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Operating VRI (Radio Interferometry)

Subject: Operating VRI Radio Interferometry
From: "W. Watson"
Date: Thu, 16 Mar 2006 23:01:21 GMT
Newsgroups: sci.astro
On another thread, we drifted off on to the topic of the Java VRI program that's on the web at <http://www.narrabri.atnf.csiro.au/astronomy/vri.html>. That thread is getting a bit long, so I thought I'd pick up on VRI here. I'm interested in completing a single example. The other thread gave me some clues how to proceed, but I thought I'd try this out here (* see note at bottom). This is pretty much an arbitrary and ficticious example based on a limited knowledge of how this works. I learned in the other post that the sequence, Source, FFT, Apply and FFT-1 is basic to the operation. The rest was obtained by playing with this.

There are four panels arranged as:

Image Panel (IP)     |  Antenna Panel (AP)
-------------------------------------------
Transform Panel (TP) |  Coverage Panel (CP)

AP has a green background with a single dish icon
CP has a background with a red point. If the nearby
In (Zoom) button is pressed, it's dashed line.

I created the acronyms for convenience here.

There are various buttons around the panel. Initially, there are three buttons to the side of each panel that are common: In (Zoom), Out and Reset. As I proceed with the example as below, a FFT and FFT-1 (inverse) will appear. Above the four panels are fill-in boxes for the source and antenna. Below the panels are fill-in boxes for the source, HA interval, frequency and bandwidth of the source, and source declination.

1. Select source as Wide Double, two point objects appear in the IP
2. Set the Lat to 45.0. Press Enter to make sure it's set.
   Use the slider bars at the bottom to set
   the Dec to 0.0 and the HAs to -5.1 (top slider) and +5.1. You may
   not get +/- 5.1, so do what you can to be close.
3. Drag the antenna icon four times to form a square with
   one icon in the center. Make each side about 1".
     A     A
        A
     A     A
   Press the Plot button and some red horizontal lines will appear
   in the CP. The FFT button appears on the left. When this button
   appears seems to vary. It may appear in an earlier step.
4. Press the FFT button and vertical grey lines appear in the FP
   with a white background, and the FFT-1 button appears on the left.
5. Press the Apply button below the CP. if The Ampl choice is set
   below the TP, then a group of dashed horizontal lines appear in the FP.
6. Press FFT-1. Two vertical rows appear of dots (circles?) in the IP.

So what have I got in the IP? Can someone offer a more meaningful example?


* I find VRT sometimes rather erratic (inconsistent) in its behavior. Sometimes buttons like FFT and FFT-1 just appear at almost a whim. When I'm forced to start again because of some missed item, I reload the page. Proceeding again often does not get quite the same responses. The behavior of this program is somewhat inconsistent. If one sets up the antennas as above, but uses the sliders before setting them, then the extreme values (5.1 vs 4.9 hrs) will be slightly different than if done afterwards. The initial default value for lat (-30.xxx) changes in the number of decimal points once you select Custom. I sometimes wonder about the effectiveness of the Reset button. Sometimes it doesn't seem to help recover from a previous bad entry.

         Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
             (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
              Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
                "Nature invented space so that everything didn't
                 have to happen at Princeton." -- Martin Rees,
                 Britain's Royal Astronomer, in a lecture at Princeton

                    Web Page: <home.earthlink.net/~mtnviews>


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