Midwave Infrared Imaging Spectrograph (MIRIS)
The Midwave Infrared Imaging Spectrograph (MIRIS) was initially developed as a proof-of-concept instrument to test the feasibility of producing a midwave infrared spectrograph using binary-optic prisms fabricated using semiconductor etching. Having successfully demonstrated the practicality of using such a prism as a dispersing element, MIRIS was developed into an operational instrument which was optimized for the study of transient events with a time resolution as short as 1 msec. The first fully operational use of MIRIS was to observe the 1998 Leonid meteor storm as part of the NASA Leonid Multi-instrument Airborne Campaign (MAC), although MIRIS had been used in earlier attempts to detect Perseid and Leonid meteors from the ground during its engineering development. Since 1998, MIRIS has been used to observe the 1998 Leonid meteor storm and in as program of 3-5.5 um spectroscopy of bright near IR stars. During 1998 and 1999 MIRIS obtained infrared detections of Leonid meteors and made observations of Leonid persistent trains.
MIRIS mounted on FISTA during the 1998 Leonis meteor storm. An intensified video camera is mounted on top of the instrument to provide visible reference images of the field of view. The gold-colored case contains the majority of the optical elements. The IR detector array is locaed in the smaller grey dewar on the left side of the case. |
Components
MIRIS consists of three modules: uncooled refractive collecting optics (camera lens); cooled reimaging optics; and an InSb detector array system (camera head). The camera head and reimaging optics can be cooled independently of each other, and the system can be operated with warm reimaging optics for alignment and testing. The vacuum spaces for the camera head and reimaging optics dewar can be configured to be open to each other or isolated.
The camera lens is made up of three refractive elements with an effective focal length of 228 mm and a speed of f/15. Two of the elements are Si and the third is Ge. The individual lens elements that make up the camera lens -- and the reimaging optics also -- are antireflection coated for the 3-5.5 micron bandpass of the instrument.
A number of different masks are available for placement in the camera lens focal plane to restrict the field of view including a conventional set of slit jaws that allows the system to be operated as a long slit spectrograph. In this mode the slit length fills the 12-degree field of view and the slit width is adjustable, but only prior to cooling the instrument. The zeroth order image of the slit is imaged on the right hand side of the detector array and the first order spectrum on the left hand side of the array. The zeroth order image of the slit can be used to guide on the source during long integrations. The system has been successfully used on the Mt. Lemmon 60-inch telescope and the Mt. Wilson 60-inch telescope in this mode of operation for astronomical observations.
Both the spectral and spatial resolution are determined by the pixel size. Using an Amber model 4256 camera head, the spectral resolution is 0.025 microns and the spatial resolution is 1 mrad. With an Amber Radiance HS camera head, the spectral resolution is 0.02 microns and the spatial resolution is 0.8 mrad. The measured single pixel system sensitivity using the model 4256 camera head in zeroth order is 1.1 x 10-14 W for a signal-to-noise ratio of 1 in 1 second. Both camera heads can be operated simultaneously. The Radiance camera is currently used in MIRIS while the 4256 camera head provides reference 2-2.5 um imagery.
