There are 4 major pieces of the MIGHTI instrument: the baffle, the interferometer, the infrared filters and the CDC camera.
MIGHTI measures the winds between 90 and 300 km altitude by observing the red and green emissions from oxygen on the Earth’s horizon (limb). However, reflected sunlight from cloud-tops, which can be over 10 km from the surface of the Earth, is much brighter than the red and green emissions from oxygen that MIGHTI is trying to measure. The long baffle (over 3 feet in length) acts to keep the bright sunlit cloud-tops out of the field of view of the instrument.
Design model of the MIGHTI instrument. Light enters though the baffle, and is modulated by a temperature stabilized interferometer. A cooled CCD camera detects the fringe patterns to retrieve the wind profiles and the multispectral band signals to retrieve the temperature profiles
The interferometer is the heart of the MIGHTI instrument. By splitting the incoming light into two channels, with different path-lengths, and mixing them back together, the MIGHTI interferometer forms what is called an interferogram, which is comprised of dark and bright stripes. The position of these changes with the Doppler shift of the emission line observed, allowing each MIGHTI unit to measure the line-of-sight wind.
Conventional Michelson interferometers require mechanical stepping of a mirror to sample four or more path-length differences. The Spatial Heterodyne Spectroscopy (SHS) approach for MIGHTI results in an improved, more rugged design, replacing the Michelson mirrors with fixed, tilted diffraction gratings. Fundamentally, each facet or groove of the tilted gratings can be regarded as a separate interferometer mirror, each representing a unique path difference, which permits the sampling of many path differences without moving interferometer parts. This eliminates the need for precision mirror stepping, which simplifies both the instrument development and on-orbit operation. The SHS technique was demonstrated by both SHIMMER and the Redline DASH Demonstration Instrument (REDDI), two of the heritage instruments [Englert et al., 2010a,b, 2012; Harlander et al., 2003, 2010]. The basic design of one of the two MIGHTI optical units is shown below (Figure 3 : Left Panel).
The band shape measurement for the temperature retrieval is achieved using a set of five narrow band filters in front of the MIGHTI CCD camera, as shown in Figure 3 (Right Panel).
To identify the small movement of the interference fringes that are formed by the MIGHTI instrument, MIGHTI employs an advanced CCD camera. Noise associated with the electronics of the camera must be kept to a minimum to prevent misidentification of the wind, and this is achieved by cooling the CCD camera to around -40 F.
Modeled airglow and calibration source signals on the MIGHTI detectors.