Appendix 3. Long Exposure (6X) Scan Databases, Catalogs and Images
4. 6x Data Acquisition
a. Scanning Strategy
Data acquisition for the deeper 6x observations followed the same frame timing and scanning protocols used in the primary survey - the only exception being that the equivalent "READ2" integration time was 7.8 s instead of the survey's standard 1.3 s. As described in Section III.1.b, each 2MASS sky exposure consists of two readouts of the array following reset. The first preserves a 51 ms integration (READ1) while the difference between the second and the first readout (READ2-READ1) provides the doubly-correlated difference frame.
In the main survey the resulting 51 ms and 1.3 s exposure time images provided increased dynamic range. The brightest unsaturated sources in the 1.3 s READ2-READ1 images were also detectable as the faintest sources in the 51 ms READ1 frames. This photometric overlap was essential in the main survey for providing contiguous and internally consistent magnitude coverage across the dynamic range of the survey. Because the 6x observations made no alteration of the 51 ms READ1 integration time while the READ2 integration time was extended to 7.8 s, the 6x observations do necessarily not have overlapping READ1 and READ2-READ1 photometry. Thus, many saturated sources in the 6x READ2-READ1 exposures are too faint to be detected in the 51 ms READ1 frames.
The effective sensitivity "gap" between READ1 and READ2-READ1 exposures in the 6x data leads to two effects that produce discontinuities in the differential point source counts constructed from 6x data (A3.2.b). Sources that are too faint to be detected in READ1 exposures yet are saturated in the longer READ2-READ1 will be omitted from the WDB and 6x-PSC, leading to incompleteness in the READ1/READ2-READ1 boundary. Saturated READ2-READ1 sources that are detected in the READ1 exposures at very low SNR will have systematically overestimated fluxes. These are illustrated in Figure 1 which shows the differential point source counts from the Lockman Hole 6x point source WDB.
The 6x WDBs and catalogs primarily exist to report the faint extension of the original survey results provided by the 6x increase in exposure time. The 51 ms READ1 extractions are also included in the 6x catalogs given their utility for variability and proper motion studies.
|Figure 1 - Differential point source counts from the 6x Lockman Hole observations showing the discontinuities at the READ1/READ2-READ1 boundaries.|
b. Sky Coverage
The 6x observations (A3.1.a) were targeted at selected fields containing objects of specific scientific interest. Targets included nearby galaxies and galaxy clusters, Galactic star clusters, star forming regions, and fields of common interest such as the Lockman Hole. The total sky coverage in the 6x mode of operation was limited and amounts to approximately 589 deg2. The location of the 6x target fields in shown in Figure 1 of A3.2.b.
6x observations were usually interspersed with normal survey observations and were scheduled at times when all available survey tiles had been observed and validated. Survey observations were extended in the Southern Hemisphere after sky coverage from the main survey was complete in order to conduct the extensive 6x observations of ~510 deg2 encompassing the Large and Small Magellanic Clouds.
Because most 6x targets were small compared with the nominal survey 6° tile length, a 1° long (in declination) "science" tile was commissioned and used for targets of appropriate angular size (for example Abell galaxy clusters). The length of the scan used to cover each 6x field is given in Table 2 of A3.1.a.
An error in the commanding script used to control the 1° 6x scans caused the telescope to start those scans from a position offset in declination from the intended starting point. This resulted in sometimes large gaps separating tiles scanned in adjacent declination bands. The worst cases are found in the Chameleon II, Hydra, Abell 754 and Abell 3420 fields where the coverage was split into two distinct fields separated by ~4° in declination. The southern components of those fields miss the primary targets altogether. In less severe cases, there are declination gaps of a few arcminutes in every other scan, as illustrated by the image of M31 in Figure 2. In 6x fields covered by 1° scans in a single declination band, the telescope commanding error resulted in the intended target being displaced towards the northern or southern edge of the field, as shown for the Abell 262 field in Figure 3.
i. 6x Tile Definitions
The 6° long 6x tiles have the same number and cover the same approximate area as the corresponding tile from the main survey. One exception is the "Test4" field that was assigned a tile number of 90312, the number corresponding to one of the main survey calibration fields. The "Test4" field actually lies adjacent to the northern edge of the 90312 calibration field.
The 1° long 6x tiles are assigned numbers in the range 30000-39999 for northern observatory scans and 333000-349999 for southern observatory scans. Within each field, the tile numbering is sequential within declination bands. The tile numbering is not continuous between fields. 1° long 6x tile numbers do not match tile numbers from the main survey.
The tile numbers corresponding to each 6x field are listed in the detailed field information summary pages accessed via A3.2.b.
|Figure 2 - 1.6°x2.0° J-band 6x Atlas Image mosaic of the M31 field that showing declination gaps in alternating scans. (Mosaic constructed using Montage courtesy of J. Good - IPAC).||Figure 3 - 1.3°x1.0° J-band 6x Atlas Image mosaic of the Abell 262 field. The nominal center of the cluster, indicated by the red circle, is displaced towards the southern edge of the field. (Mosaic constructed using Montage courtesy of A. Laity - IPAC).|
c. Photometric Calibration
Photometric calibration of the 6x measurements was carried out using photometric zeropoints derived from standard (i.e. "1x") observations of survey calibration tiles (see III.2.d). The survey zeropoints were scaled linearly to account for the increased integration time of the deeper observations. The calibration procedure employed during 6x data processing is described in more detail in A3.5.c.
Because the LMC/SMC 6x observations exclusively targeted the Magellanic Clouds, five additional calibration fields in and around the Clouds were defined prior to the start of the Magellanic Clouds campaign (see A4.1 for the location of the LMC/SMC calibration fields). Observations of these fields were interleaved with Magellanic Cloud 6x observations and with normal survey calibration observations of main survey calibration fields. Photometry of the standard stars in these "new" calibration fields was boot-strapped to the main survey calibration data and is therefore intimately tied to the primary survey calibration. The new calibration fields served as the primary source of zeropoints for the LMC/SMC campaign. Because of their proximity to the LMC and SMC, the calibration derived from new fields should be less affected by directional atmospheric transparency variations.
[Last Updated: 2008 June 11; by M. Skrutskie & R. Cutri]
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