WFIRST-AFTA Ops Concept Overview

DRAFT

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• Multiple surveys with different requirements to implement the various science programs– High Latitude Survey (HLS): ~2,400 deg2 imaging and

spectroscopic sky survey for BAO/RSD & WL (dark energy science), also used for archival studies by guest investigators

– Supernova (SN) Survey: Multiple visits to SN fields at high ecliptic latitudes to discover and track SN (dark energy science)

– Exoplanet Microlensing Survey: Multiple visits to microlensing fields near Galactic bulge to monitor planetary microlensing events

– Exoplanet Coronagraph Survey: Observe nearby stars to find and characterize both previously known and new planets

– Guest Observer: Allocated time for proposers to observe targets anywhere within the field of regard

Survey Strategy

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• The high latitude, supernova, and exoplanet microlensing surveys will all use the wide field instrument– HLS imaging survey uses the wide field imager– HLS spectroscopy survey uses the wide field grism– SN survey uses the wide field imager for discovery and the wide field IFU to type

SN, measure redshifts, and obtain lightcurves– IFU may also be used in parallel with HLS imaging or spectroscopy to support

photometric redshift calibration– Microlensing survey uses the wide field imager

• The exoplanet coronagraph survey uses the coronagraph instrument– Imager is for finding planets and for photometric characterization and the IFS is

for spectroscopic characterization

• The guest observer program can use either instrument• The wide field instrument will operate during coronagraph observations

– Needed for fine guiding, but will also take deep imaging exposures

• Coronagraph observations are not currently planned during wide field instrument observations– Are there reasons to leave the coronagraph powered on during wide field

observations for thermal or reliability reasons?

Implementation of Surveys

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• The HLS covers ~2,400 deg2 over ~2 years in both imaging (~1.3 yrs) and spectroscopy (~0.6 yrs) modes and is spread out over the 6 year mission

• HLS footprint is in regions of high Galactic latitude and is within the LSST footprint (or other deep visible survey) for photometric redshifts.

• In imaging mode, perform 2 passes over the survey footprint in each of the 4 imaging filters (J,H,F184 [for shapes] and Y [for photo-z’s]– There is a “leading” and “trailing” pass in each filter to provide roughly 180˚ roll (but exactly 180˚ not desired)

for data set self-calibration (current ops plan has ~150˚, will be optimized later)– Each pass includes four ~184 sec exposures (with five exposures in the J band, since we are attempting WL

shape measurement in this band and it has the tightest sampling requirements)– Each exposure is offset diagonally by ~slightly more than a chip gap. This pattern is repeated across the sky

in both the X and Y directions spaced by the field size.– 90% of imaging field sees ≥5 randomly dithered exposures (≥920 sec total) in Y, H, F184 bands, ≥6

exposures (≥1104 sec total) in J band– No requirement on roll alignment between passes in different filters

• In spectroscopy mode, perform 4 passes total over the survey footprint– The grism has 2 “leading” passes and 2 “trailing” passes to provide roughly 180˚ roll (but exactly 180˚ not

desired) to enable the single grism to rotate relative to the sky and provide counter-dispersion (current ops plan has ~150˚, will be optimized later)

– Each pass includes two ~362 sec exposures with a small offset to cover chip gaps– The 2 “leading” passes (and 2 “trailing”) are rotated from each other by ~5˚– 90% of spectroscopy field sees ≥6 randomly dithered exposures (≥2172 sec total)– Zero order galaxy provided in J,H or J,F184 bands by WL imaging passes

• There is no direct requirement for absolute pointing accuracy (either initial or revisit). The chief consideration is minimizing the overlap of adjacent fields needed to avoid gaps in the survey. An absolute pointing accuracy corresponding to 1% of the size of an SCA (~4.5 arcsec) is a representative value.

High Latitude Survey

HLS (Imaging) Mapping(view in slide show mode)

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Perform 1st pass mapping of a super field in the 1st filter4 exposures with a gap filling offset between each

Perform 1st pass mapping of a super field in the 1st filterPerform 1st pass mapping of a super field in the 2nd filterPerform 1st pass mapping in the remaining two filtersPerform 2nd pass mapping in the 1st filter ~6 months (or N years + 6 months) later

Perform 2nd pass mapping in the 2nd filter ~6 months (or N years + 6 months) later

Perform 2nd pass mapping in the remaining 2 filters ~6 months (or N years + 6 months) later

HLS (Spectroscopy) Mapping(view in slide show mode)

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Perform 1st pass of a super field2 exposures with a gap filling offset

Perform 2nd pass of a super field at a slight rollPerform 3rd pass of a super field ~6 months (or N years + 6 months) later

Perform 4th pass of a super field at a slight roll ~6 months (or N years + 6 months) later

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• The Type Ia supernova survey observes for a total of 6 months but is carried out over a total of 2 years in separate 1-year periods.

