Table 1 contains a summary of the basic 2MASS calibration field and combined calibration field image properties. The column titles "Properties" contains a link to web pages that summarize the detailed properties of the observations, images and extracted sources from each field. The contents of the field summaries are described below along with some of the key features found in the data. Each field summary page also contains a link to the Combined Calibration scan Data Access Page.
|Field Summary||Final Image Stack||Properties|
Notes to Table 1
A brief description of the tabular and graphical contents of the combined calibration field summary pages is given in the list below. A more detailed discussion of the field information and how it demonstrates the performance achieved by the 6x observations is given in the sections that follow. Click on the Table or Figure number in the list to go to the corresponding details in the text.
|Table 1||Calibration field centers, boundaries of source extraction area, number of scans combined, number of sources extracted|
|Figure 1||J, H and Ks extracted source counts from combined calibration images|
|Figure 2||Extracted source photometric uncertainties as a function of magnitude|
|Figure 3||Extracted source 1-sigma noise distributions|
|Figure 4||Extracted source color-magnitude diagram|
|Figure 5||Extracted source color-color diagram|
|Figure 6-8||Extracted source J, H and Ks photometric residuals with respect to the All-Sky PSC|
|Figure 9-11||Extracted source declination residuals with respect to the All-Sky PSC|
|Figure 12-14||Extracted source right ascension residuals with respect to the All-Sky PSC|
The table at the top of each summary page gives the basic properties of each calibration field and the combined images:
Clicking on the tile number in Table 1 above or in the field summary pages will display a 3-color image formed from the J, H and Ks images of the combined north-going scans of each field that are masked to the 30% coverage area. The combined J-band images are mapped into blue, H-band into green and Ks-band into red to make the 3-color images.
As discussed in A7.2.f, the effective depth also varies within the individual combined images because of the RA cross-stepping performed during calibration observation, and because of small telescope pointing differences. Depth-of-coverage maps are provided for each of the images to help account for this non-uniformity.
Most of the deep, combined images exhibit a low spatial frequency gradient in the background levels that is due to residual atmospheric OH airglow emission. The airglow emission can be very prominent in the individual scan images, particularly in the H-band, and varies both spatially and temporally on all scales. Combining hundreds to thousands of individual scan images suppresses the emission level, but does not completely eliminate it.
The images of fields that contain bright stars also show background structure due the residual presence of the stars in the sky offset corrections in the individual scan images. These illumination corrections were formed by creating -trimmed averages of all 42 dark-subtracted, flat-fielded frames in each calibration scan. Very bright stars in the images were not perfectly removed by the trimmed averaging, and their residual images left negative imprints in the sky frames when the offsets were subtracted. These residuals are normally well below the detection limit in the individual scan images, and did not adversely effect photometry in the main survey and calibration scans. However, they are prominent in many of the deep, combined images (e.g. see 90021) because of the suppression of the sky photon noise.
|Figure 1 - J, H, Ks three-color images of the central region of the 92397 calibration field. (left) from a single scan, (right) from the combination of all north-going scans).||Figure 2 - (left) J, H Ks 3-color image of a section from a single scan of the 90067 calibration field taken on 04/30/2000 UT, showing the bright asteroid (7) Iris. (right) Image of the same region in the combined north-going scans of the 90067 field showing the absence of any signature of the asteroid.|
Figures 1, 2 and 3 in each of the combined calibration field detailed property pages present the J, H and Ks distributions of differential source counts, photometric uncertainties in magnitudes and histograms of source uncertainties in flux units as a function of source brightness. Examples of these diagrams for the 90301 (b=-55.5°) and 90312 (b=-0.7°) calibration fields are shown in Figure 3 through 8 below. Similar numbers of scans were combined for these two fields (1627 and 1655, respectively). However, they represent two extremes of source surface density sampled by the calibration fields, so the illustrate the impact of confusion on the achieved sensitivity and measurements statistics in the extracted source lists.
Differential J, H and Ks source counts as a function of source magnitude are presented in the first figure in the field property pages. These figures also show the source counts from the 2MASS All-Sky PSC in the same region for comparison. The relative position of the turnover in the count distributions indicates the relative sensitivity gain over the single epoch measurements. The 90301 field counts shown in Figure 3 reach approximately 4 mag fainter than the main survey limits, as expected from the sqrt(Nscans) improvement. However, counts in the 90312 field shown in Figure 4 reach only ~1.5-2 mags fainter than the survey because of the limitation of confusion noise.
The second figure in each of the detailed field property pages shows the distribution of J, H and Ks photometric measurement uncertainties as a function of source magnitude. The characteristic brightness at which SNR=10 (=0.1086 mag) is achieved from the simple aperture photometry measurements can be read from these figures. In the 90301 field, the characteristic SNR=10 level is at J=20.3, H=19.6, Ks=19.1 mag (Figure 5 below). This is approximately four magnitudes fainter than the average SNR=10 levels ([jhk]_msnr10) for all of the individual scans of the 90301 field, J=16.4, H=15.5 and Ks=14.8 mag, consistent with the gain expected from combining 1627 scans. The characteristic magnitude for SNR=10 in the sources extracted from the combined images of the 90312 field are J=18.6, H=18.0, Ks=17.5 mag (Figure 6 below). This is only ~2.5 mag fainter than the mean SNR=10 value in the individual scans of this field (J=16.3, H=15.5 and Ks=14.7), emphasizing the limiting impact of confusion noise on the aperture photometry in high source density fields.
