As discussed in the Front-End section, the output of the square-law detector is a DC voltage. This voltage is transmitted from the detector, mounted on the telescope, to the control room along coaxial cable. The signal is then sent into a A/D (analog to digital) converter before being passed onto the PC card located in a 386 computer. Hypothetically, it is not necessary to convert the analog signal to digital; however, if the use of a computer is required to analyze the signal from the telescope, this is a necessary step.
The A/D converter has adjustable input gain which allows one to scale the input analog DC voltage. The converter itself is simply a chip which converts analog DC voltage to digital DC voltage via the manufacturer's specifications. Because the chips require +/-15VDC to operate, a power supply is located next to the A/D converter to supply this necessary voltage.
The PC card is the interface between the computer and the A/D converter. It allows the computer to communicate with the converter, thereby allowing the computer to process the incoming signals and produce meaningful results.
The telescope is always "on" so to speak, as long as power to the frontend is supplied. During this time, the telescope continually receives and transmits signals to the A/D converter. However, it is only when the computer is allowed to talk to the converter and read the transmission sent from it, will the telescope operator actually be observing. Otherwise the signal is grounded away with no record of it kept.
The operator has control over when and how often the computer reads the A/D card through the software interfacing programme radio, which is found in the C:\radio directory and can be executed by typing in radio . The documentation for the programme gives the commands available to the user during an observing session with a brief description of each command's use. The programme automatically saves the data after exiting, and the observer should transfer the data to a floppy diskette at the end of the observing session.
As noted above, the inclusion of digital components into the backend is not strictly necessary and serves only as a modern convenience to data acquisition, albeit an important convenience, as experienced by our team near the end of the project. During a system calibration run at the beginning of an observing session, the A/D converter went "dead". It is suspected that the -15VDC plug fell out of the A/D box during operation, hence bringing forth the untimely death of one or more chips. As a consequence, further observations were done using an analog chart recorder, measuring directly the DC voltage sent from the frontend.
The disadvantages to the chart recorder are obvious; the data produced are marker lines on a piece of graph paper, not digital values ready to be loaded into a spreadsheet for further analysis. However, if all that is desired is the peak signal of the source, the chart recorder is sufficient, for the signal can simply be read off the graph paper using the known scale. Data from the chart recorder were used in our observations of Cassopeia A.