IV. 2MASS Data Processing
4. Point Source Detection and Photometry
a. Frame Source Detection and Aperture Photometry
Point sources were initially detected and photometered in each instrumentally-corrected Read_1 and Read_2-Read_1 frame. These frame positions and magnitudes were used to reconstruct the relative and absolute positions of the frames on the sky. The positions and magnitudes from the Read_2-Read_1 frames were not carried further in the 2MASS pipeline, being re-estimated subsequent to frame position reconstruction using the combined frames. For those sources saturated in the Read_2-Read_1 frames, the positions and aperture magnitudes estimated from the individual Read_1 frames were combined and carried forward in the pipeline and used in the catalog.
Point sources were detected in the frames by identifying local maxima. Positions were measured for these detections using a maximum-likelihood estimator, and magnitudes were estimated using aperture photometry within a 4´´ (two camera pixel) radius aperture.
The values for all the pixels were histogrammed, and the median sky value and the noise were estimated. The noise was estimated as one-half the distance between the 15.87% and the 84.13% quantiles of the histogram.
All the pixels in the frame were examined to identify local maxima. For each local maximum exceeding the local background by more than Tpk (where Tpk=4), the following computations were performed:
The centroid of the 3×3 pixel patch centered on the local maximum was
and (xc,yc) are relative to the center of the local maximum pixel, and dij are the data values less the local background. If Fs<Tsnr (where Tsnr=10), or any of the pixels in the 3×3 patch were masked off, the local maximum was not processed further. If no saturated pixels were encountered, the position was estimated as described in the following.
The local centroid was used as the initial source position, which was
then refined using the following maximum likelihood position estimation
technique, with the cost function on each axis computed by:
P = the normalized point spread function (PSF) and Ft
is the template amplitude,
Cx and Cy were followed to their zero crossings, and then interpolated to solve for (xc,yc).
The fraction of flux in the peak pixel (fpeak) was
If the following criteria were satisfied, the fpeak value is entered into a histogram used for estimating the seeing and interpolating an appropriate PSF:
- Ft / Fap > Tfrat (0.8)
- estimated source position was within rpk (0.3) of the center of the pixel
- Read_2-Read_1 was being processed.
The 3×3 pixel patch centered on the peak pixel along with the coordinates of the peak pixel and the aperture magnitude and associated error were written to a temporary file so that the detection parameters could be re-estimated in a second pass using a PSF matched to the seeing.
- The fpeak histogram was interpolated for the 80th percentile value, which was written to a file, and used to redefine the PSF for the scan.
- The Read_2-Read_1 and Read_1 source patch files were reprocessed and the template amplitude, Ft, and the other parameters for each detection were estimated using the redefined PSF. The parameters for all extractions were written to files for Read_2-Read_1 and Read_1.
i. Bright Source (READ_1) Aperture Photometry
The photometric dynamic range of 2MASS was extended by the use of the
51 ms Read_1 exposures. Sources start to saturate on the 1.3 s Read_2-Read_1 exposures at
magnitude levels of approximately 9.5, 9.0 and 8.5 at J, H, and Ks,
respectively. For objects that were found to have one or more saturated
pixels within the measurement aperture on the Read_2-Read_1 frames, the "default
magnitude" quoted in the Point Source Catalog records was taken from the
aperture photometry from the Read_1 frames. This is indicated by a value of
"1" in the "rd_flg" parameter in the point source records, for the
appropriate bands. Positions measured from the Read_1 frames were used in the
final source position estimation only if Read_2-Read_1 profile fit results were
ii. Faint Source Detection
The fainter, and thus majority of sources found by 2MASS are detected from the Atlas Images. Each Atlas Image is convolved with a zero-sum 4´´ FWHM Gaussian over a 13-pixel sub-array. The resulting zero-sum filtered image is thresholded, and for each maximum over threshold, a detection is identified and a rough position estimate is computed from the corrected centroid. This detection process is in all essential aspects identical to that used in the FIND subprocess of the DAOPHOT II program developed by Peter Stetson (P.B. Stetson 1991). The detections list is sent to the software module that computes the running estimate of the seeing during a scan, and to the photometry routines that compute the refined estimates of flux and position. The detection threshold used is 3.0 times the estimated point source noise level for the Atlas Image. The noise level is estimated as (1/0.7) times the difference between the 32.22% and the 50% quantiles of the zero-sum filtered image histogram. This noise estimator is sensitive to confusion noise and the detection threshold (in units of flux) thus increases in areas of high source density such as the galactic plane.
