Appendix 7. Combined 2MASS Calibration Scan Images and Source Lists

1. Overview of the Combined 2MASS Calibration Scan Images and Source Lists

The 2MASS Calibration Field scans are distinguished among the 2MASS data sets in that they consist of highly redundant observations of selected small regions of the sky. Whereas the main survey and 6x observations scanned most of the sky once, each 8.5'x1° calibration field was observed hundreds to thousands of times. In addition to providing a unique multi-epoch data set for time-dependent studies (A4.1.a), the redundant and highly uniform calibration scans are well suited for image combination to achieve the deepest measurements enabled by 2MASS.

The Combined 2MASS Calibration Scan Images and Source Lists were produced by averaging together the best images of each calibration field and performing conservative source detection on the resulting images. The total exposure times accumulated for the calibration fields range from one to eight hours, so the combined calibration scan images reach 3.5 to 4.5 magnitudes deeper than single scans in the absence of confusion. Moreover, because the saturation level in the combined images remains the same as in single scans, photometry extracted from the images can be easily compared with source photometry from the survey and single calibration scan.

The Combined 2MASS Calibration Scan Images were generated by combining the images from each calibration scan produced during 2MASS pipeline data reduction. The individual scan images were spatially registered, normalized to a common photometric zero point, and combined using an inverse-noise-weighted averaging algorithm. The detailed image combination procedure is described in A7.2. Figure 1 contains an interactive comparison between the images from a single-scan of the 90004 calibration field and those produced by combining approximately 1400 scans of the field (half of the available data). This comparison illustrates how the nearly 4 magnitudes of additional sensitivity result in a dramatic increase in the number of detected sources, and in the detectability of extended emission from faint galaxies. The images also show how artifacts such as latent images, electronic and optical ghosts and flat-fielding residuals that were well below the noise limit in the single scans become clearly visible in the combined images.

Each 2MASS calibration observation consisted of six independent scans of a calibration field made in alternating north and south directions, with a 5" easterly RA displacement between scans. Images from the north-going and south-going scans were first combined separately for each field to facilitate the identification and elimination of detections of spurious artifacts (A7.3.c). Then all north- and south-going scan images were combined for maximal depth. Because of the RA displacements during the calibration observations, and because of small, random telescope pointing errors, the depth-of-coverage of each field was not uniform. The effective sensitivity rolls off towards the edges of the combined images, with a corresponding increase in the background noise. Versions of the combined each images were also produced that have pixels covered by less than 30% of the available scans in each field masked off to facilitate noise-thresholded source detection. In all, there are 21 FITS images provided for each calibration field. In each of the 3 survey bands, J, H and Ks, the images are:

Access to the deep Combined 2MASS Calibration Scan Images is provided via an on-line ftp interface in A7.4. The general properties of the deep images are described in A7.5.

Figure 1 - Comparison of a single epoch and 1400 combined-epoch images of a section of the 90004 2MASS calibration field.

The deep images are the primary products of the calibration scan data combination. Because these images reach depths that can be confusion limited in even the highest galactic latitude fields, it is anticipated that users will wish to perform their own source extraction and profile-fitting and custom aperture photometry on the objects detected in the images. A set of preliminary Source Lists from the combined calibration images is provided as part of the 2MASS Extended Mission data products. These source lists are not intended to be best possible characterizations of objects in the images. Rather they provide users with a quick, albeit conservative access point for detected source data in the fields. More importantly, the source extraction process illustrates critical factors that should be taken into account when you perform your own extractions, such as artifact identification and photometric calibration.

The source detection process, aperture photometry and astrometric and photometric calibration for the Combined Calibration Scan Source Lists are described in A7.3. Source detection was performed separately on the combined north-going and south-going images from each field that were masked down to the >30% coverage areas. Extractions were required to be detected on both north- and south-going images to filter out many of the spurious detections of image artifacts such as latent images, diffractions spikes and bright source ghosts. exclude Simple circular aperture photometry was performed on each confirming source, and the resulting J, H and Ks magnitudes were bootstrapped to the main 2MASS survey photometric system by normalizing to the measurements of the primary and secondary 2MASS standard stars in each field. The final Source Lists contain positions and 3-band photometry of sources detected down to a limit of approximately five times the effective noise level in each of the images. Also given for each extracted source are flags indicating whether the source was found to be associated with an image artifact not removed by the north-south confirmation requirement, and the identification numbers of entries in the All-Sky PSC or XSC (cntr_psc, cntr_xsc) that are positionally associated with the combined calibration scan source.

