SFXC workshop 2025 • Geodesy

Introduction

SFXC can be used to produce correlated data sets in Mark4 format for use with the HOPS post-processing package. After post-processing the results can be used for geodesy and absolute astrometry.

Data download

This tutorial uses the same N24L2 dataset as some of the previous tutorials. If you did do so already, you can download this dataset from:

https://archive.jive.nl/sfxc-workshop/n24l2

On the workshop cluster the data is available available in the /data/n24l2 directory.

For this tutorial you will need the HOPS software. This tutorial has been tested using HOPS 3.26, but earlier versions should work as well If this software is not installed already on your system you can download the HOPS source code from:

https://web.mit.edu/haystack-www/hops/hops-3.26-4329.tar.gz

On the workshop cluster you can find the HOPS source code in the /data/src directory. A pre-compiled version is available in /home/kettenis/hops/x86_64-3.26.

Building HOPS (optional)

Instructions for building HOPS are provided in the README.txt file that comes with the source code. Installing shouldn’t be difficult; the hardest part is probably setting up its dependencies, in particular the PGPLOT package. The necessary dependencies have already been installed on the workshop cluster.

mkdir hops
cd hops
tar xfz /data/src/hops-3.26-4329.tar.gz
mkdir bld-3.26
cd bld-3.26
../hops-3.26/configure
make install

Basic Correlation

In order to create Mark4 data products we’ll need to have some correlated data. You can re-use some of the data that you have correlated earlier in the workshop. But if you want to start fresh, here is an example control file that will work:

{
    "cross_polarize": true,
    "number_channels": 32,
    "data_sources": {
        "De": [
            "file:///data/n24l2/files/n24l2_de_no0005.vdif"
        ],
        "Cm": [
            "file:///data/n24l2/files/n24l2_cm_no0005.vdif"
        ],
        "Ef": [
            "file:///data/n24l2/files/n24l2_ef_no0005"
        ],
        "Hh": [
            "file:///data/n24l2/files/n24l2_hh_no0005"
        ]
    },
    "stations": [
        "Cm",
        "De",
        "Ef",
        "Hh"
    ],
    "start": "2024y144d12h47m00s",
    "stop": "2024y144d12h47m20s",
    "channels": [
        "CH01",
        "CH02",
        "CH03",
        "CH04",
        "CH05",
        "CH06",
        "CH07",
        "CH08"
    ],
    "setup_station": "Ef",
    "exper_name": "N24L2",
    "output_file": "file:///home/workshop99/n24l2-geo/N24L2_No0005.cor",
    "integr_time": 2.0,
    "slices_per_integration": 4,
    "delay_directory": "file:///home/workshop99/n24l2-geo/delays",
    "scans": [
        "No0005"
    ]
}

Put this in a file called N24L2_No0005.ctrl and make sure you adjust the output_file and delay_directory parameters to point to appropriate locations. You can use the n24l2.vix VEX file that comes with the data for this correlation. Here is a quick reminder on how to correlate:

# Set PATH and CALC_DIR
# Copy n24l2.vix to your working directory
gen_all_delay_files.py n24l2.vix N24L2_No0005.ctrl
salloc -n16
mpirun `which sfxc` N24L2_No0005.ctrl n24l2.vix
exit

Geodesy post-processing

To convert the correlator output into Mark4 format you will need to use the sfxck2mark4.py tool. This tool isn’t automatically installed, but you can run it straight from the source directory. Assuming you checked out the SFXC source code in ~/src/sfxc you can convert the data above by:

~/src/sfxc/sfxc2mark4/sfxc2mark4.py n24l2.vix N24L2_No0005.ctrl

This will write output in the 1234 subdirectory. Take a look at the contents of that directory. Can you guess what the cryptic file names mean?

Now we can use the fourfit tools from HOPS to do some fringe-fitting and produce the famous plots. First we have to activate the HOPS environment:

. ~/hops/x86_64-3.26/bin/hops.bash

This should produce a message like:

Setup HOPS v3.26 with HOPS_ROOT=/home/workshop25/hops for x86_64-3.26

On the workshop cluster this also produces a bash error that seems to be harmless.

You can now run fourfit to produce plots in test mode, using the -t option, to inspect the data. If you are on a machine with X (or have set up X forwarding), you can use the -x option to get output on your screen:

fourfit -t -x 1234

Alternatively you can create a PDF file with the plots by using the -d ps2pdf option:

fourfit -t -d ps2pdf:n24l2 1234

Phasecal

If you look at the plots generated by fourfit, you’ll see that it isn’t entirely happy and reports an H error code. One of the reasons for this is that we didn’t provide any phase cal information.

To enable phasecal extraction in the correlator you will need to add a phasecal_file parameter to the control file. This parameter should name the file where the extracted phasecal information should be written. Our convention is to give these file a .pc extension.

Unfortunately sfxc2mark4.py does not support phasecal processing for mixed-bandwidth correlation yet. Therefore you need to remove Cm and De from the stations parameter in the control file. This isn’t a major issue since the e-MERLIN out-stations don’t support phasecal anyway. But a fix is being worked on.

Now recorrelate the date, re-run sfxc2mark4.py and re-run fourfit. Do you see any difference?

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