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THE TWO MICRON ALL SKY SURVEY (2MASS)

M. F. Skrutskie,1,2

R. M. Cutri,3

R. Stiening,1

M. D. Weinberg,1

S. Schneider,1

J. M. Carpenter,4

C. Beichman,5

R. Capps,5

T. Chester,3

J. Elias,6

J. Huchra,7

J. Liebert,8

C. Lonsdale,3

D. G. Monet,9

S. Price,10

P. Seitzer,11

T. Jarrett,3

J. D. Kirkpatrick,3

J. E. Gizis,1

E. Howard,1

T. Evans,3

J. Fowler,3

L. Fullmer,3

R. Hurt,3

R. Light,3,12

E. L. Kopan,3 K. A. Marsh,5 H. L. McCallon,3

R. Tam,3

S. Van Dyk,3

and S. Wheelock3

Receivved 2004 September 27; accepted 2005 September 30

ABSTRACT

Between 1997 June and 2001 February the Two Micron All Sky Survey (2MASS) collected 25.4 Tbytes of rawimaging data covering 99.998% of the celestial sphere in the near-infraredJ(1.25m),H(1.65m), andKs (2.16m)bandpasses. Observations were conducted from two dedicated 1.3 m diameter telescopes located at Mount Hopkins,Arizona, and Cerro Tololo, Chile. The 7.8 s of integration time accumulated foreach point on the sky and strict qualitycontrol yielded a 10point-source detection level of better than 15.8, 15.1, and 14.3 mag at the J,H, andKsbands,respectively, for virtually the entire sky. Bright source extractions have 1photometric uncertainty of

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extinction and a unique sensitivity to cool objects, a near-infraredsurvey had the potential to discover new and rare objects, delin-eate the structure of the Milky Way, and establish the distributionof galaxies in nearly 4sr with a minimal zone of avoidance.

Discovering brown dwarfs required detection of

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path to each array traverses seven lenses. The first Infrasil lens,common to all bands, relays nearly collimated light to beam-splitting mirrors. Following the beam splitters, the light foreach band encounters independent identical sets of six lenses,which relay images of the sky onto the three arrays. These

calcium fluoride and Infrasil lenses form pupil images atthree Lyot stops adjacent to the band-limiting interference fil-ters. The stop diameter is 90% of the pupil diameter to permitsome freedom for misalignment of the three stops relative toone another. All lens surfaces are spherical and antireflectioncoated.

The three NICMOS3 arrays mount independently from the

cameras optical support structure. Arrays can be translated inthree dimensions with the system disassembled and warm,which enables the relative registration of the arrays to betterthan

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background flux overwhelmed the electronic read noise. Acomplete readout of the array required 51 ms. The data systemrecorded two complete reads of the array: one immediately fol-lowing the array reset andanother 1.3 s later. The pixel-by-pixeldifference between these two readouts constitutes a 1.3s doublycorrelated exposure. For 2MASS the array reset timing wasidentical to the timing for a readout. Thus, the first read-out, although it occurred immediately after reset, captured a51 ms exposure of the sky in each pixel. The 1.3 s duration

exposures saturated for stars between 89 mag. The 51 ms ex-posure extended the unsaturated regime to sources as bright as45 mag.15

2.3. Telescopes

2MASS used two custom 1.3 m equatorial telescopes. Thenorthern telescope is located at the Whipple Observatory at2306 m elevation on a ridge below the summit of Mount Hopkins,Arizona (N 31

40050B8, W 110

5204100), while the southern

telescope site is at Cerro Tololo Inter-American Observatory at2171 m elevation on a ridge below the summit of Cerro Tololo,Chile (S 30

1003B7, W 70

4801800). For each telescope, the pri-

mary mirror, composed of Corning ultra-low expansion glass, isparabolic with a radius of curvature of 5200 mm. The secondaryconic constant is 1.847 with a radius of curvature of 965.7 mm.These optics provide a Cassegrain focal ratio of f/13.5 and aCassegrain plate scale of 11B97 mm1.

Invar rods establish the primary-secondary mirror separationand minimize thermal variation of the telescope focus. Calibrationof focus change versus temperature during telescope commis-sioning permitted application of computed thermal focus cor-rections during survey observations. This automated thermalfocus correction, amounting to 5 m of secondary motion

C1,

permitted observations to proceed for weeks without real-time

feedback from the images. Seasonal adjustments of the focuszero point of order 10 m accounted for any remaining focusresidual at the largest temperature extremes.

2.4.Observatory Site Conditions

The 2MASS observing facilities operated every night thatstars were visible andtherewas no threat of rain. Feedback fromthe data processing system provided statistics on backgroundlevels, seeing/image size, achieved sensitivity, and system zeropoint (total transmission). A modest weather station providedstatistics on surface humidity, temperature, and barometric pres-sure with each observation. Figures 37 summarize the timehistory of some of these parameters for both hemispheres.Figure 3 shows the observed monthly fraction of photometrichours. The annual closure of the northern hemisphere facilityduring the Arizona summer monsoon is evident in the MountHopkins data. The dashed horizontal line represents the meanphotometric fraction for each site. Figure 4 plots background ineach band during the survey. Airglow emission dominates theJand Hbands. Significant seasonal variation is apparent in theKs-band background, since thermal emission from the telescopeoptics was usually comparable to or greater than the airglow

emission in this band. Seasonal temperature variations weremore mild at the Cerro Tololo site. Figure 5 tracks the imageFWHM, which represents the convolution of the system point-spread function (PSF) with the seeing. The 200 pixels, cameraoptics, andsamplingpattern combined to produce a system PSFof 2B5 under the best seeing conditions. Northern hemisphereimage size improved during the first years of the survey due tothe discovery and correction of a primary mirror support prob-lem. Sensitivity at the signal-to-noise ratio S/N 10 detec-tion level (Fig. 6) was evaluated hourly from the statistics of therepeated scans of calibration tiles as described in x 3.4. Back-ground, seeing, and atmospheric transparency were the primaryfactors that determined this sensitivity level. As a result,Ks-bandsensitivity experienced significant seasonal change due to the

15The Point Source Catalog includes a rd_flg column that identifies the

origin of the photometry for each band for a given source.

Fig. 3.Monthly photometric fraction vs. time at the northern and southernobservatories. The horizontal dashed lines represent the average nightly pho-tometric fraction at each site (38% north and 60% south).

Fig.4.Mean nightlyJ(blue),H(green),andKs

(red) backgrounds vs.timefor the two 2MASS observatories. Backgrounds at the JandHbands primarilyarise from airglow. The Ks background depends on both airglow and thermal

background and thus is seasonal.

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variation in thermal background, whileJand Hband were lessaffected. Zero points (Fig. 7) representthe total system transmissionincluding the Earths atmosphere. Because the 2MASS Jbandinfringes on atmospheric water absorption and is more suscep-tible to atmospheric scattering, significant seasonal variations inJ-band zero point occurred during the survey. Not surprisingly,the worst transmission losses in the northern hemisphere occuratJband and are coincident with the start and end of the summermonsoon. Zero-point variations were removed from the derived

source magnitudes via hourly observations of calibration fields(x 3.4).

3. DATA COLLECTION

3.1. Data System Configuration

Three personal computers controlled data acquisition, tele-scope pointing, and scheduling. A scheduling computer main-

tained a sky coverage database and selected regions of the skyfor observation. The scheduling computer, via an RS-232 seriallink, communicated requests for sky coverage to an instrumentcontrol computer. The instrument control computer, in turn, re-layed requests for telescope motion to a telescope control com-puter and triggered camera frame acquisition. The instrumentcontrol computer also received, displayed, and stored the returneddata. The scheduling computer could thus control a completenight of autonomous data acquisition. A telescope operator openedthe facility and initiated the scheduling program. The operatorthen supervised the progress of observations and monitored theweather, closing the facility as required.

