Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/jwst/science/moving-targets-lpsc-march2013-v3.pdf Дата изменения: Unknown Дата индексирования: Sat Mar 1 07:14:45 2014 Кодировка: Поисковые слова: solar eclipse

Operations Concept for Moving Target Observations with JWST
George Sonneborn JWST Project Scientist for Operations NASA/Goddard Space Flight Center March 17, 2013 JWST Workshop 44th Lunar and Planetary Science Conference The Woodlands, Texas
Hsieh & Jewitt (2006)
1


Topics



Pointing control for moving target observations JWST focal plane and aperture geometry Event-driven science operations and observation scheduling

2


Moving Target Ops Concept in JWST Focal Plane
Guide Star Moving Target ephemeris V3
Ecliptic

Guide Star moves along prescribed ephemeris path

NIRISS
48.5°

Target remains fixed in SI FOV

V2 Science Target Trajectory without Moving Target compensation
3


Recent Progress on Spacecraft Moving Target Capability


Mission requirements for moving target capability Capability to observe moving targets with apparent rates up to 0.030 arcsec/second


Includes Mars and beyond, but not all possible comets or near-Earth objects



Recent pointing stability simulations: <0.010 arcsec (1) for all rates



Non-linear ephemeris uplinked, stored by observation scripts, and executed by Attitude Control Subsystem th Ephemeris defined by 5 order Chebychev polynomial derived from JPL HORIZONS Commands/telemetry exchanged between observing scripts and spacecraft attitude control for autonomous execution of moving target observations. Critical Design Review for moving target capability will be in August 2013






Will include simulations of moving target pointing performance using non-linear stability analysis with JWST observatory dynamics model Cycle 1 Call for Proposals in 2017



Moving target observations will be supported in first year of JWST observing


4


Moving Target Observation Sequence


Moving target observation process (Same as Fixed Targets)/ (New for Moving Targets)


Step 1: Slew to ID Attitude; FGS ID/ACQ; Attitude Update Step 2: Specify MT ephemeris Step 3: Move guide star to Science Attitude for guide star tracking; Compute time when science exposure can start (guide star will be in position along ephemeris trajectory Step 4: FGS guide star acquisition; Attitude Update; Zero Offset Maneuver Step 5: FGS Track mode on guide star; Enable fine guidance control in ACS Step 6: Enable MT Tracking of guide star; Start science exposure Step 7: Stop science exposure; Disable fine guidance control; Update Attitude Command For offset/dither slews, Go To Step 3



5


Schematic for Moving Target Observation


MT o MT e ACS ACS ACS ACS Scrip

ffset command repositions Guide Star and initiates moving guide star tracking phemeris defines guide star position such that science target is in SI aperture iterates offset slew calculation and computes guide star position P3 at time T3 sends T3 and P3 to Scripts; Scripts sets up FGS for Track Mode TE = End executes offset slew from P2 to P3 prior to T3 waits until T3 and then starts MT tracking of guide star ts starts science exposure Science exposure ends 6
Commanded Guide Star position for ID/ACQ

of Ephemeris

Moving Target GS Track

Acquired GS position 2

1 Science exposure starts 5

Executed offset slew from P2 to P3 3 Remove offset slew error

4

32x32 track box follows guide star

GS position P3 at time T3

not drawn to scale!
TS = Start of Ephemeris
6


JWST Observatory Operations


L2 halo orbit; no Earth or Lunar eclipses Solar array power Thermal Telescope and instruments passively cooled to T~40K in shadow of sun shield


Mid-IR instrument cooled to 6.7K by cryo cooler



Communications Two 4-hour DSN contacts per day for command uplink, real-time telemetry, and data downlink Attitude control ­ two control loops Reaction wheel/gyro/star tracker observatory attitude control system Fine guidance with focal-plane camera/tracker and focal-plane fine-steering mirror Hydrazine/oxidizer propulsion system to maintain orbit and reaction wheel momentum On-board science operations are event driven, not absolute time commanded (more later) Observation ­ Series of exposures with single instrument to achieve science objective Visit ­ Exposure(s) obtained with one science instrument and a single guide star An observation may be split into more than one visit (Visit is basic unit of scheduling)





