Analysis of the Diffuse Background Components towards Efficient Use of the IBEX-lo Star Sensor for the Interstellar Flow Direction Determination
A key objective of the Interstellar Boundary Explorer (IBEX) mission is to compare the interstellar oxygen flow to the helium flow through the solar system. This comparison requires an accurate determination of the incoming energetic neutral atoms (ENAs) of the interstellar gas flow to the background stars. To obtain this type of pointing accuracy a star sensor was developed with a unique split "V" aperture and was co-aligned to the IBEX- lo collimator. Encased in the star sensor is a photomultiplier tube (PMT) that is sensitive to visual wavelengths. The magnitude range of stars that can be used for the direction determination is greatly affected by diffuse background sources including the Milky Way and the zodiacal light. Using detailed background models for subtraction increases the usable dynamic range. A preliminary background model of the sky was created assuming one color band, but analysis of Milky Way photometry maps show it is necessary to include all color bands to increase the angular resolution of the star sensor. We will present first IBEX star sensor observations in comparison with the preliminary and the improved background models. Flight data also reveal an unexpectedly strong presence of the geocorona as a background source. We will discuss these early findings and how a detailed modeling of diffuse backgrounds enhances the star sensor operation.
Precision Pointing in the Sky for IBEX Interstellar Flow Observations - Use of the Moon With the IBEX-Lo Star Sensor
The Interstellar Boundary Explorer (IBEX) is in Earth orbit and imaging Energetic Neutral Atoms (ENAs) from the heliospheric boundary and those that flow through the solar system from the interstellar medium. Two sensitive ENA cameras (IBEX-Hi and Lo) that point radially outward on the spinning and sun-pointing spacecraft obtain full-sky images over time periods of six months. With IBEX-Lo, we can identify species of the interstellar flow, such as He and O. Together with accurate knowledge of the flow direction, interstellar parameters and the interaction with the heliosphere can be deduced. In order to provide precision pointing information on the interstellar flow in astronomical coordinates, a star sensor is co-aligned with the IBEX-Lo. The star sensor is capable of distinguishing bright stars, superior planets, and the moon. The objects' known coordinates act as direction pointers for the satellite's position. The moon poses a unique problem because of its highly variable shape and distance from the satellite. New flight data allow us to calibrate the star sensor's signal to known parameters of moon passage. In comparison with laboratory calibrations and simulations, the moon's barycenter can be found by modeling the star sensor's response to a range of moon phases. As will be discussed, the correction can be accurately applied to the moon signal from flight data.