Studies of millimeter-wave atmospheric noise above mauna kea

We report measurements of the fluctuations in atmospheric emission (atmospheric noise) above Mauna Kea recorded with Bolocam at 143 and 268GHz from the Caltech Submillimeter Observatory. The 143GHz data were collected during a 40 night observing run in late 2003, and the 268GHz observations were made in early 2004 and early 2005 over a total of 60 nights. Below ≃0.5 Hz, the data time-streams are dominated by atmospheric noise in all observing conditions. The atmospheric noise data are consistent with a Kolmogorov-Taylor turbulence model for a thin wind-driven screen, and the median amplitude of the fluctuations is 280mK² rad^-5/3 at 143GHz and 4000mK ² rad^-5/3 at 268GHz. Comparing our results with previous ACBAR data, we find that the normalization of the power spectrum of the atmospheric noise fluctuations is a factor of ≃80 larger above Mauna Kea than above the South Pole at millimeter wavelengths. Most of this difference is due to the fact that the atmosphere above the South Pole is much drier than the atmosphere above Mauna Kea. However, the atmosphere above the South Pole is slightly more stable as well: the fractional fluctuations in the column depth of precipitable water vapor are a factor of smaller at the South Pole compared to Mauna Kea. Based on our atmospheric modeling, we developed several algorithms to remove the atmospheric noise, and the best results were achieved when we described the fluctuations using a low-order polynomial in detector position over the 8′ field of view. However, even with these algorithms, we were not able to reach photon-background-limited instrument photometer performance at frequencies below ≃0.5 Hz in any observing conditions. We also observed an excess low-frequency noise that is highly correlated between detectors separated by ≲(f/#)λ; this noise appears to be caused by atmospheric fluctuations, but we do not have an adequate model to explain its source. We hypothesize that the correlations arise from the classical coherence of the electromagnetic field across a distance of ≃(f/#)λ on the focal plane.