"The Radio Telescope at Wiruna - Part One" - Steve Mencinsky

As was mentioned in previous Committee News, the Committee has approved our project plan for activating the ex-Fleurs dish at Wiruna into a working radio telescope. Our proposal was based on the concept of getting a working radio telescope for an acceptable cost using proven working component modules. In other words, "minimal experimenting". Once this initial telescope is up and going, depending on its performance and on sustained interest by members, further development and enhancements could then be contemplated. Another criterion was that although simplicity is important, it must not be at the cost of limiting further enhancements. The end result of this approach was a proposal for a Meridian Transit telescope using two electronic modules, operating on a single frequency, with data collection and management done by a small personal computer.

"Meridian Transit" was chosen over "Fully Steerable" ("Tracking") for a number of reasons.

  1. First and foremost, Fully Steerable is more complex. It can be regarded as an "add on" to the working telescope design.
  2. If the tracking components are not actually removed from the dish (they haven't been!), then the working Meridian Transit scope can later be upgraded to a Fully Steerable one.
  3. Full Steering obviously requires some kind of power mechanism, just as does an optical telescope. If this be an electric motor, we immediately have a problem that the power capacity budget at Ilford is very limited. If it is mechanical (e.g. a falling weight driving some kind of clockwork or escape mechanism) then we have much design, fabrication and assembly required.
  4. Full Steering of a radio telescope requires quite sophisticated pointing capabilities - often you cannot "see" the object at which the dish is pointed.
  5. Most objects at which the dish would be pointed are of a reasonably large angular size. Thus, they would take a reasonable time to "drift" through the dish's beam. This is good enough to provide a limited imaging capability.

    The bottom line - full steering and tracking is not absolutely necessary to show a reasonable "first light" capability.

    The "single frequency" approach was chosen for the simple fact that the best performance of commercially available low-noise high-gain electronics is when those components are built for a single frequency. The analogy here (in both its good points and its limitations) is with optical filters such as hydrogen-alpha, hydrogen-beta or OIII. The electronics offers very good gain ("magnification") on the frequency in question and at the same time good noise rejection off-frequency. Just like an optical filter - it is quite "dark" to "white" light yet transmits most of the desired "colour". The limitation of the single frequency is exactly the same - a limitation in the "types" of observable objects. What can you observe with an OIII filter? It's very, very good on planetary and some other emission nebulae. But galaxies, dark or reflection nebulae, faint stars? No.

    The practical solution is to find a single frequency that is, for want of a better word, "astronomically significant". Fortunately, there are a number of these, and the best compromise seems to be that of atomic hydrogen at a frequency of 1420 MHz, equivalent to a wavelength of 21 cm. Why is this the "best" compromise?

    On one side of the scale, the higher the frequency, the better the potential resolution of a given size dish. On the other side of the scale, the higher the frequency, the better the "figure" required, since the performance of the parabolic dish, just like that of an optical mirror, depends on the surface being accurately paraboloidal to within a given fraction of a wavelength. To put this into real numbers for our dish, at a wavelength of 21 cm, a one-tenth wavelength accuracy is obviously 21mm - a reasonable target. Add to this the fact that the dish was indeed designed and built for this very frequency makes this an easy choice. Further, we were advised by a number of professional astronomers that this particular wavelength would provide the greatest number of potential targets from which we could hope to detect a signal. Again to use an optical analogy, the 21cm Hydrogen line seemed as "astronomically universal" in the radio universe as the Hydrogen Alpha line is in the optical. Finally - and this proved to be the most important practical consideration - we were able to source high performance fully built and tested equipment for this frequency at a reasonable cost.

    1420 MHz is inside the bottom edge of the "microwave" band. The electronics for this is along exactly the same design (electronically and physically) as commercial satellite dishes and electronics. It has the same set of problems to which it offers the same set of solutions. Although good noise rejection and high gain can be achieved at reasonable cost, it is not a "convenient" frequency range at which to operate ALL of the electronics, nor is it a convenient frequency at which to transfer a signal through a long cable. The solution: to convert the single fixed frequency (or strictly speaking, the very narrow band around that fixed frequency) to a more manageable lower frequency band. In our case, we have a piece of equipment known as an LNC (Low Noise Converter) that amplifies the incoming signal and then converts it from a band of frequencies around 1420 MHz to a similar band in the 144-148 MHz range. This band is one of the designated amateur radio bands, for which a whole host of equipment is easily and economically available from many electronics stores. Since the LNC is the first component, the quality and noise rejection of the entire system depends most critically on this component. We have reason to believe that the specifications of the LNC that we have purchased are indeed very good in both of these aspects.

    For further noise reduction, the stub antenna of the LNC is mounted inside a "waveguide" and behind a "choke ring", the whole assembly then being placed at the focus of the dish. These components perform essentially the same function as the tube and baffling of an optical telescope - to minimise the amount of ambient "light". What does all of this look like? In most commercial equipment, the choke ring, waveguide and LNC are contained in the one aluminium casting, usually "box shaped", ranging in size from "cigarette box" to "shoebox", that sits at the focus of a typical satellite dish. In our case, where we will be operating at the "bottom" of the microwave band, it is all slightly too big (translate: too expensive) to conveniently make as the one component. Our LNC is contained in a cast aluminium housing, bolted to the waveguide and choke ring, which will be fabricated from thin sheet metal similar in size and perhaps in shape to a Mexican sombrero. To give some idea of the size, this image shows a prototype combination feedhorn/choke designed, built and tested by the same organization that designed and built the LNC. On that image, "OD" abbreviates "outside diameter". Ours will be very similar.

    Finally, as an example of the concept mentioned in the first paragraph - the ability to enhance and further develop the radio telescope - we can "retune" the entire telescope to another band by simply placing at the focus of the dish an LNC tuned to another frequency. This is the exact radio analogy of changing filters in front of the CCD or eyeball.

    The next article in this series will look at the dish and mounting. Its current state, work required, potential enhancements.