3.2.Image Acquisition

Observations proceeded while the telescope scanned steadilyin declination at a rate of 5700 s1. During a 1.3 s exposure thesecondary mirror tilted smoothly to freeze the 8A5 ; 8A5 fieldof view relative to the focal plane despite the telescopes decli-nation motion. Aberrations introduced by the secondary tilt wereminimal compared with the systems PSF. At the end of a 1.3 sexposure the secondary rapidly tilted back to its start position,while the camera electronics read out and reset the arrays. Theframe-to-frame time interval was 1.455 s, of which 1.351 s wereused to collect data, with the remainder used for secondaryflyback and array readout. The 5700 s1 scan rate thus producedan 8300 spatial offset between framesslightly smaller thanone-sixth of the camera field of viewyielding six independent

Fig. 6.Achieved mean nightlyJ(blue),H(green), andKs(red) S/N 10sensitivity levels in magnitudes vs. time for the two 2MASS observatories. Thissensitivity is calculated from the statistics of repeated observations of stars ineach set of hourly calibration observations. The solid horizontal lines representthe surveys S/N 10 sensitivity requirement in each band:J 15:8,H 15:1,andKs 14:3.

Fig. 7.Mean nightly J(blue), H( green), and Ks (red) photometric zeropoints vs. time for the two 2MASS observatories. The zero point is measured inmagnitudes and represents the offset between instrumental and calibratormagnitudes. The zero point thus tracks the total system transmission, with morenegative values indicating poorer sensitivity.

Fig. 5.Mean nightly J (blue), H( green), and Ks (red) measured imageFWHM intensity vs. time for the two 2MASS observatories.

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images of each sky position. Following each 1.3 s exposure thecamera electronics transmitted six 256 pixel ; 256 pixel ; 16 bitframes (two images constituting the doubly correlated frames ineach of the threebands) via a fiber opticinterface to the instrumentcontrol computer, which displayed the images and stored themto hard disk. At the end of a full night of observations approx-imately 20 Gbytes of raw frame data were written to DLT tape.The tapes were subsequently express mailed to the Infrared Pro-cessing and Analysis Center (IPAC) for processing and archiving.

The 2MASS 2B0 pixel size was large relative to the typicalstellar image FWHM. The six independent images, appropri-ately distributed relative to pixel centers,mitigated undersamplingeffects. In the cross-scan direction (right ascension) subpixelstepping was achieved by rotating the arrays such that a givenstar crossed a full column while the source stepped down thearray. Along the scan direction (declination) subpixel steppingwas achieved by setting the telescope scan rate so that each stepwas a noninteger multiple of the pixel size. In practice, both ofthese adjustments were tuned to produce a well-distributed setof six centroids relative to the pixel grid for each star on eacharray. Since each array had a slightly different plate scale, only alimited range of step sizes and array rotations produced optimalresults. The step size in declination was slightly different be-tween the northern (82B6 per step) and southern (82B3 per step)hemisphere facilities.

Figure 8 compares a single 2MASS 1.3 s exposure to a finalkernel smoothed image that incorporates information from the

six dithered exposures. The figure demonstrates that the sub-pixel stepping produced stellar images that were smooth onscales smaller than the 2B0 ;2B0 pixels. Owing to the large pixelsize, however, star images on the combined frames were largecompared with a typical 100 FWHM seeing disk. The best 2MASSimages have FWHM of 2B5 (Fig. 5). Spatial undersampling alsodegraded photometric precision for bright sources. The statisticsof repeated observations of 2MASS calibration stars, discussedin x 5.2.4, demonstrate that the mean magnitude uncertainty forbright sources was typically 0.02 mag.

3.3.TilingStrategy and Scheduling

2MASS defined a scan as a basic data unit. Scans consistedof 273 (northern site) or 274 (southern site) frames acquired as

the telescope scanned steadily in declination. A scan covered aregion of sky that was one 8A5 camera field of view wide in rightascension and 6

long in declination. The celestial sphere was

divided into 59,650 slightly overlapping predefined scan-sizedsurvey tiles. The scheduling software optimally selecteda set of tiles and their order of observation each night. Thescheduling program based its tile selection on accessibility neartransit and, if a tile had been previously observed, a tiles pri-ority for reobservation. The tiles were grouped by declinationband: 0

6

, 6

12

, 12

18

, etc. Tiles overlapped in theright

ascension direction by 10% of their width (5000) at the tile endnearest the celestial equator. The convergence of lines of con-stant right ascension toward the pole produced increased tileoverlap on the poleward end of each tile. Tiles extended a fullcamera frame (8A5) in declination beyond the tiles designateddeclination boundary at the end closest to the celestial equatorin order to overlap with the adjacent declination band. Thesevarious overlaps ensured that gaps would only rarely appearbetween adjacent tiles due to telescope pointing errors. The ti leoverlap also defined the maximum size of an extended sourcethat always would appear entirely on the array in one or theother of two overlapping scans. The repeated observations ofsources in the overlap regions provided substantial feedbackon the photometric quality of the observations.

Northern hemisphere observations began in the 1218

dec-

lination band and proceeded northward. In the southern hemi-sphere, observations began in the 0

to 6

declination band

and proceeded southward. The region between 0

and 12

wasreserved for observation toward the end of survey operationssince it could be observed from either hemisphere. Despite theshorter overall operational lifetime (35 vs. 42 months in thenorth), the southern facility observed 58% of the available tiles.Figure 9 delineates the sky coverage contributions of the twoobservatories to the Point and Extended Source Catalogs.

3.4. Photometric Calibration

2MASS observed photometric calibration tiles at regular in-tervals during each night. A calibration tile observation consistedof 48 consecutive frames obtained with the same scanningmethod and integration time as was used for the 6

scans. A scan

of a calibration tile covered a field of view one camera frame wide

Fig. 8.Comparison of a single 2MASS 1.3 s J-band exposure (left) to the Atlas Image (right), which is a kernel-smoothed composite of six 1.3 s exposures.

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(8A5) in right ascension and 1 long in declination. Althougheach calibration tile was centered on a primary calibrator star(drawn largely from Near-Infrared Camera and Multi-ObjectSpectrometer [NICMOS] calibrators from Persson et al. [1998]),dozens of S/N> 20 stars within each calibration tile were usedto estimate zero-point corrections.16 A calibration observationconsisted of six consecutive scans of a calibration tile, each re-quiring about 1 minute of elapsed time. The scan direction al-ternated between northward and southward, and each scan wasoffset 500 in right ascension to mitigate any effects due to un-dersampling and bad pixels. Since each scan provided six im-ages of each star, a 7 minute calibration observation acquired36 independent 1.3 s images of every star in the tile. Subsequentprocessing reduced these images to six assessments of flux for

each starone from each of the six calibration scans.Thirty-five calibration tiles were uniformly distributed around

the sky (Fig. 10). The automated scheduling program selectedseveral tiles for calibration each night, observing at least onetile at a variety of zenithangles. Equatorial calibration tiles wereshared by both observatories. At the beginning of northernoperations two calibration tiles were observed every 2 hr dur-ing the night. After 1997 October 11 the calibration strategywas modified so that one calibration tile was observed approx-imately every hour during a night. All southern calibrationused the hourly strategy. Nightly 2MASS operations began andended with a calibration observation. The scheduling softwareadjusted the time interval between calibration observations sothat the final calibration observation occurred just prior to the

onset of morning twilight. Calibration observations thus pre-ceded and followed virtually all observations of 6

survey tiles.

The consistency of the measured zero point within a set of sixcalibration observations, as well as consistency with zero pointsmeasured throughout the night, established whether conditionswere photometric during a calibration observation. Calibrationobservations obtained both before and after a set of survey tilescans must have passed photometricity tests in order for thesurvey data to qualify as photometric.

