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We report Spitzer 3.6 and 4.5 $\mu$m photometry of 11 bright stars relativeto Sirius, exploiting the unique optical stability of the Spitzer SpaceTelescope point spread function (PSF). Spitzer's extremely stable berylliumoptics in its isothermal environment enables precise comparisons in the wingsof the PSF from heavily saturated stars. These bright stars stand as theprimary sample to improve stellar models, and to transfer the absolute fluxcalibration of bright standard stars to a sample of fainter standards usefulfor missions like JWST and for large groundbased telescopes. We demonstratethat better than 1% relative photometry can be achieved using the PSF wingtechnique in the radial range of 20--100\arcsec\ for stars that are fainterthan Sirius by 8 mag (from outside the saturated core to a large radius where ahigh 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 W2bands, (2) comparing with stars where there is accurate $K_{\text{S}}$photometry, and (3) also comparing with relative fluxes obtained with the DIRBEinstrument on COBE. These tests confirm that relative photometry is achieved tobetter than 1%.

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