Calibrating a CVZ Astrometric
Field in the LMC with ACS for
JWST
Rosa I. Diaz-Miller
Space Telescope Science Institute
September 21st, 2006
TIPS Meeting
Astrometric Field in the LMC with ACS for JWST
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Collaborators:
Jay Anderson (Rice University)
Ralph Bohlin (STScI)
Stefano Casertano (STScI)
Jerry Kriss (STScI)
Matt McMaster (STScI)
Sep 21st, 2006
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Massimo Stiavelli (STScI)
Vera Platais (STScI)
James Rhoads (UA)
Jeff Valanti (STScI)
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Rationale for Cross Calibration of JWST
Strategy for securing an astrometric field
Preliminary results
Sep 21st, 2006
Rosa Diaz-Miller
Rationale for Cross Calibration
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HST astrometric observations are essential for calibrating
JWST field distortion.
➥ Meet JWST calibration requirements
Field distortion correction within any instrument and guide
star shall not exceed 0.005 arcsec RMS
➥ This will enable the precise target positioning required for
multi-object spectroscopy with NIRSpec.
➞ MSA: (750 x 350) shutters, each (200 x 450) mas2
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➥ Enable higher quality science early in mission.
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JWST self calibration?
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➥ JWST
roll angles at one time +/- 5
➥ JWST will need more than 60 days to attempt good self
calibration of an astrometric field.
➥ Changes in stability of JWST’s mirror might have smaller
time scales.
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Added Advantages of HST-JWST cross calibration:
➥ Save JWST observing time
➥ External check on JWST astrometric solutions
➥ HST/ACS field with astrometric precision better than can likely be
obtained from JWST alone.
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How well can ACS do?
➥ Arbitrary ACS-WFC fields can achieve 5 mas accuracy.
➥ Errors less than 2 mas are possible with proper distortion correction
using multiple roll angles. Current distortion correction => 0.01 pixels
and 0.01 mag accuracies for ACS/WFC (Anderson & King 2000,
2003,2006)
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Astrometric-calibration field for JWST:
➥ High stellar density
➥ Most stars usable
➥ Stellar density variation < 2
➥ Separation ~10 FWHM
➥ S/N ~ 100 at V=24
➥ Small proper motions
➥ Field in the JWST CVZ
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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LMC is ideal. About 10 stars per 3.75x3.75 arcsec2
C. Smith, S. Points,
the MCELS Team and
NOAO/AURA/NSF
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Astrometric Calibration Strategy to get errors < 2 mas
◆ Multiple dithered observations of the same field and
different roll angles
➥ ACS-WFC filter F606W
➥ Mosaic of images to cover
a 5.4x5.4 arcmin2 region.
➥ 5 fields, 60” apart
➥ 6 points dither pattern per
field with steps of ~3-10
arcsec
➥ Large dynamic range
➥ Two roll angles separated
by 75o
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30” dithers
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Correcting for distortion
➥ Use PSF library to
determine position of each
star (Anderson & King
2005, PASP, 115, 113;
ACS ISR 2006-01)
➥ Measure all the stars in
each image.
➥ Solve for the distortion
coefficients fitting the
residuals with fourth-order
polynomials
➥ Cross identify stars in the
various images.
Anderson, J. 2005, HST Calibration Workshop
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
◆ First orientation: 30 exposures
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About 100,000 bright enough stars in the field. (J. Anderson)
Composite
image
Sep 21st, 2006
Close up
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
Identified stars in each field (field P1 in red, P2 in blue, P3 green, P5 magenta)
Close-up
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
◆RMS
residuals for first orientation fields.
◆ PSF blurrier than library ( ~10% less flux in the core.)
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Current accuracies ~0.03-0.02 pixels
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
RMS residuals along x
axis for field P1
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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RMS residual along Y axis for field P1
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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RMS residuals are derived from 6 dither pointings with
the constrain that a star should be at least in 5 of the 6
pointings
➥ Since we have large dithers => in this analysis we are missing
some stars.
➥ Need to change this constrain
➥ Need to add the short exposure
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After all geometric distortions have been removed from
the 5 fields, we are still left with a distortion that has
scales of about 1000 pixels. We need to remove this
distortion.
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
AVG= 30 pointings
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
AVG= 30 pointings
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
RMS residuals for master frame (average of 30 pointings)
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
RMS residuals for master frame (average of 30 pointings
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Second orientation: 12 pointings
➥ Stars well fit by the empirical PSF library
➥ Internal consistency with which we are
measuring positions and fluxes is better than
0.01 pixel and 0.01 mag
➥ We need to compare with first orientation
Sep 21st, 2006
Rosa Diaz-Miller
Astrometric Field in the LMC with ACS for JWST
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Summary
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First orientation: 30 pointings
➞ Current accuracies ~0.03-0.01pixels
➞ Re-derive PSF library?
➞ Add short exposures.
➞ Find extra correction to the distortion.
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Second orientation: 12 pointings
➞ Stars very well fit by my empirical library PSF
➞ Compare with first orientation
➥Expect
to get accuracies to 1mas (0.5 mas)
➞ Need to remove extra distortion first
Sep 21st, 2006
Rosa Diaz-Miller