Jessica Krick, Patrick Lowrance, Sean Carey, Jason Surace, Carl Grillmair, Seppo Laine, Schuyler Van Dyk, James Ingalls, Matthew L. Ashby, Steven Willner
We investigate differences in Spitzer/IRAC 3.6 and 4.5 μm photometry that depend on observing strategy. Using archival calibration data, we perform an in-depth examination of the measured flux densities (fluxes) of 10 calibration stars, observed with all the possible observing strategies. We then quantify differences in the measured fluxes as a function of (1) array mode (full or subarray), (2) exposure time, and (3) dithering versus staring observations. We find that the median fluxes measured for sources observed using the full array are 1.6% and 1% lower than those observed with the subarray at [3.6] and [4.5], respectively. In addition, we found a dependence on the exposure time such that for [3.6] observations, the long frame times are measured to be lower than the short frame times by a median value of 3.4% in full array and 2.9% in subarray. For [4.5] observations, the longer frame times are 0.6% and 1.5% in full and subarray, respectively. These very small variations will likely only affect science users who require high-precision photometry from multiple different observing modes. We find no statistically significant difference for fluxes obtained with dithered and staring modes. When considering all stars in the sample, the fractional well depth of the pixel is correlated with the different observed fluxes. We speculate the cause to be a small nonlinearity in the pixels at the lowest well depths where deviations from linearity were previously assumed to be negligible.
The largest source of noise in exoplanet and brown dwarf photometric time series made with Spitzer/IRAC is the coupling between intra-pixel gain variations and spacecraft pointing fluctuations. Observers typically correct for this systematic in science data by deriving an instrumental noise model simultaneously with the astrophysical light curve and removing the noise model. Such techniques for self-calibrating Spitzer photometric datasets have been extremely successful, and in many cases enabled near-photon-limited precision on exoplanet transit and eclipse depths. Self-calibration, however, can suffer from certain limitations: (1) temporal astrophysical signals can become aliased as part of the instrument model; (2) for some techniques adequate model estimation often requires a high degree of intra-pixel positional redundancy (multiple samples with nearby centroids) over long time spans; (3) many techniques do not account for sporadic high frequency telescope vibrations that smear out the point spread function. We have begun to build independent general-purpose intra-pixel systematics removal algorithms using three machine learning techniques: K-Nearest Neighbors (with kernel regression), Random Decision Forests, and Artificial Neural Networks. These methods remove many of the limitations of self-calibration: (1) they operate on a dedicated calibration database of approximately one million measurements per IRAC waveband (3.6 and 4.5 microns) of non-variable stars, and thus are independent of the time series science data to be corrected; (2) the database covers a large area of the "Sweet Spot, so the methods do not require positional redundancy in the science data; (3) machine learning techniques in general allow for flexibility in training with multiple, sometimes unorthodox, variables, including those that trace PSF smear. We focus in this report on the K-Nearest Neighbors with Kernel Regression technique. (Additional communications are in preparation describing Decision Forests and Neural Networks.)
The Spitzer Space Telescope currently operates in the "Beyond Era", over nine years past an original cryogenic mission. As the astronomy community continues to advance scientific boundaries and push beyond original specifications, the stability of the Infrared Array Camera (IRAC) instrument is paramount. The Instrument Team (IST) monitors the pointing accuracy, temperature, and calibration and provides the information in a timely manner to observers. The IRAC IST created a calibration trending web page, available to the general astronomy community, where the team posts updates of three most pertinent scientific stability measures of the IRAC data: calibration, bias, and bad pixels. In addition, photometry and telescope properties from all the staring observations (>1500 as of April 2018) are trended to examine correlations with changes in the age or thermal properties of the telescope. A long, well-sampled baseline established by consistent monitoring outside anomalies and space weather events allows even the smallest changes to be detected.
We describe our ongoing efforts to model the field distortions of the Infrared Array Camera (IRAC) during the cryogenic portion of the Spitzer Space Telescope’s operations. We have compared over two million measured source positions in ~35,000 IRAC images with their positions in Gaia Data Release 1. Fitting 3rd and 5th order polynomials to the measured offsets, we find systematic uncertainties in IRAC-measured positions that are in the 50-60 milliarcsecond range for the 3.6 micron array, and 120-150 milliarcsecond range for the 4.5 micron array. A 5th-order fit does not appear to significantly improve the results over a 3rd order fit. However, this may be due at least partly to the failure of our current centroiding technique to account for variations in the Point Response Functions across each detector. We anticipate making several improvements in our continuing analysis, including (i) the refitting of the positions and position angles of each IRAC image using the Gaia catalog, (ii) making use of a less position-sensitive centroiding algorithm, (iii) correcting where possible for the proper motions of detected sources, and (iv) significantly increasing the number of source position measurements. Once finalized, the resulting distortion corrections will be incorporated into the headers of the archived images.
