Observing Moving Targets with JWST
Ed Nelan
TIPS
April 17, 2008
Moving Targets and JWST
JWST requirements documents make numerous references to “moving
targets”, but are qualified with a “TBD”. For example:
MR-371 Strehl Ratio for Moving Targets
“ … NIRCam shall have a Strehl Ratio greater than (TBD) when tracking any
available target that exhibits an angular velocity v up to (TBD) mas/sec …’’
MR-372 Moving Target Tracking
“ … the Observatory shall track targets with velocity (TBD) mas/sec over a
total motion of (TBD) arcsec …”
NASA wanted “TBD”s associated with Moving Targets to be resolved
before Mission PDR (April 2008)
Moving Target Study Plan
• NASA HQ allocated $250K for a 3 month study in June 2007.
– Creation of the “Moving Target Working Group” (NASA, NGST, STScI)
• The objective of the study was to determine:
– A technical solution to implement MT observations
– Define departures from observations of fixed targets (requirements,
system architecture and development / verification plans)
– The cost to implement the capability
– Final Report was submitted in November 2007
• NASA considered cost and scientific benefit to determine if JWST
should be able to observe Solar System targets.
• STScI studied this issue in 2002: JWST-STScI-000398.
– But this only considered impact to S&OC.
Moving Targets, what they are:
•
Moving Targets are Solar System bodies:
–
–
–
–
–
•
Kuiper Belt Objects
planets
moons of planets
asteroids
comets
Compared to stars, they are nearby, and they move. Their
position on the sky is determined by
–
–
JWST parallax (because they are close by)
Ephemeris (because they move)
stop!
HST Legacy of Solar System
Observations
HST Observations of Solar System Targets
•
Between 2001 and 2007, 13% of all HST public out reach releases
involved Solar System observations.
•
But only ~4% of the HST program was dedicated to Solar System
observations.
galaxy
89
star
61
nebula
58
Solar System
45
miscellaneous
44
exotic
35
cosmology
26
star cluster
22
survey
18
HST Observations of Solar System Targets
•
Between 2001 and 2007, 13% of all HST public out reach releases
involved Solar System observations.
•
But only ~4% of the HST program was dedicated to Solar System
observations.
galaxy
89
star
61
nebula
58
Solar System
45
miscellaneous
44
exotic
35
cosmology
26
star cluster
22
survey
18
JWST observations of Solar System objects
• MTWG identified several science
drivers for JWST observations of
Solar System objects.
• KBOs and comets are choice
targets.
• comets originating beyond the
orbit of Jupiter have a different ratio
of hydrogen to deuterium than
Earth’s oceans (2x).
• comets in the main asteroid belt
(there are 3 known) may be the
source of Earth’s oceans.
• MIRI spectroscopy can measure
their hydrogen/deuterium.
What’s involved in
Moving Target Observations?
• Proposal Preparation, Planning & Scheduling and Execution is
more complex for moving targets:
– Target ephemeris
– JWST parallax and ephemeris
– Tracking.
• Complicates:
–
–
–
–
Proposal preparation, guide star selection system (S&OC)
AD scripts
FGS
ACS
Solar System target rates depends upon when
they are observed.
25 mas / sec
Mars
From J. Nella,JWST kickoff Meeting, 10/23/02
Solar System target rates depends upon when
they are observed.
25 mas / sec
Neptune
1 mas / sec
From J. Nella,JWST kickoff Meeting, 10/23/02
angular rates of selected objects when
observable by JWST from L2
Object
Min. Rate
(mas/sec)
Max Rate
(mas/sec)
Mars
Jupiter
Jupiter,Io
Saturn
Uranus
Neptune
Pluto *
KBO
2.5
0.070
0.004
0.040
0.020
0.004
0.160
0.002
28.6
4.5
10.2
2.9
1.4
1.0
1.0
0.5
Distance
Traveled in 10 Time to Trave l 1Υ
hrs at Min Rate at Max Rate (hrs)
(asec)
90.0
2.5
0.14
1.4
0.7
0.14
5.7
0.07
* Includes motion about Pluto-Charon barycenter
0.6
3.7
1.6
5.7
17
24
24
48
Moving Target Study: Procedure
•
NASA restricted moving target study to consider only:
–
–
–
–
–
•
tracking rates up to 3 mas/sec
linear tracks
16 mas pointing performance over 700 second intervals
only one guide star is used for the visit, no handoffs
WG was also instructed to evaluate implications of rates up to 30 mas/sec.
