This last point is simple arithmetic:
The converted and amplified signal now completes the next and last step in its journey as a signal, which is to the radio receiver. "Traditional" amateur radiotelescopes usually use an ordinary receiver which produces as output a trace on a chart recorder. We rejected this approach for a number of reasons. Firstly, it is very limiting. Once you have your paper trace, what can you do with it? (Answer: very little.) Secondly, it is old technology, with all of its limitations and none of its charm; therefore it does not take advantage of what modern technology can and does offer. Thirdly, given the resources currently available us, it is actually a more expensive alternative; and given its limitations, therefore considerably - and we mean considerably - lower value for money. Fourthly, a chart recorder has many, many moving parts, and is therefore a low-reliability item. ASNSWI members who are prepared to admit to using chart recorders in the dim distant past may be able to attest to this.
Finally, we specifically wanted a computer-compatible radio receiver. That is, we wanted a receiver that converted the signal into some kind of digital data suitable for direct input into a personal computer. Thus, the data (digitised signal) could not only be captured on site, it could also be stored on diskette, then taken off site for further processing or display. Given the trend in amateur hi-tech hobbies in general and astronomy in particular (CCD cameras, image scanning, digital image processing, high-precision pointing and tracking accessories, computerised star charts, etc, etc), we saw no reason to remain stuck in the past and every reason to surge ahead to the future. But that can only happen if we take advantage of the most modern technology for this vertical application.
Our enquiries found several competitive commercial manufacturers, and found two whose products were within our price range, those being ICOM and WinRadio. We evaluated products from both of these, on several criteria, which can be perhaps best summarised as what was, in our opinion, the best value for money within an acceptable price range. For example, we found a magnificent complete receiver tailor-made for radio astronomy, but at a list price of around A$20,000 it seemed a little outside the price envelope.
We ultimately selected WinRadio's 1500e model radio receiver. This is an integrated radio receiver and computer card, which is enclosed in an external cabinet and communicates with the computer via a standard serial (COM) connection. The receiver converts the radio signal to a digital one, and in this form it is then processed and displayed on the computer.
Arguably one of the biggest advantages of any computerised product lies not in what one has now but what might be available in the future. For example, from the WinRadio manufacturer there is already available a suite of signal processing programs that provides a number of potentially useful features such as an audio oscilloscope, audio spectrum analyser, an interface between the receiver and the computer's sound card (if it has one), a "wave" file recorder (listen to the sounds of space on your PC at home or at work!). There are also rumours (which we are vigorously pursuing) that there are some home-grown specialized radio astronomy programs especially for WinRadio.
This is all fine, but what does one actually see on the computer screen? Everything is accomplished through two displays. Image 1 shows the overall "control" screen, which has been deliberately designed to resemble the control panel of a commercial radio receiver, with the difference being that it is a "virtual" control panel in that the buttons and such are manipulated through the computer's mouse, rather than being "real" buttons. Note that the screen does indeed show 1420MHz even though the output from the Low Noise Converter has been converted to 144MHz - the WinRadio software allows for the possibility of frequency conversion.
Selecting "Spectrum Scope" from this control panel shows a second screen, shown in image 2. As can be seen, this shows the continually changing spectrum being received in real-time. A number of aspects of this spectrum can be displayed - maximum, minimum, current. More are scheduled in future releases of the software. Note that both images are "demonstration" ones, as the telescope is of course not operational yet.
This concludes the overview of the components of the radio telescope. The next article in this series will discuss issues such as "infrastructure", integration of these components, and actual operation.