We present a database of reduced data for all staring mode observations taken with the Infrared Array Camera (IRAC) during the Spitzer warm mission to monitor instrument performance, predict future instrument performance, and facilitate exoplanet and brown dwarf science. Our motivation is to be informed so that we can mitigate the impact of changing thermal conditions on science. Monitoring current trends allows us to predict future instrument performance and to adjust our recommended suite of best practices and calibrations accordingly. From this database we show that instrumental effects detrimental to high precision photometry either remain stable or improve. A uniform reduction of all IRAC light curves has never before been published, and will enable powerful science including accurate comparative studies of exoplanets and brown dwarfs. IRAC has been performing well throughout the warm mission and we expect performance to remain excellent.
The Spitzer Space Telescope is executing the seventh year of extended warm mission science. The cryogenic mission operated from 2003 to 2009. The observing proposal review process has evolved from large, week-long, in-person meetings during the cryogenic mission to the introduction of panel telecon reviews in the warm mission. Further compression of the schedule and budget for the proposal solicitation and selection process led to additional changes in 2014. Large proposals are still reviewed at an in-person meeting but smaller proposals are no longer discussed by a topical science panel. This hybrid process, involving an in-person committee for the larger proposals and strictly external reviewers for the smaller proposals, has been successfully implemented through two observing cycles. While people like the idea of not having to travel to a review it is still the consensus opinion, in our discussions with the community, that the in-person review panel discussions provide the most satisfying result. We continue to use in-person reviews for awarding greater than 90% of the observing time.
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.
The Spitzer Space Telescope Infrared Array Camera (IRAC) basic calibrated data reduction pipeline is designed to take
a single raw frame from a single IRAC detector and produce a flux-calibrated image that has had all well-understood
instrumental signatures removed. We discuss several modifications to the pipeline developed in the last two years in
response to the Spitzer warm mission. Due to the different instrument characteristics in the warm mission, we have
significantly changed pipeline procedures for masking residual images and mitigating column pulldown. In addition, the
muxbleed correction was turned off, because it is not present in the warm data. Parameters relevant to linearity
correction, bad pixels, and the photometric calibration have been updated and are continually monitored.
The Infrared Array Camera (IRAC) is now the only science instrument in operation on the Spitzer Space Telescope. The
3.6 and 4.5 µm channels are temperature-stabilized at ~28.7K, and the sensitivity of IRAC is nearly identical to what it
was in the cryogenic mission. The instrument point response function (PRF) is a set of values from which one can
determine the point spread function (PSF) for a source at any position in the field, and is dependent on the optical
characteristics of the telescope and instrument as well as the detector sampling and pixel response. These data are
necessary when performing PSF-fitting photometry of sources, for deconvolving an IRAC image, subtracting out a
bright source in a field, or for estimating the flux of a source that saturates the detector. Since the telescope and
instrument are operating at a higher temperature in the post-cryogenic mission, we re-derive the PRFs for IRAC from
measurements obtained after the warm mission temperature set point and detector biases were finalized and compare
them to the 3.6 and 4.5 µm PRFs determined during the cryogenic mission to assess any changes.
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope has been used to measure < 10-4 temporal
variations in point sources (such as transiting extrasolar planets) at 3.6 and 4.5 μm. Due to the under-sampled
nature of the PSF, the warm IRAC arrays show variations of as much as 8% in sensitivity as the center of the
PSF moves across a pixel due to normal spacecraft pointing wobble and drift. These intra-pixel gain variations
are the largest source of correlated noise in IRAC photometry. Usually this effect is removed by fitting a
model to the science data themselves (self-calibration), which could result in the removal of astrophysically
interesting signals. We describe a new technique for significantly reducing the gain variations and improving
photometric precision in a given observation, without using the data to be corrected. This comprises: (1) an
adaptive centroiding and repositioning method ("Peak-Up") that uses the Spitzer Pointing Control Reference
Sensor (PCRS) to repeatedly position a target to within 0.1 IRAC pixels of an area of minimal gain variation;
and (2) the high-precision, high-resolution measurement of the pixel gain structure using non-variable stars. We
show that the technique currently allows the reduction of correlated noise by almost an order of magnitude over
raw data, which is comparable to the improvement due to self-calibration. We discuss other possible sources of
correlated noise, and proposals for reducing their impact on photometric precision.
