Imaging Spectrograph Design and Operation
The scanning imaging spectrograph (SIS) subassembly consists of a cross track scanning mirror at the input to the telescope and spectrograph optics. At the focal plane of the spectrograph are redundant two-dimensional photon-counting detectors. The detectors employ a position sensitive anode to determine the photon event location. While not quite correct, we refer to the quantization of the position determination on the detector as defining a "pixel". The resolution of the detector can be increased by increasing the detector gain.
The imaging spectrograph builds multispectral images by scanning spatially across the satellite track as shown in the figure above. One dimension of the detector array contains 16 spatial pixels (along the spacecraft track), and the other dimension consists of 160 spectral bins over the range of 115 to 180 nm. The scan mirror sweeps the 16 spatial pixel footprint from horizon to horizon perpendicular to the spacecraft motion, producing one frame of 16 cross-track lines in 22 seconds. Simultaneous image frames are generated over the entire wavelength range in the imaging mode, but the data rate allocation limits the downlinked image data to five different wavelength intervals or "colors".
The imaging mode scan cycle consists of a limb viewing section followed by an Earth viewing section. Limb viewing pixels are collected from -72.8° from nadir (the start of scan) to -63.2° from nadir. The limb viewing section has a cross track resolution of 0.4° per pixel, and consists of 24 cross track pixels by 8 along track pixels at five wavelengths. The 8 along track pixels are formed by co-adding adjacent pixels in the 16 spatial pixel footprint. At -72.8° from nadir and a spacecraft altitude of 830 km, the spectrograph will view approximately 520 km above the horizon. One should note that the same pixel on the limb is resampled three times on each orbit due to the wide horizontal field-of-view.
The Earth viewing section has a cross track resolution of 0.8° per pixel, and always contains 16 along track pixels and five colors. The number of cross track pixels depends on whether the spacecraft is flown with a GLOB (Glare Obstructor). Currently, only the noon-midnight orbit does not fly with a GLOB. With no GLOB, a full Earth scan from -63.2° from nadir to +61.6° from nadir is performed, and contains 156 cross track pixels. If the GLOB is present, then a reduced scan from +63.2° from nadir to +42.4° from nadir is performed, with 132 cross track pixels.
In the spectrograph mode, the scan mirror is held at a fixed viewing angle (normally either the nadir direction for "ground truth" or on the limb for star calibrations). The along track dimension of the detector array is binned into 6 spatial pixels. Spectral data from all 160 bins are produced for the 6 spatial pixels every 3.0 seconds. The six spatial pixels are contained within the center most 8.88°rees of the 11.84°ree instantaneous along track field of view. The 6 along track pixels are formed by co-adding adjacent pixels in the center most 12 of the 16 spatial pixel footprint. The entire spectrum, consisting of all 160 bins, can be downlinked in the allocated spare words in the OLS data stream. We only send down the central six pixels since for this orbit (830km altitude circular) the FOV moves about one pixel in three seconds. The spectrograph mode (in which the entire spectrum is downlinked) is used predominantly during stellar calibration operations and for "ground truth" campaigns in which we will stare at the radiating volume above a ground site.
The imaging spectrograph contains three entrance slits of varying widths. The intermediate width ("MEDIUM") slit is intended for use during imaging mode operation. The widest slit would be used in imaging mode to increase the sensitivity should the optical efficiency of the system decrease over time or to minimize the statistical error for low count rate scenes such as when the FUV nightglow is to be observed. The narrowest slit improves the spectral resolution. Any slit can be used in any mode of operation. Furthermore, the slit mechanism is designed so that two motors must fail (they are independent) in a specific (i.e. "closed") mode in order for the aperature to be "shuttered". The expected failure mode would be one would leave with a fixed open slit. To date, there have been no failures of the slit mechanism.
The following table summarizes the SIS design performance characteristics. Note that normally imaging mode uses the 0.30° slit and that to reduce the size of the table we have indicated the "normal" slit for the spectrograph mode as being the narrowest (.18°) even though any one of the three slit widths can be used for either of the two modes.
|SSUSI Imaging Spectrograph Performance Characteristics|
|Parameter||Imaging (full scan)||Imaging (reduced)||Spectrograph|
|Instantaneous Field of View||cross track (normal mode)||0.30°||0.30°||0.18°|
|Pixel Field of View||Earth Limb||cross track (normal)||0.30°||0.30°||-|
|Earth Disk||cross track (normal)||0.30°||0.30°||0.18°|
|Scanned Field of View||Earth Limb||cross track||9.6°||9.6°||-|
|Earth Disk||cross track||124.8°||105.6°||-|
|Spatial resolution at nadir||cross track||10 km||10 km||2.3 km|
|along track||10 km||10 km||20 km|
|Pixel Step Period||Earth Limb||0.112 sec||0.156 sec||-|
|Earth Disk||0.112 sec||0.112 sec||-|
|Spectral Resolution||narrow slit||1.2 nm||1.2 nm||1.2 nm|
|normal slit||1.9 nm||1.9 nm||1.2 nm|
|wide slit||4.2 nm||4.2 nm||4.2 nm|
|Sensitivity (counts/sec/Rayleigh)||121.6 nm||0.016||0.016||0.019|
|Data Frame||Data Rate||3816 bits/sec||3816 bits/sec||3816 bits/sec|
|Data Frame Period||22 sec||22 sec||3.0 sec|
|Spatial pixels per frame||Earth Limb||cross track||24||24||-|
|Earth Disk||cross track||156||132||1|
|Output Word Size||Earth Limb||6 bits||7 bits||-|
|Earth Disk||6 bits||7 bits||10 bits|
The SIS consists of a cross-track scanning mirror at the input to a telescope (a 75-mm focal length off-axis parabola system with a 25 mm x 50 mm clear aperature) and a Rowland circle spectrograph. The SIS is an f/3 system with a spherical toroidal grating. The optical path incorporates baffles to prevent stray light from reaching the focal plane at the slit and the detector. The telescope mirror can not see any surfaces, other than baffle knife edges, that are illuminated by sources beyond the entrance opening of the spectrograph. The optical surfaces are coated with ARC Coating #1200 or ARC Coating #1600 to tune the system performance to the observational requirements of Table 1.
Figure 3 shows two views of the SIS. Figure 3a is a schematic side view. The scan mirror feeds the off-axis parabola and the spherical toroidal grating. Two detectors lie at the focal plane. Figure 3b shows a labeled photo of the SIS SN01 housing.