Nightly calibration solutions ( Fig. 11) were derived fromlinear fits to an entire nights data forHand Ks bands or from

hour-to-hour interpolation for J band. These fitting functionswere selected based on minimization of photometric dispersionin an analysis of the ensemble of all calibration data in whicheach individual calibration observation was treated, in turn, as ifit were survey data. TheJband experienced the largest overallvariation in zero point, largely because the bands wavelengthedges infringed on spectral regions of variable water vaporopacity. The hour-to-hour piecewise zero-point interpolationaccommodated this short-term variability.

Fig. 9.Sky coverage vs. facility. The declination range plotted covers theshared declination region observed by the two facilities. Tiles shown in blackwere observed by thenorthernhemisphere facility andincorporated into theAll-Sky 2MASS data products.

16 The 2MASS Explanatory Supplement lists these target fields and calibration

stars at http://www.ipac.caltech.edu/2mass /releases/allsky/doc/sec3_2d.tbl1.html.

Fig. 10.Distribution of 2MASS calibration tiles on the celestial sphere inequatorial coordinates. Filled squares are primary calibration tiles. Open squaresdenote additional calibration tiles created late in operations to support calibra-tion of deep LMC and SMC observations.

Fig. 11.Typical 2MASS nightly calibration solution. TheY-axis representsthe average zero-point (ZP) difference between the instrumentally derived mag-nitude and the calibrator magnitude for the ensemble of calibration standards inthetile. Clustersof points representthe six independent zero-point determinationsfrom each of six successive observations of a 2MASScalibration tile. Each set ofobservations is labeled with the calibration tile identification number and the airmass at the time of observation. The J-band photometric solution is an hour-to-hour interpolation of the observed hourly mean zero points. The H-band and

Ks -band solutions are linear fits to the entire nights data.

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Prior to final data processing the ensemble of all observationsof photometric standards were solved for minimal photometricdispersion (Nikolaev et al. 2000), creating a network of 958 self-consistent and highly precise (residuals0.01 mag) near-infraredstandards. These standards17 formed the basisfor estimation of thehourly zero point. Atmospheric extinction is small [

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successive frames and combined to create Atlas Images that hada net exposure time of 7.8 s pixel1. Tycho-2 stars appearingalong each scan tied the scan coordinate system onto the J2000.0reference system and controlled the accumulation of random walkposition error from frame to frame. Before frame combination, allJ,H, andKsframes were resampled on the same 1B00 pixel

1 co-ordinate grid using a flux-preservinginterpolation kernel. Detectorpixels that had poor responsivity, were excessively noisy, or wereaffected by transient effects, such as meteor trails, satellite tracks,or cosmic rays, were masked off in the frames prior to the inter-polation procedure. The registered and interpolated frames wereaveraged together in a continuous 6

strip. The output Atlas Im-

ages were written to FITS standard images in 512 ;1024 pixel

(1B00 pixel1

) format with a 54 pixel overlap in the declination(long) direction for convenience. Most pixels of the Atlas Imagesrepresent the average of six 1.3 s frames. Because some pixels onthe individual frames were masked and because there was somemargin in the frame overlap, a pixel in the Atlas Images may oc-casionally represent the average of zero to seven frames. EachAtlas Image has a corresponding coverage map that repre-sents, pixel by pixel, the number of samples contributing to each100 pixel.

4.4. Point-Source Detection and Photometry

The 2MAPPS point-source subsystem identified candidatepoint sources on the Atlas Images and then extracted flux es-timates from the 1.3 s and 51 ms frames using both apertures of

various sizes and profile-fit algorithms. In addition to fluxes,these algorithms reported a variety of diagnostic statistics andflags. The pipeline output populated a working database thatcontained every source extraction from every potentially pho-tometric scan of a survey tile. The 2MASS Explanatory Sup-plement (Cutri et al. 2003) provides substantial detail on thesource extraction algorithms and characteristics of the resultingdata sets. The sections that follow (xx 4.4.14.4.4) only high-light the most salient features of point-source extraction. Usersof 2MASS data should regard the Explanatory Supplement asthe only sufficient guide to productive scientific use of the 2MASSdata sets.

4.4.1. Profile-Fit Photometry

In addition to reporting fluxes obtained via different extrac-tion algorithms, the point-source subsystem populated a defaultmagnitude column by selecting the result most likely to producereliable photometry for a given source. The default magnitudefor most unsaturated sources derives from a PSF profile-fittingalgorithm. This algorithm provided the best accuracy for pointsources with S/N P 30 and in crowded regions. Sources werefirst detected on the Atlas Images (x 4.3) prior to the extractionof profile-fit fluxes. Each Atlas Image was spatially filtered usinga zero-sum 400 FWHM Gaussian. Detections were identified aslocal intensity maxima that exceeded 3 times the estimated point-source noise level in the image. This low threshold ensured ahigh level of completeness in source extraction at the expense of

Fig. 12.2MAPPS flowchart.

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triggering the detection of many false sources that were filteredfrom the released data during final catalog generation. In source-confused regions the noise estimator produced a result consistentwith the confusion noise. As a result, the detection thresholdscaled naturally to higher fluxes in response to increasing sourcedensity.

For each source detection on an Atlas Image a PSF templateindexed to the average Atlas Image FWHM was fit simulta-

neously to the source data from the six (and sometimes seven)individual 1.3 s frames covering the source. The sky backgroundwas estimated in an annular region with inner radius of 1400 andouter radius of 2000. The fit yielded an optimized position, flux,uncertainty, and2 goodness of fit. Detections that fell within 500

of each other were fit simultaneously. The uncertainties in theprofile-fit photometry were based on the a posteriori covarianceof the estimated fluxes and positions, taking explicit account ofthe PSF shape, using a measurement-noise model that includedthe effects of read noise, Poisson noise, and PSF error. Analysisof the rms statistics of repeated observations showed that forS/ N> 10 sources the profile-fit algorithm overestimated theactual uncertainty. A magnitude-dependent correction was ap-plied to the profile-fit uncertainties to make them consistent with

the empirical uncertainties from repeated observations.

4.4.2. Aperture Photometry

In the absence of confusion, aperture photometry providedbetter accuracy for brighter sources (Ks< 13) than profile-fitphotometry, since small deviations between the PSF templatesand the true source profiles dominated the PSF-fit uncertainties.The pipeline extracted both aperture and PSF-fit measurementsfor each source when possible. Aperture measurements of eachsource detected on an Atlas Image used data from the individual1.3 s frames. On each frame fluxes were measured in a seriesof circular apertures with radii from 300 to 1400 on intervals of 100.Sky brightness was estimatedin an annulus with inner andouterradii of 1400 and 2000. The source flux in each aperture was the

unweighted average of the fluxes measured on each frame. Theaperture photometry uncertainty was the rms of these individualframe measurements.

The Point Source Catalog standard aperture magnitudewas measured in a 400 radius aperture.20 The PSF of the 2MASSoptical system was broad enough that between 2% and 15% ofthe total flux from a point source fell outside this aperture, de-pending on the atmospheric seeing conditions. A curve-of-growthcorrection was applied to the standard aperture measurements tocompensate for this loss. With this correction applied the standardaperture measurement reflects the flux from an infinite sizeaperture that captures all of the light from the star image.

Magnitudes and uncertainties reported for sources that weresaturated on the 1.3 s exposures were the average and rms of the

aperture photometry measurements from the individual 51 msexposure frames. Detections that contained one or more saturatedpixels even on the 51 ms frames were estimated by fitting thenonsaturated part of their azimuthally averaged radial bright-ness profiles to one-dimensional analytic templates.

4.4.3. Total Flux Uncertainty

In addition to the default extraction uncertainty for each pointsource, the Point Source Catalog includes a column that reportsa combined uncertainty that includes zero-point uncertainty,

flat-field uncertainty, and, for sources saturated in the 1.3 s ex-posures, uncertainty in the normalization of the saturated photom-etry.21 The largest of these additional uncertainty terms, whichwere combined in quadrature with the default source extractionuncertainty, was 0.012 mag. Thus, for most sources, the com-bined uncertainty is only slightly larger than the source extrac-tion uncertainty.

