An Astrometric Approach to Measuring the Color of an Object

TL;DR

This paper introduces an astrometric method leveraging differential color refraction to measure an object's color index accurately using minimal photometric data, improving observational efficiency and calibration.

Contribution

It presents a novel astrometric technique linking differential color refraction effects to photometric color measurement, reducing reliance on traditional photometry.

Findings

Measured Himalia's color index as 0.750 1 0.004 magnitude.

Demonstrated the method's effectiveness over 8 nights and 857 CCD frames.

Identified two regions with color indices exceeding three sigma from the mean.

Abstract

The color of a star is a critical feature to reflect its physical property such as the temperature. The color index is usually obtained via absolute photometry, which is demanding for weather conditions and instruments. In this work, we present an astrometric method to measure the catalog-matched color index of an object based on the effect of differential color refraction (DCR). Specifically, we can observe an object using only one filter or alternately using two different filters. Through the difference of the DCR effect compared with reference stars, the catalog-matched color index of an object can be conveniently derived. Hence, we can perform DCR calibration and obtain its accurate and precise positions even if observed with Null filter during a large range of zenith distances, by which the limiting magnitude and observational efficiency of the telescope can be significantly improved. This method takes advantage of the DCR effect and builds a link between astrometry and photometry. In practice, we measure the color indices and positions of Himalia (the sixth satellite of Jupiter) using 857 CCD frames over 8 nights by two telescopes. Totally, the mean color index BP-RP (Gaia photometric system) of Himalia is 0.750 \pm 0.004 magnitude. Through the rotational phased color index analysis, we find two places with their color indices exceeding the mean \pm 3 \sigma.