Richard A. Kohnert

Very high x-ray efficiency from a blazed grating

The efficiency of X-ray diffraction from reflection gratings has been investigated experimentally using a gold-coated plane grating of 25 x 120 mm with a nominal blaze angle of 21 deg in a conical diffraction mount. Results show that conical diffraction is capable of producing very high dispersion efficiencies. The peak of first order at 13.3 A is 40%; at 44 A, the peak is 34%. The most immediate application of this result is to X-ray astronomy where sources are weak.

Optical design of the Ultraviolet Imaging Spectrograph for the Cassini mission to Saturn

When the Cassini spacecraft arrives at Saturn early in the next century it will carry an Ultraviolet Imaging Spectrograph (UVIS) designed and built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. Observations made with the UVIS will support a broad set of scientific investigations including spectroscopy, imaging, and occultations. The UVIS consists of two spectroscopic channels covering the wavelength ranges 56 to 118 and 110 to 190 nm. Each channel has an off-axis parabolic telescope followed by a toroidal grating spectrograph and an imaging microchannel plate-CODACON detector. Mirror coatings and detector photocathode materials optimize the sensitivity of each channel for its particular wavelength range. Spectrograph entrance slit mechanisms provide three independent spectral and spatial resolution modes for each of the three channels. A third optical train consisting of a parabolic telescope and solar blind photomultiplier tube with a CsI photocathode provides a high-sensitivity photometer mode within the UVIS. The UVIS configuration was selected as a balanced solution to a large number of engineering and scientific constraints. We describe these constraints, the optical design, and the anticipated performance of the instrument.

Science Instrumentation for the Student Nitric Oxide Explorer

The student nitric oxide explorer (SNOE) is a small satellite to be designed built and operated at the University of Colorado under the student explorer demonstration initiative from the Universitys Space Research Association (STEDI). The goal of the STEDI program is to demonstrate that low cost satellite missions can be done with large student involvement. The primary science goals of SNOE are to measure thermospheric nitric oxide (NO) and its variability over the lifetime of the mission. SNOE will also monitor the solar irradiance at soft x-ray wavelengths and the auroral energy deposition at high latitudes. Three science instruments are required to achieve the simultaneous measurements: an ultraviolet spectrometer for NO; a solar soft x-ray photometer; and a far ultraviolet photometer for studying the aurora. The instruments are designed to represent a minimum impact on the spacecraft, particularly in terms of data storage and interactions with the command and data handling system. The focus of this paper is the outline of the design of the science instruments. We discuss why these instruments are well suited for smaller, lower cost satellite missions.

SDO-EVE Multiple EUV grating Spectrograph (MEGS) optical design

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.

The EUV Variability Experiment (EVE) aboard the NASA Solar Dynamics Observatory (SDO)

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.

New Solar Irradiance Measurements from the Miniature X-Ray Solar Spectrometer Cubesat

The goal of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is to explore the energy distribution of soft X-ray (SXR) emissions from the quiescent Sun, active regions, and during solar flares, and to model the impact on Earths ionosphere and thermosphere. The energy emitted in the SXR range (0.1 --10 keV) can vary by more than a factor of 100, yet we have limited spectral measurements in the SXRs to accurately quantify the spectral dependence of this variability. The MinXSS primary science instrument is an Amptek, Inc. X123 X-ray spectrometer that has an energy range of 0.5--30 keV with a nominal 0.15 keV energy resolution. Two flight models have been built. The first, MinXSS-1, has been making science observations since 2016 June 9, and has observed numerous flares, including more than 40 C-class and 7 M-class flares. These SXR spectral measurements have advantages over broadband SXR observations, such as providing the capability to derive multiple-temperature components and elemental abundances of coronal plasma, improved irradiance accuracy, and higher resolution spectral irradiance as input to planetary ionosphere simulations. MinXSS spectra obtained during the M5.0 flare on 2016 July 23 highlight these advantages, and indicate how the elemental abundance appears to change from primarily coronal to more photospheric during the flare. MinXSS-1 observations are compared to the Geostationary Operational Environmental Satellite (GOES) X-Ray Sensor (XRS) measurements of SXR irradiance and estimated corona temperature. Additionally, a suggested improvement to the calibration of the GOES XRS data is presented.

