Appendix 7. Combined Calibration Scan Images and Source Lists

5. General Properties of the Combined Calibration Field Data

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.

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

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.

Combined Calibration Field Summary Page Contents
Table 1Calibration field centers, boundaries of source extraction area, number of scans combined, number of sources extracted
Figure 1J, H and Ks extracted source counts from combined calibration images
Figure 2Extracted source photometric uncertainties as a function of magnitude
Figure 3Extracted source 1-sigma noise distributions
Figure 4Extracted source color-magnitude diagram
Figure 5Extracted source color-color diagram
Figure 6-8Extracted source J, H and Ks photometric residuals with respect to the All-Sky PSC
Figure 9-11Extracted source declination residuals with respect to the All-Sky PSC
Figure 12-14Extracted source right ascension residuals with respect to the All-Sky PSC

a. General Field Properties

The table at the top of each summary page gives the basic properties of each calibration field and the combined images:

  1. Central position of field in equatorial (J2000) and galactic coordinates.
  2. Equatorial coordinates of the corners of the region from which sources were extracted for each field. These correspond to the area that is covered by at least 30% of all available scans of the field.
  3. Number of scans that were combined to form the north- and south-going combined images. Not all available scans of each field were used in the final combined images, so the sum of the north- and south-going scans used in each band is generally less than the total number of scans, Nscn, shown in Table 1 above. Scans with images that contained anomalies such as high noise levels or particularly bad seeing conditions were identified and omitted from the final image combination.
  4. Total number of entries in the extracted source tables for each field. This number includes extractions that are flagged as possible artifacts. Flagged artifacts comprise on average 2-3% of the extractions in each field.

b. Image Characteristics

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.

  1. Relative depth - The relative depth achieved in the different fields is a function primarily of the number of scans combined, which is tabulated in Table 1. The achieved sensitivity is tempered by confusion, which can be significant in the fields located near the Galactic Plane such as 90312, 90279, 90547, and 90808.

    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.

  2. Artifacts - Artifacts due to bright stars are prominent in all of the combined images. The image combination process reduces sky background photon noise by factors of 25-60 relative to the level in single scan images. As a result, latent images, optical and electronic ghosts, and residuals in instrumental calibration frames (i.e. sky offset corrections) are accentuated in the combined images, and faint artifacts that were well below the detection limit in the single scan images become visible. This is illustrated in Figure 1 in which a single scan and the combined north-going image of the same section of the 92397 calibration field are compared.

  3. Residual Bacgkround Structure
  4. 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.

  5. Transients - Transient events, such as cosmic ray strikes, meteor trails and fast moving sources such as asteroids are effectively suppressed by the image filtering and combination process, as described in A7.2. Examples of the process used to identify and filter anomalies in individual scans are described in A7.2.e. Figure 2 below illustrates how averaging together hundreds to thousands of images eliminates bright moving sources such as the asteroid (7) Iris which appears in six of the 3692 scans of the 90067 calibration field.

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.

c. Source Counts and Achieved Sensitivity

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.

  1. Source Counts
  2. 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.

  3. Photometric Uncertainties
  4. 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.

  5. Noise Histograms
  6. 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.

d. Photometric Properties

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 13Figure 14Figure 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.

e. Astrometric Properties

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]

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