  The dayside of HD 149026b is near the edge of detectability by the Spitzer
Space Telescope. We report on eleven secondary-eclipse events at 3.6, 4.5, 3 x
5.8, 4 x 8.0, and 2 x 16 microns plus three primary-transit events at 8.0
microns. The eclipse depths from jointly-fit models at each wavelength are
0.040 +/- 0.003% at 3.6 microns, 0.034 +/- 0.006% at 4.5 microns, 0.044 +/-
0.010% at 5.8 microns, 0.052 +/- 0.006% at 8.0 microns, and 0.085 +/- 0.032% at
16 microns. Multiple observations at the longer wavelengths improved
eclipse-depth signal-to-noise ratios by up to a factor of two and improved
estimates of the planet-to-star radius ratio (Rp/Rs = 0.0518 +/- 0.0006). We
also identify no significant deviations from a circular orbit and, using this
model, report an improved period of 2.8758916 +/- 0.0000014 days.
Chemical-equilibrium models find no indication of a temperature inversion in
the dayside atmosphere of HD 149026b. Our best-fit model favors large amounts
of CO and CO2, moderate heat redistribution (f=0.5), and a strongly enhanced
metallicity. These analyses use BiLinearly-Interpolated Subpixel Sensitivity
(BLISS) mapping, a new technique to model two position-dependent systematics
(intrapixel variability and pixelation) by mapping the pixel surface at high
resolution. BLISS mapping outperforms previous methods in both speed and
goodness of fit. We also present an orthogonalization technique for
linearly-correlated parameters that accelerates the convergence of Markov
chains that employ the Metropolis random walk sampler. The electronic
supplement contains light-curve files and supplementary figures.