MTWG developed an Operations Concept to observe moving targets.
– assumed maximum use of HST/MOSS ground system
– preserved JWST event driven operations.
– excluded solutions that require hardware changes to JWST.
•
The Operations Concept was developed into a Use Case to facilitate
the identification new requirements.
Moving Target Study, guide star usability
• In event driven operations, the visit window should be large
compared to the visit duration.
• On board scripts are provided with a list of guide star
candidates with usability windows
• Scripts select the appropriate guide star when the visit is about
to execute.
Moving Target Tracking and Offset Maneuvers
GS position after
Target Tracking is Disabled
current GS position
Combined Offset Maneuver
(GS Acq, Target Acq, Dither) and
(Moving Target Correction)
GS position moves while
Target Tracking is Enabled
GS position when Target Tracking is Enabled
Offset Maneuver 1
(GS Acq, Target Acq, Dither)
FGS Track Box after Offset Maneuvers
GS position for Target Tracking
after Offset 2 at Ts + Toff1
GS position for Target Tracking
after Offset 1 at Ts
Offset Maneuver 2 (Moving Target Correction)
compensates for the time it takes to execute the slew and re-acquire the guide star
Continuous tracking w/offsets (TA & dithers) *
11 - Science Exposure (Tracking ON)
Moving Target Tracking
(Tracking ON)
Combined Offset to Target
10 - Cmd Dither Offset
Moving Target Correction
(Tracking ENABLED)
9 - End Science Exposure
Commanded Offset
to Target
8 - Science Exposure (Tracking ON)
Moving Target Tracking
(Tracking ON)
Combined Offset to Target
Moving Target Correction
(Tracking ENABLED)
7 - Cmd Target Acq Offset
Moving Target Tracking
(Tracking ON)
Commanded Offset
to Target
1 - Acquire GS
2 - Specify GS ephem
3 - Enable GS tracking
4 - Cmd Offset to Target
6 - End Exposure
Combined Offset
to Target
5 - Target Acq Exposure (Tracking ON)
Moving Target Tracking (Tracking ON)
Commanded Offset
to Target
* from John Isaacs
Moving Target Correction (Tracking ENABLED)
New Requirements
•
•
•
Observation planning and scheduling changes
–
Very similar to moving target tracking on HST (affects APT, GSSS, etc).
–
Affects ground-to-AD interface
Only six new or modified onboard commands
–
AD sends GS ephemeris (Ps, dPs, Ts) to ACS
–
ACS sends back computed GS position P(T) to AD
–
AD sends estimate of command overhead time to ACS
–
AD requests FGS to set 8x8 Track box (instead of 8x8 Fine Guide box)
–
AD requests ACS to begin tracking moving GS ephemeris
–
AD requests ACS to stop tracking moving GS ephemeris
ACS must add capability to evaluate moving GS ephemeris
–
•
Some subtlety in keeping separate from dithers and offsets
FGS must accept new command to set 8x8 Track box
–
Very similar to setting 8x8 Fine Guide box
–
No other new requirements on FGS
Moving Target Capability Approved
A decision from today's science management council was to proceed with
including the moving target capability in JWST. Additional funds will be
provided to the program to cover these costs. The Planetary Science division
recognized this capability's value and has agreed to pay for it. We are
definitely going to do this.
I would like to thank George Sonneborn and the crew at STScI who worked on
this study. It was an easy sell because of all the excellent groundwork they
did.
Tuesday, Feb 07, 2008
Eric Smith, JWST Program Scientist