Accurate Photometry of Saturated Stars Using the Point-Spread-Function Wing Technique with Spitzer

TL;DR

This paper presents a method using Spitzer's stable PSF wings to achieve high-precision photometry of saturated bright stars, improving calibration for stellar models and future telescopic missions.

Contribution

It introduces a novel PSF wing technique leveraging Spitzer's optical stability to perform accurate photometry of saturated stars, enhancing calibration standards.

Findings

Achieved better than 1% relative photometry accuracy.

Validated the method through multiple independent comparisons.

Extended the calibration of bright stars for future astronomical missions.

Abstract

We report Spitzer 3.6 and 4.5 $\mu$m photometry of 11 bright stars relative to Sirius, exploiting the unique optical stability of the Spitzer Space Telescope point spread function (PSF). Spitzer's extremely stable beryllium optics in its isothermal environment enables precise comparisons in the wings of the PSF from heavily saturated stars. These bright stars stand as the primary sample to improve stellar models, and to transfer the absolute flux calibration of bright standard stars to a sample of fainter standards useful for missions like JWST and for large groundbased telescopes. We demonstrate that better than 1% relative photometry can be achieved using the PSF wing technique in the radial range of 20--100\arcsec\ for stars that are fainter than Sirius by 8 mag (from outside the saturated core to a large radius where a high signal-to-noise profile can still be obtained). We test our results by (1) comparing the [3.6]$-$[4.5] color with that expected between the WISE W1 and W2 bands, (2) comparing with stars where there is accurate $K_{\text{S}}$ photometry, and (3) also comparing with relative fluxes obtained with the DIRBE instrument on COBE. These tests confirm that relative photometry is achieved to better than 1%.