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					  National Research   Conseil national
  Council Canada      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
             Airplanes




                                    Other
                         Weather
                                    Observatories




                         Failures

Satellites
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

9
8
7
6
5
4
3
2
1
0
    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 Cycle
                                    Laser Clearing House Permitted Laser Operation   SAO65711 from Lick site


                          2
Laser On = 1 or Off = 0




                          1




                          0
                              0        1             2                  3        4              5
                                                         Time (hours)
                                  Slightly Better Than Typical
                                  Case
                                   Laser Clearing House Permitted Laser Operation   RA14:28 Dec 43:24 Lick site


                          2
Laser On = 1 or Off = 0




                          1




                          0
                              0            1                 2                3                  4
                                                        Time (hours)
                                  Good Case
                                   Laser Clearing House Permitted Laser Operation for Star 231 from the MMT


                          2
Laser On = 1 or Off = 0




                          1




                          0
                              0     1          2            3          4           5           6
                                                       Time (hours)
Typical: 28 ‘On’ events, 41% Off time;
20 minute max. On ; 1 minute median Off
                                      Typical Permitted Laser Operation (Star 46158 from Lick)


                          2
Laser On = 1 or Off = 0




                          1




                          0
                              0   1           2                  3         4             5
                                                  Time (hours)
      Histogram of Permissible Time
      Durations “Laser On”
               Distribution of Laser On-Time

200
180
160
140
120
100
 80
 60
 40
 20
  0




        IN
         0

         0

         0

         0

         0

 >1 00
        00

        00

        00

        00

        00

        00

        00

        00

        00

      .0

      .0

      .0

      .0

      .0



     M
      .
     1.

     2.

     3.

     4.

     5.

     6.

     7.

     8.

     9.
   10

   11

   12

   13

   14

   15

   5
                         Time Minutes
                                                                            (1       cum) slot 
Probability of “Laser On” vs.                                                    0.0375
                                                                                            m



Duration > T, ( Avoid Satellites)                                                0.0173
                                                                            2.886·10 -3


                                      Probabilit y vs. Length of t ime on
                  1
   100%


         10%
           0.1




                                                             exp(-T/ +bT2-ct3)
P(t>T)




           1%
            0.01



                          exp(-T/)
          1  10
              3
         0.1%


0.01%
   1  10
          4
                      0        20     40            60             80         10 0              12 0
                   0          20      40           60            80          100         Minutes
                                               Time T -minutes
      Queue Scheduling Increases
      Odds of Finding a Time Slot.
Probability of
                                 T bT 2 cT 3 
                                                  
                 Pr( t  T )  e 
1 slot of
length t>T                                         
Probability of
success with
more targets
                 Pr( suc)  1  (1  Pr( t  T ))   N

in queue.


                     ln( 1  Pr( suc ))
                 N
Solve for
Queue length
needed:             ln( 1  Pr( t  T ))
          Queue Length - vs.
          Integration Time >T
            1 Event            Probability of success
Int Time    P(t>T)         0.5            0.6          0.7    0.8    0.9   0.95     0.99
          5       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       26
         15        8.1%      8             11           14     19     27      35       55
         20        4.4%     15             20           27     35     51      66     101
         25        2.7%     26             34           44     59     85    110      170
         30        1.8%     39             52           68     91   130     170      261
         35        1.2%     56             74           98   130    187     243      373
         40        0.9%     76           100          132    176    252     328      505
         45        0.7%     99           130          171    229    327     426      655
         50        0.6%   124            164          216    289    413     537      826
         55        0.4%   154            204          268    359    513     667    1026
         60       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
                               Useless for
           CW       Pulse     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                Hrs
Adaptive Optics Module              528
Beam Transfer Optics                1107
Laser Launch Telescope             25000
SALSA ( satellite, aircraft, laser 5085
safety system )
Laser ( minimum MTBF for          >48
<10% downtime from failures )
 Gemini South MCAO MTBF            42
Overall reliability calculated from
subsystems’ Mean Time Between Failure.
                                          1
                               1 
              MTBFtotal  
Reciprocal
of sum of
                                    
reciprocals
                           i MTBFi 
• 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               50000
Relay Optics                      6249
M6, M7 Slow tip/tilt & steering   8333
Diagnostics                       20000
5 Fast steering mirrors           2000
Rotator                           50000
Electronics                       30000
        Overall BTO reliability    1107
     Sensitivity Analysis:
     MTBF & Downtime vs. Laser Reliability
hr   350                                             30
                                                      %
     300                                             25

     250
                                                     20              % Downtime
                         Overall
     200                                                             due to failures
                         MCAO
                                                     15
     150                 reliability

                                                     10
     100

      50                                              5

       0                                              0
            25   40   48.3   100   200   1000 5000
                                                          25   40   48.3   100   200   1000 5000
           Laser MTBF Hours                                    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.
                   Slew,    NGS       Laser     Science    Queue   Lost
                  Acquire   Astro-   Launch,    Integra-    Size   Time
 Scenario         PWFS      metry    Acquire      tion       N      %

Full overhead      2.0       2.0      0.47        30       150     12.9
 Astrometry +      1.3        -       0.47        30       150     11.8
 Partial setup
 Partial setup,    0.3        -       0.47        30       150     9.1
no astrometry

Calib’n. while     2.0       2.0      0.47      3 x 15     TBD? 8.3
     Laser off                                               ?
    Dwell on       2.0       2.0     n x 0.47   n x 6.6      1  43%
 same Object
     % Time Sky Useful for Astronomy,
     But MCAO Not Available.
         Availability Calculation           *** Optimistic
                                            Case:
Failures from MTBF Calculation      10.0%   Do Sky
                                            calibrations,
Cirrus Losses for pulsed laser      13.9%   change filters
                                            and gratings
Satellite Interference ***          8.3%    while laser off.
                                            (~38.5% of
Other Observatory Interference       2%     time)

  Total Lost time best case         26%
     Availability Calculation:
     Combining lost-time factors

    Pr( losstotal )  1   (1  Pr( lossi ))
                               i

                               A. Convert each lost-time
                               factor to a Probability of
                               Success. Multiply together
B. Subtract ‘up-time’ from 1   to get overall ‘up-time’
to get total percentage of     percentage.
time lost due to all factors
    Lost Time: Overhead, Clouds,
    Satellites, Failures
                                                         Total Down time
                                                           Laser Format
                                             Satellite            CW
  Satellite Interference Scenario                         Pulse
                                              Loss                (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%
   PDR Agenda

Friday, 5/25
0800 Laser System             1300   Cost and schedule
0900 CTIO Sodium Studies      1400   Committee session
0915 Control System           1700   Committee report
0945 Break                    1800   Adjourn
1000 RTC Electronics
1045 Safety System
1100 Availability analysis
1130 Closed vendor sessions
1200 Lunch

				
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posted:12/17/2011
language:English
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