Small python package to get the instrumental polarization of the VLTI. For details on the model and how the values are achieved see the paper Polarization analysis of the VLTI and GRAVITY
run:
cd VLTIpol
pip install .The package can be used to show the polarization properties of the VLTI. A Mueller or Jones matrix can be created for each telescope position:
import VLTIpol as vp
v = vp.CalibVLTI()
az = 10 # deg
el = 80 # deg
# Mueller
M = v.polmat_VLTI(az, el)
print(M)
[[ 1.00000000e+00 2.19203803e-02 -6.16133748e-03 1.38354392e-02]
[ 2.65928446e-02 8.53388556e-01 -2.34065865e-01 4.65766507e-01]
[-1.48820507e-03 5.21049075e-01 3.78736731e-01 -7.64433568e-01]
[ 7.00277534e-04 2.54027106e-03 8.95041752e-01 4.45179083e-01]]
# Jones
J = v.polmat_VLTI(az, el, jones=True)
print(J)
[[-0.11073747+0.69367468j 0.18918713-0.05982619j]
[-0.03124834+0.18939207j -0.65362468+0.20427131j]]For the Jones matrices each telescope is available with its individual properties. To get the matrix for a single telescope one can use:
J = v.polmat_VLTI(az, el, jones=True, telescope=1)With telescope ranging for 1 - 4 for UT 1 - 4. If telescope is not given (or None) or for Mueller matrix, the averaded properties are given.
Ultimatively the package is created to correct the instrumental effect in GRAVITY observations. This can be done as follows
import numpy as np
from astropy.io import fits
import VLTIpol as vp
# Values that one gets from the header of the GRAVITY files
header = fits.open('gravity.fits')[0].header
azimuth = header['ESO ISS AZ']
elevation = header['ESO ISS ALT']
paralactic_angle = (header['ESO ISS PARANG START'] +
header['ESO ISS PARANG END'])/2
# HWP and K-Mirror angle have to be calculated from encoder positions:
km_angle = np.mean([(header[f'ESO INS DROT{i} START']
+ header[f'ESO INS DROT{i} END'])/2
for i in range(1,5)])
hwp_angle = np.mean([(header[f'ESO INS DROT{i+4} START']
+ header[f'ESO INS DROT{i+4} END'])/2
for i in range(1,5)])
# VLTI & GRAVITY mueller matrix:
M = vp.calib_all(azimuth, elevation,
km_angle, hwp_angle, paralactic_angle)This gives you the full Mueller matrix of the VLTI & GRAVITY. The way how to apply it will then depend on your way on how to process the observing data. Examples can also be found in the connected paper