• The imager is used for SN discovery and the IFU spectrometer is used to type SN, measure redshifts, and obtain lightcurves

• Supernova observations take place with a five-day cadence, with each interval of observations taking a total of 30 hours of combined imaging and spectroscopy.

• Fields are located in low dust regions ≤20˚ off an ecliptic pole• Example 3-tiered survey with each tier optimized for a different redshift range

– Tier 1 for z<0.4: 27.44 deg2 in Y and J bands– Tier 2 for z<0.8: 8.96 deg2 in J and H bands– Tier 3 for z<1.7: 5.04 deg2 in J and H bands– Tier 3 is contained in Tier 2 and Tier 2 is contained in Tier 1

• Need SN mapping strategy– Gap filling– Revisit accuracy TBD– Dither with 30 mas accuracy

• IFU exposure times are tailored for each individual supernova• Final revisit for each target for spectroscopy after SN fade for galaxy subtraction,

some may occur after the dedicated 2 year period, but are accounted for in the example observing scenario

• Weekly interaction with the ground after visiting discovery fields to schedule follow ups on SN candidates

Type Ia Supernova Survey

Supernova Field Mapping

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Example Weekly Discovery and Follow up Timeline

Sun Mon Tues Wed Thurs Fri Sat

3/4 Day Cadence5 Hour SNe Discovery

2 Hour Candidate SN Spectroscopic Follow-Up

Data Downlink/Uplink

Data Downlink Only

Weekly Obs. &New SNe Spec. Obs. New SNe Spec. Obs.

Prior concept for slit spectrometer ops on JDEM Probe. I expect there will be a similar plan for discovery and follow up with an IFU, but need inputs from SN group.

• The exoplanet microlensing survey observes 10 fields in the Galactic bulge for continuous 72-day seasons, interrupted only by monthly lunar avoidance cutouts (~4 days/month). The plan includes 6 seasons, with >2 years between the first and last season.

• The Galactic Bulge is observable for two 72-day seasons each year.

• In each season, the 10 fields are revisited on a 15 min cadence, viewing in a single wide filter (W149) for light curve tracking. For one exposure every 12 hours, a narrow, blue filter (Z087) is used to measure the color of the microlensing source star.

• Fields are revisited to an accuracy of ~110 mas (1 pixel rms) TBD; no precise dithers

• Data latency for notifications to other assets?

Exoplanet Microlensing Survey

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Detecting Planets with a Microlensing Survey

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• Current example observing schedule has observations scheduled in 26 blocks of 2 weeks each, interspersed throughout the mission.

• Portions of each block are dedicated to detecting a planet (imaging) and characterizing a planet (spectroscopy).

• The targets are around nearby stars (within ~10 parsecs) and are distributed around the field of regard.

• The current operations planner assesses the availability of each potential target star during each block of coronagraph observing time. Out of a catalog of 239 potential target stars, in each of the 26 coronagraph observing blocks we find at least 24 to be continuously viewable over the full 2-week period with no violations of the Earth, Moon, or Sun pointing constraints.

Exoplanet Coronagraph Survey

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• Acquire a dark hole on a nearby bright star– Acquire star in tip/tilt– Measure and remove low order WFE– Measure and remove high order WFE– Verify dark hole

• Slew to target star– Thermal loads change in this process

• Acquire target– Imager is coarse sensor– LOWFS is the fine sensor– Apply tip/tilt control to set the target on occulter– 1st order correction of low order wavefront

• Integrate– Each band and each sensor

Strawman Observing Scenario

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• Allocated time throughout the 6-year mission for proposers to observe targets anywhere within the field of regard

• The GO program by definition cannot be “allocated” at this stage in the project.

• The scheduling of HLS observations can be re-organized based on the content of the GO program. Currently, we have simply required that the time not used for other programs be ≥1.25 years, and that all portions of the sky are visible in multiple years during otherwise-unallocated time.– In computing the unallocated time, we subtract the penalty for a

typical 90° slew from each unallocated window. (This way, the slewing penalty between any two programs is charged against the time allocation of one of the programs but not both.)

