Clampin/GSFC JWST

Mark Clampin (GSFC)

John Stansberry (STScI)

Mark Clampin (GSFC)

John Stansberry (STScI)

JWST Operations

Clampin/GSFC JWST

JWST Observatory StatusJWST Observatory StatusJWST Observatory StatusJWST Observatory Status

Clampin/GSFC JWST

Image QualityImage QualityImage QualityImage Quality• 150 nm @ NIRCam focal plane: 2 m 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 ( < 2.3 m): 32 mas/pixel

NIRSpec pixel size ( > 2.4 m): 100 mas/pixel

/D (0.7 m) ~ 22 mas

MIRI pixel size (imaging/prism): 110 mas/pixel

/D (5.0 m) ~ 0.16 mas

• Contrubuting factors

• Image stability

Clampin/GSFC JWST

Image Quality: MIrrorsImage Quality: MIrrorsImage Quality: MIrrorsImage Quality: MIrrors

Clampin/GSFC JWST

Image QualityImage QualityImage QualityImage QualityF

070W

LinearScale

F11

5W

F20

0WF

444W

LogScale

Diffraction Limited: Strehl > 0.8 (WFE ≤ 150 nm)

Clampin/GSFC JWST

OTE Thermal Stability: IOTE Thermal Stability: IOTE Thermal Stability: IOTE 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

Clampin/GSFC JWST

OTE Thermal Stability: IIOTE Thermal Stability: IIOTE Thermal Stability: IIOTE 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

➠

Clampin/GSFC JWST

OTE Thermal Stability: IIIOTE Thermal Stability: IIIOTE Thermal Stability: IIIOTE 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

Clampin/GSFC JWST

Wavefront Sensing & ControlWavefront Sensing & ControlWavefront Sensing & ControlWavefront 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

Clampin/GSFC JWST

Image MotionImage MotionImage MotionImage 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 InstrumentIM Allocation

(mas)NIRCam 6.6NIRSpec 6.7

MIRI 7.4NIRISS 6.8

Clampin/GSFC JWST

Reaction WheelsReaction WheelsReaction WheelsReaction 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)

Clampin/GSFC JWST

Spatial Scanning OptionsSpatial Scanning OptionsSpatial Scanning OptionsSpatial 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

Clampin/GSFC JWST

Calibration OptionsCalibration OptionsCalibration OptionsCalibration 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

Clampin/GSFC JWST

JWST Exposure NomenclatureJWST Exposure NomenclatureJWST Exposure NomenclatureJWST Exposure Nomenclature

Clampin/GSFC JWST

JWST MULTIACCUM PatternsJWST MULTIACCUM PatternsJWST MULTIACCUM PatternsJWST MULTIACCUM Patterns

Clampin/GSFC JWST

Data Rate and StorageData Rate and StorageData Rate and StorageData 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

Clampin/GSFC JWST

Max Uninterrupted Exposure DurationMax Uninterrupted Exposure DurationMax Uninterrupted Exposure DurationMax 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

Clampin/GSFC JWST

Max Uninterrupted Exposure DurationMax Uninterrupted Exposure DurationMax Uninterrupted Exposure DurationMax 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

Clampin/GSFC JWST

High-gain Antenna ProposalHigh-gain Antenna ProposalHigh-gain Antenna ProposalHigh-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

Clampin/GSFC JWST

Event-driven OperationsEvent-driven OperationsEvent-driven OperationsEvent-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

Clampin/GSFC JWST

Observation PlanningObservation PlanningObservation PlanningObservation 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

Clampin/GSFC JWST

Exposure Time CalculatorExposure Time CalculatorExposure Time CalculatorExposure 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

Clampin/GSFC JWST

What time is it?What time is it?What time is it?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