4.4.4. Band-Merging

Although point-source detection and photometry was con-ducted independently for the J, H and Ks detections in eachscan, the catalog contains a single entry for each source. Theindividual band detections were positionally merged intosingle source entries using a nearest-neighbor algorithm withlogic to break merging conflicts and chains. No informationother than positional data guided the merging algorithm. Thefinal position quoted for each point source was derived fromthe flux-variance-weighted average of the position from eachband detection.

4.5. Extended Source Identification and Characterization

The 2MASS Extended Source Catalog contains sources that

are extended with respect to the instantaneous PSF, such asgalaxies and Galactic nebulae. The 2MASS Explanatory Sup-plement, as well as Jarrett et al. (2000), describes the extendedsource component of the 2MASS pipeline in detail. Briefly,point /extended-source discrimination was conducted for eachband-merged point-source detection by comparing a variety ofradial shape, surface brightness, image moments, and symme-try parameters with characteristic stellar parameters using anoblique decision tree classifier. The classification tests includedfilters to exclude double and triple stars, which were one of themain contaminants in high source density regions. Stellar pa-rameters were measured empirically as a function of time ineach scan to compensate for variations in the atmospheric see-ing using the aggregate properties of band-merged point-source

extractions.Once identified, extended sources were then extracted directly

from the Atlas Images. Photometry was performed in a series offixed circular apertures (5007000), circular and elliptical aperturesdefined by the sourcesJandKs 20 and 21 mag arcsec

2 and Kronisophotes, and multiple apertures yielding extrapolated totalmagnitudes. Elliptical fits were made to the isophotal contours,yielding basic source shape parameters such as semimajor axes,axial ratios, and position angles. Background compensation forthe extended source photometry was made by fitting and sub-tracting a low-order two-dimensional polynomial to the entireAtlas Image.

The extended source processing algorithm was not designedto extract the parameters for the largest galaxies on the sky. Al-

though not a primary objective of 2MASS, high-quality infra-redfluxes andspatial parameters forgalaxies largerthan 12000 insize have been reconstructed and deliveredin the 2MASS LargeGalaxy Atlas (Jarrett et al. 2003), which is included in the Ex-tended Source Catalog.

4.6. Position Reconstruction

Source positions were reconstructed in the International Ce-lestial Reference System (ICRS) via the Tycho-2 ReferenceCatalog ( Hog et al. 2000). Position reconstruction involved three

21Catalog columnsj,h,k_cmsigdenote the default source extraction uncer-

tainty, while columnsj,h,k_msigcomcontain the combined uncertainty.

20 The standard aperture measurements appear in Point Source Catalogcolumnsj,h,k_m_stdap.

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steps. First, point-source extractions in overlapping frames wereused to determine the relative frame offsets. Second, astrometricreference stars were identified among the extractions, and theirscan positions were used to tie together the three bands in both theshort 51 ms and standard 1.3 s images simultaneously. Doing soconstrained the random walk errors in the frame offsets along ascan and mapped the scan reference frame onto the astrometricgrid. During preliminary 2MASS data processing, source posi-

tions were reconstructed using the Astrographic CatalogTycho-1(ACT) Reference Catalog because Tycho-2 was not yet avail-able. Tycho-2 was used as the primary astrometric reference inthe final data processing. The third position reconstruction step,performed only in final data processing, used the position differ-ences of sources appearing in the overlapping regions of adjacentscans to constrain astrometric solutions across scan boundaries.Doing so improved the uniformity of astrometric solutions onlarger spatial scales, particularly for scans with relatively fewor poorly distributed Tycho-2 reference stars. Astrometric solu-tions for each scan were incorporated into theWorld CoordinateSystem information in each 2MASS Atlas Image.

The 2MASS position uncertainties incorporate contributionsfrom four components. Uncertainties associated with source ex-

traction were computed by fitting a template to the stack of over-lapping 1.3 s exposures of the source from a single bands scandata. The dispersion about the fit determined the extraction po-sition uncertainty. This single-band position measurement wastransformed to a multiband coordinate system that incorporatedinformation from each of the three bands. Uncertainties in reg-istration of single-band coordinates to the multiband coordinatesystem were determined empirically from the dispersion in po-sition discrepancy of astrometric standard stars across the dif-ferent single-band systems. In determining a single position anduncertainty for a band-merged source, the two uncertainties abovewere root-sum-squared for a given source to arrive at the un-certainty in the multiband coordinate. When band detectionswere merged, their independent positions were combined via

inverse-variance weighting to compute the refined position, andthe refined uncertainties were computed accordingly. Ultimately,positions were transformed to the celestial coordinate system.Uncertainties in position on the celestial sphere were determinedempirically from position differences of sources observed in thescan overlap regions and from the position residuals of Tycho-2astrometric reference stars as a function of positionalong the scan.The resulting uncertainties were then added in quadrature to thoseabove.

4.7. Artifact Identification

Likely spurious extractions triggered by artifacts of brightstars were automatically identified in the point- and extended-source lists using geometric and brightness-based algorithms.

Possible artifacts were selected by searching template areasaround bright stars that were known to be affected by scatteredlight, diffraction spikes, dichroic glints, latent images, and elec-tronic cross-talk banding. The extent of the affected area wasa function of the stars brightness and the scanning step size.Discrimination between spurious detections and real sourceswas attempted via the assignment of a probability of contami-nation. Probable spurious extractions were excluded from therelease catalogs. The list of identified artifacts was maintained,however, in order to create image overlays for the Atlas Imagesso that users could readily distinguish likely artifacts from realsources in the images.

During the final reprocessing of the data, meteor trails, whichtypically appear on only one of the six frames covering a given

sky location, were removed prior to the construction of the AtlasImages. This procedure eliminated virtually all of the spurioussource extractions along the trails andcleaned the trails from theAtlas Images.

Artifact identification, although efficient, was not perfect.The surveys requirements allow 10 in any one band to be artifacts. The Point Source Cat-alog contains 321 million sources with S/ N>10, and the require-

ment allows for more than 100,000 unidentified artifacts in thissubset. Section 5.2.8 discusses the characterization of the achievedreliability of the Point Source Catalog.

4.8. Quality Assurance

Verification and validation of pipeline data quality used anautomated software system that generated a set of HTML re-ports summarizing characteristics such as telescope tracking andscanning stability, detector noise levels, nightly photometric sta-bility from the calibration solutions and the photometric con-sistency of sources extracted from overlapping regions of scans,atmospheric seeing and background levels, astrometric solutionperformance, and the incidence of transient events such as air-craft passages, bugs on the camera windows, etc. Each scan was

assigned a quality score depending on these quantitative per-formance criteria. Quality assurance scientists reviewed the auto-mated reports for each night, confirming or adjusting the resultingquality scoring as necessary. During nightly operations theobservatory scheduling software incorporated these quality scoresto establish the nightly priority for tile observation. For final pro-cessing, the quality assurance scoring drove selection of scans forinclusion in the 2MASS All-Sky Data Release products.

4.9. Final Product Generation

The final data processing generated working source data-bases that contained 1,314,981,867 and 2,590,500 extractionsof point and extended sources, respectively, from 70,712 scans

of survey tiles. The number of sources in the working databaseswas substantially larger than the number of sources in the finalcatalogs because the working databases contained (1) multipledetections of sources scanned more than once because they fellin tile overlap regions or because they were in tiles that werereobserved and (2) extractions of faint, often unreliable, sourcesresulting from the low detection thresholds used to ensure com-pleteness. The data release requirements dictated that the finalcatalogs meet a higher standard of uniformity and reliabilitythan the contents of the working databases. The final Point andExtended Source Catalogs included a subset of the contents ofthe working databases based on the following procedures:

1. Select the best scan of any multiply observed tile, basedon net photometric sensitivity.

2. Select a single apparition of any multiply detected sourcefalling in a tile overlap region using a purely geometric algo-rithm that chose the apparition falling farthest from a tile edge.Basing this choice only on source position minimized spatialbias in the catalogs from the tile overlap regions.