7


Spectroscopic Apertures and Slits

Channel NIRSpec IFU NIRSpec Fixed Slits

Wavelength (µm) 0.6 ­ 5 0.6 ­ 5

Spatial Slice (arc sec) 0.1 0.2, 0.4, 1.6 0.2 x 0.46 0.176 0.277 0.387 0.644 0.6

Field of View (arc sec) 3.0x3.0
0.2 x 3.3 0.4 x 3.8 1.6 x 1.6

Number of Slices or Apertures 30
3 1 1

NIRSpec Microshutters 0.6 - 5 MIRI IFU 1 MIRI IFU 2 MIRI IFU 3 MIRI IFU 4 MIRI Low Res Slit 5 - 7.8 7.7 ­ 11.9 11.8 ­ 18.3 18.2 ­ 28.5 5 - ~14

200 x 200
4 x (97 x90)

~250,000
4 x 365() x 171(X)

3.7 x 3.7 4.7 x 4.5 6.2 x 6.1 7.7 x 7.9 0.6 x 5.5

21 17 16 12 1
8


IFU geometry for Uranus

NIRSpec IFU geometry

MIRI IFU geometry

Ecliptic

0.6 ­ 5.0 µm 3.0 x 3.0 arcsec

5.0 ­ 7.8 µm 3.7 x 3.7 arcsec

Uranus image (Keck Observatory) from 12 July 2004, solar elongation 133°

9


NIRSpec Fixed Slit Geometry for Neptune
Neptune image (Keck) from 27 July 2007, solar elongation 163°

0.2 x 3.3 arcsec slit

Ecliptic

10


MIRI IFU geometry for Comet Tempel 1

5-7.8 µm channel 3.7x3.7 arcsec

HST Image (Feldman & Weaver)

11


NIRSpec Microshutter "Long Slit" Concept
Microshutter pseudo long slit All shutters in column opened (~90 arcsec long in one quadrant) Each shutter is 0.2x0.46 arcsec

Spectral dispersion

HST Image of Comet Holmes (2007)
12


Event-Driven Science Operations


On-board scripts autonomously control the execution of the science timeline Scripts respond to actual slew completion and command execution


Scripts also respond to interrupted or failed visit, moving on to next valid visit Visits scheduled with overlapping windows to avoid idle time



Visit execution begins between earliest and latest start times within Visit window (nominally ~24 hrs)



Increases observing efficiency by at least 10% (compared to fixed-time scheduling) Can switch instruments in adjacent visits (all instruments on, all the time) Supports time-critical observations, Targets of Opportunity (48 hour minimum response time)
Set of observations scheduled in a "Timed Command Sequence" of visits starting at times t1, t2, t3, and t4 t2 t3 t4 t Visit 1 Same set of observations in a sequence of visits specified by an Earliest (E) and Latest (L) Start Time and a Latest Finish Time (F) E L F Visit 1 E L Visit 2 Visit 2 Visit 3 Visit 4 Visit 2 fails GS Acq & Observatory starts next visit earlier within window Lost Time E Visit 3 E Visit 4 L Visit 3 L F Visit 4
13

t

1

5

F

F

Visit 2 fails GS Acq & Observatory must wait to start the next visit

Lost Time


Guide Stars for Moving Target Observations


Event-driven operations provide flexibility in use of Guide Stars for moving targets Multiple sets of guide stars defined to cover complete visit scheduling window Guide star selection software development at STScI nearly complete Observations with different instruments require separate guide stars (and visits)
E Visit E L F Guide Star 1 Window Guide Star 2 Window Guide Star 3 Window Visit Start Time, can use Guide Star 2 or 3 for tracking Guide Star 4 Window Guide Star 5 Window Guide Star 6 Window L F Nominal Visit Scheduling Window

E = Earliest Start Time, L = Latest Start Time, F = Latest End Time

14


Time-Critical Moving Target Observations




Some observations need to execute in a narrow time window Fully supported by JWST planning and scheduling system Earliest and Latest Start Times define beginning of scheduling window Latest End Time defines end of window Observations with different instruments require separate guide stars (and visits) Back-to-back observations of a target with different instruments depends on GS availability
E L Visit E L F Guide Star 1 Window Guide Star 2 Window Visit Start Time, can use Guide Star 1 or 2 for tracking Guide Star 3 Window F Visit Window

E = Earliest Start Time, L = Latest Start Time, F = Latest End Time
15


Moving Target Development for JWST Ground System


STScI is nearly finished with three key ground system features for moving targets


Guide star selection software


Multiple guide stars uplinked to cover entire plan window, but only one of these guide stars will be used for the visit (which one is determined in real time on board by scripts) MOSS has been a cornerstone of HST's Solar System observation planning for over 20 years Will be a major element of the MT observation planning for JWST



Conversion of Moving Object Support System (MOSS) from Fortran to C++




Implement polynomial ephemeris in Visit Scheduling System, derived from JPL HORIZONS ephemerides


The ephemeris used by flight software to track the guide star will be a 5th-order polynomial.



