IV. 2MASS Data Processing

4. Point Source Detection and Photometry

d. The Default Magnitude

2MASS point source photometry is out carried using different algorithms depending on the brightness regime in which a source falls. The J, H and K_s default magnitudes (j_m, h_m, k_m) listed for each object in the PSC represent what are the best available measurements in each band, as determined by the automated processing. The 3-character rd_flg specifies the origin of the default magnitude in each band. Because the J, H and K_s fluxes for a given source can be in different brightness regimes the default magnitudes can be from different measurement algorithms. Table 2 in I.6.b gives the frequency of occurrence for each value of the rd_flg per band in the PSC.

i. Non-Saturated "Read 2-Read 1" (J>9, H>8.5, K_s>8 mag)

Profile-fit Photometry (rd_flg="2")

The majority of sources in the PSC have brightnesses below the saturation threshold of the 1.3 s "Read 2-Read 1" (R2-R1) exposures. R2-R1 saturation occurs near J~9, H~8.5 and K_s~8 mag, but can vary by up to ~0.5 mag depending on the atmospheric transparency and seeing, and the location of a source relative to pixel centers. Most default magnitudes for non-saturated R2-R1 sources in the PSC are measured using profile-fit photometry (see IV.4b) performed simultaneously on the all six (and sometimes seven) individual 1.3 s (R2-R1) exposures covering the source. These sources have rd_flg="2" in the appropriate bands. For computational efficiency, profile-fit photometry is carried out using a PSF that is small relative to the full extent of a source, so to place the profile-fit magnitudes onto the true photometric scale of the 2MASS system, they are normalized to curve-of-growth-corrected aperture photometry (see IV.4c) using an empirical correction factor that is determined for each Survey scan.

Aperture Photometry (rd_flg="4")

Aperture photometry (see IV.4c) is also performed for all non-saturated sources on the R2-R1 exposures. The source brightness is measured in a series of apertures ranging from 3" to 14" radius using a sky reference annulus with inner and outer radii of 14" and 20", respectively. The multi-aperture measurements are used to derive aperture curves-of-growth which provide correction factors that convert measurements in any aperture to the "total flux" of a source. The standard aperture magnitudes and uncertainties provided for each non-saturated PSC source ([jhk]_m_stdap and [jhk]_msig_stdap) are the curve-of-growth-corrected 4" aperture measurements. The curve-of-growth correction assumes that a source has a profile that follows the point response function of the system. Therefore, if a source is extended or otherwise not-point-like, the standard aperture magnitudes will not capture the total light from the source.

As discussed above, the curve-of-growth-corrected aperture photometry is used primarily to normalize the instrumental profile-fit photometry magnitudes to the absolute photometric scale of the system. On rare occasions, the profile-fitting photometry routines fail for sources in crowded environments or in regions with complex backgrounds. If an aperture magnitude is available, it will be listed in the default magnitude field in the appropriate band, and the rd_flg="4" in the affected bands. However, these magnitudes are highly uncertain due to the confused environment.

ii. Non-Saturated "Read 1" (4.5<J<9, 4<H<8.5, 3.5<K_s<8 mag)

Aperture Photometry (rd_flg="1")

Sources brighter than J~9, H~8.5 and K_s~8 mag will saturate in the 1.3 s R2-R1 exposures. Up to brightnesses of J~4.5, H~4 and K_s~3.5 mag, these sources are non-saturated on the short 51 ms "Read 1" (R1) exposures. The default magnitudes for such sources are taken from aperture photometry (see IV.4a) on the 51 ms R1 exposures, and the rd_flg="1" in the appropriate bands. The R1 aperture magnitudes are measured in a 4" radius aperture using a sky reference annulus with inner and out radii of 14" and 20", respectively. The small-aperture magnitudes derived from the R1 exposures are also tied to the absolute 2MASS photometric scale through normalization (see IV.4c) to the curve-of-growth-corrected R2-R1 aperture magnitudes.

iii. Saturated "Read 1" (J<4.5, H<4, K_s<3.5 mag)

