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How Do You Know Where You're Pointing?
by Malcolm Mallette, WA9BVS (mallettem @

If you do not know where your SETI dish is pointed, any discovery has no meaning as it cannot be confirmed. Confirmation of reception of a signal is impossible unless the radiotelescope doing the confirmation can be pointed to the Right Ascension and Declination that the signal comes from.

Our radiotelescope at the University of Indianapolis is doing wide band Radio Astronomy. However, the pointing problem is the same as an Argus station. With a 15 ft dish at 3.8 GHz, we have a one degree half power beamwidth. Accurate pointing is a primary consideration.

We first tried using an angle finder for rough setting. Then we moved the dish in what we hoped were fractions of a degree until the drift scan observation showed that a natural source was centered. We often moved the dish up and down while an object was in transit while watching the receiver output on a computer monitor and on a analog meter to set the dish right on. The maximum signal position was right on the source. Of course, we were pointing south and setting the elevation.

We then tried a potentiometer driven by a belt with teeth that was in turn driven by the rotation of the dish up and down. That worked fine for accuracy up to 1.5 degrees but was not accurate enough for our operation.

We finally used a digital level. The digital level reads out in tenths of a degree. The dish was centered on Taurus A by moving the dish up and down while Taurus A was transiting. The digital level has a magnetic mount as an option. Using the magnetic mount, the digital level was fastened to a selected steel bar that supports the dish and moves with the dish. The place the magnet on the digital level's mount is placed on the bar is marked so the digital level can always be placed on the same spot.

The digital meter was read and its reading compared with the actual elevation, which we knew as we knew that the dish was centered on Taurus A. An offset was then determined. For example, if the actual elevation is 72.3 degrees and we read 72.5 degrees on the digital level, we know that we must subtract 0.2 degrees when we read the digital level.

Using the digital level, we can set the dish elevation, and therefore the declination at which it is pointed, to within about 0.2 degrees.

To set the dish's azimuth heading to exactly south, merely observe transits of a natural object and move the dish slightly until the peak signal coincides with the time the object should transit the south meridian.

I am assuming that you will use the drift scan method of observation for a SETI search. By setting the dish on an exact elevation using a known natural source, you can determine the offset for a digital level and set the dish accurately for elevations at which there are no strong natural sources.

We will probably go to a more complex system that remotely reads the dish position for computer aiming and star tracking. However, for an individual's Argus station used in drift scan operation, the digital level is adequate, costs about $130 with magnetic mount, and is easy to use and calibrate.

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