used in DESDM called coadd_grabmatches. I first ran it to match R vs I band and then used

that match catalog to match against z band and so on.

A code to do this is at /home/moon/shantanu/desdm/devel/ImageProc/trunk/src/coadd_grabmatches_singleband.c

There is one line in the code (below # 266) which needs to be changed depending upon number of coluns

in input catalog.

Once this is done you need to make a file with the matched magnitudes in all four bands

(its possible to automate this script with some effort so that it does not need to be changed)

h3. Step 3 Doing the stellar locus

One you have the catalogs all you need to do is to do a fit by eye or quantitative to SED fits provided by Joerg.

Here is my 2nd order polynomial  curve fit done to these relations

1) b-R vs R-I

b -R = -0.2904*x^2+1.319x +0.2941

(where x=R-I)

2) R-I vs I -z

R-I = -1.016*x^2+ 3.532*x +0.1294

where x=I-z

3) b-R vs I-z

R-I = -2.20378*x^2+ 3.96368*x +0.386878

You can either use (1) & (2) or (2)  & (3) to get the zeropoints.

I did the plotting in ROOT, but probably it can be done in any software.

h4. Step 4 Rerun sextractor with the new zeropoints

Same as (1) but use the new zeropoints now.

h3. Direct calibration of I-band

Joerg pointed out that I band can be directly calibrated using

I_BESS = a0 + a1 * F814W + a2 * (F606W - F814W)

with

a0 = -1.009

a1 =  1.067

a2 = -0.001

!{width: 10%}Izeropoint.png!

This allows us to get direct calibration of I-zeropoint= 28.37+/- 0.02 (where error = sigma of fit/sqrt(no of points))

We then recalibrated (b-R) vs (R-I)

!{width: 10%}plotslrfit.png!

Also a histogram wrt stellar locus

!{width: 10%}slrhisto.png!

the zeropoint for b band = 26.88

h1. Results for other clusters

h2. SPT-CLJ0102-4915

I band zero-point = 28.52 +/- 0.0137