3. Remove sources with high probability of being an artifact.4. Select point sources with S/N > 7 in at least one band or

S/ N> 5 in at least one band but detected in all three bands.5. Select extended sources with S/ N> 7 in at least one band.

Additional diagnostic/summary information was generatedfor each source selected into this catalog subset. These post-processing value-added data columns included such quanti-ties as the separation and position angle of a given source from

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its nearest neighbor in the catalog. Another Point Source Catalogcolumn combined S/N, 2, detection repeatability, scan back-ground noise, and detection upper limits band by band into asingle shorthand three-character photometric quality flag.22

5. ACHIEVED PERFORMANCE

5.1. Data Product Summary

Final product generation created a Point Source Catalogcontaining 470,992,970 sources and an Extended Source Cat-alog of 1,647,599 sources, along with 4,121,439 FITS imagesfrom the 59,731 unique scans from which these sources wereselected. These products along with several ancillary tablesconstitute the 2MASS All-Sky Data Release23 made availableto the public in 2003 March. Figure 13 graphically illustratesthe contents of the primary catalogs with all-sky projections re-constructed from the entries in the Point and Extended SourceCatalogs.

5.2. Point-Source Performance

The 2MASS Science Team carried out a number of analysesto validate and characterize the All-Sky catalogs. These anal-yses operated on the catalogs themselves, as well as on data setssuch as the calibration data, which were processed in a manneridentical to the primary data. The calibration data, in particular,contained thousands of repeated apparitions of sources. Sta-tistics of these repeated observations formed the basis for em-pirical estimates of uncertainties, completeness, and reliability.Table 2 summarizes the primary characteristics of the 2MASSPoint Source Catalog determined by these analyses.

5.2.1. Sky Coverage

2MASS imaged 99.998% of the celestial sphere in all threebands, missing 0.64 deg 2 owing to 50 mispointed tiles that leftnarrow uncovered gaps relative to neighboring tiles. Since sourceextraction required that a point source be >1000 from a tile edge,the uncovered regions of the Point Source Catalog amountto 1.152 deg 2.24 The area lost to these gaps is small comparedwith the area lost by the exclusion of sources in a magnitude-dependent radius around bright stars. These avoidance areas,invoked to minimize the incorporation of false detections dueto artifacts, reduce the catalogs sky coverage to 99.5%.

5.2.2. Dynamic Range

2MASS photometry has a dynamic range of >20 mag owingto the source extraction softwares ability to address differentexposure regimes. Unsaturated sources in the nominal 1.3 s ex-posures (mag 817) and in the 51 ms integrations created bythe first readout of the array following reset (mag 48) were

extracted using aperture and/or profile-fit photometry techniques.Flux estimates for sources that saturated the 51 ms exposureswere provided by template fitting to the unsaturated scatteredlight in the wings of the saturated star image (mag 4).25 The

brightestKs-band source extracted by the pipeline was OrionisatKs 4:4 mag, while the faintest common Ks extractionsappearing in the Point Source Catalog have Ks 16:2 mag.Figure 14 presents source counts at the north Galactic pole fromthe 2MASS Point Source Catalog in all three bands spanningthe full dynamic range.

5.2.3. Photometric Sensitivity

Figure 15 shows the rms flux uncertainty versus magnitudefor each star in one set of six consecutive observations of a2MASS calibration tile. This statistic establishes the S/N 10limiting magnitudes for that set of observations and thus for theconditions of seeing and background that prevailed at the timeof observation. Evaluation of such statistics for the ensemble ofcalibration scan data established a relationship for the S/N 10limiting fluxes of any given survey scan as a function of the back-ground and seeing. Survey scans that fell below the sensitivityrequirements were scheduled for reobservation.

The histograms in Figure 16 document the S/N 10 sensi-tivity for every scan used in the 2MASS All-Sky Data Releaseas established by the scans background and seeing. Althoughthe 2MASS sensitivity specification was J 15:8, H 15:1,

andKs 14:3 at S/N 10, Figure 16 demonstrates that the bulkof the data are substantially more sensitive than these goals,particularly in the Jband. Because sensitivity varied from tileto tile, the source counts in the raw Point Source Catalog arespatially nonuniform, with structure that reflects the tile pattern.Since the source counts are >99% complete at the S/N 10limits in the absence of confusion, the catalog becomes uniformat high Galactic latitude at magnitudes brighter than the S/N 10specifications above. At low Galactic latitude source confusionraises the flux level at which the completeness requirement ismet (x 5.2.9).

5.2.4. Photometric Accuracy

All except two of the 59,731 survey tiles were observed

under photometric conditions according to quality assurancemeasures. Thin clouds were detected in small portions of thescans of two tiles during final processing, and sources in theseregions were assigned appropriately large photometric uncer-tainties and correspondingly poor photometric quality flags.

The statistics of repeated observations served to validate theflux uncertainties assigned to individual sources by the extrac-tion algorithms (Fig. 15). As discussed in x 4.4.1, thebulk of thedefault photometry in the Point Source Catalog arises from theprofile-fit algorithm. This algorithm consistently overestimatedthe true flux uncertainty, and the extracted uncertainties werescaled to match the empirical uncertainties from the repeatedobservations. Figure 17 illustrates the locus of flux uncertaintyforKs-band sources across the entire dynamic range of 2MASS.

At faint magnitudes (Ks > 13) uncertainties rise due to thedominance of background noise. For 8:5< Ks < 13 default un-certainties are consistently in the range 0.020.03 mag. Thislimiting profile-fit uncertainty is attributable to PSF mismatchin the profile-fit algorithm, driven by undersampling due to thecoarse 2B0 pixel size.

Fluxes for stars with 4 < Ks < 8:5 were measured with aper-ture photometry in the six or seven independent 51 ms exposuressince their images saturated the 1.3 s frames. Since aperture pho-tometry provides a more consistent flux estimate than profile-fitphotometry for brighter stars, these stars have mean uncertainties(light blue points) that are smaller than the profile-fit extractionuncertainties. Stars whose images were saturated even on the51 ms exposures (Ks < 4; dark blue points) have much larger

22 This photometric quality flag, ph_qual, readily identifies sources withS/N > 10 in one or more bands via the appearance of one or more As in theentry. The surveys requirements in Table 1 for completeness, reliability, anduniformity apply to the 321 million sources in the Point Source Catalog meetingthis condition.

23See http://www.ipac.caltech.edu/2mass /releases/allsky/.

24The locations of these coverage gaps are documented at http://www.ipac

.caltech.edu/2mass/releases/allsky/doc/sec3_2c.html.25

The Point Source Catalog rd_flgcolumn reports, band by band, the pho-tometry method used to populate the default magnitude column.

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Fig. 13.Full-sky distribution of point (top) and extended (bottom) sources. Point sources are presented in Galactic coordinates centered on b 0

andl 0

. Theextended source map is presented in equatorial coordinates and centered at 180

and 0

. The faint blue band in the extended source map traces the Galactic

plane as represented by the Point Source Catalog. Intensity is proportional to source density. The images are a color composite of source density in the J(blue),

H(green), and Ks(red) bands.

TABLE 2

2MASS Point Source Catalog Performance

Category Performance

Dynamic range................................... >20 mag (4 through 16 mag atKs )

Sensitivity in unconfused regions ..... 100% of coverage better than S/N = 10 atJ= 15.9, H= 15.0, and Ks = 14.3 mag; 50% of coverage better than S/N = 10

at 16.4, 15.5, and 14.8 mag, respectively; most sensitive individual tile S/ N = 10 at 16.8, 16.0, and 15.3 mag

Photometric precision ........................ 99.95% for sources with S/N > 10 in any one band

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Fig. 14.2MASS Point Source Catalog source counts ( per 0.2 mag bin) forseveral hundred square degrees near the north Galactic pole. Blue, green, andred histograms representJ,H, andKscounts, respectively.

