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July 11, 2014

Annual Checkout Makes for Great Pluto Preparation

Kimberly EnnicoKimberly Ennico
New Horizons deputy project scientist

You walk up to the Restaurant at the End of the Solar System, ready to try that slice of “Pluto on ice” that you heard amazing things about. The chef behind the counter asks, “So, how would you like your data?”

Without hesitation you reply, “well-calibrated.”

Pretty pictures or spectral measurements make no sense without context. For images, we need to know how many kilometers are in a single pixel and for each raw digitized value, a mapping from bits to energy units (like magnitude or intensity). For spectra, we need to know how much spatial information is covered per pixel, as well as what each pixel’s response to wavelength and brightness is. For particle instruments, we need to know the energy and direction of each ion or dust grain.

Before launch, every New Horizons instrument underwent intensive laboratory characterization called preflight calibration. They were subject to spatial targets, integrating spheres, laser pulses and particle accelerators – to name a few good “known” sources – to get “translations” from bits stored to disk to “real” units like wavelength, flux energy, intensity and the like. After launch, such translations were verified with in-flight calibrations, where, for example, instead of a lab source, the instruments stared at stars or inspected Jupiter and its moons.

Each year, the team executes an ACO, or annual checkout, where instrument performances are trended and the mission team looks for changes. Additional observations provide information to remove unwanted “artifacts” like high-responsive pixels, smearing and ghosts.

This summer is ACO-8, our eighth annual checkout since launch. It showcases our last calibrations prior to the 2015 Pluto encounter. It’s jam-packed with observations that are done yearly for trending, but also some new ones to make sure the New Horizons instrument suite is indeed “well-calibrated.” Highlights include new radiometric calibrations for the LEISA infrared spectrometer, a long stability test for the REX radio experiment, and a test for revised thresholds for PEPSSI, the high-energy particle detector. More calibration data is taken during the 2015 Pluto flyby, and together, these data sets are placed in the data-reduction pipeline to translate bits to “real” values. Resources and time aboard the spacecraft to execute these observations are limited, so a series of reviews and assessments are done prior to each checkout.

The team is eager to get the data from ACO-8. We woke up on June 15 and payload calibrations continue through August. It may not be the Pluto flyby, but this summer’s data will play a big role in the science return from New Horizons next year!

Artist's impression of New Horizons in the void of space

Demonstration of read-out smear removal, preserving the photon count, in LORRI’s calibration pipeline. The data in the smear is caused by imperfections in the CCD readout when illuminated by a lot of light. The source of the photons is from the object being imaged, so we need to correctly relocate the information. Data without good calibration is messy.

As a New Horizons deputy project scientist, Kimberly Ennico manages instrument readiness and calibration aspects of the mission. Her expertise includes instrument development, space qualification and calibration; optical/infrared astronomy; optical/infrared detectors, optics, cameras and spectrometers; and science communication.