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Simulating Imaging of Large Scale Radio Arrays on the Lunar Surface
Chapters
Summary July 30th, 2020
A simulation framework for testing the imaging capabilities of large-scale radio arrays on the lunar surface is presented. Major noise components are discussed, and a software pipeline is walked through with details on how to customize it for novel scientific uses.
Transcript
This package extends an industry standard simulation package for radio astronomy CASA for the use of lunar arrays, a re-emerging field of interest with many scientific possibilities. Before beginning a simulation, navigate to the Deep Blue Data website and download the software package. The software has only been tested in a Unix environment and may not fully function in other environments.
To customize the createarrayconfig. py script, provide a list of longitude and latitude coordinates for each antenna to select the configuration of the array and change the lunar path variable in the script to reflect the new download location of the digital elevation model containing the elevation data of the lunar surface. Use the command to run the createarrayconfig.
py script to use the lunar digital elevation model to solve for the elevation at each longitude and latitude for each antenna. Save the longitude, latitude, and elevation to files and print to the screen for easy copying and pasting into the next script, then make figures showing the array configuration on top of the local lunar topography. To customize the eqrovertimeearth.
c script, copy the longitude, latitude, and each antenna elevation output into the corresponding lists in the script and update the numspacecraft variable with the number of receivers and corresponding coordinates. Update the lunar_furnsh. txt included in the package with the new path names for the required frame and ephemeris files and specify the set of dates on which the observations should occur to inform the ephemerides within SPICE to accurately track where the Earth and sun are in relation to the defined array for those dates.
Specify the targeted area of the sky for the array to track and image. Next, use the GCC command to compile the eqrovertime. c script and change the paths to reflect where the cspice libraries are located.
Use the command to run the equatorial array over time executable to obtain a number of files, each with a set of variables in them. Most important are the XYZ position of each antenna in the J2000 coordinates and the right ascension and declination coordinates of the targeted area of the sky, then save output variables to txt files containing the data for all the requested dates. To customize the lunarearthpickfreakintegration.
py script, specify the observing frequency for the array at which to make an image and specify a CASA compatible truth image with jansky pixel values for the array to reconstruct. Change the constants in the code to reflect the size and resolution of the input truth image. Use the command to run the lunarearthpick.
py script. The negative numsc flag is used to inform the code how many antenna and/or receivers are being used and helps unpack the data from the txt files containing the receiver coordinates. To customize the noisecopies.
py script, set the system equivalent flux density and set the bandwidth being integrated over in variable noise line 200 to 500 kilohertz. Set the integration time in variable noise line 200 and use the command to run the noisecopies. py script.
The script will first create an image from the noiseless visibility data, calling standard ratio astronomy algorithm clean to create the image. The script will then create copies of the measurement set and add the appropriate noise level to the complex visibility data before using clean to image the data for a range of integration times up to 24 hours and over several robust weighting scheme values. Depending on the configuration of the array, the image quality may vary with the choice of data weighting schemes.
Running createarrayconfig. py as demonstrated should create an elevation map similar to that presented in which the configuration of the defined array is plotted on top of the local topography of the lunar surface as derived from the lunar reconnaissance orbiter Lunar Orbiter Laser Altimeter derived digital elevation model. Using CASA to simulate an array response should result in a similar output to that observed here which can be used to calculate the visibility data.
Data imaging can then generate noiseless and noisy images with the noisy images appearing less clear than the noiseless images. This protocol utilizes a combination of astronomical charts from NASA's SPICE package alongside elevation maps of the moon surface using lunar reconnaissance orbiter data to accurately simulate any array on the moon.
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