Extreme Ultra Violet Imaging Spectrograph (EUV)
EUV – Science
The primary objective for the EUV profiler is to determine the altitude profile of the dayside ionospheric density through limb imaging in the extreme ultraviolet range.
One of the most prominent terrestrial airglow features is the OII 83.4 nm triplet emission. The strongest “initial source” of photons is solar photoionization of neutral atomic oxygen around 160-175 km [Meier, 1991]. The Sun also emits 83.4 nm photons, serving as a secondary source whose impact on the dayside airglow increases with altitude above the ionospheric peak. The 83.4 nm photons propagating upward from the initial source or downward from the Sun undergo resonant scattering by O+, the dominant positive ion within the F-region, and capture the shape of the F-region ion distribution. This is illustrated in Figure 1.
Figure 1 – The ICON EUV instrument measures the limb profiles of 61.7 nm and 83.4 nm airglow to determine the altitude profile of the daytime ionospheric density.
EUV – Heritage
This EUV instrument follows the heritage of four UCB “EURD-class” astrophysical space flight instruments (Figure 2). These astrophysical instruments were designed for detecting the same wavelengths as the ICON-EUV instrument, but for signals that are an order of magnitude less bright than the Earth’s airglow. The interpretation of the EUV dayglow builds on the recent success of the SSULI and RAIDS missions.
EUV – How it works
The ICON EUV spectral profiler is illustrated in Figure 3. The EUV instrument measures these emissions with a straightforward grating spectrograph that provides spectral information in one dimension (horizontal) on the 2D detector, and altitude distribution of the emission brightness in the other dimension (vertical).
The EUV uses a simple “vertical push broom” one-dimensional imaging spectrograph of UCB design that consist of a single diffraction grating that directly views a wedge of sky through a fixed slit aperture. The large difference in the vertical and horizontal curvature of the grating (toroidal design) permits the simultaneous vertical focusing of the scene located at great distances outside of the instrument and the horizontal focusing of the input slit located at the instrument entrance on the detector. The view wedge is dispersed and imaged into a spectrum per field angle on a photon-counting detector as shown in Figure 3. The simplicity of a single-optic design results in a highly efficient instrument and eases alignment and calibration issues compared to multi-optic designs.
Figure 3 – The ICON EUV profiler.
EUV – The Details
The ICON EUV features an enclosed aluminum structural cavity withstanding 1 atm. vacuum that acts as the optical bench and contains the diffraction grating and the open-faced microchannel plate detector. The instrument cavity can be hermetically sealed with a one-time operable flap valve actuated in flight by a shape memory alloy mechanism. A field-of-view entrance baffle extends beyond the optical entrance to eliminate solar panel glints and minimize sun-pointing constraints. To reject entrance of low-energy ions into the optical cavity that could be accelerated into the MCP detector and cause excessive background noise, the hermetic optical cavity uses a low-voltage electric field applied to the slit and to fine metal grids placed over the entrance baffle, the space-venting aperture, and the detector face inside the cavity.
The horizontal field (12°) and the instrument scale set the ruled width of the grating to 40mm. A 40mm tall entrance slit, the imaging-direction pupil, and the imaging angle (16°) sets the 90mm grating height. The EURD-type optical theory, given a 3000 l/mm ruling, estimates the toroidal figure of the grating (R=176 mm, ρ=336 mm), incident angle (13.7°), and the optimum slit-to-grating distance and detector-to-grating incident angles (177 and 171 mm, respectively). Numerical ray tracing shows that the spectral resolution requirements allow a slit width of 890 µm which yields the per pixel geometric factor to 6.3×10-4 cm2 sr while maintaining a clean separation between 58.4 nm and 61.7 nm.
The glass diffraction grating uses straight holographic rulings blazed for 61.7nm. Similar to one of the previous EURD-Class instruments, SPEAR, the grating surface will be coated with chromium for ruling acceptance testing, and then over coated with boron carbide (B4C). Like SPEAR, the grating is adhesively mounted to a titanium block that interfaces to the structure via a compliant three-point mount. The points use spherical bearings on threaded mount rods to allow for remote, mechanized, optical alignment in an UV vacuum test chamber during ground tests. A set of internal baffle plates are arranged to block zero order radiation and stray diffraction grating orders from bright airglow lines.
The detector will use an open microchannel plate z-stack with a crossed delay line (XDL) detector (< 150 μm electronic resolution). We have flown these detectors on numerous missions including several rockets. The instrument electronics include a high voltage power supply attached to the cavity housing. A detector electronics module containing front end amplifiers and dual (X,Y) time delay to digital converters (TDCs) and a digital interface card are located in the XDL electronics stack. We use an existing electronics module that has been developed through the PKB-ALICE, LRO-ALICE, SSULI (DMSP), and JUNO-UVS flight programs.
EUV – References
Edelstein, J. et al., EURD observations of interstellar radiation, Astrophysics and Space Science, 276, 177-185, 2001.
Meier, R.R., Ultraviolet Spectroscopy and Remote Sensing Of The Upper Atmosphere, Space Sci. Rev., 58, 1, 1991.