What the Spectrometer Does. First off, understand that in using the Spectrometer, the astronomer is after a "power" spectrum of the object they're studying. This is basically a plot with power noted on a vertical axis and frequency, usually labeled velocity or wavelength, on the horizontal. The Spectrometer's correlator produces this output in two steps:
Step one. The hardware produces what is termed the "correlation function" of the input signal. This is basically a measure of how similar the input signal is to a delayed (hence the term lag) version of itself. Each "lag" corresponds to one step of delay.
Step two. To produce a power spectrum (what the astronomer wants), the results of the correlation function produced above must be Fourier Transformed. Fourier transform refers to a mathematical tool used by scientists and engineers (and many others) to alter a problem into one that can be more easily solved. In our case looking at astronomical data, this means converting time domain data to a frequency domain data to produce a spectrum. The mathematics involved in this process is very complex. You can refer to published literature on this subject if you want a deeper understanding of the processes involved. As a final note, this Fourier transform is done on an analysis computer and not in the Spectrometer itself.
The following describes in simple terms how a signal "can" flow through the Spectrometer and it's various components. With 40 inputs, the Spectrometer can be configured in a multitude of ways with varying signal paths depending on the configuration chosen. Given this, only a very basic example is provided here.
A signal coming to the Spectrometer is initially routed to a 1.6 GHz Sampler. From here, it can be split to provide a signal to four separate 100 MHz Samplers. Each of these 100 MHz Sampler in turn, split the signal and send it to each of the four Sampler Distributor Cards. The Sampler Distributor Cards take the signal and split it again with one path going to a Prompt Memory Card, the other to a Delayed Memory Card. The signals from the memory cards are then sent to a Correlator Card. From there, the signal is sent to a Long Term Accumulator (LTA) followed by the Interface Card. The Interface Card finally sends it to the VME Computer.
The GBT operator is responsible for assisting astronomers in carrying out their observing programs using the Spectrometer. Refer to the GBT Operations Manual, procedures and checklists for specific guidance on setting up and operating this complex system.
Monitoring the state, status, and controlling certain electronic set-ups of the Spectrometer is done via Control Library for Engineers and Operators (CLEO) applications. These applications are run from the operator's control room workstation. They can also be accessed through other workstations on site under specified conditions. In addition to its primary monitoring function, this software package can also be used to manually control observing functions. Specific information concerning CLEO applications is included in an on-line CLEO Manual.
Observing with the GBT and using its variety of backend instruments will be done interactively through graphical control screens or by running pre-planned observing tables. This interface uses a "windows" type environment. Setting up various GBT instrument and backend devices may be accomplished in both automated and manual modes depending on the observers plan. Specifics' concerning this software, and how it can be used in connection with the Spectrometer, is discussed in the software's users manual. Please refer to it as necessary.