6 December, 2000

The magic that makes AST/RO work.

Like most things, the processes that allow AST/RO to collect and analyze light from molecular clouds are hidden, and can seem like magic. This magic happens inside the dewars and computers that first analyze the light.

From the sky, the light gets reflected into one of four dewars. Inside the dewar, the light from the skys first stop is into a feedhorn which then directs the light into a mixer.

Since the signals from the sky are extremely weak, we need to amplify the signal. To do this, we need mix it with another known signal. But before we can do this, we need to change the light into an electrical signal that we can easily manipulate. This is done with a SIS junction- two superconductors sandwiching an insulator. Superconductors are substances that have no electrical resistance, and thus transmit an electrical signal very well. With the SIS junction set up correctly, an incoming photon will cause an electron to cross this junction, thus creating an electrical current.

Once we have an electrical signal, we amplify and combine it with a known signal. We can change this combined signal so that it is in a frequency range that is easily analyzed by our computers, changing the signal from hundreds of gigahertz to tens of Hertz. Then, the known signal can be taken away, leaving our original signal from the sky in a frequency range that we can analyze easily.

It is important to mention that the superconductors need to be kept cold ­ very cold ­ in order for them to be a superconductor. Typically, this is only a few Kelvin, a few degrees above absolute zero. This is accomplished by keeping liquid helium in the dewar, which boils at a temperature of 4 Kelvin (4K).

AST/RO has four receivers: Major Dobbins (230 Gigahertz, GHz), FLaMR (490 GHz), Wanda (490 GHz and 810 GHz), and PoleSTAR (4-channel 810 GHz). More detailed information about how the receivers work can be found at the University of Arizona, where they are built: http://soral.as.arizona.edu -- the overview section is a good place to start.