An Efficient Method for Measuring the Color of a Natural Satellite with Astrometric Applications

Abstract The Gaia DR3 catalog released in 2022 contains precise broadband photometry for more than one billion stars in the G , BP, and RP passbands, which is very useful for calibrating the differential color refraction (DCR) effect in ground-based observations. It is a pity, however, that the catalog does not provide color indices (e.g., BP − RP) for Solar System Objects, such as natural satellites. In this study, we try to measure the Gaia standard color of a natural satellite by transforming the instrumental color index to a Gaia standard one using a local Johnson–Cousins photometric system ( BVRI ). This method requires only a few CCD frames and a few reference stars in each frame. To verify its effectiveness and applicability, we measured the colors of three natural satellites: Himalia (JVI), Phoebe (SIX), and Triton (NI). We found that the results are quite good, with a measurement uncertainty that can be less than 0.01 mag. Based on the measured color indices, we corrected the DCR effect in our observations, significantly improving astrometric accuracy and precision. For example, the standard deviations of the positional ( O − C ) residuals (observed minus computed) for Himalia improved from 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 084 to 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 021 in R.A., and for Phoebe, from 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 031 to 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 012.