The ICON mission is led by the University of California-Berkeley (Space Sciences Laboratory). In the frame of this mission the Space Center of Liege was involved in the optical design optimization and related analysis, and VUV on ground calibration.
ICON FUV is a two channel spectrographic imager that measures intensity and spatial distribution of oxygen (135.6 nm) and molecular nitrogen (157 nm) of the ionosphere. As those wavelengths are strongly absorbed by the atmosphere, the optical elements of the system have to be tested inside vacuum chambers. Prior to the instrument alignment and calibration, two 3600 gr/mm gratings were characterized. The primary focus is the measurement of the diffraction efficiencies; while the second objective is to select the best grating and to define which is the flight and the spare. A dedicated setup has been developed to assess the grating optical performances under vacuum. A 1 cm diameter collimated beam is generated using an off-axis parabola and a UV source at its focal point. The grating is placed at the center of two rotation stages collinearly aligned. One detector is placed on a rotating arm, deported from its rotation center. A PMT detector records diffracted light intensity with respect to its angular position and its wavelength. Angular incidence on the grating is tuned with the help of the second rotation stage. The grating efficiency homogeneity and scattering properties are measured through a Y-X scan.
The optical calibration of the ICON-FUV instrument requires designing specific ground support equipment (GSE). The ICON-FUV instrument is a spectrographic imager that operates on two specific wavelengths in the UV (135.6 nm and 157 nm). All the operations have to be performed under vacuum UV light. The optical setup is based on a VUV monochromator coupled with a collimator that illuminates the FUV entrance slit. The instrument is placed on a manipulator providing fields pointing. Image quality and spectral properties can be then characterized for each field. OGSE, MGSE, optical calibration plan and vacuum alignment of the instrument are described.
In the frame of the ICON (Ionospheric Connection Explorer) mission of NASA led by UC Berkeley, CSL and SSL Berkeley have designed in cooperation a new Far UV spectro-imager. The instrument is based on a Czerny-Turner spectrograph coupled with two back imagers. The whole field of view covers [± 12° vertical, ± 9° horizontal]. The instrument is surmounted by a rotating mirror to adjust the horizontal field of view pointing by ± 30°. To meet the scientific imaging and spectral requirements the instrument has been optimized. The optimization philosophy and related analysis are presented in the present paper. PSF, distortion map and spectral properties are described. A tolerance study and alignment cases were performed to prove the instrument can be built and aligned. Finally straylight and out of band properties are discussed.
The Mesosphere Lower Thermosphere (MLT) is the most inaccessible and least understood region of the Earth's atmosphere. AtmophericGravity waves play a substantial role in the dominant processes of energy and momentum transport in this region. The WAVES mission (proposed as a NASA Midex) will be the first mission dedicated to studying atmospheric gravity waves, their sources and how they affect the MLT. An instrument, the Multi-spectral Limb Photometer (MLP) will make limb viewing observations to support the WAVES mission. This instrument will observe airglow variations caused by longer wavelength waves passing through the region and will make measurements of temperature and composition in various regions in the MLT. The MLP instrument images the earth atmosphere in limb view in the orbital plane. The resolution in the vertical dimension (altitude) is about 2 km. In the horizontal dimension the MLP collects an averaged intensity over a region of 250 km in width. Vertical imaging vs. horizontal non imaging is realized by cylindrical lenses. The stray light baffling design is specially adapted to allow for day and night observation. The MLP is a single optical channel instrument using a CCD sensor. We propose to use a grille filter spectrometer consisting of a telecentric imager in which a set of narrow vertical strip interference filters are included. The image of the limb is projected onto these strip filters preserving the imaging qualities (vertical dimension). With the interference filter it is possible to realize a spectral function fitting with the multiple spectral bandpass of the emitting species. The full wavelength range is 555-892 nm where about 10 emission lines are to be resolved. The instrument sensitivity is adapted to the intensity and spectral spacing (with respect to neighboring emission lines) of each line: spatial and spectral width of each interference filter strip are independently optimized. This is unique compared to spectrograph using grating technology where spectral resolution and sensitivity are undesirably coupled through the slit width. Finally, the CCD sensor captures a composite image where columns depict the vertical limb imaging and rows indicate the spectral signature.
The FUV Spectrographic Imager for IMAGE is simultaneously imaging auroras at 1218 and 1358 angstrom. It is designed to efficiently reject the Lyman-(alpha) emission line at 1215.7 angstrom. This paper describes the optical calibration. The content is: 1) field of view calibration: detector pixels location with respect to the reference optical cube; distortion matrix used to computer the TDI. b) Radiometric calibration: detector response and linearity; instrument throughput according to its clear aperture and mirror reflection lost; response vs. wavelength and band-rejection certification.
The FUV Spectrometer Imager for IMAGE is designed to simultaneously take aurora images at 1218 and 1356 angstrom. This paper describes the alignment procedure and performance results. The Spectrograph alignment requires to efficiently reject the Lyman-(alpha) line at 1216 angstrom. The imager alignment requires to tune optical components until finest imaging.
A system of three identical CCD-cameras was developed enabling stereoscopic auroral observations. An image intensifier allows for real-time imaging of auroral arcs with interference or broad-band filters. The combination of a small-angle optics with a CCD-chip of 756 by 580 pixels provides spatial resolutions of auroral small-scale structures down to 20 m. The cameras are controlled by personal computers with integrated global positioning (GPS) modules enabling time synchronization of the cameras and providing the exact geographical position for the portable cameras. Calibration with a standard light source is the basis for quantitative evaluation of images by image processing techniques. The current technical development is the combination with local operating networks (LON) for monitoring camera parameters like voltage and temperature and remote control of parameters like filter positions, mounting tilt angles and camera gain.
The mathematics of emission computed tomography is applied to the three-dimensional reconstruction of the optical emission within an auroral arc. According to general experimental conditions, a very limited angular range and a small number of observers require an iterative back projection method. Parameters for the quantitative correspondence between the original and reconstructed volumes and between the images are defined and using this method, a theoretical arc can be reconstructed with root- mean-square errors of the images of less than 2%. The reconstruction accuracy of the volume can be improved with an increasing number of observers to root-mean-square errors of about 15%. Different geometries are tested but the best results are obtained as long as one observer looks along the magnetic field line through the auroral arc. The calculations confirm the range of suitable observation geometries to within 20 km from the field line through one of the observers.
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