The term radio telescope often conjures the image of a large dish. This dish is only one part of a radio telescope, being equivalent to the light-collecting mirror of an optical telescope. In this section, the construction of the telescope's dish and the rest of the components that make up the antenna, as well as the devices which are needed to point the antenna to different positions on the sky are discussed.

The antenna was donated by the Sunshine Electronics Company. Due to the large size of the dish, it was first disassembled for transportation and reassembled on the roof of the Science B building at the University of Calgary. A mount for the antenna was constructed in the university's machine shop according to the design of the telescope technician, Fred Babbott. Before the dish could be secured on the mount, the mount was weighted down with lead and guy wires were attached to prevent the wind gusts from blowing the dish onto the roof of the nearby engineering building.


The securing and levelling of the base.



The mount for the underside of the antenna dish.



Attaching the dish to mount.


The antenna was initially intended for the reception of satellite TV signals; however, for our purposes, it was modified and given an alt-azimuth mounting so that it could be more easily pointed. Still, pointing limitations mandated that the telescope only be used for observations at transit-- observations of sources as they drift across the celestial meridian. A subsequent addition of an azimuth stabilization arm will remove this restriction.

For pointing in altitude, a plumb bob and protractor device was affixed to the mount. During the process of attaching the protractor to the mount, Fred actually permitted Jason to do some hands-on work--drilling holes through the protractor.


Preparing the protractor to be secured to the telescope.

By the end of the observing run, the machine shop had finally completed the arm which will allow for pointing in azimuth. This, as well as the back of the protractor can be seen in the figure below.


Pointing mechanisms in altitude and azimuth.

The initial calibration in altitude of this device was done using the lunar transit of March 27, 1996. This calibration was then refined by interpolation based on several transits of Taurus A, also known as the Crab Nebula. The declination of this supernova remnant is 21:59:49 (21.9969°). The best transit was obtained at a protractor reading (PR) of 66.2°.

Therefore, the final transformation equation is:

PR(°) = declination(°) + 44.2°

for objects crossing the celestial meridian in the south. A similar calibration remains to be performed for pointing to the north. It should also be noted that due to the situation of the telescope on the roof of Science B which has a high wall at the edge, it is not possible to point to the south at declinations less than ~15°. A similar constraint will be placed on pointing to the north due to the roof of the building.

The pointing precision was limited by the protractor scale to approximately 0.3° for single measurements, barring the effects of wind. It should be noted that this equation is valid only for objects observed at transit. Since the azimuth positioning arm is in place, observations at positions other than transit are now possible. However, further calibrations must first be performed.

Before the feed could be secured to the telescope, it was necessary to ensure that the feed be positioned exactly at the focus of the dish. Since the dish is parabolic, it has only one focal point. The distance from the centre of the dish to the focal point is called the focal length. Our antenna has a focal length of 1.19m.


Dish schematic.


Once the frontend of the telescope was constructed, the horn was affixed to the support arms such that the feed was located at the focal point of the dish (proceed to the Front-End section for more detail).