Transcript NGAO Astrometry PPT

Current astrometric error is as low as 0.25 mas.
Individual stars in one night
• Galactic Center observed with
NIRC2 on Keck
• astrometric accuracy is a
function of atmospheric
conditions.
Nightly averages for 3 different nights.
• astrometric floor at ~0.25 mas
What sets the floor?
Slides from UCLA Galactic Center Group (Ghez, Lu)
Astrometric accuracy is a function of location in the field.
Use 1 night of GC data:
• LGSAO, NIRC2
• 107 exposures
• tint = 60 sec each
• Strehls: 0.3 - 0.4
• FWHM: 53 - 63 mas
• WFE ~ 345 nm (Marechal)
Simultaneous
MASS/DIMM (estimated
at zenith):
• r0 = 10 - 15 cm
• theta0 = 1.5’’ - 3.0’’
Courtesy of Matthew Britton:
http://eraserhead.caltech.edu/keck/galactic_cent
er/turbulence_plots/turbulence_plots.html
Slides from UCLA Galactic Center Group (Ghez, Lu)
Galactic Center data shows strong isoplanatic effects
dependence on astrometric error.
• In each frame, measure the
position relative to the
reference star, which is close to
the laser spot.
• Take the RMS from all frames.
• The RMS positional error can
be decomposed into radial and
tangential components relative
to the laser position (or
reference source).
Slides from UCLA Galactic Center Group (Ghez, Lu)
PSF elongation seems strongest in elevation direction.
• PSF degrades due to
both isoplanatic and
isokinetic effects.
• Isoplanatic effects
dominate… isokinetic still
under investigation.
Differential tip-tilt jitter
masked by larger effects?
• anisoplanatism
• chromatic differential
atmospheric refraction
along elevation?
Slides from UCLA Galactic Center Group (Ghez, Lu)
Need an astrometry
experiment with the tiptilt star and a separate
guide star in the FOV.