The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2009, incorporates a suite of instruments
including the EUV Variability Experiment (EVE). The Multiple EUV Grating Spectrograph (MEGS) channels use
concave reflection gratings to image solar spectra onto CCDs that are operated at -100°C. MEGS provides 0.1nm
spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-
year mission. Characterizations and selection testing of the CCDs and the thin foil filters for SDO EVE have been
performed with both in-band and visible illumination. CCD selection was based on results from testing in LASP facility
Calibration and Test Equipment (CTE3) as well as results from at testingMIT. All CCDs meet the requirements for
electronics gain, flat field, Quantum Efficiency (QE), dark current, reverse clock, CTE, bad pixels and the -120°C
survival test. The thin foil filters selection was based on tests performed at LASP facilities and NIST. All filters provide
>106 attenuation of visible light with the proper EUV transmission needed for order sorting capabilities and are free of
critical pinholes.
The Extreme-Ultraviolet Variability Experiment (EVE) is a component of NASA's Solar Dynamics Observatory (SDO)
satellite, aimed at measuring the solar extreme ultraviolet (EUV) irradiance with high spectral resolution, temporal
cadence, accuracy, and precision. The required high EUV quantum efficiency (QE), coupled with the radiation dose to
be experienced by the detectors during the five year mission (~1 Mrad), posed a serious challenge to the charge-coupled
device (CCD) detectors. MIT Lincoln Laboratory developed the 2048 × 1024 pixel CCDs and integrated them into the
detector system. The devices were back-side thinned and then back surface passivated using a thin, heavily boron-doped
silicon layer grown by molecular beam epitaxy (MBE) at less than 450°C. Radiation-hardness testing was performed
using the Brookhaven National Laboratory's National Synchrotron Light Source (BNL/NSLS). The MBE-passivated
devices were compared against devices with back surfaces passivated with a silver charge chemisorption process and an
ion-implant/furnace anneal process. The MBE devices provided both the highest QE at the required (-100°C) operating
temperatures, and superior radiation hardness, exceeding the goals for the project. Several flight-ready devices were
delivered with the detector system for integration with the satellite.
The highly variable solar extreme ultraviolet (EUV) radiation is the major energy input into the Earth’s upper atmosphere and thus impacts the geospace environment that affects satellite operations and communications. The Extreme ultraviolet Variability Experiment (EVE) aboard the NASA Solar Dynamics Observatory (SDO, to be launched in 2008) will measure the solar EUV spectral irradiance from 0.1 to 105 nm with unprecedented spectral resolution (0.1 nm), temporal cadence (10-sec), and accuracy (10%). The EVE program will provide solar EUV irradiance data for the Living With the Star (LWS) program, including near real-time data products to be used in operational atmospheric models that specify the space environment and to assist in forecasting for space weather operations. The EVE includes several instruments to cover the full EUV range. The Multiple EUV Grating Spectrographs (MEGS) has two grating spectrographs. The MEGS-A is a grazing-incidence spectrograph to measure the solar EUV irradiance in the 5 to 37 nm range with 0.1 nm resolution, and the MEGS-B is a normal-incidence, dual-pass spectrograph to measure the solar EUV irradiance in the 35 to 105 nm range with 0.1 nm resolution. The MEGS channels have filter wheel mechanisms, holographic gratings, and cooled CCD detectors. For in-flight calibration of the MEGS, the EUV SpectroPhotometer (ESP) measures the solar EUV irradiance in broad bands between 0.1 and 39 nm, and a MEGS-Photometer to measure the bright hydrogen emission at 121.5 nm. In addition, underflight rocket experiments are planned on about an annual basis to assure that the EVE measurements have an absolute accuracy of better than 25% over the five-year SDO mission. This paper will describe the optical design of the EVE instrumentation and the plans for pre-flight and in-flight calibrations.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2008, incorporates a suite of instruments including the EUV Variability Experiment (EVE). The EVE instrument package contains grating spectrographs used to measure the solar extreme ultraviolet (EUV) irradiance from 0.1 to 105 nm. The Multiple EUV Grating Spectrograph (MEGS) channels use concave reflection gratings to image solar spectra onto CCDs that are operated at -100°C. MEGS provides 0.1nm spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-year mission. MEGS-A utilizes a unique grazing-incidence, off-Rowland circle (RC) design to minimize angle of incidence at the detector while meeting high resolution requirements. MEGS-B utilizes a double-pass, cross-dispersed double-Rowland circle design. MEGS-P, a Ly-α monitor, will provide a proxy model calibration in the 60-105 nm range. Finally, the Solar Aspect Monitor (SAM) channel will provide continual pointing information for EVE as well as low-resolution X-ray images of the sun. In-flight calibrations for MEGS will be provided by the on-board EUV Spectrophotometer (ESP) in the 0.1-7nm and 17-37nm ranges, as well as from annual under-flight rocket experiments. We present the methodology used to develop the MEGS optical design.
