Configure the azimuth tilt meter.

Arguments:

Double af_offset

The zero offset tilt of the aft-fore tilt meter. See the context section below, for the sign of this argument.

Double af_scale

The scale factor of the aft-fore tilt meter. See the context section below, for the sign of this argument.

Double lr_offset

The zero offset tilt of the left-right tilt meter. See the context section below, for the sign of this argument.

Double lr_scale

The scale factor of the left-right tilt meter. See the context section below, for the sign of this argument.

Double stime

The 10-90% rise time (hours) of the low-pass filter that is applied to the tilt-meter readings, before they are used to correct the telescope pointing.

Example:

In the following example, the 1st and 2nd arguments tell the control-system that the aft-fore tilt meter has no zero offset, and that a +1 volt increase in the output of this tilt-meter, corresponds to a +1 arcminute increase in the elevation of the telescope bore-sight.

  conf_tilt_meter 0.0, 0:1:00,  0.0, -0:1:00,  0:0:2.7

The 3rd and 4th arguments tell the control-system that the left-right tilt meter also has no zero offset, and that a +1 volt increase in the output of this tilt-meter, corresponds to -1 arcminute change in the clockwise rotational tilt of the azimuth platform, around the horizontal vector that points outwards from the azimuth axis towards the current pointing azimuth.

Finally, the 5th argument in the example, tells the control system to apply a low-pass filter with a 10-90% rise time of 2.7 seconds, to smooth the tilt-meter readings that are used to dynamically correct the telescope pointing.

Context:

Telescopes generally have a pair of orthogonal tilt meters on their azimuth platforms, which are used to measure the tilt of the azimuth platform relative to the local gravitational vertical. These tilt meters are analog devices that output voltages that are proportional to the measured tilts. This command configures the voltage to degree conversion factors and offsets of the tilt meters. These should be the values from the manufacturer's data-sheet. The resulting outputs will then be further calibrated using calibration terms provided via the cal_tilt_meter command.

Smoothing of the tilt meter readings

Since the tilt meters are used to dynamically correct the telescope pointing for tilts in the azimuth platform, rapid changes in their readings have been observed to provoke slow oscillations in the servo, particularly at high and low elevations. The reason for these oscillations is the roughly half-second response time of the servo to transients in the commanded positions, and the fact that the accelerations of these delayed responses briefly change the azimuth tilt. In the absence of any software delay between tilt-meter transients and there use to correct the pointing, a single transient can produce roughly 1Hz ringing or oscillations.

In practice, since each one-second of telescope motion is calculated one second in advance, the tilt meter reading that is used to calculate a given second of motion, is taken from 1 second before the start of that motion. So the actual frequency of the ringing in the current system is slower than 1Hz.

To prevent the tilt meters from introducing instabilities, it is necessary to smooth their readings, to suppress the transients that provoke the oscillations. This is done using a simple exponential filter whose time constant can be set with the conf_tilt_meter command. The time constant is defined as the time taken by the filter to reach 90% of its final value, after a step in the tilt-meter readings.

The Aft-Fore Tilt

The aft-fore output voltage of a tilt meter measures the tilt of the azimuth platform in the direction of increasing elevation. The output voltage is converted to an angle in degrees, using the following equation:

	    af_tilt = af_offset + af_scale * volts
	  

The signs and values of af_offset and af_scale must be chosen such that positive values of af_tilt correspond to tilts that raise the elevation of the telescope bore-sight, and negative values to tilts that lower the elevation of the telescope bore-sight.

The Left-Right Tilt

The left-right output voltage of a tilt meter measures the tilt of the azimuth platform around the azimuth vector. The output voltage is converted to an angle in degrees, using the following equation:

	    lr_tilt = lr_offset + lr_scale * volts
	  

The signs and values of lr_offset and lr_scale must be chosen such that positive values of lr_tilt correspond to tilts that turn the azimuth platform clockwise around the horizontal vector that points outwards from the azimuth axis towards the current azimuth.

The Temperature Sensor

The output voltage of the temperature sensor is converted to celcius using the following equation:

	    temp_tilt = temp_offset + temp_scale * volts