National ResearchCouncil Canada

Conseil nationalde 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.

Lost time due to Satellite Interference

0123456789

0.1 0.2 0.3 0.4 0.5 0.6 >0.7

No. of stars vs. fraction of time lost

Worst Case Laser Duty CycleLaser Clearing House Permitted Laser Operation SAO65711 from Lick site

0

1

2

0 1 2 3 4 5

Time (hours)

Lase

r O

n =

1 or

Off

= 0

Slightly Better Than Typical Case

Laser Clearing House Permitted Laser Operation RA14:28 Dec 43:24 Lick site

0

1

2

0 1 2 3 4

Time (hours)

Lase

r O

n =

1 or

Off

= 0

Good Case

Laser Clearing House Permitted Laser Operation for Star 231 from the MMT

0

1

2

0 1 2 3 4 5 6

Time (hours)

Lase

r O

n =

1 or

Off

= 0

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

Typical Permitted Laser Operation (Star 46158 from Lick)

0

1

2

0 1 2 3 4 5

Time (hours)

Lase

r O

n =

1 or

Off

= 0

Histogram of Permissible Time Durations “Laser On”

Distribution of Laser On-Time

020406080

100120140160180200

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.0

011

.00

12.0

013

.00

14.0

015

.00

>15

MIN

Time Minutes

Probability of “Laser On” vs. Duration > T, ( Avoid Satellites)

1 cum( )slot

m

0 20 40 60 80 100 1201 10

4

1 103

0.01

0.1

1Probability vs. Length of time on

Time T -minutes

P(t

>T

)

exp(-T/)

exp(-T/ +bT2-ct3)

Minutes0 20 40 60 80 100

10%

1%

0.1%

0.01%

100%

Queue Scheduling Increases Odds of Finding a Time Slot.

e TcTbTTt

32

)Pr(

))Pr(1ln(

))Pr(1ln(

Tt

sucN

NTtsuc ))Pr(1(1)Pr(

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 >T1 Event Probability of success

Int Time P(t>T ) 0.5 0.6 0.7 0.8 0.9 0.95 0.995 37.9% 1.5 1.9 2.5 3.4 4.8 6.3 9.7

10 16.5% 4 5 7 9 13 17 2615 8.1% 8 11 14 19 27 35 5520 4.4% 15 20 27 35 51 66 10125 2.7% 26 34 44 59 85 110 17030 1.8% 39 52 68 91 130 170 26135 1.2% 56 74 98 130 187 243 37340 0.9% 76 100 132 176 252 328 50545 0.7% 99 130 171 229 327 426 65550 0.6% 124 164 216 289 413 537 82655 0.4% 154 204 268 359 513 667 102660 0.36% 192 253 333 445 637 828 1273

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 1997-1999

MCAO

CW Pulse Useless for any Science

1997 46% 32% 21%

1998 49% 31% 19%

1999 41% 26% 15%

Average 46% 30% 18%

Useful Science Time Lost to MCAO - Caused by Thin Clouds

Fraction of Science time lost

CW Pulse

Classical 33.3% 13.9%

Queued 18.9% 8.9%

Failures

Mean Time Before Failure (MTBF)

Reliability MTBF HrsAdaptive Optics Module 528Beam Transfer Optics 1107Laser Launch Telescope 25000SALSA ( 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.

11

i itotal MTBF

MTBFReciprocal 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 50000Relay Optics 6249M6, M7 Slow tip/tilt & steering 8333Diagnostics 200005 Fast steering mirrors 2000Rotator 50000Electronics 30000

Overall BTO reliability 1107

Sensitivity Analysis: MTBF & Downtime vs. Laser Reliability

0

50

100

150

200

250

300

350

25 40 48.3 100 200 1000 5000

%

Laser MTBF Hours

hr

Overall MCAO reliability

% Downtime due to failures

Laser MTBF Hours

0

5

10

15

20

25

30

25 40 48.3 100 200 1000 5000

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 2.0 2.0 0.47 30 150 12.9

Astrometry + Partial setup

1.3 - 0.47 30 150 11.8

Partial setup, no astrometry

0.3 - 0.47 30 150 9.1

Calib’n. while Laser off

2.0 2.0 0.47 3 x 15 TBD??

8.3

Dwell on same Object

2.0 2.0 n x 0.47 n x 6.6 1 43%

% Time Sky Useful for Astronomy, But MCAO Not Available.

Availability Calculation

Failures from MTBF Calculation 10.0%

Cirrus Losses for pulsed laser 13.9%

Satellite Interference *** 8.3%

Other Observatory Interference 2%

Total Lost time best case 26%

*** Optimistic Case:

Do Sky calibrations, change filters and gratings while laser off. (~38.5% of time)

Availability Calculation:Combining lost-time factors

i

itotal lossloss ))Pr(1(1)Pr(

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 ScenarioSatellite Loss

Pulse CW(MCAO)

Full Setup + Astrometry 12.9% 30% 38%

Partial Setup + Astrometry 11.8% 29% 37%

Partial Setup, no Astrometry 9.1% 27% 34.9%

Part Setup- no astr'y. – cal. if laser off 8.3% 26% 34.3%

Dwell on Same Object 43% 54% 59%

Laser Format

Total Down time

PDR Agenda

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