Carl Grillmair, Sean Carey, John Stauffer, Mark Fisher, Ryan Olds, James Ingalls, Jessica Krick, William Glaccum, Seppo Laine, Patrick Lowrance, Jason Surace
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.
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.
The telescope time allocation process for NASA's Great Observatories involves a substantial commitment of time and
expertise by the astronomical community. The annual review meetings typically have 100 external participants. Each
reviewer spends 3-6 days at the meeting in addition to one-two weeks of preparation time, reading and grading
proposals. The reviewers grade the proposals based on their individual reading prior to the meeting and grade them again
after discussion within the broad, subject-based review panels. We summarize here how the outcome of the review
process for three Spitzer observing cycles would have changed if the selection had been done strictly based on the
preliminary grades without having the panels meet and discuss the proposals. The changes in grading during the review
meeting have a substantial impact on the final list of selected proposals. Approximately 30% of the selected proposals
would not have been included if just the preliminary rankings had been used to make the selection.
This paper discusses the Spitzer Space Telescope General Observer proposal process. Proposals, consisting of the scientific justification, basic contact information for the observer, and observation requests, are submitted electronically using a client-server Java package called Spot. The Spitzer Science Center (SSC) uses a one-phase proposal submission process, meaning that fully-planned observations are submitted for most proposals at the time of submission, not months after acceptance. Ample documentation and tools are available to the observers on SSC web pages to support the preparation of proposals, including an email-based Helpdesk. Upon submission proposals are immediately ingested into a database which can be queried at the SSC for program information, statistics, etc. at any time. Large proposals are checked for technical feasibility and all proposals are checked against duplicates of already approved observations. Output from these tasks is made available to the Time Allocation Committee (TAC) members. At the review meeting, web-based software is used to record reviewer comments and keep track of the voted scores. After the meeting, another Java-based web tool, Griffin, is used to track the approved programs as they go through technical reviews, duplication checks and minor modifications before the observations are released for scheduling. In addition to detailing the proposal process, lessons learned from the first two General Observer proposal calls are discussed.
The Infrared Array Camera (IRAC) on Spitzer Space Telescope includes four Raytheon Vision Systems focal plane arrays, two with InSb detectors, and two with Si:As detectors. A brief comparison of pre- flight laboratory results vs. in-flight performance is given, including quantum efficiency and noise, as well as a discussion of irregular effects, such as residual image performance, "first frame effect", "banding", "column pull-down" and multiplexer bleed. Anomalies not encountered in pre-flight testing, as well as post-flight laboratory tests on these anomalies at the University of Rochester and at NASA Ames using sister parts to the flight arrays, are emphasized.
KEYWORDS: Calibration, Infrared telescopes, High dynamic range imaging, Space telescopes, Stray light, Infrared cameras, Stars, Infrared radiation, Space operations, Telescopes
We describe the astronomical observation template (AOT) for the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (formerly SIRTF, hereafter Spitzer). Commissioning of the AOTs was carried out in the first three months of the Spitzer mission. Strategies for observing fixed and moving targets are described, along with the performance of the AOT in flight. We also outline the operation of the IRAC data reduction pipeline at the Spitzer Science Center (SSC) and describe residual effects in the data due to electronic and optical anomalies in the instrument.
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.
The Marshall Space Flight Center, Alabama, in a teaming arrangement with the University of Florida, Gainesville, and the Joint Astronomy Center, Hawaii, has completed a comprehensive investigation into the feasibility of a low-cost infrared space astronomy mission. This mission would map the emission of molecular hydrogen in our galaxy at two or three previously inaccessible mid-IR wavelengths, and provide information on the temperatures. The feasibility of the low-cost mission hinged on whether a thermal design could be found which would allow sufficient passive cooling of the telescope to elimiate the need for a large, expensive dewar. An approach has been found which can provide telescope temperatures on the order of 50 K, which makes the mission feasible at low cost in low-Earth orbit.
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