Mark Clampin (GSFC) John Stansberry (STScI)

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Mark Clampin (GSFC)John Stansberry (STScI)
JWST Operations
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JWST Observatory Status
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Image Quality

• 150 nm _at_ NIRCam focal plane 2 mm diffraction
  limit
• Note performance specified to short wavelength
  cameras
• WFE budget contributors include
• OTE, ISIM, Instrument (including stability),
  jitter pointing
• Sampling is an issue (see talks tomorrow)
• NIRCam pixel size (l lt 2.3 mm) 32 mas/pixel
• NIRSpec pixel size (l gt 2.4 mm) 100 mas/pixel
• l/D (0.7 mm) 22 mas
• MIRI pixel size (imaging/prism) 110 mas/pixel
• l/D (5.0 mm) 0.16 mas
• Contrubuting factors
• Image stability

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Image Quality MIrrors
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Image Quality
Diffraction Limited Strehl gt 0.8 (WFE 150 nm)
F115W
F200W
F444W
F070W
LinearScale
LogScale
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OTE Thermal Stability I

• Primary concern for transit spectroscopy/imaging
  is the stability of the image from observation to
  observation and over time
• JWST will be a very stable telescope
• Function of thermal timescales for observatory
  elements
• Req.
• Sets Wavefront Sensing Control (WFSC) cadence
  of 14 days
• Most important for short wavelength instruments

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OTE Thermal Stability II

• Note that requirement is determined against the
  worst case
• Slew from cold-soak to hot-soak with 14 day hold
• Facilitates relatively simple computing case for
  complex models

?
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OTE Thermal Stability III

• For a real-world science operations Design
  Reference Missions contain a distribution of
  pointing durations and sun-angles e.g. typical
  pointings 103 secs
• WFE drift driven by the observations in long
  duration pointing tail
• Typical WFE drift over 14 days will be lt
  requirement
• Studies by Gersh-Range et al. and JWST thermal
  team
• Further options exist for WFE drift mitigation
  via scheduling
• Phase curves on same targets over several days
  will cause most significant WFE drift

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Wavefront Sensing Control

• Wavefront Sensing and Control
• Measured every two days Issue for phase curve
  observations?
• Fine-tuned every 14 days
• Cadence for WFSC will be reviewed during
  commissioning
• Observatory Project Science is planning
  thermal-slew test during commissioning
• Slew over pre-determined angle
• Monitor drift in image quality over several days
  
• Correlates thermal models and thermal
  measurements
• Feeds into Cycle 1 Science

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Image Motion

• Current Image Motion Requirement
• OBS-2031 The RMS of the difference between the
  "offset-adjusted image position" and its mean
  (for an observation period of up to 10,000
  seconds) shall be less than or equal to the
  values shown below (per axis) over any 15 second
  interval of fine guidance.

Science Instrument IM Allocation (mas)
NIRCam 6.6
NIRSpec 6.7
MIRI 7.4
NIRISS 6.8
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Reaction Wheels

• JWST has 6 reaction wheels
• Reaction wheels control momentum in order to
  orient telescope
• Solar radiation pressure on sunshield is a major
  factor for momentum management of JWST
• Using push-through algorithm for zero-crossing
  events
• Vendor Rockwell Collins Deutschland GBMH
  (Formerly Teldix)
• Heritage
• 11 yrs - Chandra, 8 yrs - EOS Aqua and 6 yrs -
  Aura
• 12 years on Life test unit (MSFC)

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Spatial Scanning Options

• Moving Target
• Requirement When commanded, the ACS shall
  compensate for the apparent motion of a moving
  target which exhibits an angular velocity between
  0 and 30 mas/sec with respect to a guide star
  that remains within a single Fine Guidance Sensor
  field of view
• Upper limit to rate 60 mas/sec
• Sine or repeating pattern would be required
  across FOV
• HST-like scan could be accomplished at higher
  rates using slews employed for small angle
  maneuvers
• Not operating under fine guide jitter 16 mas
• Sine or repeating pattern would be required
  across FOV
• FSW change for slew patterns would have to be
  added, together with ground support

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Calibration Options

• Is it possible to calibrate the structure of
  detector pixels on-orbit to facilitate jitter
  decorrelation for science instruments
• Added option for Fine Steering Mirror (FSM) to
  step a star around a detector pixel under fine
  guidance
• Known as FSM-offsets
• Small-angle maneuvers use reaction wheels and are
  limited in precision
• FSM offsets have precision of few mas and allow
  an image to be stepped around a single pixel to
  map out pixel response functions
• Efficient small angle dithering
• Especially useful for MIRI where
  ground-calibration is not feasible