Remote sensing of the atmosphere and the surface of the earth is performed by the Imager and Sounder instruments onboard the GOES (Geostationary Operational Environmental Satellite) Satellites. By employing large PV Hg1CdTe focal plane array (FPA) detectors, instruments like the Advanced Baseline Imager (ABI) and Advanced Baseline Sounder (ABS) will provide improved update times, resolution, and sensitivity. However, uniformity in the pixel geometry across the array must first be demonstrated in order to maintain the accuracy of weather products at each spot on the ground. This uniformity is particularly important in weather products involving radiance subtractions and ratios from multiple spectral bands employing different detectors. Measurement ofthe spatial response associated with a pixel is important in determining both ground resolution and the effect ofradiance from outside the pixel field-of-view. Therefore, a high precision test set-up has been developed at Lincoln Laboratory to measure both the modulation transfer function (MTF) associated with each pixel in the array and the cross-talk from pixel to pixel. Details of the test set up and initial results of the testing will be discussed.
Remote sensing of the atmosphere and surface of the earth is performed by the Imager and Sounder instruments onboard the GOES (Geostationary Operational Environmental Satellite) Satellites. The current versions of these instruments have two and four detectors per band, respectively, that are scanned across the earth. Large photovoltaic, Hg1-xCdx Te Focal Plane Arrays (FPAs) will permit faster coverage, improved resolution, and improved sensitivity for future designs like the Advanced Baseline Imager (ABI) and the Advanced Baseline Sounder (ABS). However, the transition away from the current small number of detectors requires a technology demonstration of the same or better radiometric precision and uniformity across available FPAs. These measurements, using appropriate flux levels, f-numbers, and readout rates for GOES, are underway at MIT Lincoln Laboratory. Both corrected response from pixel to pixel (residual spatial non-uniformity) and temporal stability of each pixel during the calibration period are required to better than 0. 1 K NEdT. The test set-up and the measurements of dark current and signal performance will be discussed for two arrays.
The Emergency GOES Imager study responds to the potential need for a small, back-up imager for weather observations in the event of failure of one or more of the current GOES satellites. The Emergency GOES Imager (EGI) is designed to be compact and lightweight. Minimal spatial resolution is required in the visible and IR band for the purpose of synoptic forecasts. The ground resolution requirement is 16 km for the 10.2 to 11.2 micrometers IR band and 4 km for the 0.5 to 0.7 micrometers visible band. Due to the small size of the instrument, the EGI has the potential to be deployed either alone on a small launcher or as an auxiliary payload on a larger satellite. The overall size of the EGI is dependent on the orientation of the satellite because of the dependence on amount of solar shielding required for the cooler, and the choice of coolers for specific satellite orientations. Although the EGI design is for an emergency system, the design utilizes recent technology in the form of both a linear IR focal plane array, in front of its constant-motion mirror, and a visible CCD array with a staring-format. The IR array has the potential to present a technical challenge to array manufacturers in the area of calibration, assuming a 0.1 K NEDT. We discuss the means by which the emergency requirements are met with this small and simple system, define the limiting technologies in the design, and explore modifications necessary to expand these requirements.
NOAA has commissioned a solar x-ray imager to be built for use on the GOES spacecraft. The mission of the SXI is to provide soft x-ray imagery of the Sun. The current instrument design employs a microchannel plate detector stack to convert the incident x-rays to an electrically detectable signal. In this paper, we discuss the SXI performance improvements possible by replacing the detector with a back-illuminated, x-ray sensitive CCD fabricated using technology developed at MIT/LL. In addition to a description of the x-ray sensitive CCD, we discuss possible improvements in data quality, reduction in instrument mass and power requirements, and simplified instrument handling.
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