Guest Observer Program

Central Line of Sight (LOS) Field of Regard (FOR)

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+54˚

Keep-Out Zone

Observing Zone

Keep-Out Zone

SNe Fields

SNe Fields

+126˚

Galactic Bulge (Available twice

yearly)

SNe Fixed Fields be ±20˚ off one of the Ecliptic Poles,

Microlensing can observe Inertially Fixed Fields in the Galactic Bulge (GB) for 72 days twice a year

WL/ BAO-RSD/ GO/Coronagraph Surveys can be optimized within the full Observing Zone

Observing Zone: 54˚-126˚ Pitch off Sun Line360˚ Yaw about Sun Line±15˚ Roll about LOS(off max power roll)

GB

The LOS cannot point within 33° (TBR) of the limb of the Earth and 8° (TBR) of the Moon.

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Galactic Bulge Viewing

Looking down on the ecliptic plane, ~72 day seasons available to view the bulge

Galactic Bulge

36˚

36˚

Ecliptic Plane

Galactic PlaneGalactic Bulge

Observing timeline with constraints in GEO orbit; initial inclination 28.5°, initial RA of ascending node 228° (over 6 years, precesses to inclination=26.4°, RAAN=188°).

Example Observing Schedule

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• This timeline is an existence proof only, not a final recommendation.• Unallocated time is 1.43 years (includes GO program)• High latitude survey (HLS: imaging + spectroscopy): 1.96 years

– 2401 deg2 @ ≥3 exposures in all filters (2440 deg2 bounding box)• 6 microlensing seasons (0.98 years, after lunar cutouts)• SN survey in 0.62 years, field embedded in HLS footprint• 1 year for the coronagraph, interspersed throughout the mission

Example Observing Schedule: Properties

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High LatitudeSurvey Area

MicrolensingFields

Ecliptic PlaneCelestial Equator

• 1.2 Gbps continuous Ka downlink capability from GEO– Preliminary analysis indicates at least 16x microstepping

required to keep jitter down during observations– Wide Field Instrument produces ~600 Mbps, assumes 2x

lossless compression– Coronagraph data volume 30 Gbits/day plus <1 Gbits/day

LOWFS

• 2 Ka antenna on-board, only one radiating at a time• Ground architecture similar to SDO

– 2 ground stations at White Sands, both within the beam width of the Ka antenna

Downlink

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• Slew/settle accuracy for each program (when can we start next obs)

• FGS ops• Orbit maintenance• Momentum unloading• Ground system

– Assumes MOC is 8x5

Additional Info to Include

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• Blind Acquisition – No guide stars picked out• Directed Acquisition – Pre-planned guide stars• Field – The projected field of view on the sky• Super Field – Multiple deg2 portion of the sky mapped at one

time• Slews

– Dither– Gap filling– X-Field– Y-Field

• Frame Time – The length of time to read out a single detector• Integration Time – The effective observation time between

the first and last read frame of each pixel in a detector without any resets in between.

Definitions

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Continuous Viewing Zone Analysis

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• Baseline Cycle 4 orbit is circular GEO– Initial inclination = 28.5˚, RAAN = 236˚– Precession (perturbations from Sun, Moon, and Earth oblateness

through ℓ=4)

• Field of regard– Pitch range ±36˚ (angle from Sun = 54-126˚)– Telescope LOS to Earth center ≥ 41.7˚ (TBR) (33˚ (TBR) from Earth

limb from Len Seale’s stray light analysis + 8.7˚ Earth radius)– Chris ignored the Moon for this analysis (moves at 13˚/day, can

avoid with minimal impact)– No constraints on radiator angles

• Eclipses– Earth eclipse seasons at b<9˚– Also 15 Moon eclipses outside these seasons in 6 year mission

(short, can avoid – not discussed here)

Sky Availability-Basic Assumptions(courtesy Chris Hirata)

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Earth Cutouts

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Earth Cutout Geometry

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Sun Cutouts

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+54˚

Keep-Out Zone

Observing Zone

Keep-Out Zone

SNe Fields

SNe Fields

+126˚

Galactic Bulge (Available twice

yearly)

GB

Combined Cutouts

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Beta Angle over the Mission

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Fraction of Sky Available versus b

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• The following 7 charts show the combined Earth + Sun viewing constraints at 1 month intervals.

• The Sun viewing constraint is periodic every 6 months since the pitch limit is symmetric under positive pitch (away from Sun, up to +36˚) and negative pitch (toward Sun, down to -36˚). Thus the last chart is the same as the first.

• In each hemisphere, the region between the gray curves is allowed by the Sun, and within the blue circles is allowed by the Earth.

• Maximum viewing fraction (satisfying both constraints) is at low b (April or October).

Combined Cutout Charts

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