"Radio Telescope at Wiruna - Part Two" - Steve Mencinsky

As promised, this article, the second in this series, will look at the dish and mounting, its current state, work required and potential enhancements. In a sentence, the dish and mounting are "useable as they are, but...". We have it from several people who worked on the mount in its original location that it is over-engineered for the dish, but as this is not a portable instrument that appears to have only good points.

The mounting appears to be oriented close enough to north-south to be serviceable. (We are not trying to measure positions of radio sources to within microseconds!) All moving parts move reasonably freely after much cleaning and lubrication. We have scraped off loose rust and painted over corroded surfaces with zinc-rich paint as much as we possibly can. In a couple of cases the corrosion is so bad that it has prised apart some surfaces, but we believe that we have sealed all of these surfaces and that the structural integrity of the dish seems to be intact. The only tasks to be done here seem to be simple ones:

  1. To rearrange the attachment of the counterpoise springs to take the weight off the declination shaft when the dish is pointing away from the zenith.
  2. To attach a larger wheel to turn the declination shaft; although if the previous step is very successful this step may be unnecessary.
  3. To restore the declination circle/pointer and accurately repoint it, to within a degree will be sufficient for our purposes. Arrange a dim light over it for night use.
  4. To scribe or engrave some registration marks on the RA shaft/gear/wheel assembly to ensure that the meridian pointing can be restored in case of accidental (or malicious!) movement.

    One final comment here relates to the limit of movement of the whole assembly. In its original configuration, the equatorial head was on a mounting nearly three metres tall which allowed it to be tilted "almost" to the northern horizon. The current mounting is only about one metre high, thus placing a northern limit of approximately +30 degrees in declination. Regrettably, this means that listening to any signal from Vega will be beyond our capabilities.

    The status of the dish itself is a much more open issue. Much of the mesh is corroded, but it would appear that the better thing to do here is to attempt to repair (sandpaper and resoldering) what is already there rather than to attempt to replace any of the mesh.

    The ribs are in good condition. We mounted an expedition to Fleurs to examine the dishes in their original condition. In that expedition we secured replacements for the ribs that have been cut. We also decided that the modifications for the mounting for the south-east and south-west ribs was a poor compromise made necessary by the original use of the dish. The Fleurs dishes were originally part of an "aperture synthesis" telescope which required that they be able to track an object for more than twelve hours at a time. This obviously required the ability to point almost everywhere in the observable hemisphere. Since our requirements will be considerably less severe, even in a fully tracking environment, we have designed an alternative that is simpler, stronger, more rigid and easier to align. Its only weakness is that it will limit east-west movement to perhaps +/- 2 hours away from the meridian. This will be acceptable for our purposes even for a fully tracking design.

    The shape of the dish is also questionable. Whilst the dish is useable as is, we have learnt that the performance of the whole system greatly depends on the dish accuracy. As was alluded to in the first article, it seems that +/- one centimetre would appear to be the required accuracy; and a simple visual inspection of the tangential components alone show that the accuracy is clearly nowhere near that.

    The task of refiguring the disk to this order of accuracy would appear to be our currently greatest challenge. We have received advice and instructions from a number of professionals (both radio astronomers and radio communication people) on procedures for doing this; and the procedures seem to be quite straightforward, but the logistics of actually doing it appear to be quite formidable. Fortunately, this can be done in an incremental fashion: a little at a time, every time helping a little more. And not doing it is not a show stopper: it will merely limit the performance of the dish somewhat.

    The last component here is the mounting of the electronics at the prime focus. Previous implementations of Fleurs dishes had either a single pole or a quadrupole mounting. Neither seems wholly satisfactory. Following a suggestion from Ron Hawkins, we will have built a rigid triangulated frame (visualise if you will a tiny "Eiffel Tower") which will then be securely bolted to the centre section. The electronics will then be mounted on a plastic assembly (since plastic is transparent to microwaves) on top of this frame. Further plastic arms (e.g. 60mm water pipe) could then be connected between this frame and the perimeter of the dish to further improve the rigidity of the dish if required. This design will also allow easy removal and accurate replacement of the electronics if the committee deems that electronics modules may not be left unattended on the dish for security reasons.

    Finally, let us look to the future. The dish has a fully functional worm and wheel on the polar axis, requiring only some kind of motorization to allow tracking of a source as the Earth's rotation carries it across the sky. As previously discussed, the main limitation to motorizing would appear to be the limited availability of electrical power. To use an electric motor to provide all of the torque would appear at this stage to be utterly beyond the solar-charged batteries that we now have. One suggestion that shows promise is to mount a pulley and ratchet mechanism on the worm shaft, to which is attached a heavy weight via a cable, so that the whole assembly provides perhaps 90 to 95% of the torque required to turn said shaft. Thus, the electrical requirements for the drive motor would reduce to merely providing the remaining 5 - 10% of the torque (and therefore power consumption) and regulating that the whole mechanism turns at the same constant weight.

    The polar alignment would then also need closer attention. Fortunately, the way that the equatorial head attaches to the mount allows for adequate movement in both azimuth and altitude which should make the actual alignment a reasonably straightforward task. As to knowing which direction to move things, it would appear to be simple enough to (temporarily) attach a small refractor to the mounting and then ask for assistance from the astrophotographers to achieve an accurate polar alignment.

    Another avenue of potential upgrade is to examine more than the single 1420MHz frequency. Lower frequencies can be accommodated by the dish since their requirements of accuracy of paraboloid shape are more modest. Higher frequencies will be limited by the accuracy of the dish. One reasonable avenue of pursuit is the emission from hydroxyl (OH) molecules. According to professional astronomers with whom we have discussed this, this emission is the "next most interesting" after the one that we have chosen. At a frequency of 1612MHz, it is at a wavelength (and therefore dish accuracy) only 20% away from what we have now. Therefore, this should be well within the accuracy of the dish; and the electronics will also require a minimum of a venture into the unknown.

    The next article in this series will discuss the "radio" component of our radio telescope.