I. Introduction

1. Mission Overview


The Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010, AJ, 140, 1868) is a NASA Medium Class Explorer mission that conducted a digital imaging survey of the entire sky in the 3.4, 4.6, 12 and 22 μm mid-infrared bandpasses (hereafter W1, W2, W3 and W4). WISE will produce and release to the world astronomical and educational communities and general public a digital Image Atlas covering the sky in the four survey bands, and a reliable Source Catalog containing accurate photometry and astrometry for over 300 million objects. The WISE Catalog and Atlas will enable a broad variety of research efforts ranging from the search for the closest stars and brown dwarfs to the most luminous galaxies in the Universe. WISE science data products will serve as an important reference data set for planning observations and interpreting data obtained with future ground and space-borne observatories, such as JWST.

WISE conducted its survey using a 40 cm cryogenically-cooled telescope equipped with a camera containing four mid-infrared focal plane array detectors that simultaneously imaged the same 47x47 arcmin field-of-view on the sky (Figure 1). The spacecraft flew in a 526x531 km x 97.5°, 6pm ascending node, sun-synchronous polar orbit. The WISE telescope scanned continuously at near constant Ecliptic longitudes while a scan mirror temporarily froze the sky on the focal planes. Sets of exposures (7.7 sec in W1 and W2 and 8.8 sec in W3 and W4) were acquired at each point on the sky as the telescope scanned from pole-to-pole. The FOV of each successive exposure set overlaps the previous one by 10%, and the scan paths of adjacent orbits overlap by approximately 90% on the Ecliptic because of the WISE orbital precession. The number of independent exposures accumulated on each point on the sky was typically 12 or more on the Ecliptic and increases to several hundred at the Ecliptic poles.

WISE was launched on a Delta II 7320 rocket from Space Launch Complex 2 at Vandenberg Air Force Base at 14:09:33 UTC on December 14, 2009 (Figure 2). The cryogenic telescope cover was ejected on December 29, 2009, two weeks into the in-orbit checkout (IOC) period (Figure 3). Nominal survey observations were started on at 11:45:53.4 UTC on January 14, 2010 following the successful completion of the one month IOC. Cryogenic survey operations continued uninterrupted through September 30, 2010 when the last of WISE's solid hydrogen cryogen sublimated. After cryogen exhaustion, the two short wavelength detectors remained operational at near cryogenic survey sensitivity. WISE continued to survey the sky through February 1, 2011, funded by the NASA Planetary Division, in the NEOWISE post-cryo mission phase to continue the pursuit of new solar system objects. Data collection with WISE was halted after collection of the final post-cryo survey image at 11:03:01.3 UTC on February 1, 2011 (Figure 4). Final contact with the WISE flight system was made on February 17, 2011. WISE remains in orbit, hibernating in an inertial pointing mode with the telescope at right angles to the Sun.

During its 413 day IOC and operational phase, WISE orbited the Earth 6240 times, collecting 2.7 million exposures containing 7.9 trillion pixels of information. 15.6 TB (8.6 TB compressed) of image and engineering data were transmitted to the ground via the TDRSS network, and ingested and processed to produce calibrated image and extracted source data products. All components of the WISE system operated flawlessly. No survey time was lost due to unplanned safe holds or other unexpected behavior, and fewer than 100 image frames were lost due to downlink errors.

Prof. Edward Wright of the University of California Los Angeles is the WISE Principal Investigator. The WISE program is overseen by the NASA Explorer Office at Goddard Space Flight Center. Overall project management, project science and mission operations is the responsibility of the Jet Propulsion Laboratory, California Institute of Technology. Ball Aerospace and Technologies Corporation is the prime contractor for the WISE spacecraft, and managed the Integration and Test for the program. The Space Dynamics Laboratory at Utah State University is the prime contractor for the WISE payload that includes the cyrogenic telescope, optics and detector assemblies. The Infrared Processing and Analysis Center at Caltech is responsible for WISE data ingest, processing, archiving and distribution to the user community. Education and Public Outreach for WISE is conducted by the University of California Berkeley.

