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Create Calibration files
There are currently three calibration steps needed to define the polarization calibration. The required data is now stored in polcal files in the data/ directory.
The files currently have the format of wavelength, half wave plate efficiency, position angle, quarter wave plate efficiency. In the future, the files should have the following formats: For files with multiple lines (for example wavelength dependance), they each should have their own file for each observation date they are made on. So they should have the following format:
RSSpol_Linear_Efficiency_20151205.txt
If the data consists of a single value which is measured regularly, they should be kept in their own file which is appended to with the format of observations date, value, and error.
RSSpol_Linear_PAZeropoint.txt
Each calibration data set that requires a separate observation is to have its own file.
Three calibration files are currently required for linear polarization. These describe
- the RSS half-wave plate efficiency and axis wavelength dependence,
- the linear polarization zero-point and its wavelength dependence (the “instrumental polarization”), which is assumed to originate in the SALT primary mirror and SAC, and
- the overall position angle absolute zeropoint, which is the sum of offsets in the halfwave axis zeropoint, the rho-drive mechanism, and the telescope azimuth mechanisms.
This assumes that there are not any corrections needed for the calibration of data due to pupil variations.
Half-wave plate efficiency/axis measurements are made by making observations with a lamp through polaroids and can be done during the day. The blocks describing the red and blue efficiency observations are available in 2015-2-COM-001. These tests only need to be run when major changes are made to the wave plates.
Filename: RSSpol_HW_Calibrations_yyyymmdd.txt
Sample:
# RSS halfwave plate calibration data
# Ang heff PA (deg, instrumental coordinates)
3000 1.00 117.00
3100 1.00 118.69
3150 1.00 119.95
…..
Data required:
900 l/mm Red and Blue daytime efficiency runs (2015-COM-001)
Reduction:
Polarization pipeline, with specpolfinalstokes flag:
HW_Cal_override = True
specpolfinalstokes will also override both corrections below, and will produce stokes.fits files in instrumental coordinates.
File creation:
RSSpol_HW_Calibration.py yyyymmdd <blueeff_stokes.fits redeff_stokes.fits ...>
Creates a calibration file in the cwd, which may be copied to the pipeline data directory.
Observations of an unpolarized standard are required in order measure the linear polarization zeropoint as a function wavelength. These are required monthly.
Filename: RSSpol_Linear_PolZeropoint_yyyymmdd.txt
Sample:
# Telescope zero point linear polarization
# Ang P (%) PA (deg, primary mirror coordinates)
3000 0.1 117.00
3100 0.1 118.69
3150 0.1 119.95
…..
Data required:
900 l/mm Red and Blue observations, Linear-Hi pattern, of one or more unpolarized standards.
See, eg, 2015-2-COM-003.
Reduction:
Polarization pipeline, with specpolfinalstokes flag:
Linear_PolZeropoint_override = True
specpolfinalstokes will produce stokes.fits files assuming zero instrumental polarization, in telescope coordinates (relative to the tracker x-y axis).
File creation:
RSSpol_ Linear_PolZeropoint.py yyyymmdd <blueunpol_stokes.fits redunpol_stokes.fits ...>
Creates a calibration file in the cwd, which may be copied to the pipeline data directory.
The position angle zeropoint is measured by observing a known, non-variable polarized standard and correcting the observed position angle to the known value. This should be measured monthly.
Filename: RSSpol_Linear_PAZeropoint.txt
Sample:
# RSS-Telescope PA offset
# date dPA(deg)
20151219 0.00 # this date and earlier
20160304 0.00
Data required:
Observation (300 or 900 l/mm, covering V or R-Band), Linear-Hi pattern, of one or more unpolarized standards. See, eg, 2015-2-COM-003.
Reduction:
Polarization pipeline, with specpolfinalstokes flag:
PAZeropoint_override = True
specpolfinalstokes will produce stokes.fits files assuming zero PA offset, in equatorial coordinates.
File creation:
RSSpol_ Linear_PAZeropoint.py yyyymmdd <polstd1_stokes.fits polstd2_stokes.fits ...>
Prints out a single text line which may be added to the pipeline data directory current version.
Circular polarization calibration files are still to be determined.