Figure 2 shows near infrared color-magnitude and color-color diagrams generated using the Source Lists from the 90272, 90301 and 90330 high galactic latitude 2MASS calibration fields. These fields have depths ranging between 3.5 and 4.0 magnitudes deeper than single 2MASS scans. The improved sensitivity provides both a deeper sampling of the stellar main sequence as well as the detection of the faint, red galaxy population with Ks> 15 mag and J-Ks>1 mag that is just barely detected at the depth of the main 2MASS survey. As illustrated in Figure 3, extragalactic sources begin to dominate near infrared counts at high latitudes at Ks>15.5 mag. The deepest combined calibration scan images reach limiting magnitudes of Ks~19, and thus have the sensitivity to probe L* galaxies out to z~0.8-1.0.

The format of the Combined Calibration Scan Source List tables is described in A7.3.d. Access to the extracted source lists from each field is provided in A7.4, and their general properties are described in A7.5.

Figure 2 - Color-magnitude (top) and color-color diagrams extracted from the combined 90272, 90301 and 90330 high galactic latitude calibration field images. The primary 2MASS survey was complete to Ks<14.3 mag. Figure 3 - Cumulative Ks source counts from the 90272 combined calibration field extracted source list (crosses) compared with the high latitude counts from the 2MASS All-Sky PSC and XSC (points). Dashed lines show power law fits to the main survey point and extended source counts.

a. Inventory of Calibration Scans

Thirty-five calibration fields were observed during the main survey, and five additional fields were observed from late 2000 to early 2001 to support the long exposure "6x" observations of the Large and Small Magellanic Clouds. The location on the sky of the survey and LMC/SMC calibration fields is shown in Figure 4, and Table 1 contains a listing of the calibration fields, their mean centers in equatorial and galactic coordinates, and the total number of scans taken of each in photometric conditions. Click the "Tile" number in the first column of Table 1 to view a preview 3-color JPEG image of the combined north-going images for each field.

Figure 4 - Equatorial sky map showing locations of the 2MASS survey (filled squares) and LMC/SMC (open squares) calibration fields.

Under the "Final Image Stack" columns of Table 1 are the number of north-going and south-going scans that were incorporated into the final combined calibration scan images for each band. As discussed in A7.2.b, calibration scans whose images contained anomalies such as residual meteor trails, severe airglow emission, extremely poor seeing and even ladybug trails were excluded from the final combined image "stacks." Hence the sum of number of scans used in the north- and south-going combined images is less than the total number of scans available for each field. The improvement in sensitivity relative to single scans realized for high galactic latitude fields is very close to the square root of the number of scans combined. At lower latitudes where confusion becomes increasingly important, the sensitivity improvement can be less than the sqrt(N).

The final column of Table 1 contains a link to a page summarizing the general properties of the images and extracted source lists for each field. A description of the contents of these summary pages, and a discussion of the overall properties of the combined calibration fields is given in A7.5.

Table 1 - 2MASS Combined Calibration Field Summaries
Field SummaryFinal Image StackProperties

Notes to Table 1

b. Cautionary Notes

  1. General Combined Calibration Scan Notes
  2. Combined Calibration Scan Images
  3. Combined Calibration Scan Extracted Source Lists

Because the Combined Calibration Field Products are generated from the 2MASS Calibration Scan Atlas Images, they exhibit most of the same characteristics and limitations as the data from the individual scans. Users are therefore encouraged to familiarize themselves with the Cautionary Notes to the Calibration Scan Working Databases and Atlas Images before using the Combined Calibration Images and Source Lists.

The sections below describe features and caveats that are unique to the Combined Calibration Scan data products and repeat a few of the important features that are characteristic of the calibration scan data in general.

i. General

ii. Combined Images

iii. Extracted Source Lists

[Last Updated: 2006 October 30; by R. Cutri]

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