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We present several corrections for point-source photometry to be applied to data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These corrections are necessary because of characteristics of the IRAC arrays and optics and the way the instrument is calibrated in flight. When these corrections are applied, it is possible to achieve a [image]2% relative photometric accuracy for sources of adequate signal-to-noise ratio in an IRAC image.

We present several corrections for point-source photometry to be applied to data from the Infrared Array Camera (IRAC) on theSpitzer Space Telescope.These corrections are necessary because of characteristics of the IRAC arrays and optics and the way the instrument is calibrated in flight. When these corrections are applied, it is possible to achieve a∼2% ∼ 2 % relative photometric accuracy for sources of adequate signal-to-noise ratio in an IRAC image.

The Infrared Array Camera (IRAC) on theSpitzer Space Telescopeis absolutely calibrated by comparing photometry of a set of A stars near the north ecliptic pole to predictions based on ground‐based observations and a stellar atmosphere model. The brightness of point sources is calibrated to an accuracy of 3%, relative to models for A‐star stellar atmospheres, for observations performed and analyzed in the same manner as for the calibration stars. This includes corrections for the location of the star in the array and the location of the centroid within the peak pixel. Long‐term stability of the IRAC photometry was measured by monitoring the brightness of A dwarfs and K giants (near the north ecliptic pole) observed several times per month; the photometry is stable to 1.5% (rms) over a year. Intermediate‐timescale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC was in nominal operations; the intermediate‐term photometry is stable, with a 1% dispersion (rms). One of the secondary calibrators was found to have significantly time‐variable (5%) mid‐infrared emission, with a period (7.4 days) matching the optical light curve; it is possibly a Cepheid variable.

We present infrared photometry of all 36 potential JWST calibrators for which there is archival Spitzer IRAC data. This photometry can then be used to inform stellar models necessary to provide absolute calibration for all JWST instruments. We describe in detail the steps necessary to measure IRAC photometry from archive retrieval to photometric corrections. To validate our photometry we examine the distribution of uncertainties from all detections in all four IRAC channels as well as compare the photometry and its uncertainties to those from models, ALLWISE, and the literature. 75% of our detections have standard deviations per star of all observations within each channel of less than three percent. The median standard deviations are 1.2, 1.3, 1.1, and 1.9% in [3.6] - [8.0] respectively. We find less than 8% standard deviations in differences of our photometry with ALLWISE, and excellent agreement with literature values (less than 3% difference) lending credence to our measured fluxes. JWST is poised to do ground-breaking science, and accurate calibration and cross-calibration with other missions will be part of the underpinnings of that science.

We present observational constraints on the stellar populations of two ultra-diffuse galaxies (UDGs) using optical through near-infrared (NIR) spectral energy distribution (SED) fitting. Our analysis is enabled by new \\(Spitzer\\)-IRAC 3.6 \\(\\mu\\)m and 4.5 \\(\\mu\\)m imaging, archival optical imaging, and the prospector fully Bayesian SED fitting framework. Our sample contains one field UDG (DGSAT I), one Virgo cluster UDG (VCC 1287), and one Virgo cluster dwarf elliptical for comparison (VCC 1122). We find that the optical--NIR colors of the three galaxies are significantly different from each other. We infer that VCC 1287 has an old (\\(\\gtrsim7.7\\) Gyr) and surprisingly metal-poor (\\([Z/Z_{\\odot}]\\lesssim-1.0\\)) stellar population, even after marginalizing over uncertainties on diffuse interstellar dust. In contrast, the field UDG DGSAT I shows evidence of being younger than the Virgo UDG, with an extended star formation history and an age posterior extending down to \\(\\sim3\\) Gyr. The stellar metallicity of DGSAT I is sub-solar but higher than that of the Virgo UDG, with \\([Z/Z_{\\odot}]=-0.63^{+0.35}_{-0.62}\\); in the case of exactly zero diffuse interstellar dust, DGSAT I may even have solar metallicity. With VCC 1287 and several Coma UDGs, a general picture is emerging where cluster UDGs may be \"failed\" galaxies, but the field UDG DGSAT I seems more consistent with a stellar feedback-induced expansion scenario. In the future, our approach can be applied to a large and diverse sample of UDGs down to faint surface brightness limits, with the goal of constraining their stellar ages, stellar metallicities, and circumstellar and diffuse interstellar dust content.

Airborne low-resolution 16–65 μm spectra of 19 oxygen-rich and 3 S stars, and simultaneous 100–200 μm photometry of the brightest objects are presented. The survey includes K and M giants, AGB stars, the post-AGB star AFGL 4106, and the hypergiant IRC+10420. Oxygen-rich stars with cold shells have emission bands at 24, 28, 33, 42, and 60 μm, caused by thermal emission from grains of crystalline silicates and water ice. Spectra of most supergiants and some giants are featureless at wavelengths greater than 20 μm, but supergiants with very high mass loss rates have very weak emission features from crystalline silicates. AGB stars with the 13 μm, feature have an emission feature centered at 29 μm. Weak, narrow emission features at 20.7, 22.4, 29.0, and 31.6 μm appear in some of the oxygen-rich stars. The shape of the 20 μm feature in VY CMa is variable. The S stars have a broader emission band that peaks near 30 μm. A simple model with several grain species is fitted to the spectrum of RX Boo. The 13 μm, feature probably arises from hot corundum grains. Other features can be fitted with combinations of amorphous and crystalline bronzite, olivine, clino-enstatite, and γ-Ca2SiO4. The calibration system, which is based on Mars, Uranus, and Callisto, is independent of ground-based photometric calibrations and the IRAS photometric calibration. Except for very bright objects, there is excellent agreement at 25 and 60 μm between this work and the IRAS Point Source Catalog.

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.

The dominant non-instrumental background source for space-based infrared observatories is the zo- diacal light. We present Spitzer Infrared Array Camera (IRAC) measurements of the zodiacal light at 3.6, 4.5, 5.8, and 8.0 {\\mu}m, taken as part of the instrument calibrations. We measure the changing surface brightness levels in approximately weekly IRAC observations near the north ecliptic pole (NEP) over the period of roughly 8.5 years. This long time baseline is crucial for measuring the annual sinusoidal variation in the signal levels due to the tilt of the dust disk with respect to the ecliptic, which is the true signal of the zodiacal light. This is compared to both Cosmic Background Explorer Diffuse Infrared Background Experiment (COBE DIRBE) data and a zodiacal light model based thereon. Our data show a few percent discrepancy from the Kelsall et al. (1998) model including a potential warping of the interplanetary dust disk and a previously detected overdensity in the dust cloud directly behind the Earth in its orbit. Accurate knowledge of the zodiacal light is important for both extragalactic and Galactic astronomy including measurements of the cosmic infrared background, absolute measures of extended sources, and comparison to extrasolar interplanetary dust models. IRAC data can be used to further inform and test future zodiacal light models.

We present several corrections for point source photometry to be applied to data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These corrections are necessary because of characteristics of the IRAC arrays and optics and the way the instrument is calibrated in-flight. When these corrections are applied, it is possible to achieve a ~2% relative photometric accuracy for sources of adequate signal to noise in an IRAC image.