William Joseph Glaccum

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

Absolute Calibration of the Infrared Array Camera on the Spitzer Space Telescope

ABSTRACT The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is 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 intermediat...

In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 μm in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC.

Photometry using the Infrared Array Camera on the Spitzer Space Telescope

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 ratio in an IRAC image.

Repeatability and Accuracy of Exoplanet Eclipse Depths Measured with Post-Cryogenic Spitzer

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.

A Spitzer/IRAC Measure of the Zodiacal Light

The dominant non-instrumental background source for space–based infrared observatories is the zodiacal light (ZL). We present Spitzer Infrared Array Camera (IRAC) measurements of the ZL at 3.6, 4.5, 5.8, and 8.0 μm, taken as part of the instrument calibrations. We measure the changing surface brightness levels in approximately weekly IRAC observations near the north ecliptic pole 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 ZL. This is compared to both Cosmic Background Explorer Diffuse Infrared Background Experiment data and a ZL 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 ZL 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 ZL models.

Active star formation in NGC 2264

The region of NGC 2264 near the cone nebula is the site of active star formation in a rotating ring seen nearly edge on as a two lobed source. Allens infrared source (IRS 1) surrounds a B3V star still embedded in the southern lobe of the cloud. The northern lobe, IRS 2, also probably contains young stars.

Pointing effects and their consequences for Spitzer IRAC exoplanet observations

Spitzer observations of exoplanets routinely yield accuracies of better than one part in 10,000. However, there remain a number of issues that limit the attainable precision, particularly for long duration observations. These include initial pointing inaccuracies, pointing wobble, initial target drift, long-term pointing drifts, and low and high frequency jitter. Coupled with small scale, intrapixel sensitivity variations, all of these pointing issues have the potential to produce significant, correlated photometric noise. We examine each of these issues in turn, discussing their suspected causes and consequences, and describing possible and planned mitigation techniques.

Absolute photometric calibration of IRAC: lessons learned using nine years of flight data

Significant improvements in our understanding of various photometric effects have occurred in the more than nine years of flight operations of the Infrared Array Camera aboard the Spitzer Space Telescope. With the accumulation of calibration data, photometric variations that are intrinsic to the instrument can now be mapped with high fidelity. Using all existing data on calibration stars, the array location-dependent photometric correction (the variation of flux with position on the array) and the correction for intra-pixel sensitivity variation (pixel-phase) have been modeled simultaneously. Examination of the warm mission data enabled the characterization of the underlying form of the pixelphase variation in cryogenic data. In addition to the accumulation of calibration data, significant improvements in the calibration of the truth spectra of the calibrators has taken place. Using the work of Engelke et al. (2006), the KIII calibrators have no offset as compared to the AV calibrators, providing a second pillar of the calibration scheme. The current cryogenic calibration is better than 3% in an absolute sense, with most of the uncertainty still in the knowledge of the true flux densities of the primary calibrators. We present the final state of the cryogenic IRAC calibration and a comparison of the IRAC calibration to an independent calibration methodology using the HST primary calibrators.

Calibration and data quality of warm IRAC

We present an overview of the calibration and properties of data from the IRAC instrument aboard the Spitzer Space Telescope taken after the depletion of cryogen. The cryogen depleted on 15 May 2009, and shortly afterward a two-month- long calibration and characterization campaign was conducted. The array temperature and bias setpoints were revised on 19 September 2009 to take advantage of lower than expected power dissipation by the instrument and to improve sensitivity. The final operating temperature of the arrays is 28.7 K, the applied bias across each detector is 500 mV and the equilibrium temperature of the instrument chamber is 27.55 K. The final sensitivities are essentially the same as the cryogenic mission with the 3.6 μm array being slightly less sensitive (10%) and the 4.5 μm array within 5% of the cryogenic sensitivity. The current absolute photometric uncertainties are 4% at 3.6 and 4.5 μm, and better than milli-mag photometry is achievable for long-stare photometric observations. With continued analysis, we expect the absolute calibration to improve to the cryogenic value of 3%. Warm IRAC operations fully support all science that was conducted in the cryogenic mission and all currently planned warm science projects (including Exploration Science programs). We expect that IRAC will continue to make ground-breaking discoveries in star formation, the nature of the early universe, and in our understanding of the properties of exoplanets.