Flux Calibrator Models - Data Formats

Conventions and Data Formats

--CASA Developer--

This section describes the conventions, the formats as well as the locations of the data of the flux density calibrator models used in setjy. The detailed descriptions of specific flux standards and list of the calibrators are found in Flux Calibrator Models in the Reference Material section.

Extragalactic flux calibrator source models

The spectral flux density models are expressed in a polynomial in the form

\begin{equation}
log S[Jy] = a + b*log\nu + c*log^2\nu + …
\end{equation}

where $\nu$ is a frequency either in MHz or GHz depending on the standard. In setjy, the point source model is constructed as a componentlist scaled by the spectral flux density model. For the standards, Baars, Perley 90, Perley-Taylor 95, Perley-Taylor 99, Perley-Butler 2010, and Stevens-Reynolds 2016,the polynomial coefficients are hard-coded in the code.

For Perley-Butler 2013 and Scaife-Heald 2012, the coefficients are stored in CASA tables called PerleyButler2013Coeffs and ScaifeHeald2012Coeffs, respectively located in ~/nrao/VLA/standards/ in the CASA data directory(from CASA prompt, you can find the data root path by typing casa[‘dirs’][‘data’]). The separation of the data from the flux calibration code makes the maintenace easy and enable a user to acces the informaiton directly. Your can access these tables using the table tool (tb) and browsetable task. The list of the column header for PerleyButler2013Coeffs is shown below:

CASA <8>: tb.colnames
--------> tb.colnames()
Out[8]:
['Epoch',
'3C48_coeffs',
'3C48_coefferrs',
'3C138_coeffs',
'3C138_coefferrs',
'3C147_coeffs',
'3C147_coefferrs',
'3C286_coeffs',
'3C286_coefferrs',
'3C123_coeffs',
'3C123_coefferrs',
'3C295_coeffs',
'3C295_coefferrs',
'3C196_coeffs',
'3C196_coefferrs']

The coefficients of each source are stored in a column as a vector and the corresponding errors are stored in a seperate column. The different row represents the corresponding coefficinets at that epoch for the time variable sources while for the steady sources each row contains identical information. The frequency is assumed in GHz.

The list of the column header for ScaifeHeald2012Coeffs is shown below:

CASA <11>: tb.colnames
---------> tb.colnames()
Out[11]:
['Epoch',
'3C48_coeffs',
'3C48_coefferrs',
'3C147_coeffs',
'3C147_coefferrs',
'3C196_coeffs',
'3C196_coefferrs',
'3C286_coeffs',
'3C286_coefferrs',can b
'3C295_coeffs',
'3C295_coefferrs',
'3C380_coeffs',
'3C380_coefferrs']

 The reference frequnecy for Scaife-Heald 2012 is 150MHz. 

 

Solar System objects

For the solar system object used as a flux calibrator, setjy contstruct a model visibiity of the calibrator with the (averaged) brightness temperature model and ephemeris data of the sources as described in ALMA Memo #594. While the older Bulter-JPL-Horizons 2010 standard, hard-coded the brightness temperature models in the code, the models for Butler-JPL-Horizons 2012 are tabulated in ASCII files (SSobject_name_Tb.dat) located in the CASA data directory under ~/alma/SolarSystemModels. With an exception of Mars, the data for the brightness temparature models are stored in a simple format: 1st column - source frequency in GHz; 2nd column - the brightness temperature in Kelvin.  The follow example script shows how it can be plotted for Titan.

import numpy as np

rootdatapath=casa['dirs']['data']
source='Titan'
datapath=rootdatapath+'/alma/SolarSystemModels/'+source+'_Tb.dat'
data=np.genfromtxt(datapath)
data=data.transpose()

freq=data[0]
temp=data[1]
pl.plot(freq,temp)
pl.title(source+' Tb model')
pl.xlabel('Frequency (GHz)')
pl.ylabel('Tb (K)')

 

And the following is the output plot by executing the script above.

 

The Tb model for Mars (Mars_Tb.dat) is calculated as a function of time and frequency, with tabulations every hour and at frequencies of: 30, 80, 115, 150, 200, 230, 260, 300, 330, 360, 425, 650, 800, 950, and 1000 GHz. The first line of the file contain frequencies in GHz. The data starts at the second line of the file with the format:    YYYY MM DD HH MJD Tb for at each frequency sepearated by a space.

 

New Asteroid models

Ceres_fd_time.dat, Luthetia_fd_time.dat, Pallas_fd_time.dat, and Vesta_fd_time.dat contain thermophysical models by Th. Mueller (private communication). These time variable models are already converted to flux densities and are tabulated for 30, 80, 115, 150, 200, 230, 260, 300, 330, 360, 425, 650, 800, 950, and 1000 GHz. Time intevals are 1 hr. for Ceres and 15min. for Luthetia, Pallas, and Vesta with the data available from 2015 01 01 0UT to 2021 01 01 0 UT.  In setjy task,these models are automatically selected for the data with the observation dates falls within this time range.