Optical design of the ultraviolet imaging spectrograph for the Cassini mission to Saturn

When the Cassini spacecraft arrives at Saturn early in the next century it will carry an UltraViolet Imaging Spectrograph (IJVIS) designed and built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. Observations made with the UVIS will support a broad set of scientific investigations including spectroscopy, imaging, and occultations. The UVIS consists of three spectroscopic channels covering the wavelength ranges 55—i 15 nm, 1 15—190 nm, and 160— 320 nm. Each channel has an off-axis parabolic telescope followed by a toroidal grating spectrograph and an imaging microchannel plate-CODACON detector. Mirror coatings and detector photocathode materials optimize the sensitivity of each channel for its particular wavelength range. Spectrograph entrance slit mechanisms provide four independent spectral and spatial resolution modes for each of the three channels. A fourth optical train consisting of an off-axis parabolic telescope and solar blind photomultiplier tube with a CsI photocathode provides a high sensitivity photometer mode within the UVIS. The UVIS configuration was selected as a balanced solution to a large number of engineering and scientific constraints. We describe these constraints, the optical design, and the anticipated performance of the instrument.

Colorado Ultraviolet Transit Experiment: a dedicated CubeSat mission to study exoplanetary mass loss and magnetic fields

Abstract. The Colorado Ultraviolet Transit Experiment (CUTE) is a near-UV (2550 to 3300  Å) 6U CubeSat mission designed to monitor transiting hot Jupiters to quantify their atmospheric mass loss and magnetic fields. CUTE will probe both atomic (Mg and Fe) and molecular (OH) lines for evidence of enhanced transit absorption, and to search for evidence of early ingress due to bow shocks ahead of the planet’s orbital motion. As a dedicated mission, CUTE will observe ≳100 spectroscopic transits of hot Jupiters over a nominal 7-month mission. This represents the equivalent of >700 orbits of the only other instrument capable of these measurements, the Hubble Space Telescope. CUTE efficiently utilizes the available CubeSat volume by means of an innovative optical design to achieve a projected effective area of ∼28  cm2, low instrumental background, and a spectral resolving power of R∼3000 over the primary science bandpass. These performance characteristics enable CUTE to discern transit depths between 0.1% and 1% in individual spectral absorption lines. We present the CUTE optical and mechanical design, a summary of the science motivation and expected results, and an overview of the projected fabrication, calibration, and launch timeline.

The Colorado Ultraviolet Transit Experiment (CUTE): a dedicated cubesat mission for the study of exoplanetary mass loss and magnetic fields

The Colorado Ultraviolet Transit Experiment (CUTE) is a near-UV (2550 - 3300 Å) 6U cubesat mission designed to monitor transiting hot Jupiters to quantify their atmospheric mass loss and magnetic fields. CUTE will probe both atomic (Mg and Fe) and molecular (OH) lines for evidence of enhanced transit absorption, and to search for evidence of early ingress due to bow shocks ahead of the planet’s orbital motion. As a dedicated mission, CUTE will observe ⪆ 60 spectroscopic transits of hot Jupiters over a nominal seven month mission. This represents the equivalent of > 700 orbits of the only other instrument capable of these measurements, the Hubble Space Telescope. CUTE efficiently utilizes the available cubesat volume by means of an innovative optical design to achieve a projected effective area of ∼ 22 cm2 , low instrumental background, and a spectral resolving power of R ∼ 3000 over the entire science bandpass. These performance characteristics enable CUTE to discern a transit depth of ⪅1% in individual spectral absorption lines. We present the CUTE optical and mechanical design, a summary of the science motivation and expected results, and an overview of the projected fabrication, calibration and launch timeline.

Photomultiplier tube detector performance and stability for the Earth Observing System's SOLSTICE II instrument

The goal of the Earth Observing System (EOS) SOLar STellar Irradiance Comparison Experiment II (SOLSTICE II) is to measure the solar ultraviolet irradiance (115 nm - 320 nm) to within 5% of its absolute value with a 0.5% per year relative accuracy over the course of a minimum mission lifetime of five years. Most detectors degrade over time while studying the sun. The SOLSTICE instrument design is such that detector and optical system degradation is tracked by routinely observing a series of stable early-type stars. Any changes in the system may then be removed from the solar irradiance. Detector performance and stability lies at the heart of SOLSTICE experimental success. The SOLSTICE detectors are Hamamatsu R2078 PhotoMultiplier Tubes (PMTs). We have developed an integrated PMT package [PMT, PMT housing, (mu) -metal magnetic shield, high voltage divider, and pulse-amplifier discriminator (PAD)] that will achieve our performance objectives. We report here on both the design of the integrated detector package and the laboratory measurements of the operational lifetime performance characteristics of SOLSTICE detectors. These include pulse height distribution, quantum efficiency, photocathode surface uniformity, and magnetic susceptibility.