JWST Operations
Mark Clampin (GSFC)
John Stansberry (STScI)
JWST
Clampin/GSFC
JWST Observatory Status
JWST
Clampin/GSFC
Image Quality
•
150 nm @ 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 < 2.3 mm): 32 mas/pixel
➡ NIRSpec pixel size (l > 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
JWST
Clampin/GSFC
Image Quality: MIrrors
JWST
Clampin/GSFC
Image Quality
F115W
F200W
F070W
F444W
Diffraction Limited: Strehl > 0.8 (WFE ≤ 150 nm)
Log
Scale
JWST
Linear
Scale
Clampin/GSFC
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
JWST
Clampin/GSFC
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
➠
JWST
Clampin/GSFC
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 < requirement
➡ Studies by Gersh-Range et al. and JWST thermal team
JWST
•
Further options exist for WFE drift mitigation via scheduling
•
Phase curves on same targets over several days will cause most
significant WFE drift
Clampin/GSFC
Wavefront Sensing & Control
•
Wavefront Sensing and Control
➡ Measured every two days: Issue for phase curve observations?
➡ Fine-tuned every 14 days
•
Cadence for WFS&C 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
JWST
Clampin/GSFC
Image Motion
•
Current Image Motion Requirement:
➡ OBS-2031 The RMS of the difference between the "offsetadjusted 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
NIRCam
NIRSpec
MIRI
NIRISS
JWST
IM Allocation
(mas)
6.6
6.7
7.4
6.8
Clampin/GSFC
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)
JWST
Clampin/GSFC
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
JWST
Clampin/GSFC
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
JWST
Clampin/GSFC
JWST Exposure Nomenclature
JWST
Clampin/GSFC
JWST MULTIACCUM Patterns
JWST
Clampin/GSFC
Data Rate and Storage
•
•
JWST Solid state recorder (SSR) daily limit
➡
57.5 GB/day for science data (NOTE: on-board compression doesn’t 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) :
➡
➡
Subarray-mode readout (single output per detector)
➡
•
JWST
0.8 MB/sec/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 < 1.7 days)
➡
Detector resets reduce these rates somewhat
Clampin/GSFC
Max Uninterrupted Exposure Duration
•
Four basic limits
➡ High-gain antenna re-pointing
➡ Nominally requires visits to be < 9000 seconds duration
➡ Nominally occurs during visit breaks
➡ PROPOSAL: Allow transits to observe through HGA repointings
➡ Momentum unloads (cadence could be ~doubled by momentum
biasing)
➡ Worst-case: 5 day cadence
➡ Off-Nominal: 10 day cadence
➡ Nominal: 25 day cadence
JWST
Clampin/GSFC
Max Uninterrupted Exposure Duration
•
Four basic limits (continued)
➡ Max # integrations = 2^16 = 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
JWST
Clampin/GSFC
High-gain Antenna Proposal
•
HGA re-pointing is the most severe constraint on exposure length
➡ HGA re-pointing causes small, short disturbances
➡ < 70 mas pointing disturbance
➡ < 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
JWST
Clampin/GSFC
Event-driven Operations
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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
JWST
Clampin/GSFC
Observation Planning
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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
JWST
Clampin/GSFC
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
JWST
Clampin/GSFC
What time is it?
•
Time (UTC) flows from ground  JWST S/C  ISIM  Science Data
➡ S/C clock stable to < 2 sec / 40 hours (14 ppm)
➡ S/C clock corrected every contact (~12 hr nominal contact interval)
➡ Accuracy requirement < 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
JWST
Clampin/GSFC