On-Sun Observing
- Tim Williams
Focused sunlight from the primary mirror at the TEOA present a thermal hazard. Approximately 15kW of energy is concentrated in a small area. This can and has caused damage to equipment mounted at the top end can and has resulted in lost time.
Safety Function | On-Sun Observing |
Hazard | reduce hazards of focused sunlight on equipment |
Triggering Event | Telescope sightline >1.5 solar radii from Sun |
Priority |
|
Modes | always active |
Reaction | Block light path |
Safe State | Aperture Cover closed M1 Mirror Cover closed |
Required Integrity | SIL2 PLc |
Initial Design
The initial safety function was built around small telescopes (with neutral density filters) using two-dimensional position-sensitive detectors (PSD). PSDs are optoelectronic position sensors that utilize photodiode surface resistance. Unlike discrete element detectors such as CCDs, PSDs provide continuous position data, high position resolution, and high-speed response.
This approach had some drawbacks. The mounting locations meant they had a limited field-of-view. Second, because these devices are inherently less accurate with decreased input, when a substantial cloud passed in front of the Sun, they initiated the safety function. The time to re-open the Aperture Cover and M1 Cover was deemed to adversely affect observing efficiency and they were disabled.
Current Design
The current design relies on a Solar Position Algorithm to calculate the Sun’s position and compare it with the Telescope pointing provided by the Allen-Bradley Safety Encoders. This require accurate time information for calculating the algorithm. In addition the PLC was upgraded to an L8 series processor to allow for double precision floating point calculations.