Fig. 15.Sensitivity vs. magnitude distributions obtained from the rms ofthesix independently derivedmagnitudes of each star appearingin oneset of sixcalibration scans. The black crosses are the measurements of individual stars;red bars are the trimmed average rms values for all stars in 0.5 mag wide bins.The horizontal blue lines in each panel indicate the S/N 10 levels, and thevertical blue lines show the surveys S/N 10 requirements.

Fig. 16.Histograms of the distribution of the S/N 10 sensitivity leveldivided by hemisphere for each of the 59,731 scans that comprise the 2MASScatalogs.

Fig. 17.Reported default uncertainty (k_cmsig) as a function of magni-tude for theKsband. Faintward ofKs 9 a Hess diagram represents the densityof occurrence of different values ofk_cmsigin the Point Source Catalog. For4 < Ks < 9, light blue points represent the uncertainties for individual sourcesthat were saturated in the 1.3 s standard exposure time but were unsaturated inthe 51 ms exposures. The dark blue individual points forKs < 4 represent theuncertainties for sources that saturated the 51 ms exposures whose fluxes wereestimated from fits to the unsaturated wings of the stellar profile.

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uncertainties because fluxes were estimatedfrom a fit to the wingsof the saturated stellar profile. The magnitude boundariesbetweenthese regimes vary by a few tenths of a magnitude depending onthe wavelength band and seeing and background conditions.

5.2.5. Photometric Uniformity

Large-scale drifts in the 2MASS photometric system wereminimized using the global photometric calibration procedure

of Nikolaev et al. (2000) described in x 3.4. Photometric solu-tions were established independently for each hemispheres data.Calibration stars shared by both hemispheres exhibit 1:0 reflects actual differences in extragalactic populations in the local vs. global samples.

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sensitivity at the time of observation for each tile to enable ro-bust statistical analysis of the whole sky accounting for the vary-ing completeness threshold. Since virtually all of the 2MASSscans have S/N 10 sensitivity better than the requirements of

15.8, 15.1, and 14.3 mag forJ,H, andKs , respectively (Fig. 16),the Point Source Catalog is >99% complete above these limitsfor nearly the entire unconfused sky.

5.2.8. Reliability

A reliable source is one that can be detected repeatedly,within the constraints of possible proper motion or flux vari-ability. In practice, reliability represents the ratio of the numberof real astronomical sources to the total number of sources inthe data set. The surveys requirements dictated that Point SourceCatalog reliability should exceed 99.95% for sources detectedat or above the S/N 10 threshold in any one band. The PointSource Catalog contains 321 million sources meeting this single-band S/ N 10 criterion. The 99.95% reliability requirement

allows for 160,000 spurious extractions with S/N >10 in oneband in the Point Source Catalog.

The reliability of the Point Source Catalog was assessed viaboth external and internal comparisons. The sensitivity of thevisible-wavelength SDSS exceeds that of 2MASS for the de-tection of all but the intrinsically reddest objects. Comparisonwiththe SDSS Early Release Areayielded a reliabilityof 99.981%for the Point Source Catalog for sources with S/N > 10 in anyone band. Internal tests for reliability used repeated observa-tions of sources near the celestial poles, where the tiles con-verged and thus overlapped heavily. Fields with at least sixindependent observations yielded differential reliability in the0.5 mag wide bins above the S/N 10 limits of 15.8, 15.1, and

Fig. 20.Astrometric comparison between 2MASS and other catalogs as a function of 2MASS Ks magnitude. The solid black curve indicates rms positiondifferences in the cross-scan (right ascension) direction between 2MASS and the subset of Tycho-2 stars that appeared in Tycho-1 [Tycho-2(1)], while the blue curve isfor sources appearing in Tycho-2 [Tycho-2(2)]. This latter subset did not appear in Tycho-1 and thus includes fainter stars with poorer position estimates. Positionresiduals with UCAC are indicated in red. The green line represents internal 2MASS position differences between adjacent tiles. Analysis of the component of positionerror along the scan direction yields similar results.

Fig. 21.Completeness vs. magnitude for a set of 549 observations of cali-bration field 90161 where the J-band S/N 10 sensitivity was near the medianvalue for all scanscontributingto the Point Source Catalog,J 16:4. Each pointrepresents the detection percentage in the 549 trials for a given star. Visual exam-ination of star images corresponding to points that lie well away from the overalllocus reveals that these are close pairs at the threshold of the resolution of thesurvey and sources near bright stars.

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14.3 mag atJ,H, andKs , respectively, of 99.98%, 99.98%, and99.96%.

These tests revealed unreliable source extractions and thusgeneral causes of unreliability within the catalogs. Sources de-tected in only one band had the greatest potential for beingunreliable, particularly those detected only in the intermediate-wavelength Hband. A limited number of unstable pixels cre-ated single-band unreliable sources. The worst of these bad

pixels were identified, and their artifacts were removed from thecatalogs. The surviving hot pixel sources are weakly distin-guished from real sources, and attempts to remove them wouldhave adversely affected completeness.

Bright stars created significant scattered light artifacts intheir vicinity. Most of these were geometrically identified andremoved from the catalogs. A small number of otherwise un-identifiable artifacts remain as unreliable sources. Other sourcesof unreliability include detections of sources triggered by sat-ellite trails, meteoroids, and even insects. The majority of theresulting unreliable sources were removed from the catalog,and only a small residual contamination remains. Finally, minorplanets with known ephemerides were identified and flagged inthe Point Source Catalog (and cross-identified in a separate

minor planet ancillary table). Unidentified minor planets con-tribute to unreliability, despite being real sources, since they donot adhere to the requirement of repeatable detection at the sameposition.

5.2.9. Confusion

The nominal 2MASS sensitivity limits apply to regions wherethe sky background dominated the noise. In regions of highstellar density such as dense clusters and the Galactic planenear the Galactic center, confusion from unresolved sourcesdominated the background Poisson noise. Source count versusmagnitude histograms reveal the effects of confusion, s ince theconfusion noise statistics dictate the limiting magnitude forcompleteness. Figure 22 shows the all-sky distribution of lim-

iting magnitude for Ks-band source counts. The figure dem-onstrates that confusion restricts the sensitivity of 2MASS foronly a small region of the inner Galactic plane atKsband. Sinceinterstellar extinction is at a minimum in theKsband, theJandH bands are less affected by confusion. The primary areas ofconfusion are (1) longitudes 75

from the Galactic center and

latitudes 1

from the Galactic plane and (2) within an approxi-mately 5

radius of the Galactic center.

5.3. Extended Sources

Since the 2MASS millijansky flux limits favor the detec-tion of Galactic stars over extragalactic sources (cf. Glazebrooket al. 1994), the 1.6 million object 2MASS All-Sky ExtendedSource Catalog, which itself is dominated by galaxies, containsonly one source for every 300 counterparts in the Point SourceCatalog. These 1.6 million extended sources adhere to simi-lar high standards of uniformity, completeness, and reliability.Fixed-aperture measurements of repeated observations of ex-tended sources demonstrate S/N 10 sensitivity limits fainterthan 15.0, 14.3, and 13.5 mag atJ,H,andKsband, respectively.The mean color difference of the high Galactic latitude extendedsource population is90%for sources more than 30

from the Galactic plane. This level

of completeness is validated both through log (N) versus log (S)source counts and through comparison with external catalogs(Fig. 23). Extensive visual inspection of 2MASS extended sourcesand comparison with SDSS Early Data Release classifica-tions yield an extended source reliability of >99% for Galac-tic latitudes more than 20

away from the Galactic plane. For

the Extended Source Catalog, reliability refers to the correctidentification of a catalog entry as representing a truly diffuseobject as opposed to, for example, two or three merged star im-ages. Jarrett et al. (2000) discuss, in detail, the performance ofthe extended source extraction pipeline and its effectiveness in

Fig.22.Full-sky projection illustrating thespatial distribution of source confusion in theKsband. The color coding reflects the magnitude of the peak of the sourcecountsvs. magnitude histogramfor each directionin thesky.Althoughsome overallvariabilityin limiting magnitude arisesfrom thevariationin seeingand backgroundlevel, confusion noise dominates the noise near the Galactic plane and thus raises the limiting flux for detection significantly.

