National Aeronautics and Space Administration Pluto Astrometry From Table Mountain Observatory William M. Owen, Jr. Jet Propulsion Laboratory California Institute of Technology Copyright © 2010 California Institute of Technology. Government sponsorship acknowledged.

National Aeronautics and Space Administration Why? Pluto has been observed for less than half a revolution Its period and distance are uncertain New Horizons approach optical navigation gives the B plane but does not help the time of close approach until very late Therefore encounter sequences are driven by the radial uncertainty in the ephemeris

National Aeronautics and Space Administration Who? Steve Synnott, George Null and I researched high-accuracy astrometry in the early ’90s –Improve ephemerides of Solar System bodies –Beacon asteroids for Deep Space 1 –Optical astrometry for laser-bearing s/c Students do a lot of the observing

National Aeronautics and Space Administration Where? JPL’s Table Mountain Observatory –East end of San Gabriel Mountains –7500 ft elevation

National Aeronautics and Space Administration What? 24-inch f/16 Ritchey-Chrétien telescope 4K x 4K CCD, 15 microns/pixel Scale about 1/3 arcsec/pixel Field of view about 1/3 degree

National Aeronautics and Space Administration When? First light in 1996 We’ve been observing Pluto somewhat regularly since 2001 –Note: with seeing typically 2″ we’re actually measuring the photocenter of Pluto+Charon

National Aeronautics and Space Administration How? Oversampled images give accurate ( x, y ) Large field of view means lots of ref stars Routine use of Eichhorn’s overlapping plate method –Even more reference stars are used –Changes in star patterns as function of ( x, y ) give better camera model –“A star has only one position at a time”

National Aeronautics and Space Administration Traditional approach Standard practice has been to treat each picture separately: –Assume ( ,  ) for the plate center –Map the observed ( x, y ) into ideal ( ,  ) using reference stars –Use the results to convert target’s ( x, y ) into ( ,  ) and thence to ( ,  )

National Aeronautics and Space Administration Better way Camera model includes pointing explicitly: Rotate apparent vector to camera system: Project into focal plane: Add differential refraction too

National Aeronautics and Space Administration Better way 2 Take several pictures of the target, with a different telescope pointing for each Identify all reference stars (and target) Create ad hoc catalog of all unidentified stars Throw everything into a least squares fit

National Aeronautics and Space Administration Better way 3 Solution parameters: –( ,  ) for everything (ref stars constrained by catalogued position sigmas at epoch of obs) –Three pointing angles for each picture –Scale and lower-order distortions for each picture –Higher-order distortions across all pictures Calibration mosaic of a star cluster provides a priori distortion model

National Aeronautics and Space Administration Asteroid occultations We observed Hertha. Not Anastasia.

National Aeronautics and Space Administration Pluto Results