CTA Group @ Physik-Institut, Uni Zürich
Currently, an important part of our daily work consists of redesigning and testing actuator devices that are part of
an Active Mirror Control System (AMC).
This is done in the framework of a design study for CTA. CTA stands for "Cherenkov Telescope Array" which
will be a future observatory for high energy gamma rays.
The picture below shows the teststand for the AMC modules on the rooftop of our physics institute.
Live-View from the rooftop of the Physics Institute
(last update: May 10 2017 18:52:27)
Long-Term Actuator Test
A possible location of La Palma (Canary Islands), implies a night duration between astronomical twilight
(sun is at least 18 deg below local horizon) of 7 hours to 11 hours.
For the following calculations, 9 out of 24 hours per day are assumed to be observation time,
on 365 days a year. We also assume, that a re-orientation of the mirror segments is required
once every 5 minutes.
This will add up to 108 movements per night.
For an estimated lifetime of 10 years, this adds up to 394200 movements.
At our teststand setup, we command both AMC actuators to extend 3mm and immediately retract 3mm.
We call this a 'cycle'.
A cycle repeats every 24 seconds, which actually includes some slack time to allow for a busy control
We assume that one telescope movement, done for re-orientation, implies one 'cycle' per actuator.
This yields an acceleration factor of 3600/108 = 33 1/3. That is, 3 days in the teststand are
equivalent to 100 days of real-life telescope operation.
Thus, in order to simulate 10 years of real-life lifetime, 110 days in the teststand are required.
- Each actuator drives 150 cycles per hour, 3600 cycles per day
- Teststand operates 24 hours non-stop
Simulation of Mechanical Load
An hexagonal-shaped aluminum mirror dummy (2.5m2, 33kg) is used to simulate
the expected static load on the actuator modules.
It is carried by a 3-point gimbal. Two of the three mount points are actively driven by actuators.
Currently, the tilt angle
of the aluminum plate is 30 degrees, which results in half the weight (162 N) appearing as longitudinal load
on the AMC actuators. For a single actuator, this is reduced by a factor of 3 to a static load of 54 N.
Additionally, weather data including wind blasts are recorded in order to investigate the
effect of the dynamic loads on the mechanics.
The dynamic load is proportional to the square of the wind speed.
Assuming a CW-coefficient of 1.5 and
a wind gust of 10 m/sec results in an additional longitudinal load of 66 N.
|Latest Weather Data Measurement
| Date (measurement)
| Time (measurement)
| Humidity (control box)
|| 95 %
| Temperature (control box)
|| 8.1 °C
| Humidity (outside)
|| 98 %
| Temperature (outside)
|| 9.4 °C
| Air Pressure (relative)
|| 1014.2 hPa
| Average Wind Speed (5min)
|| 0.3 m/s
| Maximum Wind Speed
|| 0.8 m/s
| Wind Direction
Current Actuator Status
One 'cycle' command includes a 3mm actuator extension and an immediate 3mm retraction
- 4984236 cycles completed
- equal to 46150.3 days
- or 126.4 years of real-life telescope operation