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discriminating against false sources and separating stars fromgalaxies.

6. EXPLANATORY MATERIALSAND DATA DISTRIBUTION

All of the topics addressed in this paper are analyzed in depthin the 2MASS All-Sky Release Explanatory Supplement (Cutriet al. 2003). In addition to describing the details of the obser-vation hardware, data processing, and the characteristics of thecatalogs, this supplement provides substantial guidance in theastronomical use of the catalogs and describes potential pitfallsawaiting an uninformed user. The Explanatory Supplement is aliving HTML document that is updated with new informa-tion as it becomes available.

Formal release of the 2MASS Point and Extended SourceCatalogs and Ancillary Tables occurred in 2003 March via thedistribution of five double-sided single-layer DVD-ROM diskscontaining 43 Gbytes of compressed ASCII bar-delimited

tables formatted for direct loading into a computer database. ThisAll-Sky Data Release supplants all previous incremental re-lease data, replacing the preliminary source extractions and im-ages from the incremental release products with ones that havebeen processed through the final version of the data pipeline. TheAll-Sky data have been made publicly available via the internetat the Infrared Science Archive (IRSA)26 where they can be ma-nipulated online through Structured Query Language (SQL)commands. The volume of the 2MASS All-Sky Image Atlasdatabase (10 Tbytes uncompressed) prohibited an initial mediarelease of Atlas Images. Instead, the IRSA site initially providedaccess to quick-look images stored with 20:1 lossy compres-

sion using the HCompress algorithm (White et al. 1992). Sub-sequently, mass storage has become available to serve theseimages without compression loss.

7. DEFINING CHARACTERISTICS

A large project can be characterized by a number of definingfeatures and innovations. Even small decisions can have signif-icant leverage on the overall success of a large enterprise. Atthe risk of repeating some of the discussion of 2MASS charac-teristics above, this section collects and summarizes a numberof the factors contributing to the quality and uniformity of the2MASS data products.

7.1.Role of Prototyping

The 2MASS project benefited from three years of prototypingof the surveys hardware and observational techniques. During thisperiod data were acquired in the same freeze-frame scanningformat as the survey itself, butwith a cameracontaining a single

NICMOS3 array ( Beichman et al. 1998). Prototyping validatedthe efficacy and efficiency of the freeze-frame scanning tech-nique, and the single-channel camera optical design was iter-ated during the prototyping campaign to become the basis forthe final survey camera design. The prototyping exercise per-mitted initial development of the data processing pipeline inadvance of the start of formal observations and, more importantly,validated assumptions regarding the end-to-end performance ofthe system. This knowledge of the likely sensitivity, complete-ness, and reliability during operations enabled the developmentof a robust set of testable requirements for success (outlined inTable 1) that, if met, would support the pursuit of the surveysprimary science. These requirements remained unaltered and un-compromised throughout the survey and thus became a driving

Fig. 23.TheJ(blue),H(green), andKs(red) source counts vs. magnitude for the Extended Source Catalog. The blue triangles and dashed line represent data fromGlazebrook et al. (1994) forKs > 13:5 with an extension toKs 12 using their no-evolutionmodel. The yellow dotted line represents source counts fromGardneret al. (1997).

26See http://irsa.ipac.caltech.edu.

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factor in maintaining the quality and uniformity of the final dataproducts.

7.2.Telescope and Camera Features

The 2MASS telescope and camera designs were straight-forward implementations of existing technologies. This sim-plicity contributed substantially to the successful fabricationof the facilities and to their smooth operation, not to mention

the timely completion of observations and the uniformity of thedata products between hemispheres. The 2MASS cameras, forexample, could have incorporated the then newly available1024 ; 1024 format arrays or could have multiplexed severalNICMOS3 arrays per band. Doing so, however, could have addedsubstantial risk to the project schedule and/or complicated day-to-day operations and data processing.

7.2.1. Optical Design

The 2MASS camera optical design itself was simple in that itwas identical, in triplicate, to the successful and well-characterizedprototype optical design. More importantly, this design produceduniform image shape and minimal distortion across the entire 8A5field of view, simplifying the frame combination and profile-fitphotometry algorithms. Incorporating dichroic mirrors into theoptical design permitted simultaneous imaging of a field in allthree colors, thus streamlining many aspects of the data pipelineand avoiding band-to-band systematics from variable seeing andtelescope pointing.

7.2.2. Focus Stability

Invar focus spacers between the primary and secondarymirrors reduced the thermal variation in telescope focus by anorder of magnitude compared with steel. The telescope controlcomputer adjusted the secondary mirror focus as a function oftelescope frame temperature to compensate for the residual fo-cus variation attributable to the Invar. This active focus control

eliminated the need for real-time focus feedback from the in-coming images. After data tapes arrived at IPAC, analysis of theslight astigmatism introduced into the cameras by the dichroicmirrors permitted monitoring of any long-term accumulation offocus error. Seasonal adjustments of 10 m in the secondarymirror zero position compensated the focus for the largest tem-perature swings.

7.2.3. Freeze-Frame Scanning

The 2MASS secondary mirror tilted to freeze the focal planewhile the telescope scanned in declination, representing the firstlarge-scale implementation of freeze-frame scanning in astron-omy. This technique was extremely efficient. Frame-to-framesettling time was dominated by the inertial mass and resonant

frequency of the secondary mirror rather than that of the entiretelescope. Of the 1.455 s cycle time, only 51 ms was requiredfor secondary mirror flyback and settling; during the remainingtime the focal plane image was frozen in place and the array wasintegrating on sky. The regular exposure timing enforced sub-stantial uniformity on the individual image frames, which inturn was one of the key factors that enabled the automated datapipeline to combine frames accurately and extract consistentresults.

7.2.4. Subpixel Sampling

Freeze-frame scanning provided a natural means of recov-ering spatial information from the undersampled 2B0 percamerapixels. Because the telescope was scanning smoothly in the dec-

lination direction and because successive frames were offset8300, simple rotation of an array provided precise subpixelstepping in the direction perpendicular to the scan direction.Similarly, given the fixed frame-to-frame time interval, the tele-scope scan rate was adjusted so that a star would land withconsistent subpixel offset from frame-to-frame in the directionalong a scan. These two independent substeps were tuned toproduce sampling that optimally recovered a smooth system

PSF (Fig. 8). The data processing pipelines quality assuranceoutput monitored the subpixel stepping pattern for each scan.

7.2.5. Hardware Redundancy and Modularity

The northern and southern observatories, cameras, and dataacquisition systems were nearly identical. Because of this sim-ilarity, construction of the northern facility prior to the southerntelescope provided valuable experience that made the commis-sioning of the remote southern facility uneventful. The nearlyidentical nature of the two facilities streamlined operations andscheduling and ultimately aided in delivering uniform data prod-ucts. Modular spare parts were available at both sites for most ofthe data acquisition hardware and for the more vulnerable com-ponents of the telescope hardware. These spares were rarely used.

During the years of operation of the two facilities only 5 dayswere lost to equipment failure when a dome drive gear failed atCerro Tololo and a part had to be shipped from the United States.A gradually failing array in the northern camera was replacedwithout loss of operations time during the monsoon shutdownperiod in 1999 August.

