UNDER CONSTRUCTION
The long exposure scans taken during Survey operations will be included in a new round of processing as part of the 2MASS Extended Mission. These data underwent very preliminary processing immediately after they were acquired during the survey. This preliminary processing was used to assess basic data quality to charge target area coverage. All 1391 6x scans will undergo a new processed, regardless of whether they were deemed non-photometric during the earlier preliminary processing. This will allow a new assessment of photometric quality based on the improved pipeline and quality assurance techniques, and introduces only a very small overhead to the processing since <5% of the scans are found to be non-photometric.
The links below describe the 6x target fields and give a detailed inventory of the numbers of Tiles observed and scans made of those Tiles.
Photometric calibration of the 6x data is made using the nightly observations of calibration fields in much the same way as was done for the primary survey. The calibration scans were taken at the normal survey exposure time, and the only adjustement made in the calibration process was to adjust the instrumental magnitudes used for each band/observatory by a factor of 2.5log10(6) = 1.9454 mags to account for the longer exposure time of the 6x data. This approach assumes that the detectors are linear over the regime sampled by the 1x and 6x exposures.
We use the traditional nightly calibration approach rather than bootstrapping the photometric calibration to the PSC and XSC for several reasons:
Comparisons between the 6x and 1x calibrated photometry of point
and extended sources on the scan and nightly basis has been integrated
into the night Quality Assurance diagnostics for 6x processing.
Position reconstruction of the 6x data is conducted using the 2MASS PSC as the primary astrometric reference catalog. This differs from the approach used for the primary survey where the Tycho 2 catalogs was the primary reference. The use of the PSC was necessary especially for the 1° LCA scans where there are usually very few if any Tycho 2 reference stars available. The use of the PSC has a number of advantages including the very high density of reference stars in each Tile and the well-characterized accuracy of the astrometry in the PSC. An alternative high density reference catalog might have been the UCAC, but that catalog is not year available for all of the regions covered by 6x processing.
As with the photometry, comparisons between 6x and 1x positions of point and extended sources on the scan and nightly basis has been integrated into the night Quality Assurance diagnostics for 6x processing. Comparisons will also be made with UCAC where available to provide an independent estimate of the 6x positions reconstruction accuracy.
Last Updated: 18 April 2004
R. Cutri - IPAC