ANIR: Atacama near-infrared camera for the 1.0 m miniTAO telescope

We have developed a near-infrared camera called ANIR (Atacama Near-InfraRed camera) for the University of Tokyo Atacama Observatory 1.0 m telescope (miniTAO) installed at the summit of Cerro Chajnantor (5640 m above sea level) in the north of Chile. The camera provides a field of view of 5'.1 x 5'.1 with a spatial resolution of 0 ''.298 pixel(-1) in the wavelength range of 0.95 to 2.4 mu m, using Offner relay optics and a PACE HAWAII-2 focal plane array. Taking advantage of the dry site, the camera is capable of narrow-band imaging observations at the hydrogen Paschen-alpha (Pa alpha, lambda = 1.8751 mu m in air) wavelength ground-based observations of which have been quite difficult due to deep atmospheric absorption, mainly from water vapor. We have been successfully obtaining Pa alpha images of Galactic objects and nearby galaxies since the first-light observation in 2009 with ANIR. The throughputs at the narrow-band filters (N1875, N191) including the atmospheric absorption show larger dispersion (similar to 10%) than those at broad-band filters (a few percent), indicating that they are affected by temporal fluctuations in precipitable water vapor (PWV) above the site. We evaluate the PWV content via the atmospheric transmittance at the narrow-band filters, and deduce that the median and the dispersion of the distribution of the PWV are 0.40 +/- 0.30 and 0.37 +/- 0.21mm, for the N1875 and N191 data respectively, which are remarkably smaller (49% +/- 38% for N1875 and 59% +/- 26% for N191) than radiometry measurements at the base of Cerro Chajnantor (an altitude of 5100 m). The decrease in PWV can be explained by the altitude of the site when we assume that the vertical distribution of the water vapor is approximated at an exponential profile with scale heights within 0.3-1.9 km (previously observed values at night). We thus conclude that miniTAO/ANIR at the summit of Cerro Chajnantor indeed provides us an excellent capability for a ground-based Paa observation.