HAT-P-20b is a giant metal-rich exoplanet orbiting a metal-rich star .
We analyze two secondary eclipses of the planet in each of the 3.6- and 4.5 \mu m bands of Warm Spitzer .
We have developed a simple , powerful , and radically different method to correct the intra-pixel effect for Warm Spitzer data , which we call pixel-level decorrelation ( PLD ) .
PLD corrects the intra-pixel effect very effectively , but without explicitly using - or even measuring - the fluctuations in the apparent position of the stellar image .
We illustrate and validate PLD using synthetic and real data , and comparing the results to previous analyses .
PLD can significantly reduce or eliminate red noise in Spitzer secondary eclipse photometry , even for eclipses that have proven to be intractable using other methods .
Our successful PLD analysis of four HAT-P-20b eclipses shows a best-fit blackbody temperature of 1134 \pm 29 K , indicating inefficient longitudinal transfer of heat , but lacking evidence for strong molecular absorption .
We find sufficient evidence for variability in the 4.5 \mu m band that the eclipses should be monitored at that wavelength by Spitzer , and this planet should be a high priority for JWST spectroscopy .
All four eclipses occur about 35 minutes after orbital phase 0.5 , indicating a slightly eccentric orbit .
A joint fit of the eclipse and transit times with extant RV data yields e \cos { \omega } = 0.01352 ^ { +0.00054 } _ { -0.00057 } , and establishes the small eccentricity of the orbit to high statistical confidence .
HAT-P-20b is another excellent candidate for orbital evolution via Kozai migration or other three-body mechanism .