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Observing Moving Targets with JWST

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

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Observing Moving Targets with JWST

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  1. Observing Moving Targets with JWST Ed Nelan TIPS April 17, 2008

  2. 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)

  3. 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.

  4. 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)

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  6. HST Legacy of Solar System Observations

  7. 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.

  8. 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.

  9. 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.

  10. What’s involved inMoving 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

  11. Solar System target rates depends upon when they are observed. Mars 25 mas / sec From J. Nella,JWST kickoff Meeting, 10/23/02

  12. Solar System target rates depends upon when they are observed. Neptune 1 mas / sec 25 mas / sec From J. Nella,JWST kickoff Meeting, 10/23/02

  13. angular rates of selected objects when observable by JWST from L2 * Includes motion about Pluto-Charon barycenter

  14. 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.

  15. 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.

  16. 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

  17. 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 7 - Cmd Target Acq Offset Moving Target Correction (Tracking ENABLED) Moving Target Tracking (Tracking ON) Commanded Offset to Target 6 - End Exposure 1 - Acquire GS 2 - Specify GS ephem 3 - Enable GS tracking 4 - Cmd Offset to Target 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)

  18. 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

  19. 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

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