Pointing and calibration

In the A.R.G.H. project

• Equatorial mount with digitally controlled DC actuators.
• Controller: Arduino or Raspberry Pi with a motor shield.
• Tracking software: Stellarium with plugins, GPredict for satellites, SkyChart.
• Communication: serial over USB between the main computer and the motor controller.

With a 5–6° beam, high-precision tracking systems are not required, and small mechanical errors are tolerated.

What the SRT teaches us

Knowing where the telescope points

To track objects accurately, the computer must know where the dish is pointing. On the SRT this is achieved by taking the telescope to a reference position (pointed exactly at true north) and resetting the controller. True north is found with a compass, knowing the local magnetic declination (which can be calculated on the NOAA website).

For an equatorial mount the same principle applies: the alignment of the polar axis and the reference position must be defined carefully and checked on known objects, such as the Sun.

Travel limits

The SRT uses travel limits, both limit switches and electronic limits in the controller. A.R.G.H. should also provide safety limits, both physical and in software, before the dish is moved.

Calibration with an absorber

Calibration is more accurate if the telescope points at an absorber. There are three possibilities:

• an absorber held by hand in front of the feed: it blocks most interference and HI emission, but reflects some LNA noise;
• an absorber on an actuator that moves it in front of the feed;
• a tree or bush on the horizon to point into, which does not reflect LNA noise.

The absorber foam must be protected from weather and from animals, which might use it as a nest.