Near-term work includes Update/define solar system use cases for operations development


Science planning, on-board scripts, target acquisition, science data processing



Target acquisition for spectroscopic observations (fixed and moving targets) Proposer interface to define observations and to input observer-supplied ephemerides

16


Observation Planning and Time Accounting Philosophy


Treat all observers equally Help observers craft programs that run efficiently on the Observatory The combination of exposure times plus computed and statistical overheads should reasonably describe the actual time for the Observatory to execute an Observation.


Computed overheads include all deterministic activities to acquire and setup a series of science exposures that form an Observation.


GS and target acquisitions, instrument mechanisms, offset slews, detector setup, etc These overheads depend on the exact instrument configuration and type of observation. Instrument internal overheads will be determined during cryo testing in 2014-15 Acquisition and offset slew times will be estimated by high-fidelity simulations



Statistical overheads include the major Observatory slew to the initial target attitude There is no "moving target tax" applied to MT programs



Policies not yet developed for how to handle other types of Observatory overheads


Activities not included in any observing program (e.g. momentum unloads, wavefront sensing & control, orbit maintenance, calibration observations) Resource-intensive observations, such as rapid TOOs (e.g. SNs, GRBs,) or high data volume
17




BACKUP

18


Apparent Rates of Motion Within JWST Field of Regard
Minimum rate (mas/sec) 2.5 1.0 0.07 0.04 0.02 0.004 0.16 0.35 0.22 Maximum rate (mas/sec) 28.6 18.4 4.5 2.9 1.4 1.0 1.0 0.89 0.56 Time to move 2 arcmin at min rate (hrs) 13.3 33.3 476 833 1667 8333 208 95 152 Time to move 2 arcmin at max rate (hrs) 1.2 1.8 7.4 11.4 24 34 34 37 59

Object Mars Ceres Jupiter Saturn Uranus Neptune Pluto Haumea Eris

Note: Times for some objects to move 2 arcmin at minimum rate (e.g. Uranus and Neptune) exceed the time when the object is moving at the minimum rate. However, durations are significantly longer than plausible observing windows (24-48 hours) for all objects except Mars when near minimum rates of motion.
19


Spatial Resolution for a 2 µm Diffraction Limit (0.07")

Object

Angular Diameter (arcsec) 7 37 17 3.5 2.2 0.1

Diameter (km)

2 µm Spatial Resolution (km) 68 270 490 1020 1590 1600

IFU size (km) 3"x3" 2900 11,600 21,180 43,700 68,180 72,000

Mars Jupiter Saturn Uranus Neptune Pluto @ 35 AU

6800 143,000 120,000 51,000 50,000 2300

Angular diameter shown for planet at quadrature

20


JWST attitude control system diagram
Integrated Science Instrument Module (ISIM)
Science Instrument (SI) observes science target Fine Guidance Sensor (FGS) observes guide star
Focal plane

Optical Telescope Element (OTE) Primary Mirror Tertiary Mirror

Secondary Mirror

SI FGS Fine Steering Mirror (FSM)

FGS Control
Guide star centroid 16 Hz Star Trackers observe reference stars (roll control)

FSM Control
FSM offset to correct Observatory pointing

Photons
Reaction Wheels correct Observatory pointing

Fine Guidance Control Gyros Star Trackers

FSM command to position guide star

Observatory Pointing Control

Reaction Wheels

Spacecraft Bus

Guide star centroid Commanded position

Guide star subarray FGS detector
21


22


23


24


JWST Field of Regard
North Ecliptic Pole Continuous Viewing Zone North



85°
5° 45°

Observatory thermal design defines the allowed Solar orientations


135°


Solar elongation 85° to 135° (like Spitzer) Roll ±5° about line of sight


360°

JWST can observe the whole sky every year while remaining continuously in the shadow of its s uns hield.


Continuous Viewing Zone South



Field of Regard is an annulus covering 35% of the sky The whole sky is covered each year with small continuous viewing zones at the Ecliptic poles

25


NIRSpec Microshutter "Long Slit" Concept

NIRSpec microshutter pseudo long slit shutters opened on diagonal Each open shutter is 0.2x0.46 arcsec

HST Image of Comet Holmes (2007)
26