Figure 1 - WISE flight system in the payload processing facility at VAFB. The domed cover was ejected two weeks after launch. Figure 2 - WISE launch from Vandenberg AFB at 14:09:33 12/14/09 UTC (credit: United Launch Alliance) Figure 3 - WISE "first-light" image taken on 12/31/10 during IOC. This is a four-band color composite image showing a region in the constellation Carina. Figure 4 - WISE "last-light" image taken on 2/1/2011. This is two-band color composite image showing a region in the constellation Perseus.


NEOWISE (Mainzer et al. 2011 ApJ, 731, 53) is an enhancement to the primary WISE mission data processing and archive system, funded by the NASA Planetary Science Division, that enhances the Solar System science return of WISE data. Dr. Amanda Mainzer of the Jet Propulsion Laboratory, California Institute of Technology is the NEOWISE Principal Investigator. The NEOWISE program consists of two parts. The first, the WISE Moving Object Pipeline System (WMOPS), exploits the WISE observing cadence that images repeatedly the same region of sky multiple times to detect and characterize moving solar system objects, with special emphasis on the search for Near Earth Objects. The second component of NEOWISE provides an archive of image and source data from individual WISE exposures that will enable enable pre-covery of WISE detections of objects that are discovered after the end of the WISE mission.

NEOWISE/WMOPS is a software system that links non-inertial detections made on the single-exposure WISE images made near the same point on the sky to form candidate moving object tracklets (Figure 5). WMOPS reported candidate tracklets to the IAU Minor Planet Center (MPC) within 10 days of the midpoint of the WISE observations. The MPC computed initial orbits and posted objects on their websites for follow-up and confirmation by ground-based observers. The first tracklet was reported to the MPC on January 9, 2010. Since then, over 279,000 tracklets were reported, and nearly two million individual vetted WISE mid-infrared detections of over 158,000 unique small bodies are available from the MPC. WISE/NEOWISE has been credited with the discovery 135 previously unknown NEOs and 17 comets, along with over 30,000 main belt asteroids (Figure 6).

The archive component of the NEOWISE program provides for the release of WISE single-exposure images and extracted source working database to the science community in conjunction with the WISE primary mission data releases. These products are accessed via the on-line services of the NASA/IPAC Infrared Science Archive (IRSA). This enhancement also provides an interface query capability to the IRSA services that enables searching for a WISE single-exposure image that covered the predicted position of a moving object, using the object name or orbital elements. This search capability will soon be extended to the database of single-exposure source detections.

Under the auspices of the NEOWISE program, the NASA Planetary Division funded the WISE post-cryo survey phase to enable the continuation of the WISE Moving Object search. NEOWISE post-cryo operations took place from October 1, 2010 through February 1, 2011.

Figure 5 - WISE 4-band color-composite of the Tadpole Nebula illustrating moving object tracklets. This image was formed by combining multiple independent WISE exposures of the region without pixel outlier rejection, so that moving objects appear multiple times. The rectangles outline the tracklets produced by asteroids 1719 Jens and 1992 UZ5. The circled streak near the top of the image is a residual trail left by a satellite detected in one of the WISE exposures. Figure 6 - Comet P/2010 B2 (WISE), the first of 17 comets discovered by the NEOWISE program, on 1/22/2010. This is a 3-band (4.6-12-22μm) color composite image formed by coadding 15 independent exposures on the position of the comet.

