same star taken at different airmass, all stellar photospheric absorption features should directly cancel, leaving only those absorption features due to the earth’s atmosphere. If we assume that the opacity of the telluric absorption is directly proportional to airmass we derive: $$\tau(\sigma,X = 1.0) = \frac{1}{(X_{high}-X_{low})} \times ln[ \frac{I(\sigma,X_{low})}{I(\sigma,X_{high})}]$$ where $\tau$ is the atmospheric opacity, $X_{low}$ is the low airmass value, and $X_{high}$ is the high airmass value. A typical opacity map derived in this way for the H–band is shown in Figure \[fig2\]. Several of the features in this map identified with known constituents of the earth’s atmosphere such as water vapor, methane, and carbon dioxide, are denoted in Figure \[fig2\]. Again if the atmospheric opacity varies linearly with airmass we can simply scale the opacity for each star so that $\tau(\sigma,X) = X \times \tau(\sigma,X=1.0)$. Using this technique we corrected the spectra to zero–airmass; $$I(\sigma,X=0.0) = I(\sigma,X) \times e^{\tau(\sigma,X)}$$ We used the highest signal–to–noise A0 standard star spectra ($SNR > 100$) with the largest $\Delta X$ to define the opacity. We found some residual telluric absorptions, possibly due to water vapor which do not vary strictly with airmass. Such features severely complicate the reduction of the J–band spectra.

