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Observing Efficiency and Reliability of Gemini South MCAO

Observing Efficiency and Reliability of Gemini South MCAO. Glen Herriot Herzberg Institute of Astrophysics National Research Council Canada. Observing Losses for MCAO. Airplanes. Other Observatories. Weather. Failures. Satellites. Purpose.

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Observing Efficiency and Reliability of Gemini South MCAO

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  1. Observing Efficiency and Reliability of Gemini South MCAO Glen Herriot Herzberg Institute of Astrophysics National Research Council Canada

  2. Observing Losses for MCAO Airplanes Other Observatories Weather Failures Satellites

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

  4. 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.

  5. Lasers Must Not Interfere With Satellites

  6. Fraction of Time Laser Must Be Shut Down.

  7. Worst Case Laser Duty Cycle

  8. Slightly Better Than Typical Case

  9. Good Case

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

  11. Histogram of Permissible Time Durations “Laser On”

  12. Probability of “Laser On” vs. Duration > T, ( Avoid Satellites) 100% 10% exp(-T/ +bT2-ct3) 1% exp(-T/) 0.1% 0.01% 20 100 80 40 60 0 Minutes

  13. 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:

  14. Queue Length - vs. Integration Time >T

  15. 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.

  16. Fraction of Year Lost to Clouds -CTIO 1997-1999

  17. Useful Science Time Lost to MCAO - Caused by Thin Clouds

  18. Failures

  19. Mean Time Before Failure (MTBF)

  20. 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.”

  21. Subsystem reliability: e.g., Beam Transfer Optics MTBF

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

  23. 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.

  24. Observing Scenarios for Satellite Interference Studies.

  25. % 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)

  26. 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

  27. Lost Time: Overhead, Clouds, Satellites, Failures Total Down time Laser Format Satellite Loss CW (MCAO) Satellite InterferenceScenario Pulse Full Setup + Astrometry 12.9% 38% 30% Partial Setup + Astrometry 11.8% 37% 29% Partial Setup, no Astrometry 9.1% 34.9% 27% Part Setup- no astr'y. – cal. if laser off 8.3% 34.3% 26% Dwell on Same Object 43% 59% 54%

  28. Friday, 5/25 0800 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 PDR Agenda

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