c. WISE At-A-Glance

Table 1 - WISE Description
Launch and Orbit
LaunchDecember 14, 2009; 6:09:33 PST from Vandenberg SLC2W
Launch VehicleBoeing Delta II 7320-10C
Orbit526 x 531 km x 97.5°; 6am/pm sun-synchronous
Orbital Rate3.78 arcmin/sec
Orbital Period5720 sec (95.33 min); decreasing to 5700 sec at end of on-orbit ops
Orbital Precession Rate0.986°/day = 3.92'/orbit at 95.33 min period
Spacecraft and Payload (see III.1 and III.2)
Total Mass 459.1 kg
Total Power 317.2 W
SpacecraftBATC RS300
Star TrackersTwo Ball CT-633
Solar PanelsFixed, 2.6 m2
High-Gain AntennaFixed, 0.6 m diameter, Ku-band
Telescope Diameter40 cm
Number of Imagers and FOV4 all reflective. Simultaneously image same 47'x47' FOV on four detectors.
Number of Detectors4; 2 Teledyne HgCdTe, 2 DRS Si:As
Detector Format1024x1024 18μm pixels
BandsW1: 3.4 μm, W2: 4.6 μm, W3: 12 μm, W4: 22 μm
Angular Pixel Size (IFOV)2.75" (W1-W3), 5.5" (W4 binned 2x2 on-board)
Image FWHMW1: 6.1", W2: 6.4", W3: 6.5", W4: 12.0"
Detector Read-out ModeSample-up-the-Ramp (SUTR)
Exposure Time7.7 sec (W1 and W2), 8.8 sec; 8x1.1 sec samples (1st sample dropped for W1/W2)
Exposure cadence11 sec
Image Data Rate4.65 Mbps to Spacecraft
50.6 GB/day uncompressed
~20.6 GB/day with lossless Rice compression
Downlink ServicesTDRSS Ku-band; 20 W transmitter + SSA; 100 Mbps downlink rate
Contacts Schedule54 minutes/day @ 51 MBps effective link; 4 contacts/day on average
Survey Strategy (see III.4)
Data Acquisition ModeFreeze-frame scanning. Near-zenith-pointing telescope scans continuously while scan mirror freezes sky on focal planes for 11 sec.
Frame-to-Frame (in-scan) Overlap5.39' (11.49%) at 11 sec cadence
Survey Scan Angular Velocityω = (0.0125,3.7977,0) arcmin/sec
Longitude step between orbits+30.65' / -22.15'
Orbit-to-Orbit (cross-scan) OverlapAverage of 42.68' (90.9%) on Ecliptic plane. Increases towards poles.
Sky CoverageTypically 12 independent exposures near the Ecliptic Plane, increasing towards Ecliptic poles.
Photometric System (see IV.3.g)
Band Center for Vega-like SpectrumW1: 3.35 μm, W2: 4.60 μm, W3: 11.56 μm, W4: 22.09 μm
Band width for Vega-like SpectrumW1: 0.66 μm, W2: 1.04 μm, W3: 5.51 μm, W4: 4.10 μm
Flux for 0 Magnitude (Vega)W1: 306.68 ± 4.60 Jy, W2: 170.66 ± 2.56 Jy, W3: 29.045 ± 0.436 Jy, W4: 8.2839 ± 0.2899 Jy
Data Release and Primary Products (see I.2)
Data Release SchedulePreliminary: April 14, 2011      Final: Spring 2012
Preliminary Release Scope~57% of sky, survey data from January 14 to April 29, 2010
Preliminary Release Image Atlas10,464 Atlas Image Sets - FITS format, 4 bands x intensity, depth-of-coverage, uncertainty images
Preliminary Release Source CatalogSource Catalog - positions and photometry for 257,310,278 objects

Table 2 - WISE Mission Events
Date (UTC)Event
12/14/2009 Launch from Vandenberg AFB. IOC Start.
12/29/2009 Cover eject
12/31/2009 First light. Start of Provisional Data Processing
01/07/2009 Near survey quality observations begin
01/13/2009 End of IOC
01/14/2009 Start of 4-band cryogenic survey operations
04/20/2010Start of First-pass Data Processing
07/17/2010 First sky coverage completed
08/05/2010 Secondary cryogen tank exhaustion
09/29/2010 Primary cryogen tank exhaustion
10/01/2010 Start of post-cryo survey operations
01/09/2011 Second sky coverage completed
02/01/2011 End of post-cryo survey operations
02/17/2011 Flight-system decommissioned
04/14/2011 Preliminary Data Release
April 2011Start of Second-Pass Data Processing
Spring 2012 Final Data Release

