Interferometer

Now assuming you've passed the RFI and real estate requirements, it's time to have some fun with the dark art of interferometry! You'll notice the technical level goes up a couple of notches from here on. You can skip the theory and jump right into the "doing it" section if you like but everything will make more sense with a bit of theory.

The original goal of Simple was to open interferometry up to anyone that wanted to give it a try. The theory of interferometry is quite complicated. I'll give you a quick overview here plus recommend some heavier sources for when you want to explore it further.

Theory time!

In order for a telescope to develop a clear image or "resolve" an object, the diameter of the collecting area must be a lot greater than the wavelength of the radiation it's collecting. This applies equally to both optical and radio telescopes. For an optical telescope, the wavelengths of visible light are in the range of 380 to 780 nanometres (thousand millionths of a metre) so the collecting mirrors are large in comparison. This means the optical telescope can resolve the details of the object and produce a clear, focused image.

For a radio telescope, the wavelengths can be tens of metres down to millimetres. A single dish antenna would have to be kilometres in diameter to focus the longer radio waves and would be too difficult to build.

Radio interferometry provides high [angular] resolution by using two or more small antenna elements to simulate the collective power of one large antenna. As an example we'll look at a simple two element telescope and a single object or "point source" that radiates radio waves.

The radio waves from the source alternately arrive in and out of phase as the Earth rotates. This causes slight changes in the difference in the path length between the two elements. When the signals from the two elements are combined they form interference patterns in the same way as for an optical interferometer. For a point source the difference in the path length to the elements varies across the source. The resulting fringes show the radio intensity or "brightness" across the sky.

Diagram of a simple 2 element interferometer

Changing the distance between the antennas (the baseline) allows us to explore the structure of the object we're studying. Long baselines increases the resolution and allows us to study small features. Short baselines allows us to study large scale structure.

To use a common optical example, low resolution (short baseline in RA) would allow us to see the moon as a whole, noting it's overall shape, distribution of craters etc. Switching to high resolution (long baseline RA) would allow us to study an individual crater more closely.

Lets have a look at some antenna patterns, so you can see the difference in coverage:

The following images are courtesy of David Brodrick

20m Baseline Antenna Pattern
150m Baseline Antenna Pattern

So, what difference does that make to the resulting waveforms?

Resulting waveforms from correlation of 20m (Blue), 45m (Pink) and 60m (Red) baselines

That's a pretty brutal summation of a huge topic like radio interferometry. I thoroughly recommend you dig further!

There's a nice (free) Web based tutorial from MIT here:
http://web.haystack.mit.edu/urei/tutorial.html

A very detailed account of interferometry is given by Thompson, Moran and Swenson in the excellent book, Interferometry and Synthesis in Radio Astronomy [Wiley-Interscience; 2nd edition (April 2001) ISBN: 0471254924]. This book is one of the RA "bibles" but it's very expensive.

 

Yeah, yeah, so what do you get with a "Simple"?

Example of 2 element Simple interferometer fringes (David Brodrick, Simple Too) [LST]

Here's a few notes from Dave on the above set of fringes after inital testing of his Simple Too interferometer:

"You may note the cental lobe for the CenA fringes is at ~13:20, exactly the LST when CenA is at transit. Likewise the Galactic fringes have a central lobe at ~17:45 LST, exactly corresponding to the transit of Saggitarius A, the center of the Milky Way. Seems the phases are well calibrated."

(Just wait until we get the imaging working... kawwww)

OK, on with the juicy details!