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 Summary						Final Image Stack			Properties
Tile	ra(J2000)	dec(J2000)	glon	glat	Nscn	NJ(N/S)	NH(N/S)	NK(N/S)
90021	6.10619	-1.97294	107.36651	-64.02523	2025	875/873	873/872	907/906	P
90294	8.31622	-39.40154	318.91629	-77.15726	2621	1208/1208	1208/1207	1207/1207	P
90299*	11.25260	-70.58353	303.70812	-46.53497	468	217/217	217/217	217/217	P
90004	28.66074	+0.71693	154.11677	-58.27527	2977	1393/1391	1404/1405	1411/1412	P
90301	51.72678	-39.84268	244.63466	-55.50638	3430	1627/1628	1627/1628	1627/1628	P
90247	53.01617	+37.34386	155.04346	-15.31752	1962	885/900	885/899	888/904	P
90533	55.26392	+6.93647	179.51313	-36.67609	1839	851/847	851/847	860/855	P
90191	66.58918	+3.62342	190.90892	-29.77810	2086	979/973	979/973	979/974	P
90400*	74.90247	-65.73341	276.28631	-35.95631	378	166/166	166/166	166/166	P
90401*	78.62001	-71.00065	282.10956	-33.37346	432	75/75	75/75	75/75	P
90013	89.28447	+0.01890	206.63167	-12.04885	3515	1631/1626	1632/1624	1631/1624	P
90402*	93.56589	-69.66665	279.95935	-28.52288	156	196/197	196/197	196/197	P
90121	97.37444	-59.65713	268.95105	-25.88052	562	267/266	266/267	266/267	P
90161	105.22232	+48.48935	168.28907	+21.45529	2593	1170/1171	1169/1171	1205/1209	P
90312	126.40319	-39.09847	257.57412	-0.66895	3501	1655/1651	1654/1650	1654/1649	P
92026	128.12790	-1.57084	226.54812	+21.54656	2158	1019/1019	1018/1018	1017/1017	P
90067	132.81203	+11.84773	215.63879	+31.90474	3692	1698/1706	1695/1705	1695/1705	P
90091	145.75214	+59.06160	154.88828	+44.63064	789	369/369	369/369	369/369	P
92397	170.45775	-13.22047	271.78770	+44.16553	2585	1195/1199	1193/1197	1193/1197	P
90217	180.44070	-50.05148	294.81518	+12.03580	1687	796/795	796/794	796/795	P
90266	183.61047	+35.59855	163.59937	+78.43114	2776	1299/1299	1299/1299	1305/1305	P
90860	185.41757	-0.12034	287.00945	+61.82763	2452	1087/1078	1075/1068	1075/1066	P
90867	220.24529	-0.45767	351.08190	+51.86698	949	453/447	453/446	453/446	P
90273	224.21932	-44.81900	325.15505	+12.57220	1780	832/841	832/840	832/841	P
90272	224.64303	+37.14173	61.47581	+61.44777	1973	898/901	898/901	923/926	P
90868	225.11368	-0.65787	356.36663	+48.36291	2186	1029/1027	1029/1027	1029/1027	P
90565	246.68168	+5.87185	20.52082	+34.69775	3396	1545/1551	1545/1550	1588/1593	P
90009	246.80780	-24.68901	352.96954	+16.58465	1582	749/752	749/752	748/752	P
90330	247.89420	+30.14552	50.25044	+42.07085	1192	564/564	564/564	564/564	P
90279	267.09736	-45.42783	346.06515	-8.92593	977	461/464	462/464	461/464	P
90182	279.89627	+49.09363	78.07393	+22.00907	1703	728/728	727/729	755/755	P
90547	282.82780	-4.27488	29.11141	-1.91959	671	270/278	270/277	279/286	P
90808	285.48438	-4.48794	30.12525	-4.37662	1876	848/850	847/850	850/853	P
90234	307.83812	-49.64775	349.60727	-36.21631	2076	981/981	981/981	981/981	P
90813	310.27504	-5.06339	41.33281	-26.64299	1570	699/708	699/707	719/721	P
92409	330.11998	+20.84962	77.72803	-26.65088	1439	654/650	654/650	666/662	P
92202	331.40247	-11.07477	47.13268	-47.91669	2802	1318/1318	1318/1318	1318/1318	P
90893	349.54575	+0.54857	80.12434	-54.38199	2619	1200/1199	1200/1199	1225/1223	P
90290	352.64369	+38.31591	105.94466	-21.87369	1141	564/565	564/565	567/568	P
90298*	356.63061	-74.50079	308.68978	-41.89381	108	52/52	52/52	52/52	P

Notes to Table 1

• Tile - Calibration Field number. An asterisk denotes special LMC/SMC calibration tiles. Clicking on the Tile number will link to a 3-color preview JPEG image formed from the masked, north-going image stacks.
• ra(J2000) - Average center right ascension of all scans of this Tile (decimal degrees)
• dec(J2000) - Average center declination of all scans of this Tile (decimal degrees)
• glon - Average center galactic longitude of all scans of this Tile (decimal degrees)
• glat - Average center galactic latitude of all scans of this Tile (decimal degrees)
• Nscan - The total number of photometric scans available for stacking
• Image Stack/NJ,NH,NK(N/S) - Number of north-going (N) and south-going(S) scans used in the final, filtered scan stacks for this Tile, for each band. The number of scans can differ between bands because of noise and anomaly filtering differences (see A7.2.e).
• Properties - Link to a page summarizing properties of the extracted sources in this field (A7.5).

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 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

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 K_s 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 K_s-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

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.

4. 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 K_s 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

Differential J, H and K_s 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.

2. Photometric Uncertainties

The second figure in each of the detailed field property pages shows the distribution of J, H and K_s 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, K_s=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 K_s=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, K_s=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 K_s=14.7), emphasizing the limiting impact of confusion noise on the aperture photometry in high source density fields.

3. Noise Histograms

The third figure in the individual field properties pages shows histograms of the J, H and K_s 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-K_s~0.7-0.8 mag in the color-magnitude diagram, and faint, red galaxies with K_s>15 mag and J-K_s>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-K_s~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 K_s>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 K_s 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.

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]