Appendix 3. Long Exposure (6X) Scan Databases, Catalogs and Images

5. Data Processing

c. Photometric Calibration

2MASS 6x photometric calibration is based on the same hourly observations of calibration fields used by the main survey (III.2d). Measurements of standard stars in the calibration fields are used to derive nightly photometric transformations that are applied to images and source extractions (IV.8). Because all calibration observations are made with the same READ1 and READ2 exposure times used for the main survey, the instrumental zero point magnitudes applied to the 6x source extractions are adjusted to compensate for the longer 6x READ2 exposure times. The regular 2MASS calibration fields were augmented with five "special" fields located in and around the Large and Small Magellanic Clouds to support the 6x LMC/SMC observations from the southern 2MASS observatory (see below).

i. Instrumental Zero Point Adjustments

The instrumental zero point magnitudes used for 6x photometric calibration, M0,inst, are given in Table 1, along with the values used for the main survey for reference (IV.8). The 6x READ1 zero points are the same for survey and 6x observations because there was no change to the READ1 exposure time. There is only one set of northern H-band zero points are given in Table 1 because the 6x observations were made only with the "new" northern H-band array detector (III.1.b.iii).

All 2MASS calibration observations, including those covering the special LMC and SMC fields, were made using the standard survey READ1 and READ2 exposure times, 51 ms and 1.3 s, respectively. Compensation for the different 6x and calibration READ2 exposure times is nominally by a factor based on the ration of the exposure times: 2.5*log10(6) = 1.945 mag. However, initial tests of 6x photometry calibrated using just the exposure time correction showed that the resulting photometry was systematically fainter than survey photometry of the same stars in some bands. The survey-6x bias was approximately 0.03 mags in the southern H and Ks bands and 0.01 mags in the northern H band. There was no noticeable bias in the J-band in either camera, or the northern Ks band data. The northern H-band and southern H and Ks band 6x instrumental zero point magnitudes were adjusted so that the mean 6x-survey biases are removed. The adjusted values are shown in red in Table 1.

Table 1 - Instrumental Zero Point Magnitudes

ii. Special LMC/SMC Calibration Fields

The original 35 survey calibration fields were augmented with five new fields defined to support the 6x observation campaign of the Large and Small Magellanic Clouds at the southern 2MASS observatory. The location of these new fields is given in Table 1 of A4.2.a. Three are located in and around the LMC, and are designated calibration tile numbers 90400, 90401 and 90402. The remaining two are located in the SMC and are designated tile numbers 90298 and 90299.

The LMC/SMC calibration fields were introduced into the normal calibration observation strategy so that one of the five was observed hourly during the 6x observations of the LMC and SMC to improve survey efficiency and to minimize calibration errors due to direction-dependent atmospheric transparency variations. Three or more normal survey calibration fields were also observed each night during the LMC/SMC 6x campaign, to provide direct tie-ins to the survey photometric system, and to allow monitoring of the LMC/SMC calibration fields relative to the survey fields. All calibration fields observations were made using the normal survey READ1 and READ2 exposure times of 51 ms and 1.3 s, respectively.

A network of secondary standard stars within each of the LMC/SMC calibration fields was selected by searching the 2MASS All-Sky PSC for stars that satisfy the following criteria:

Photometry for the candidate secondary stars was calibrated by reducing their observations with respect to regular survey calibration using observations from between five and fifteen nights during which three or more regular survey fields were scanned. This typically yielded 50-100 independent measurements of each candidate. Three-sigma trimmed average magnitudes and RMS residuals were computed for each star, and any object with a residual greater than 0.04 mag was eliminated from the network.

Table 2 contains a listing of the positions and mean and RMS brightnesses for the final list of secondary standards in each of the LMC/SMC calibration fields.

Table 2: LMC/SMC secondary standards by field

[Last Updated: 2007 January 8; by R. Cutri & S. Wheelock]

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