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JWST Exposure Nomenclature
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JWST MULTIACCUM Patterns
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Data Rate and Storage

• JWST Solid state recorder (SSR) daily limit
• 57.5 GB/day for science data (NOTE on-board
  compression doesnt work)
• Ensures downlink can keep up w/ data production
• H2RG data rates (continuous readout, i.e. upper
  limit)
• Full-frame/stripe-mode readout (4 parallel
  outputs per detector)
• 0.8 MB/sec/detector
• Subarray-mode readout (single output per
  detector)
• 0.4 MB/sec/detector
• NIRCam is the only real potential problem
• 2 detectors full-frame 138 GB/day (2.4x
  allocation)
• 2 detectors subarray 35 GB/day (w/in
  allocation for observations lt 1.7 days)
• Detector resets reduce these rates somewhat

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Max Uninterrupted Exposure Duration

• Four basic limits
• High-gain antenna re-pointing
• Nominally requires visits to be lt 9000 seconds
  duration
• Nominally occurs during visit breaks
• PROPOSAL Allow transits to observe through HGA
  re-pointings
• Momentum unloads (cadence could be doubled by
  momentum biasing)
• Worst-case 5 day cadence
• Off-Nominal 10 day cadence
• Nominal 25 day cadence

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Max Uninterrupted Exposure Duration

• Four basic limits (continued)
• Max integrations 216 65536 (ASIC hardware
  limit)
• Max exposure durations for integrations with 2
  frames per ramp
• Full-frame 586 hrs (32.2 sec/integration)
• 64 x 64 subarray 2.7 hrs (148
  msec/integration)
• 2048 x 64 stripe 18.6 hrs (1.02
  sec/integration)
• 2048 x 64 subarray 73.1 hrs (4.02
  sec/integration)
• Wavefront Sensing (2-day cadence)
• WFS visits can presumably be shifted /- a day
• 7 WFS visits (nominal) between control activities

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High-gain Antenna Proposal

• HGA re-pointing is the most severe constraint on
  exposure length
• HGA re-pointing causes small, short disturbances
• lt 70 mas pointing disturbance
• lt 1 min disturbance duration
• FGS will remain in fine-guide through the
  disturbance
• There is some flexibility in specifying timing of
  HGA re-points
• HGA Ops Proposal for Exoplanet Transits
• Allow exoplanet observations to continue through
  HGA re-points
• Small effects on photometry may result, but
  probably better than stopping and restarting
  exposures (data gaps response drifts)
• This mitigates data-volume issues because data
  can be downlinked as it is acquired

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Event-driven Operations

• Fixed-time constraints are allowed
• PHASE constraint allows any of several transits
  for a given system to be observed at a specified
  orbital phase (scheduling flexibility)
• Start of exposures uncertain to 5 minutes
• Timeline Scenarios (TBR checking w/ Wayne
  Kinzel)
• Failed visit(s) prior to transit
• Over-long visit(s) prior to transit

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Observation Planning

• Requirements for APT implementation are under
  discussion
• Coronagraphy super-template concept one
  possibility
• APT holder with special qualities
• Allows organization of, and application of
  special constraints to, groups of normal
  observing templates
• Could flag observations as, e.g.
• Allowed to proceed through HGA re-points
• Allowed to use 2-detector mode (NIRCam)
• Could group observations of a target, e.g.
• Single folder for multi-band, multi-instrument
  observations of one target
• Multiple events to increase SNR

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Exposure Time Calculator

• No dedicated exoplanet ETC is in the works
• Normal SNR predictions on host star can be used
  to estimate detection limits for transit
  signatures
• There is currently no additional knowledge
  available to enable a more precise ETC
  implementation

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What time is it?

• Time (UTC) flows from ground ? JWST S/C ? ISIM ?
  Science Data
• S/C clock stable to lt 2 sec / 40 hours (14 ppm)
• S/C clock corrected every contact (12 hr nominal
  contact interval)
• Accuracy requirement lt 0.5 sec
• Correction is applied gradually, not as a jump
• ISIM tags data w/ time last pixel in a group gets
  read out
• Data groups get stacked before delivery to SSR
• Only the time tag for the last group in a stack
  is retained on the SSR
• FITSWRITER reconstructs time for individual
  groups
• Frame-time algorithms from instrument teams
• Precision probably 30 msec