The third figure in the individual field properties pages shows histograms of the J, H and Ks source measurement noise, in flux units. The peaks of these distributions given the characteristic source measurement noise values that is analogous to the "point source noise" computed for each survey scan that is listed in the [jhk]_msnr10 columns of the Survey Scan Information Table. Figures 7 and 8 below compare the noise histograms for the 90301 and 90312 fields, illustrating the elevated noise due to confusion in the high density field.
|Figure 3 - 90301 calibration field (l,b=245.6°,-55.5°)||Figure 4 - 90312 calibration field (l,b=257.6°,-0.7°)|
|J, H and Ks source counts from the combined images of two calibration fields. All-Sky PSC source counts in the same approximate areas of sky are shown in the shaded regions.|
|Figure 5 - 90301 calibration field||Figure 6 - 90312 calibration field|
|Photometric measurement uncertainty vs. magnitude for sources extracted from two combined calibration field images.|
|Figure 7 - 90301 calibration field||Figure 8 - 90312 calibration field|
|Histogram of extracted source measurement noise (in flux units) as a function of source flux in two combined calibration fields.|
The relative, band-to-band photometric performance in the combined calibration scan measurements is illustrated by the color-magnitude and color-color diagrams shown in the fourth and fifth figures in each field summary page. Examples of these diagrams for the 90301 and 90312 fields are shown in Figures 9-12 below. 90301 is located at high galactic latitude (b=-55.5°) and is free from foreground extinction. The sources detected in this field are a mixture of predominantly dwarf stars which comprise the vertical ridge of points at J-Ks~0.7-0.8 mag in the color-magnitude diagram, and faint, red galaxies with Ks>15 mag and J-Ks>1 mag. The main sequence is visible in the lower left of the color-color diagram in Figure 11, with a concentration corresponding to the location of late-K and early-M dwarfs near J-Ks~0.8 mag and J-H~0.6 mag. Most of the faint galaxies detected in this field are below the sensitivity limit of the main 2MASS survey, but they dominate the number counts in the combined calibration images for Ks>16 mag. They form the diagonal ridge in the center of the color-color diagram, and may include objects with redshifts up to z~0.7-0.9. By contrast, most sources detected in the combined images of the 90312 calibration field, which lies on the Galactic plane, are dwarf and giant stars. The locus of points in the color-color diagram of 90312 is elongated in the direction of the reddening vector, indicating the presence of considerable foreground extinction. There are few if any galaxies detected in this field.
|Figure 9 - 90301 calibration field||Figure 10 - 90312 calibration field|
|Color-magnitude diagrams for the 90301 and 90312 combined calibration field extracted source lists. Red points denote individual sources and the contours trace the density of points.|
|Figure 11 - 90301 calibration field||Figure 12 - 90312 calibration field|
|Color-color diagrams for the 90301 and 90312 combined calibration field extracted source lists. Red points denote individual sources and the contours trace the density of points.|
The sixth, seventh and eighth diagrams in the detailed field summary pages show the differences between J, H and Ks photometry from the All-Sky PSC and source photometry extracted from the combined calibration fields plotted as a function of brightness in the combined calibration fields. Examples of the photometric residual plots for the 90161 calibration field are shown below in Figures 13-15. There is little net photometric offset over most of the brightness range because the combined calibration source photometry was calibrated using the survey calibration stars in each field. The photometry of bright sources that saturate the 1.3 s READ2-READ1 exposures from which the Atlas Images are formed is systematically too faint in the combined calibration data relative to the PSC. PSC photometry for bright objects is drawn from the non-saturated 51 ms exposures. A similar bias is visible for the faintest sources, but in this case the PSC reports brighter magnitudes because of statistical flux overestimation for low SNR detections.
|Figure 13||Figure 14||Figure 15|
|Difference between All-Sky PSC and combined calibration source list photometry for objects in the 90161 field. (left) J-band, (center) H-band, (right) Ks-band.|
Figures 9-11 in the detailed field summary pages show the declination residuals between astrometry in the 2MASS All-Sky PSC and the same sources measured in the combined calibration fields. Figure 9 shows the declination offsets as function of declination, Figure 10 shows the offsets as a function of RA, and Figure 11 shows a histogram of offsets. Figures 12-14 in each field summary page show the equivalent plots for the RA residuals.
Position reconstruction for the calibration scan data used USNO-A2.0 as the primary astrometric reference catalog, rather than the Tycho-2 Catalog that was used as the reference for the main survey. Because of small systematic astrometric differences between those two catalogs, the resulting calibration scan astrometry has systematic biases with respect to the main survey.
The mean position offset between the calibration scan and survey Atlas images was at least partially corrected during the image combination process by determining the average offsets between sources extracted from the individual calibration scans and their counterparts in the All-Sky PSC, and then applying those offsets to each calibration scan image before final combination. However, this procedure did not remove right ascension or declination offset structure within each scan. An example of this remaining structure can be seen in Figure 16 below which shows the declination residuals for the 90161 field.
The largest remaining astrometric residuals occur in the combined calibration image and source lists from the 90004 field, as shown in Figure 17 below. Because of the large "raw" astrometric offsets with respect to the All-Sky PSC in this field, many of the positional correlations between the individual scan extractions that were needed to measure the offsets were missed. This resulted in an undercorrection of the mean offsets.
|Figure 16 - 90868 calibration field||Figure 17 - 90004 calibration field|
|Source declination residuals between the All-Sky PSC and the combined calibration scan images plotted as a function of declination for two different fields.|
[Last Updated: 2008 February 18; by R. Cutri]