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Re: Interferometry of a Monotonic Source?

Subject: Re: Interferometry of a Monotonic Source?
From: "W. Watson"
Date: Thu, 09 Mar 2006 12:53:50 GMT
Newsgroups: sci.astro
Martin Brown wrote:

W. Watson wrote:

Let me ask some questions about the Java program. It's been quite a while since I thought about Fourier transforms, but I have a reasonable idea of what they are about. I've looked at the documentation and guide. After I formulated the questions below, I happened to glance at the end of the guide, and see there's more documentation.


Best to read the documentation too, but to play with it and get a feel for what an aperture synthesis instrument does it is invaluable as a simulator (mirrors exist at Jodrell Bank UK).
As it turns out, that's all the documentation there is. The above reference was just back to stuff they had referenced earlier.

The display is divided into four area. Upper left is the image, Upper right is some sort of positional display of antennas. The lower right is uv-coverage. The lower left is the uv components such as the real and imaginary part, phase and magnitude.


Top Left is the real space image (x,y)
(choose an object - wide double is good)
Click FFT and it will show you the visibility
Visibility? real, imag, phase, mag in the lower left? Ah, I suspect you mean what the selection looks like, e.g., two stars widely separated. Using wide double and FFT for ampl, the lower right shows vertical (hazy, unsharp) lines.

Bottom Left is now the FFT of the image (u,v)

Top Right allows you to position your observatory antennae
(place them at equal separations in a straight horizontal line)
I'm using ATCA and spread the six antenna out on a line equadistantly. The central area shows some small circles along the innermost lines.
Bottom Right are the measurements they will make (u,v)
(set observation range HA from -6h to +6h,
and dec to -90 for nice easy circular baseline tracks)

Press "Reset" to make the observations (set of red rings)
Which reset? I do see now some semi-circular rings in the uv-coverage. Ah, it looks like it was cranking through and now has red circles? Initially the uv coverage showed a line of red dots or dashes.

Now press apply to mask off the FFT of the true sky to leave only the measurments that your interferometer would have made.
The bottom left now contains concentric circles that are slighly broken up. What did I mask off?

Press "FFT-1" and you have the raw image from aperture synthesis.
Pressing FFT-1 doesn't produce anything -- yet. Five minutes later I don't see any change anywhere in the four areas.


It looks like this program is set up to operate for several specific observatories, ATCA, Merlin, etc. Suppose I want to define my own two station observatory. How can that be done?

What is uv coverage? If I play around the controls of that area, I see what look like red dots. They are distributed differently for the obs choice.


u-v coverage is where the baselines have measured the spatial frequencies of the sky brightness distribution.
So this is sort of like getting some idea of the noise for the entire sky? Are those red dots in the initial horizontal line associated with the array pattern?

What do the buttons in the lower right corner do? Apply, Add, Accumulate and Clear. Blue, red, hide? How about at the upper right? Default and Station Lock.
????

How does this actually operate? I guess I give it an object like a disk or point, and somehow start an observing session. If it simulates a period of operation (if specifiable), what do I actually end up with?


A good idea of what the image from aperture synthesis would look like.
I'm thinking in terms of what steps really occur to make that happen. I suspect I need a book like 'Apert Synth for Dummies". Anything on the web? I have this vague notion that I start with with an image, mess it with noise, convert it to the u-v domain, take observations some how that are in the u-v comain, fiddle with collecting them over a 12 hour period, and the convert a mess of stuff back to the spatial domain.

What should I have expected from above? The wide doubles as seen by the particular array given some sky noise? Is there a resulting image in one of the panels?


Once you have set up the model you can load different target sources and pretend to observe each one with VRI by clicking

Source
FFT
Apply
FFT-1

Hope this makes some sense. You can play with much more complex locations of antennae but interpretaion becomes harder. VLA style Y is fun.
Which choice is VLA?

Regards,
Martin Brown



         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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