1-d Radial Profile Fitting (rd_flg="3")

Stars brighter than approximately J~4.5, H~4 and K_s~3.5 mag saturate even the 51 ms exposures. A new feature of the 2MASS All-Sky Release PSC is that brightness and accurate position estimates derived directly from 2MASS data are now provided for very bright stars. Photometry for saturated R1 stars is performed using a 1-d radial profile fit to the azimuthally-averaged image profile on the R1 exposures, and the rd_flg value for the appropriate band is "3".

iv. Upper Limits

Non-Detections (rd_flg="0")

Sources that are detected in only one or two of the 2MASS bands have 95% confidence brightness upper limits provided in the default magnitude field in the non-detected band(s). In these cases, rd_flg="0" in the non-detected band(s). The upper limits are derived from aperture measurements made on the Atlas Images using a 4" radius aperture centered on the position of the source in the detected bands. The reported upper limit is two times the noise level if no flux is detected in the aperture measurement, or the measured flux plus two times the noise level if positive flux is measured. The uncertainties in the default magnitudes ([jhk]_cmsig and [jhk]_msigcom), are null in these cases.

Inconsistently Deblended Sources (rd_flg="6")

Multiple sources with separations near the resolution limit of 2MASS may not be split or deblended consistently in all detected bands. In these cases, the deblended magnitudes will be listed for each component in the resolved band(s). In the unresolved bands, the default magnitude will be the 95% confidence brightness upper limit measured in a 4" aperture on the Atlas Image at the location of the source in the resolved band(s). The non-resolved bands are denoted with rd_flg="6". Since there are detections in the non-resolved bands, the upper limits given in the non-resolved bands provide useful constraints on real source color.

An example of this is given in Figure 1, which shows the close source pair 2MASS J14352630-2615587 and 2MASS J14352601-2615582 (sep=4.0"). A subset of the PSC source information for these stars is shown in Table 1. Both components of this double were detected in the K_s band, and fit simultaneously during the profile-fitting procedure. The double was not resolved in the J and H bands, even though the image is clearly extended in those bands. Thus, the source listings for these two stars show deblended profile-fit measurements (rd_flg="2" and bl_flg="2") in the K_s band, and inconsistent deblend upper limits (rd_flg="6" and bl_flg="0") in the J and H bands. The upper limits given in the J and H bands capture much of the flux from two components, though, so they contain useful photometric information. Inconsistent deblend upper limits (rd_flg="6") should not be confused with non-detection upper limits (rd_flg="0").

Table 1 - Source attributes for the partially resolved double shown in Figure 1

|      designation|   j_m|j_cmsig|   h_m|h_cmsig|   k_m|k_cmsig|ph_qual|rd_flg|bl_flg| prox|
|             char|double| double|double| double|double| double|   char|  char|  char|doubl|
|                 |   mag|    mag|   mag|    mag|   mag|    mag|       |      |      |arcse|
 14352630-2615587  12.346    null 11.984    null 13.131   0.041 UUA     662    002      4.0
 14352601-2615582  11.717    null 11.406    null 11.328   0.035 UUA     662    002      4.0

Figure 1

v. Detections with Failed Measurements (rd_flg="9")

In a very small number of cases in the PSC, valid measurements could not be extracted on any of the exposures for detections passed to the photometry routines. In such cases, the default magnitude is null and rd_flg="9" in the affected band, indicating there is technically a detection at that position, but no useful measurement can be made. In practice, this occurred in confused environments, and sometimes near the edges of the focal plane where significant fractions of the sky annuli or measurement apertures fell off the frame.

The northern observatory K_s detector exhibited very high noise levels in its central column of the 51 ms exposures taken prior to 1997 December 25 UT (an update to the northern camera controller electronics solved this problem). This noisy column was masked off, so R1 detections that fell on or near the column often had failed measurements (see III.1b). Such sources will usually show a good photometry in J and H, but a detection with no valid measurement in K_s.

[Last Updated: 2003 February 13; by R. Cutri]

Previous page. Next page.
Return to Explanatory Supplement TOC Page.