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.
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.
[Last Updated: 2007 January 8; by R. Cutri & S. Wheelock]
Observatory Band Survey 6x
READ1 READ2 READ1 READ2
North J 17.44 20.93 17.44 22.88
H 16.85 20.34 16.85 22.282
Ks 16.54 20.03 16.54 21.98
 
South J 17.39 20.88 17.39 22.83
H 16.93 20.42 16.93 22.340
Ks 16.40 19.89 16.40 21.808
Table 2: LMC/SMC secondary standards by field
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