ALMA Near Field holographies support

Current support for ALMA near field holographies is limited. It is possible to produce aperture images but the near field phase effects are not yet fully corrected, making it impossible to produce panel adjustments from them.

ALMA near field ASDM to .holog.zarr filler

Information on the usage of the ALMA NF ASDM filler can be found in the etc section of our github page.

In case of pip install you have to visit the etc section of our github page and download the .py files from there.

In case of instalation from source the .py files can be found in `<astrohackdir>/etc/alma-nf-filler’.

After using the filler you will be left with a .holog.zarr file that contains the data necessary to produce the gridded beam image and the apertures.

Producing apertures

In astrohack the function that produces apertures from .holog.zarr files is called holog. This function grids the beam image from the data and then Fourier transforms (FT) it to produce the apertures. In the case of near field holographies after the first FT subsequent transformations are applied to correct for the non-fresnel terms followed by a step of correction of the aperture phase from the NF effects.

The apertures can be produced and then subsequently plot with a simple python script. In this script we call holog with the parameter alma_osf_pad set to the pad on which the antenna was positioned during the holography measurement. We set to parallel to False as holog parallelizes holographies per antenna and ALMA nf holographies contain a single antenna.

After the call to holog we are left with an object that contains the gridded beam images as well as the apertures. This object is reflect on disk by a .image.zarr file. From this object we can acces the plotting functions, plottextunderscore beams and plottextunderscore apertures.

from astrohack import holog

image_mds = holog(holog_name='almanf.holog.zarr',
              image_name='almanf.image.zarr',
              alma_osf_pad='TF03',
              parallel=False,
              overwrite=True # denpending on your preferences
              )

# Plotting gridded beam images
image_mds.plot_beams('almanf_plots', angle_unit='asec', display=False)
# Plotting aperture images
image_mds.plot_apertures('almanf_plots', polarization_state=['I'], display=False)

If after a while you want to interact once more with the image_mds object you can use the open_image function.

from astrohack import open_image

image_mds = open_image('almanf.image.zarr')

For more information on the API of holog and other astrohack functions can be found here. More detailed instruction on how to use the plotting functions can be found on our visualization tutorial.

Caveat

Currently the correction of the apertures for the NF effects is not working as it should. Aperture phases contain significant ringing that is not yet understood.