Table 3 - Preliminary Release Achieved Performance
 Level 1 RequirementAchieved
Sky Coverage
≥4 independent exposures in each filter on at least 95% of the sky≥4 coverages on >99% of the sky, all bands.
95% of sky has 10.0/9.9/9.0/9.4 coverages in W1/W2/W3/W4
(Preliminary Data Release covers 57% of the sky)
Photometric Sensitivity
SNR=5 at 0.12/0.16/0.85/4.0 mJy (16.0/15.1/11.3/8.3 Vega mag) at W1/W2/W3/W4 in unconfused regions with 8 coverages0.05/0.10/0.73/5.9 mJy (17.0/15.6/11.5/7.9 Vega mag) based on comparison with external measurements
Photometric Accuracy
RMS error in relative accuracy <7% accuracy for SNR>100 RMS = 0.02 mag (W1/W2/W3) and 0.03 mag (W4)
Astrometric Accuracy
RMS position error with respect to the 2MASS PSC <0.5" for SNR>20 in at least one bandRA and Dec RMS~0.2" for W1<13.0 mag. Increasing fraction of fainter sources exhibit a systematic declination bias of 0.2-1.0". Approx. 20% of sources with W1<14.5 mag are affected.
Catalog Completeness
>95% for SNR>20 in unconfused regions> 95% for sources with W1<16.6, W2<16.0, W3<10.8, W4<6.7 Vega mag in unconfused regions with ≥12 frame depth-of-coverage
Catalog Reliability
>99.9% for SNR>20 in unconfused regions> 99.9% for sources with W1<15.6, W2<14.3, W3<10.2, W4<6.5 mag in unconfused regions with ≥12 frame depth-of-coverage.

d. The WISE Preliminary Data Release

The WISE Preliminary Data Release is the first release of a large quantity of WISE image and extracted source data to the scientific and educational community. The Release includes data from the first 105 days of the WISE cryogenic survey, covering approximately 57% of the sky. An executive summary of the Release data products is given in section I.2. The goal of this early release is to provide a significant data set to the scientific community as quickly as possible following the end of mission operations that will introduce users to the WISE data, and will enable a broad range of investigations. The Preliminary Data Release also enables the WISE Project Team to exercise and refine data preparation and release processes prior to the final release in 2012.

The WISE Preliminary Release products are not the ultimate realization of the WISE data. The Preliminary Release products are generated from the results of first-pass WISE data processing that started just three months following the survey start. Processing was done with an early version of the Wise Science Data System (WSDS) that was based on pre-launch and very early survey calibrations and reduction algorithms. Detection thresholds and source selection criteria for the Preliminary Release Source Catalog were conservative with the emphasis put upon providing a reliable yet representative sample of WISE data. As a result of the rapid deployment necessary to meet the schedule for the Preliminary Data Release, the Preliminary Release products did not benefit from much of the experience gained over the full mission lifetime. Consequently, the Preliminary Release products contain a number of known features and limitations that are documented in I.4. Users are strongly recommended to familiarize themselves with the known idiosyncracies of the WISE data products documented therein.

All WISE cryogenic survey data will undergo a planned, complete second-pass processing using an updated version of the WSDS that incorporates corrections to deficiencies in first-pass processing, and improved calibrations and processing algorithms developed from analysis of the full mission data set. The results of the second-pass processing will be used to generate the WISE Final Data Release products that will be issued in the Spring of 2012.

e. About this Document

This Explanatory Supplement provides users of WISE products with a description of the overall WISE survey and data products. An executive summary of the Preliminary Data Release products is available in Section I.2, and known features and limitations in Section I.4. Section II contains a more detailed user's guide to the products, with information about their content and characteristics. In Section III, the WISE flight system, operations and survey strategy are presented. Detailed descriptions of the functions and algorithms of the WSDS are described in Section IV. Generation of the final Image Atlas and Source Catalog are described in Section V, and detailed descriptions of the achieved performance and quality of the Preliminary Release data products are presented in Section VI.

This Explanatory Supplement is a living document that will be updated frequently to provide the most current information for users of the WISE data products. Please see the Document Change History for a record of the latest updates.

Last update: 2011 August 17

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