AntConfig Tutorial
- Ver 0.1 - original (mdv and jmh)
- Ver 0.2 (2006-08-24) - updated for new lambda instead of nCreateGain etc (jmh)
Purpose
This tutorial is a good place to start when using the AntConfig program for the first time. See also the "Tips" tab for help on specific issues as well as the "Glossary" tab for explanation of terms.
Overview
AntConfig is a program to assist with array layout design for radio astronomy interferometric telescopes (i.e. arrays of receiving elements, rather than single element). The program allows you to specify physical layouts, easily move antennas around (either manually or using built-in optimization routines) and view the performance on the array through looking at a number of performance metrics and images (e.g. point spread function, effect on loaded sky scene, performance metrics, etc.).
Array layouts may be loaded from files, generated from standard configurations or even created manually within the program using mouse positioning.
The built-in optimization of antenna positions works by the user specifying the ideal point spread function and the program then iteratively creating new layouts which approach the desired performance, starting from the currently selected layout.
A lot of effort has been put into the user interface in order to quickly and easily convey a lot of information to the user. The program is very powerful with many options, but the basics are quite straightforward and the user should be able to make sense of most of it, by playing with the user interface and working through this introductory tutorial and the available tips.
Introductory Tutorial
Follow the instructions below and try to end up with an antenna layout similar to that shown in the FindKAT.ant file:
View the FindKAT.ant layouts
Load the FindKAT.ant layout file by going to the menu "File" -> "Open Layout Document..." and select FindKAT.ant from the files on the harddrive. If you don't have this file with your distribution, simply proceed anyway with a new layout document. Various views of the layout may be shown including physical configuration, uv-coverage, etc. Click on the "PSF", "UV" or "Img" buttons in the toolbar at the top for some standard view combinations or right click on a specific viewing pane and select the view you wish to have displayed from the available options. Note that the panes at the bottom of the screen allow you to switch between different layouts within the layout file (and those created subsequently) and also show layout performance parameters. Tip: use Pg-Up and Pg-Down in the "Layout Selection" pane to move between layouts (else mouse select).
Open new layout document
Now that you have seen a possible "answer", start a new layout document by going to the menu "File" -> "New Layout Document". A new set of windows will open which are blank since no antenna positions have been defined yet. Note that the FindKAT layout document will still be viewable if you left it open by selecting it from the top menu item "Window".
Add a compact core of 10 antennas
Add 10 antennas in an east west line, specifying a max baseline of 250 meters by right clicking on the "Antenna configuration" pane -> "Add new layouts...". Select "Style" -> "EW Line", "N antennas" -> 10, "Max baseline" -> 250". Your 10 antennas should become visible in the "antenna configuration" pane. Use mouse wheel to zoom in and out, right button drag to move the view around, left button drag to select antennas within rectangular area, double click to add antenna at location, single left click to select antenna or uv spanshot sample (if displayed), right click for options. Similar mouse functionality is available in the other view panes as well.
Optimize the initial 10 antennas for uniform UV coverage
This helps later for imaging in that the lower spatial fequencies are already well sampled so that additional antennas only need longer baselines and not short ones as well for Gaussian coverage. To perform this optimization of the layout for uniform UV coverage, start by clicking on the "UV" view in the toolbar since this will show the PSF profile and UV density histogram windows which are instructive for the optimization. Now go to the 'Ideal' tab in the "experiment" pane on the left and set "Baseline sigma" to 5000 and max baseline to 250 and the min baseline at 40. Please see the "Glossary" tab next to the "Tips" tab for explanation of parameters and labels such as "baseline sigma", "ideal", "derived from ideal", etc.
Under the "Site" tab, set the (array) latitude at -30 degrees, the (source) declination to -15 (the average case for optimization) and duration to 0 hours under the "Timing" tab. Take a look at thhe parameters under the "Optimization" tab. "Lambda" is a parameter which controls the amount of freedom that the optimization algorithm has. A small value means less perturbation of antenna positions between iterations and usually more stable convergence (though it could more easily get stuck in a local minimum). Usually "Lambda" will need to be smaller when there are fewer degrees of freedom (e.g. fewer antennas) else the optimization may become unstable.
Check that the "Major iterations" are set to 100 and "Linesearch iter" are set to 15 and press the "L" button on the main toolbar to optimize. The mouseover and status bar tips for the "L" optimization informs you that, with this option, the program will determine a reasonable value of "Lambda" itself and optimize the layout using this. If you wish to specify your own "Lambda", use one of the "Q" optimizers instead. After optimization uusing "L", you should end up with a roughly circular layout where the antennas may be slightly paired. Right click the antenna configuration view and choose 'protect this layout' to prevent it from being deleted or overwritten (a red colour will appear for this configuration in the "Layout Selection" pane and also in the title of the antenna configuration window. Select all the antennas and make them unmovable. To do this drag rectangle over the antennas then right click and select appropriate option. Unmovable antennas are shown with a square overlay. All going well, you should now have a layout with reasonably uniform UV coverage in snapshot observation mode.
Add 10 more antennas
Add 10 more antennas in east west line style with max baseline 1000m to the current layout (set "N layouts" to 0 in the 'add new layouts dialog box' to add to the existing layout). Because you protected the existing layout, a copy of it is made before the additional antennas are added (see the "Layout Selection" pane). Increase "Duration" to 4 hours under the "Timing" tab. Under the "Ideal" tab set "Ideal sigma" to 80 and then increase "Max baseline" to 1000m. Press "L" in the top toolbar to quick optimize. The optimization will take longer than previously and the progress is reported in the status bar at the bottom. You should end up with roughly have a filled Reuleaux triangle layout. Protect this layout and ensure that all the antennas are unmovable by selecting the appropriate ones (make use of the control key to invert selections) and choosing the correct right click menu option.
Add 20 more antennas
Add 20 more antennas in a circle spanning 2500m to this layout. Once again, the layout is cloned automatically because it is protected. Set "Ideal sigma" to 40 and increase "Max baseline" to 2500 under the "Ideal" tab. Optimize using the "L" button as before.
Load image
Load the iris256.fits image under the "Sky" tab and look at how the different layouts produce different results by selecting between them in the "Layout Selection" pane. To switch to the default set of views for images, click on the "Img" button in the top toolbar.
Compare your new layout with the original FindKAT solution
Hopefully, these should look similar :-) Note that repeating exactly the same steps will not produce the same array layout each time due to a time-varying random element which has been deliberately introduced into the optimization process.
Conclusion
There are many powerful features built into AntConfig. This tutorial gives you a taste of some of them. Don't forget to take a look through the built-in Tips and Glossary. Enjoy!!
