  We examine the repeatability, reliability, and accuracy of differential
exoplanet eclipse depth measurements made using the InfraRed Array Camera
(IRAC) on the Spitzer Space Telescope during the post-cryogenic mission. We
have re-analyzed an existing 4.5 {\mu}m data set, consisting of 10 observations
of the XO-3b system during secondary eclipse, using seven different techniques
for removing correlated noise. We find that, on average, for a given technique,
the eclipse depth estimate is repeatable from epoch to epoch to within 156
parts per million (ppm). Most techniques derive eclipse depths that do not vary
by more than a factor 3 of the photon noise limit. All methods but one
accurately assess their own errors: for these methods, the individual
measurement uncertainties are comparable to the scatter in eclipse depths over
the 10 epoch sample. To assess the accuracy of the techniques as well as to
clarify the difference between instrumental and other sources of measurement
error, we have also analyzed a simulated data set of 10 visits to XO-3b, for
which the eclipse depth is known. We find that three of the methods (BLISS
mapping, Pixel Level Decorrelation, and Independent Component Analysis) obtain
results that are within three times the photon limit of the true eclipse depth.
When averaged over the 10 epoch ensemble, 5 out of 7 techniques come within 60
ppm of the true value. Spitzer exoplanet data, if obtained following current
best practices and reduced using methods such as those described here, can
measure repeatable and accurate single eclipse depths, with close to
photon-limited results.
