TIPS/JIM
19 July 2007
TEL Group
Improving HST
Pointing & Astrometry
M. Lallo
TIPS/JIM 19 July 2007
Why, What, Who
TIPS/JIM
19 July 2007
TEL Group
Why Improve HST Pointing & Astrometry?
➡ Absolute astrometry increasingly important:
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multi-mission archives, virtual observatories, coordinated observations
“Fundamentally important property of HST data” – STUC, 2002
➡ Improved initial pointing supports most efficient COS acquisition scenario.
➡ Well-calibrated FOV entering SM4 simplifies SMOV calibrations & operations.
What Comprises the Initiative?
➡ Use of GSC2 in operations
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Utilized since cycle 15
Reduces catalog error from ~0.7˝ to 0.25˝ (1σ absolute) and 0.38˝ to 0.15˝ (1σ relative across FOV)
➡ Retroactive improvements in archive astrometry
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Updated astrometry keywords by identifying GSC2 objects in HLA images. (ISR-ACS-2005-06, Koekemoer et al)
Possibility of improving all archive data by applying GSC1 – GSC2 coordinate delta to CRVALs (initial scoping)
➡ More frequent attention to SI & FGS alignment calibration
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as in the late ‘90s, FOV alignment calibrations or checks are again included in our calibration program
error on determinations of FGS & SI positions: ~0.05˝ , orientations: ~ 0.03º - 0.004º (well below GSC2 error)
Who was/is involved?
B. McLean, B. Owens, D. Taylor, M. Reinhart, R. Whitman, SMSB, OPB, STSB
➡ GSC2 ops:
HLA Team (A. Koekemoer, B. McLean, B. Whitmore, W. Miller, H. Jenkner), others
➡ Archive astrometry:
➡ HST FOV cals & updates: Lallo, Nelan, Cox, E. Kimmer (GSFC), J. Mo (PRDB), others in INS & ESS
HST Pointing
TIPS/JIM
19 July 2007
TEL Group
V3
V1
Roll is established to within ~10˝ of commanded angle using FHSTs
V2
Dominant guidestar is positioned in FGS space in order to place and
keep the science target at the requested reference point via pure
translations.
'(43S
)
0”
20
.1
'(43
.g
(e
$04
45*4
8'$
V2
(-U2)
/*$.04
"$4
'(4
V3
(-U3)
(adom,ddom),(V2dom,V3dom), Rollcommanded → (a,d) (V2,V3)
Three basic calibrations relate FGS & SI parameters to the common vehicle frame V2,V3
➡ FGS distortion solutions & other FGS-specific calibrations transform raw measurements of guidestars:
(qa, qb, & Fine Error Signal) → FGS X,Y
➡ FGS alignment matrix: applies rotation & offset to transform FGS X,Y → vehicle V2,V3
➡ SI orientations & specifiable reference pixels are characterized in V2,V3 and used for pointing & dithers
Since the PCS’s measurement errors are comparatively very low, the error in the pointing
solution is dominated by the accuracies of the operational catalog and of the three types
of calibrations listed above.
Alignment Trends & Effects on Pointing
TIPS/JIM
19 July 2007
TEL Group
SIs & FGSs migrate in the HST field of view
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evolution decreases roughly exponentially with time on-orbit.
FGSs exhibit factor 10 greater motion than SIs (total translation & rotation ≲5˝ & ≲1º versus ≲0.5˝ & ≲0.1º).
an uncalibrated displacement of a dominant FGS of DV2,DV3 offsets SI target by same DV2,DV3.
an uncalibrated angle change in a dominant FGS of Dq about its (central) reference point produces a spread of offsets
at the SI ranging from DV2,DV3 = 0 to +/–570*tan(Dq), depending on guidestar position in the FGS.
Past work by the STIS team (L. Dressel) clearly illustrates FGS trends for 3 years after
SMOV3A (FGS2r installation)
➡ V2 vs V3 offset plot (left) shows stability and accuracy of calibrated FGS positions going as their time on orbit.
➡ Offset vs. time plot for FGS2r makes asymptotic behavior clear.
➡ These data were used to assist in the determination of new FGS alignment matrices installed operationally in late 2002.
Median Pointing Offset for FGS2R in V2 & V3
Offset from Reference Point (arcseconds)
1.5
1.0
V3
0.5
0.0
–0.5
–1.0
V2
–1.5
2000.0
2000.5
2001.0
2001.5
2002.0
2002.5
2003.0
These secular changes cannot be readily modeled by time-dependent functions, so must
be monitored and calibrated periodically, with special attention in early post-SM4.
