StepIP

class sofia_redux.instruments.hawc.steps.stepip.StepIP

Bases: StepParent

Remove instrumental polarization from Stokes images.

This step subtracts instrumental polarization in the detector frame, either with a different value for each detector pixel, or with a uniform value assigned to all pixels.

Input for this step is DataFits containing Stokes and Error images for each of I, Q, and U, as well as COVAR Q I and COVAR U I images, a bad pixel image, and a table of data.

This step is typically run after sofia_redux.instruments.hawc.steps.StepWcs and before sofia_redux.instruments.hawc.steps.StepRotate. Output from this step contains the same extensions as the input, with values modified for the Stokes Q and U images, along with their associated error and covariance images.

Notes

The correction is applied as

\[Q' = Q - q' I\]

\[U' = U - u' I\]

and propagated to the associated error and covariance images as

\[\sigma_Q' = \sqrt{\sigma_Q^2 + (q' \sigma_I)^2 + 2q'\sigma_{QI}}\]

\[\sigma_U' = \sqrt{\sigma_U^2 + (u' \sigma_I)^2 + 2u'\sigma_{UI}}\]

\[\sigma_{Q'I} = \sigma_{QI} - q' \sigma_I^2\]

\[\sigma_{U'I} = \sigma_{UI} - u' \sigma_I^2\]

\[\sigma_{Q'U'} = -u' \sigma_{QI} - q' \sigma_{UI} + qu\sigma_I^2.\]

Note that the input \(\sigma_{QU}\) is assumed to be identically zero, since this correction is applied before rotation of the Q and U parameters.

Methods Summary

fill_nan_median(array)	Fill in NaNs in an array with its median value.
read_file_ip(fileip)	Read IP values from a file.
read_ip()	Read IP q and u values from the parameters.
run()	Run the data reduction algorithm.
setup()	Set parameters and metadata for the pipeline step.

Methods Documentation

fill_nan_median(array)

Fill in NaNs in an array with its median value.

Parameters:

arrayarray-like

The array to process.

Returns:

array-like

The array with NaN values replaced.

read_file_ip(fileip)

Read IP values from a file.

Extract correct arrays given the observing wavelength, and return them. For waveband X, the file must contain extensions ‘IP q band X’, ‘Error IP q band X’, ‘IP u band X’, and ‘Error IP u band X’.

Parameters:

fileipstr

Path to an IP FITS file.

Returns:

qiarray-like

IP q value matching the input data.

uiarray-like

IP u value matching the input data.

eqiarray-like

Error on the IP q value.

euiarray-like

Error on the IP u value.

bandstr

Filter band name.

read_ip()

Read IP q and u values from the parameters.

The parameters are expected to be defined as a list, with one entry for each HAWC band. The correct value for the input data is selected from the list.

Returns:

qifloat

IP q value for the input data.

uifloat

IP u value for the input data.

run()

Run the data reduction algorithm.

Because this step is single-in, single-out (SISO), self.datain must be a DataFits object. The output is also a DataFits object, stored in self.dataout.

The process is:

1. Determine the IP correction from the parameters.

2. Apply it to Q and U images, and propagate errors and covariances.

setup()

Set parameters and metadata for the pipeline step.

Output files have PRODTYPE = ‘ip’, and are named with the step abbreviation ‘IPS’.

Parameters defined for this step are:

qinstlist of float

Fractional instrumental polarization in q (one value for each waveband).

uinstlist of float

Fractional instrumental polarization in u (one value for each waveband).

fileipstr

If set to ‘uniform’, then the qinst and uinst values will be applied uniformly to all pixels. Otherwise, this parameter is expected to be a path to a FITS file containing IP values for each pixel, at each waveband.