National Research Council Canada Conseil national de recherches Canada Observing Efficiency and Reliability of Gemini South MCAO Glen Herriot Herzberg Institute of Astrophysics National Research Council Canada

Observing Losses for MCAO Weather Satellites Failures Other Observatories Airplanes

Purpose This analysis estimates the overall observing efficiency of MCAO, which is NOT specified in the requirements documents.

Top Level Requirements Lose no more than 10% of scheduled time to failures CoDR book assumed that half the nights in a year will be scheduled for MCAO, (180 x 10 hr). But this is a classical scheduled spec. We have taken it to mean 1800 hours per year, queue-scheduled. Maximum object setup time (excluding telescope slew and acquire PWFS stars) < 120 seconds Telescope Slew and PWFS2 acquisition < 120 seconds.

Lasers Must Not Interfere With Satellites

Fraction of Time Laser Must Be Shut Down.

Worst Case Laser Duty Cycle

Slightly Better Than Typical Case

Good Case

Typical: 28 ‘On’ events, 41% Off time; 20 minute max. On ; 1 minute median Off

Histogram of Permissible Time Durations “Laser On”

Probability of “Laser On” vs. Duration > T, ( Avoid Satellites) exp(-T/  ) exp(-T/  +bT 2 -ct 3 ) Minutes % 1% 0.1% 0.01% 100%

Queue Scheduling Increases Odds of Finding a Time Slot. Probability of 1 slot of length t>T Probability of success with more targets in queue. Solve for Queue length needed:

Queue Length - vs. Integration Time >T

Thin Cirrus Clouds - Blind LGS Multiple CW LGS suffer from “fratricide” unless sky is photometric. Pulsed laser may tolerate ~20% loss on both upward and downward path. < ~1/3 magnitude extinction.

Fraction of Year Lost to Clouds -CTIO

Useful Science Time Lost to MCAO - Caused by Thin Clouds

Failures

Mean Time Before Failure (MTBF) Reliability MTBF Hrs Adaptive Optics Module 528 Beam Transfer Optics 1107 Laser Launch Telescope SALSA ( satellite, aircraft, laser safety system ) 5085 Laser ( minimum MTBF for <10% downtime from failures ) >48 Gemini South MCAO MTBF 42

Overall reliability calculated from subsystems’ Mean Time Between Failure. Reciprocal of sum of reciprocals Total MTBF is poorer than any individual subsystem’s reliability. Total MTBF dominated by “weakest link in a chain.”

Subsystem reliability: e.g., Beam Transfer Optics MTBF Hours Shutter & Beam Dump Relay Optics 6249 M6, M7 Slow tip/tilt & steering 8333 Diagnostics Fast steering mirrors 2000 Rotator Electronics Overall BTO reliability 1107

Sensitivity Analysis: MTBF & Downtime vs. Laser Reliability % Laser MTBF Hours hr Overall MCAO reliability % Downtime due to failures Laser MTBF Hours

Interference With Other Observatories, Aircraft Gemini South neighbours at CTIO and SOAR have right of way for natural guide star observations. Estimate ~1 incident per night where MCAO must shut down, costing ~2% lost time. Interference problem is Rayleigh “light sabres” clashing, not the 90 km beacon itself.

Observing Scenarios for Satellite Interference Studies. Scenario Slew, Acquire PWFS NGS Astro- metry Laser Launch, Acquire Science Integra- tion Queue Size N Lost Time % Full overhead Astrometry + Partial setup Partial setup, no astrometry Calib’n. while Laser off x 15TBD? ? 8.3 Dwell on same Object 2.0 n x 0.47n x %

% Time Sky Useful for Astronomy, But MCAO Not Available. *** Optimistic Case: Do Sky calibrations, change filters and gratings while laser off. (~38.5% of time)

Availability Calculation: Combining lost-time factors A. Convert each lost-time factor to a Probability of Success. Multiply together to get overall ‘up-time’ percentage. B. Subtract ‘up-time’ from 1 to get total percentage of time lost due to all factors

Lost Time: Overhead, Clouds, Satellites, Failures Satellite Interference Scenario Satellite Loss Pulse CW (MCAO) Full Setup + Astrometry12.9% 30% 38% Partial Setup + Astrometry11.8% 29% 37% Partial Setup, no Astrometry9.1% 27% 34.9% Part Setup- no astr'y. – cal. if laser off 8.3% 26% 34.3% Dwell on Same Object43% 54% 59% Laser Format Total Down time

PDR Agenda Friday, 5/ Laser System 0900 CTIO Sodium Studies 0915 Control System 0945 Break 1000 RTC Electronics 1045 Safety System 1100 Availability analysis 1130 Closed vendor sessions 1200 Lunch 1300 Cost and schedule 1400 Committee session 1700 Committee report 1800 Adjourn