The list of detections from each image is passed to a subroutine which estimates
the seeing shape parameter which is then passed along with the list of detections
to the profile fit photometry subprocess described in the next
section of this documentation.
iii. Very Bright Source (Saturated READ_1) Detection and Photometry
Stars brighter than approximately magnitude=4 to 5 saturated even the 51 ms Read_1 exposures. Special techniques have been utilized for photometry and astrometry on these objects, extending the dynamic range another ~8 magnitudes to the brightest objects in the 2MASS sky. The corresponding "rd_flg" value for these measurements is "3". The photometry and astrometry for these objects is of reduced accuracy compared to the unsaturated objects. 2MASS photometric and astrometric accuracy for these objects were required to be sufficient to allow reliable identification of associated bright star artifacts, but accuracies achieved may be sufficient for additional purposes. It should be noted that many of the brightest near infrared stars are variables and may also have high proper motions.
Photometry on these stars has been done using a 1d radial profile fit to the unsaturated wings of the point spread function, out to a snr limited radius (or 60 arcseconds for the brightest objects). Positions in the individual data frames were estimated by computing the flux weighted centroid of the pixels over a snr threshold The single frame positions for each band were combined during position reconstruction in a manner analogous to that used for unsaturated Read_1 measurements.
The brightest star in the 2MASS Catalog is alpha Orionis (M2 Ia), with J= -2.989, H=-4.007, Ks= -4.378 mag.
Saturated Read_1 Frame Position Estimation
The positions for stars saturated in the Read_1 frames were estimated using a flux weighted centroid. When a saturated pixel was encountered in an Read_1 frame during Read_1 source extraction, connected pixels over a threshold (500dn) were examined and the flux weighted centroid over those pixels was computed. A second centroid was also computed for that subset of pixels also over a second higher threshold (20000dn). If more than 25 pixels were counted over the second threshold, the second centroid was used for the source position, if less, the first centroid was used. The second centroid avoided problems due to asymmetries in the outer halo of very bright stars, and allowed measurements to be made close to frame edges for those stars. These 2 centroids were found to be essentially identical for sources with 15 to 50 pixels over the second threshold. This technique gives good astrometry for sources from those just over the Read_1 saturation threshold to the brightest sources in the 2MASS Catalog.
Combination of the frame positions and subsequent astrometric processing is discussed in the position reconstruction section of this documentation.
Saturated Read_1 Photometry
Photometry for a star saturated in Read_1 was done using a 1d radial profile
fit to the combined data from all 6 frames covering that star in a scan.
When saturated pixels were encountered in an Read_1 frame during
source extraction, the frame data was written to a temporary file for
later examination. After all the Read_1 sources were identified in the single
data frames, all the data for each saturated star in the scan was registered
and converted to radial profiles. The radial profile data was fit to
models for the camera and band being processed. The radial profile
models are in the following form:
The logarithm of that portion of the radial profile data that was unsaturated and of sufficient signal to noise ratio was fit to the logarithm of the model profile using least-square minimization of the errors. The square root of the variance of the residuals from this fit was converted to a magnitude and is reported as the magnitude sigma. It should be noted that this is derived from the fit residuals and is not a formal error.
The bright source model profiles do not account for profile variations due to seeing. A corrective model for estimated magnitude dependence on seeing has been developed but was not applied during 2MASS processing. Later analysis found some deficiencies in the corrective model for seeing. Seeing corrections can be as large as several tenths of a magnitude.
A detailed description of these algorithms and the model parameters used may be found here.
Although there are issues with the seeing corrections for the saturated Read_1 magnitudes, accuracies of 0.1-0.2 magnitudes have been demonstrated throughout most of the range of 8 magnitudes, as well as astrometry on the order of 170 mas.
The confusion radius is larger for bright stars than for the rest of the catalog. Although many saturated doubles are separated down to 7 arcsec, a few very bright saturated doubles with larger separations were not resolved. The worst example known is alpha Centauri AB with components 17.7 arcsec apart which were not separated in the 2MASS catalog. A small number of other bright stars are known to be missing measurements in one or two bands or missing entirely from the 2MASS catalog due to processing problems.
[Last Updated: 2003 Feb 27; by R. Cutri & E. Kopan]
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