a. Photometric Zeropoint Evaluation and Extinction Coefficients:
The basic transformation between instrumental and calibrated 2MASS magnitudes applied to all point and extended sources is:
Mcal = Minst + c1 - c2(X-1.0)
Each of the coefficients is a function of wavelength. Note that no color coefficients are included in the 2MASS photometric transformations, so all photometry is reported in the "2MASS system."
The photometric zeropoint in each band, c1(J,H,Ks), was evaluated for each night's observations by constructing the average difference between the catalog and extinction-correction instrumental magnitudes for all primary and secondary standards measured during the night:
c1 = < Mcat - Minst´ >
Minst´ = Minst - c2(X-1.0)
As mentioned earlier, the values of the atmospheric extinction coefficients used for the photometric solution are based on longer term average values and are not derived on a nightly basis. The extinction coefficients used to calibrate the 2MASS Spring 1999 photometry are:
c2(J) = 0.109 mag/airmass
c2(H) = 0.031 mag/airmass
c2(Ks) = 0.061 mag/airmass
Section III.1.c contains a discussion of the range of photometric zero points measured during the Survey (see Figure 4 in that section). The zeropoint offsets for the nights included in the 2MASS Spring 1999 data release fall in the range of -0.2 to +0.2 mags. The H and Ks zeropoints can vary night-to-night by up to 0.1 mag, and the J zeropoints can vary by as much as 0.2 mags. The zeropoints also exhibit longer term, seasonal trends. In particular, the J-band atmospheric transparency drops significantly in the July-August period at Mt. Hopkins, a period that corresponds to the southern Arizona "monsoon" season when atmospheric water vapor content is high.
The photometric uncertainties quoted in the 2MASS Spring 1999 Point and Extended Source Catalogs have the contribution of the nightly Photometric Calibration RMS uncertainties incorporated. For the PSC, the calibration uncertainty is combined in quadrature with the measurement errors (msig) and other systemmatic terms in the "msgicom" fields. The uncertainties quoted for all magnitudes in the XSC have the calibration uncertainty combined in quadrature with measurement and other systemmatic errors.
[Last Update: 1999 May 11; by R. Cutri]