7.3.Automated Data Acquisition and Intelligent Scheduling

The 2MASS telescope operators supervised the automaticoperation of the facilities. The telescope operators had respon-sibility for opening and closing the facility, monitoring theweather for conditions that might be hazardous to the telescope,maintaining the instrument cryogens, and writing and deliver-ing the raw data tapes. The 2MASS computers, accounting forfeedback from previous sky coverage, selected tiles for obser-vation, pointed the telescope, and acquired data throughout thenight. The scheduling software arranged for continuous obser-vations from dusk to dawn spaced such that observations beganand ended with standard star tiles. The scheduling algorithmaccounted for the availability of each sky tile near its optimumair mass for observation, increasing priority for tiles that wouldsoon be lost for the season. As a result, the telescopes did notexperience periods when no observable tiles were available untilthe last several months of operations.

7.4.Dynamic Range Extension

2MASS developed a robust set of flux extraction algorithms

that were effective even for heavily saturated objects. By ad-justing the reset timing of the arrays to match the frame timing,both readouts constituting a doubly correlated frame con-tained calibratable information on 1.3 s and 51 ms timescales.The brightest unsaturated sources in the 1.3 s exposures over-lapped with the faintest detectable sources in the 51 ms ex-posures providing continuous unsaturated flux estimation fromthe 17 mag detection limit to the 4 mag saturation limit ofthe 51 ms frames. As part of the evolution of the data reductionpipeline, a module was added for final processing that estimatedfluxes with 10%20% accuracy from the wings of sources thatsaturated even the 51 ms exposures. This algorithm successfullyestimated the flux of the brightest infrared stars (mag 4).The dynamic range of 2MASS spanned more than 20 mag.

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7.5. Source Detection Redundancy

2MASS benefited substantially from multiple observationsof sources on a variety of timescales. Every point in a 2MASSscan was imaged six, and occasionally seven, times in succes-sion separated by the 1.455 s frame interval. For sources brightenough to be detectable in the 1.3 s single-frame exposure time,these multiple detection opportunities provided a robust meansto discriminate between real sources and spurious events such

as cosmic rays or unstable pixels.The survey tiles overlapped 5000 in the right ascension di-

rection and 8A5 in declination. The independent measurementsof sources in these overlap regions provided (1) checks forclouds during the intervals between calibration observations;(2) general tests of photometric stability, completeness, and re-liability spanning the duration of the survey; and (3) a means oftransferring astrometric information across tiles, which was ofparticular value for tiles containing few astrometric referencestars.

Calibration tile scans provided hundreds, and in some casesmore than 3000, independent observations of 35 1

;8A5 fields.

Each calibration tile contained hundreds of stars. The tiles weredistributed uniformly around the sky and sampled a wide va-

riety of astrophysical environments including low and high stardensity fields, star-forming regions, dark clouds, and galaxy clus-ters. The large number of repeated observations provided robuststatistics for validating the source uncertainties assigned by theprofile-fit and aperture photometry algorithms, as well as for es-timating the surveys overall completeness.

Most importantly, calibration tile observations were obtainedin sets of six consecutive scans. The rms statistics for thestars ineach set provided a real-time characterization of the sensitivityof the observing system as a function of sky background, see-ing, and system throughput. The sensitivity of the primary sur-vey tiles was then established using the seeing and backgroundrelationships derived from the calibration observations.

7.6. Quality Assurance Feedback

The surveys large data rate and volume made it impossiblefor a human to validate every image. Instead, an automatedquality assurance system produced feedback to support bothsurvey planning and verification of the scientific quality of thedata products. This system collected status outputs from each ofthe data reduction pipeline subsystems and produced a concise,web-based report for human review. For each scan the reportsummarized the achieved photometric and astrometric perfor-mance, as well as a number of telescope and camera metrics. Aquality assurance scientist reviewed each nightly report andverified the overall numerical quality score that the pipelineassigned to each survey scan.

Because a portion of the sky might rapidly become in-accessible following observation, rapid feedback of the qualityscore to the observatory scheduling algorithm was essential.Before thepipeline was capable of processing full nights of dataat the rate they were arriving from the telescopes, a quick lookquality feedback pipeline processed each nights standard startile observations (10% of the data volume) and identified poor-quality data to be queued for reobservation. In the last year ofoperations the 2MAPPS pipeline became sufficiently maturethat quality assessment could be conducted rapidly on the sur-vey scans themselves, permitting direct assignment of a qual-ity score for each completed scan and enabling reobservationof any tiles that fell below the requirements for sensitivity orimage quality.

7.7.EvolvingPipeline and Incremental Data Releases

Although the prototyping campaign provided a well-informedstarting point for the development of the 2MASS data reductionpipeline, thesurveys requirementswere farmore rigorous (Table 1).Recognizing that achieving the high standards of spatial unifor-mity, completeness, and reliability would require substantial ex-perience with significant amounts of real survey data, 2MASSplanned for phased pipeline development and data product re-

leases. Observations began with a preliminary version of the datareduction pipeline in place that provided basic source extrac-tion data for analysis of the systems performance. The pipelinewas allowed to evolve during the survey, incorporating improv-ing knowledge of the observing systems, atmosphere, and near-infrared sky. Now obsolete incremental public data releaseswere generated from the output of this evolving pipeline. Thesedata releases provided substantial experience in selecting highlyreliable and complete subsets from the larger databases of sourceextractions and enabled scientists from the broader communityto exercise these preliminary data products.

By the end of observations the data reduction pipeline hadevolved to a final stable version. This final version incorporatedinstrumental calibrations and algorithms for photometry, as-

trometry, and image artifact identification that were optimizedusing all survey data. Final photometric calibration, for exam-ple, made use of a network of primary and secondary standardstars that were developed using the results of the preliminaryprocessing. Similarly, the Tycho-2 catalog, which was not avail-able during most of the survey, was available for use in the finalpipeline. The entire survey data set was reprocessed using thisfinal version of the pipeline, and the images and source extrac-tions from final processing comprise the Atlas and Catalogs ofthe 2MASS All-Sky Data Release.

7.8. Extended Mission Products

The working databases fromwhich the release catalogs weredrawn contain 1.3 billion sources, many of which are redundantdetections from repeated scans or scan overlaps, faint sourcesthat fell below the thresholds for inclusion into the catalogs, orspurious sources extracted from the noise. Following the releaseof the primary products the 2MASS project entered an extendedmission phase with the objective of delivering the contents ofthe working databases, calibration databases, and all Atlas Im-ages to the public. These databases contain multiple detectionsof sources that range from duplicate detections of sources in theoverlap between tiles to more than 3000 independent observa-tions of sources in some calibration tiles. The extended missionwill compute merged source statistics for every multiple detectionproviding a best estimate of flux and position for each multiplydetected source. During the final year of 2MASS operations, adeep observing program filled idle time when all of the avail-able sky had been observed. The exposure time for these deepobservations was 46.8 s, or 6 times the standard 2MASS expo-sure time, producing data that were about 1 mag more sensitive.The deep observations covered 580 deg2 toward star-forming re-gions, galaxy clusters, M31, M32, the Pleiades, and the LockmanHole (Beichman et al. 2003), as well the entire Large and SmallMagellanic Clouds. Atlas Images and extracted source databasesfrom these deep observations will also be released as part of the2MASS extended mission.

8. SUMMARY

2MASS has produced a fiducial image of the near-infraredJ- (1.25 m), H- (1.65 m), and Ks-band (2.16 m) sky with

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millijansky sensitivity supported by precise pipeline-extractedphotometry and astrometry from a catalog of 471 million ob-jects. The point-source S/ N 10 limit is achieved at or fainterthan J 15:8,H 15:1, and Ks 14:3 mag for virtually theentire sky. For sources at or above the S/N 10 threshold the2MASS Point Source Catalog is highly complete (>0.99) and re-liable (>0.9995). Bright source photometricaccuracy is