Crowded-Field Astrometry with SIM PlanetQuest. II. An Improved Instrument Model

TL;DR

This paper enhances an astrometric instrument model for SIM PlanetQuest by including spectral dispersion, multi-pixel detectors, and fringe binning, leading to more accurate bias estimates and improved understanding of instrumental effects.

Contribution

It introduces a more comprehensive instrument model that accounts for spectral dispersion, pixelation, and fringe binning, refining previous bias estimations in crowded-field astrometry.

Findings

Pixellation of dispersed fringes causes systematic differences from earlier models.

The improved model helps identify ways to reduce measurement confusion.

Provides a better basis for evaluating instrumental effects in future astrometric observations.

Abstract

In a previous paper we described a method of estimating the single-measurement bias to be expected in astrometric observations of targets in crowded fields with the future Space Interferometry Mission (SIM). That study was based on a simplified model of the instrument and the measurement process involving a single-pixel focal plane detector, an idealized spectrometer, and continuous sampling of the fringes during the delay scanning. In this paper we elaborate on this ``instrument model'' to include the following additional complications: spectral dispersion of the light with a thin prism, which turns the instrument camera into an objective prism spectrograph; a multiple-pixel detector in the camera focal plane; and, binning of the fringe signal during scanning of the delay. The results obtained with this improved model differ in small but systematic ways from those obtained with the earlier simplified model. We conclude that it is the pixellation of the dispersed fringes on the focal plane detector which is responsible for the differences. The improved instrument model described here suggests additional ways of reducing certain kinds of confusion, and provides a better basis for the evaluation of instrumental effects in the future.