Aerospace Nightglow Imager (ANI)
ANI provides intensity and temperature measurements of the atmospheric OH (hydroxl) emission. It combines a near-infrared camera with a custom wide-angle lens together with a 256 x 256 HgCeTe detector array to provide images over a 80 x 80 o region of the sky. The detector is also a NICMOS3 device. The wide-angle lens, which consists of ten separate AR coated elements, provides an f/5, telecentric output. This output is passed through a 4-position filter wheel, to select individual OH lines or the nearby continuum. The light then enters the cryogenic enclosure where a 6-element re-imaging system (also AR-coated and optimized for transmission between 1.2 and 1.8 microns) reduces the image size to the 1-cm dimensions of the array. At the detector surface the optics are operating at f/1.3. Within the camera cell is a broad bandpass astronomical "H" filter (1.55-1.75 microns) which transmits the output of the external filterwheel but removes most of the thermal background.
The heart of ANI, the cryostat that houses the infrared detector, camera optics, and refrigeration device that cools the detector to approximately 80 K. The other principal part of ANI, which is not shown, is the wide field infrared lens system that forms an image of the sky at the large entrance window shown in the figure. |
In order to facilitate remote observations for extended periods when no servicing is possible, the entire optics/detector train is cooled by a commercial cryocooler. The device uses the free expansion of gas (Joule-Thompson effect) to provide an essentially vibrationless cooling head. A thermal switch links the cooling head to the detector mount. It, in turn, is connected through a thermal insulator to the optics. This arrangement provides for the two very different temperatures needed to optimize performance while minimizing the cooling requirements (Tdetector = 80 K, Toptics = 152 K). Two narrowband filters (FWHM = 0.004 microns) isolate the first two OH P-branch transitions of the n = 4 to n = 2 vibrational band (at 1.6026 and 1.6124 microns), while a third at 1.5750 microns samples the continuum.
