1. Mark Clampin (GSFC) John Stansberry (STScI)
JWST Operations
Mark Clampin (GSFC) John Stansberry (STScI)

2. JWST Observatory Status

3. 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
Contrubuting factors
Image stability

4. Image Quality: MIrrors

5. Diffraction Limited: Strehl > 0.8 (WFE ≤ 150 nm)
Image Quality
Diffraction Limited: Strehl > 0.8 (WFE ≤ 150 nm)
F115W
F200W
F444W
F070W
Log Scale
Linear Scale

6. 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

7. 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 ➠

8. 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
Further options exist for WFE drift mitigation via scheduling
Phase curves on same targets over several days will cause most significant WFE drift

9. 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

10. Image Motion Current Image Motion Requirement:
OBS‑ 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

11. 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)

12. 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
FSW change for slew patterns would have to be added, together with ground support

13. 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

14. JWST Exposure Nomenclature

15. JWST MULTIACCUM Patterns

16. 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) :
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 < 1.7 days)
Detector resets reduce these rates somewhat

17. 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 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

18. Max Uninterrupted Exposure Duration
Four basic limits (continued)
Max # integrations = 2^16 = (ASIC hardware limit)
Max exposure durations for integrations with 2 frames per ramp:
Full-frame: hrs (32.2 sec/integration)
64 x 64 subarray: 2.7 hrs (148 msec/integration)
2048 x 64 stripe: hrs (1.02 sec/integration)
2048 x 64 subarray: 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

19. 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

20. 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

21. 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

22. 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

23. 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