Appendix 4. 2MASS Calibration Scan Working Databases and Atlas Images
4. Calibration Scan Data Processing
2MASS calibration scan data were reduced using the same version of the 2MASS Automated Production Pipeline (2MAPPS v3.0) that was used for final processing of all of the primary survey data. The basic processing steps were the same as those shown in Figure 1 of IV.1. Calibration scan data reduction produced a calibrated Image Atlas and point and extended sources working databases (Cal-PSWDB, LMC/SMC Cal-PSWDB, Cal-XSWDB and LMC/SMC Cal-XSWDB) that are identical in format with those from survey data processing. "Catalogs" were not extracted from the calibration scan WDBs.
Three aspects of calibration scan data reduction were handled differently than they were for survey scan data processing:
The images and extracted source photometry from each calibration scan were calibrated by applying the instantaneous photometric zero point offsets derived directly from the calibration observation containing the scan rather than the offset derived from the fits to the nightly photometric solution. The zero point offset for the six scans in a calibration observation is the mean offset measured using all primary and secondary standard stars in all six scans. The uncertainty in the zero point is given by the RMS error in the mean. The mean zero points RMS errors for each calibration scan are given by the [jhk]_zp_ap and [jhk]_zperr_ap entries in the calibration scan metadata table.
Exploiting the in situ measurements of the primary and secondary standard stars available in every calibration scan has two specific advantages over the strategy used to calibration the survey measurements. First, this strategy reduces the contribution to the total photometric error from uncertainties in the nightly zero point solution fits and from real transparency variations that occur on timescales shorter than modeled by the nightly photometric zero point solutions. Second, the in situ calibrations minimize errors that might result from using an incorrect atmospheric extinction corrections because the standards are observed at identical airmasses and times as the data to be calibrated. Extinction corrections become third-order terms because they account only for airmass differences between the six scans in a calibration observation that are observed at nearly identical airmasses within a time span of approximately 6 minutes.
The photometric calibration accuracy of the calibration scans is slightly better than in the survey scans. Figure 1 shows the distribution of J, H and Ks zero point RMS errors for all calibration scans. The most common uncertainties are in the range 0.003-0.004 mags, and the 68th percentile (1-sigma) for the distributions are 0.005, 0.005 and 0.006 mag at J, H and Ks, respectively. The estimated characteristic calibration residuals for survey scans discussed in IV.8.d are 0.011, 0.007 and 0.007 mag in J, H and Ks, respectively.
One disadvantage of the calibration scan photometric calibration approach is that it ties all measurements of sources within a given calibration field only to the standard stars within that field. Thus, any systematic biases in the "true" magnitudes of standards in a given field relative to those in other fields will be imprinted on the photometry of all sources in the field. This limitation is mitigated by the global 2 minimization procedure that was used to derive internally consistent "true" magnitudes for the 2MASS standard star network (Nikolaev et al. 2000, AJ, 120, 3340). Systematic biases of the standards in any one field relative to the network are measured to be <1-2% (VI.3.a.ii).
|Figure 1 - Distributions of RMS measurement errors in the ] J, H and Ks photometric zero point offsets for all 2MASS calibration scans.|
This was necessary because there are frequently very few and sometimes no Tycho-2 reference stars within the areas covered by the 1° long 2MASS calibration scans, making accurate astrometric reconstruction impossible. The USNO-A2.0 catalog provides a high density of reference stars with good internal precision within all of the calibration fields. However, because there are small, spatially-systematic biases between USNO-A2.0 and Tycho-2 astrometry, there are corresponding systematic offsets between positions in in the 2MASS calibration scan Cal-PSWDBs and Cal-XSWDBs and the All-Sky PSC and XSC.
Figures 2 and 3 show the RA and Declination position residuals for point sources in the 90021 calibration field between the Cal-PSWDB and All-Sky PSC. The bias variation with Declination is typical among the calibration fields, although the amplitude and spatial structure of the bias varies from field-to-field. The maximum bias amplitude is 0.6-0.7" (in Declination), found in the 90004 field (Figure 4). For reference, Table 1 contains a listing of the average and standard deviation of the position offsets between calibration scan point sources and All-Sky PSC sources for each calibration field. However, these mean offset values are not sufficient to correct the calibration scan astrometry to the main survey reference frame because of the the systematic bias variations with position.
Calibration scans were not subjected to the same rigorous quality verification process that was applied to 2MASS survey data.
As described in IV.10, survey scans were assigned an integer quality "score" from 0 to 10, based upon photometric quality at the time of the observation, the achieved sensitivity as gauged by atmospheric conditions, the seeing stability, and the quality of astrometric reconstruction. Calibration scans were assigned a binary quality score (10 or 0 = pass or fail) that was based only on the photometric quality of the data. Only data from scans receiving q=10 were loaded into the calibration scan Atlas Image archive and point and extended source WDBs. The requirements for receiving a passing score of 10 were:
- The standard stars in the scan could be unambiguously identified
- Photometric zero points could be derived from the standard star measurements
- The scan was part of a six scan a calibration observation that was observed under photometrically-acceptable conditions. This means that the dispersion of the instantaneous zero point offset within the set of six scans was less than 0.05 mag, and the zero point offset was consistent with a photometric solution in at least part of the night.
Calibration scan data were not required to surpass sensitivity thresholds in the context of atmospheric seeing and background levels, as were survey data. In addition, small astrometric errors were acceptable in the calibration scans, as long as standards could be identified and their photometry extracted. As a result, the calibration Image Atlas and WDBs contain data from scans that do not necessarily meet all of the quality criteria imposed on the main survey data.
[Last Updated: 2008 February 18; by R. Cutri]
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