ziotom78 / grasp2alm

Package supporting conversion from GRASP beam format to spherical harmonic coefficients for CMB experiments

grasp2alm

This package supports the conversion from beam data calculated using GRASP for CMB experiments to spherical harmonic coefficients ($a_{lm}$) based on the HEALPix framework. The code is designed based on Beam, which is part of LevelS, the pipleline of the Planck experiment.

Instllation

git clone https://github.com/yusuke-takase/grasp2alm
cd grasp2alm
pip install -e .

Class Descriptions

Class: BeamGrid

Class to hold the data from a beam grid file of GRASP.

Methods

• to_polar(self, copol_axis="x"): Converts the beam grid to polar coordinates.
• plot(self, pol='co', color_resol=20, figsize=6, cmap="jet", return_fields=False): Plots the beam grid.

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Class: BeamCut

Class to hold the data from a beam cut file of GRASP.

Methods

• to_polar(self, copol_axis="x"): Converts the beam to polar coordinates.
• plot(self, pol='co', color_resol=20, figsize=6, cmap="jet",return_fields=False): Plots the beam.

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Class: BeamPolar

Represents beams of Stokes parameters. Type to store Stokes parameters of a beam on a spherical polar ($\theta\phi$) grid. This type is an input to the spherical harmonic transform. Internally, this package uses the 'Ludwig 3' definition for thepolarisation basis with the co-polar (positive Q) direction aligned with the y-axis.

Methods

• stokes_rotate(): Rotates $Q$ and $U$ Stokes parameters from the co-cross basis to the polar basis. The $Q$ and $U$ Stokes parameters are usually represented in the co-cross basis, where the co-polar direction is aligned with the y-axis (consistent with Ludwig 3 convention). For the purposes of extracting the spherical harmonic coefficients, it is more useful to represent them in the polar basis. Unlike the original Fortran method, this function operates on a deepcopy of the input BeamPolar, so it does not perform destructive actions.
• to_map(nside, nstokes=3, outOftheta_val=0.0, interp_method="linear"): Convert the beam to a map. Supported interp_methods are 'linear', 'nearest', 'slinear', 'cubic', 'quintic' and 'pchip'.
• plot(stokes="I", color_resol=20, figsize=6, cmap="jet", return_fields=False): Plots the beam.

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Class: BeamMap

Represents a beam map.

Methods

• to_alm(lmax=None,mmax=None,iter=3,pol=True,use_weights=False,datapath=None,gal_cut=0,use_pixel_weights=False,): Converts the beam map to spherical harmonic coefficients, which is often called $b_{lm}$.
• to_alm_lsq(lmax,mmax,pol=True,tol=1e-10,maxiter=20) Converts the beam map to spherical harmonic coefficients by using healpy.map2alm_lsq.

Function Descriptions

Function: grasp2alm

Convert a GRASP file to a spherical harmonic coefficients of BeamMap by using healpy.map2alm.

Arguments

• filepath (str): Path to the GRASP file.
• nside (int): Resolution parameter for the output beam map.
• lmax (int): The desired lmax parameters for the analysis.
• mmax (int): The desired mmax parameters for the analysis.
• outOftheta_val (float): Value to assign to pixels outside the valid theta range.
• interp_method (str): Interpolation method to use. Default is 'linear'. Supported are 'linear', 'nearest', 'slinear', 'cubic', 'quintic' and 'pchip'.
• copol_axis (str, optional): Axis of the co-polarization component. Defaults to 'x'.
• iter (int, scalar, optional): Number of iterations (default: 3).
• pol (bool, optional): If True, assumes input maps are TQU. Output will be TEB alm's. (input must be 1 or 3 maps) If False, apply spin 0 harmonic transform to each map. (input can be any number of maps) If there is only one input map, it has no effect. Default: True.
• use_weights (bool, scalar, optional): If True, use the ring weighting. Default: False.
• datapath (None or str, optional): If given, the directory where to find the pixel weights. See in the docstring above details on how to set it up.
• gal_cut (float [degrees]): Pixels at latitude in [-gal_cut;+gal_cut] are not taken into account.
• use_pixel_weights (bool, optional): If True, use pixel by pixel weighting, healpy will automatically download the weights, if needed.

Function: grasp2alm_lsq

Convert a GRASP file to a spherical harmonic coefficients of BeamMap by using healpy.map2alm_lsq.

Arguments

• filepath (str): Path to the GRASP file.
• nside (int): Resolution parameter for the output beam map.
• lmax (int): The desired lmax parameters for the analysis.
• mmax (int): The desired mmax parameters for the analysis.
• outOftheta_val (float): Value to assign to pixels outside the valid theta range.
• interp_method (str): Interpolation method to use. Default is 'linear'. Supported are 'linear', 'nearest', 'slinear', 'cubic', 'quintic' and 'pchip'.
• copol_axis (str, optional): Axis of the co-polarization component. Defaults to 'x'.
• pol (bool, optional): If True, assumes input maps are TQU. Output will be TEB alm's. (input must be 1 or 3 maps) If False, apply spin 0 harmonic transform to each map. (input can be any number of maps) If there is only one input map, it has no effect. Default: True.
• tol (float): The desired accuracy for the result. Once this is reached, the iteration stops.
• maxiter (int): Maximum iteration count after which the minimization is stopped.