Calibrator PSF residual width (in pixels) vs. atmospheric conditions. The residual widths are calculated as follows: (1) each column (i.e., spectral coordinate; see Fig. 3) of the detector is fitted with a Gaussian to estimate the width of the PSF. (2) The fitted widths are then fitted with a first-order polynomial (ideally, λ/D). (3) The fitted polynomial is then subtracted from the fitted widths to generate a residual widths vector -which represents the nonlinear deviations of the fitted widths. Left Panel: PSF residual width vs. airmass during observation for the water vapor and ozone bands. Right Panel: PSF residual width vs. precipitable water vapor (PWV) during observation for the water vapor and ozone bands.