Calibration Goals & Accuracy
TIPS/JIM
19 July 2007
TEL Group
Blue circle in figure at 1˝ radius represents the 1s scatter in HST pointing that has been seen, to
a greater or lesser degree, throughout HST’s mission life.
➡ as described, such scatter is due to uncalibrated misalignments and celestial coordinate error (in this case coming mostly
from GSC1) rather than any basic limitation of HST’s ability to achieve much more precise pointing.
The yellow circle at radius 0.3˝ is our goal for total 1s pointing error from Cycle 16 onward.
➡ GSC2 reduces catalog error to 0.25˝ (absolute) and 0.15˝ (relative)
➡ dedicated alignment calibrations, when performed, are good to ~ 0.05˝ for shifts, and between 0.03º and 0.004º
The outermost ring represents the COS ~1.25˝ aperture radius (actual aperture edge is fuzzy)
➡ 0.3˝ 1s pointing scatter will enable COS to operate without time-consuming ACQ-Search (map)
➡ target would fall directly into aperture with enough margin for the centering algorithm, with a success rate at the 3s level.
After SM4, alignment calibration frequency would vary to maintain accuracy goals given the
more rapid trending of the new FGS & SIs.
0.5˝
1.0˝
1.5˝
Overlaid COS PSF courtesy Makidon
GSC2 operational catalog, combined with reasonably attentive FOV calibrations to readily
achievable accuracies, can reduce the HST pointing scatter by better than factor 3.
Updates, Monitoring & Maintenance
TIPS/JIM
19 July 2007
TEL Group
Large astrometric field, spanning the HST FOV, is utilized for FGS and SI alignment
➡ Open cluster M35 (McNamara & Sekiguchi 1986) as reprocessed by HST PASSOPS had been used throughout HST
mission
➡ This cycle we tested performing such calibrations with UCAC2 catalog. UCAC2 & GSC2 are on the ICRS frame. Much
greater sky coverage gives us better scheduling/target flexibility. UCAC2 now chosen as default.
Basic Process
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1. update FGS magnification (this linear distortion term continues to trend significantly for many years on-orbit)
2. perform fits of guidestars and astrometry stars to obtain zero point and rotation of the three FGSs
3. with the resulting current FGS calibration, determine accurate pointing solution for SI alignment observations
4. make operational updates to SIAF, FGS matrices, FGS magnifications and related products.
Figures illustrate preferred observing strategies for FGS and SI alignment
CAL-OTA 11021 obtained data which are being analyzed to determine FGS and SI zero
points and angles.
➡ We have a number of results at this time and intend to update the operational FGS matrices and the SIs’ SIAF values this
fall. Currently assessing results obtained via multiple methods and inputs. A future presentation will describe and quantify
operational updates once performed.
➡ Computed FGS alignment changes since 2002 are indicating roughly ~ 0.2˝ displacement for FGS2 and ~ 0.1˝ for FGS1.
Orientation changes are ~ 0.04º. ACS shifts ~0.4˝ over 5 years with no or little rotation detected beyond our
measurement error.
Updates, Monitoring & Maintenance
TIPS/JIM
19 July 2007
TEL Group
Tests of new FGS alignment quality
➡ Differencing the separation between dominant & subdominant guidestars in astrometric celestial coordinate space against
the separation obtained from FGS measurements transformed to V2,V3 give an indicator of the quality of the FGS
alignment if the astrometric coordinate error is known and/or if the same guidestar pair is compared.
➡ SI-FGS observations utilize same guidestar pair at ~10 different locations in the FGSs.
➡ For these cases, the current operational FGS matrices give 55 mas average separation error +/– 37 mas
➡ The newly determined FGS matrices gave 17 mas +/– 4 mas
“Passive” monitoring (between alignment calibration visits or programs)
➡ Post SM4, COS could provide missed-distance statistics from its centering slews in the same way STIS has done.
➡ Another interesting possibility for monitoring guidestar separation uncertainties would be the routine extraction of this
value, along with the FGS numbers, from the corresponding header keywords in the jitter files. If possible, this will have
the advantage of large number statistics.
➡ Other FGS calibration data may be suitable in some cases to check the evolution of the magnification term
Additional FOV alignment orbits are being considered for cycle 16 supplemental calibration,
though it is possible they will not be needed.
Programs similar to 11021 will be planned and executed periodically in cycle 17 onward, again
with the goal to calibrate below GSC2 error.