Larry D. Gardner

Stray light, radiometric, and spectral characterization of UVCS/SOHO: laboratory calibration and flight performance

The Ultraviolet Coronagraph Spectrometer is one of the instruments on board the Solar and Heliospheric Observatory spacecraft, which was launched in December, 1995. The instrument is designed to make ultraviolet spectrometric measurements and visible polarimetric measurements of the extended solar corona. Prior to launch laboratory measurements were carried out to determine system level values for many of the key performance parameters. Further measurements on instrument performance have been carried out since launch. Presented are descriptions of measurement techniques and representative results.

Stray-light suppression in a reflecting white-light coronagraph

An analysis of stray-light suppression in the white-light channel of the Ultraviolet Coronagraph Spectrometer experiment for the Solar and Heliospheric Observatory is reported. The white-light channel consists of a reflecting telescope with external and internal occultation and a polarimeter section. Laboratory tests and analytical methods are used to perform the analysis. The various stray-light contributions are classified in two main categories: the contribution from sunlight that passes directly through the entrance aperture and the contribution of sunlight that is diffracted by the edges of the entrance aperture. Values of the stray-light contributions from various sources and the total stray-light level for observations at heliocentric heights from 1.4 to 5 solar radii are derived. Anticipated signal-to-stray-light ratios are presented together with the effective stray-light rejection by the polarimeter, demonstrating the efficacy of the stray-light suppression design.

Efficiency variations of UVCS/SOHO based on laboratory measurements of replica gratings

We have carried out measurements of efficiency as functions of position across the surfaces of replica grating made from the same masters as the UVCS/SOHO flight units. Variations in first order efficiency which significantly affect the interpretation of UVCS data are found along the direction perpendicular to the grooves. Variations are also found along the direction parallel to the grooves, but these do not seriously affect UVCS data interpretation. The measurements and their application to the radiometric calibration of UVCS/SOHO are discussed.

Grating stray light analysis and control in the UVCS/SOHO

The Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric (SOHO) mission has been developed for spectroscopic and polarimetric determinations of temperatures, densities and flow velocities in the extended solar corona. The instrument consists of a visible light (VL) polarimeter, and two ultraviolet (UV) spectrometers, optimized for the H I Lyman (alpha) (Ly-(alpha) ) line ((lambda) 1216 angstrom) and for the O VI doublet ((lambda) (lambda) 1032/1037 angstrom). The stray-light profile of the Ly-(alpha) holographic grating has been measured, in a laboratory set-up, in both the spatial and spectral directions. The observed profile has been found to be a combination of two components: the scattering from the grating surface, and the Fraunhofer diffraction due to the vignetting of the grating. An analytical expression for the scattering component of the grating point spread function (PSF) has been derived from a simple model of the grating surface roughness. The stray-light profile generated by the analytical expression of the grating PSF gives a good fit of stray-light profile measured in the laboratory. This instrument function has been used in the analysis of in-flight UVCS observations of the profile of electron scattered Ly- (alpha) from the solar corona. These observations have resulted, for the first time, in the most direct measurement of the coronal electron temperature.

Advanced Solar Coronal Explorer mission (ASCE)

The Advanced Solar Coronal Explorer (ASCE) is one of five missions selected for a Phase A Concept Study in the current round of proposed MIDEX missions. ASCEs instrument complement is supported by a SPARTAN 400 reusable carrier. The spacecraft is carried into orbit and deployed by the Space Shuttle; at missions end, nominally 2 years later, it is retrieved and returned to earth for post-flight calibration. ASCE comprises two instrument modules, the Spectroscopic and Polarimetric Coronagraph (SPC) and the Extreme Ultraviolet Imager (EUVI). The external occulter for the coronagraph is supported on a boom, which is extended 10 meters beyond the instrument apertures once the spacecraft is on station. Large aperture optics can therefore be used, and this, in combination with improvements in optical and photon detection efficiencies, will provide spectroscopy of the extended solar corona with unprecedented sensitivity and spatial resolution, routine measurements of the electron temperature, and polarimetry of the H I Lyman lines. SPC also extends the short wavelength limit to 28 nm. As a consequence, SPC will be able to perform the first He II 30.4 nm and He I 58.4 nm spectroscopy of the extended corona. In the visible part of the spectrum (450 - 600 nm), SPCs Large Aperture Spectroscopic Coronagraph (LASCO) channel will provide polarimetric images with 1.8 arc second resolution elements, which will allow the determination of polarized brightness of the coronal plasma. In a separate parallel channel LASCO will also provide images at single minor ion line wavelengths from which can be determined the shapes and Doppler shifts of those lines. The distant external occulter provides for major improvement in stray light suppression. The EUVI instrument will take high cadence images of the full disk and low corona at four selectable wavelengths with 0.9 arc second resolution elements. A description of the instrument design and performance capabilities is presented.

Polarimetry of the UV solar corona with ASCE

The Advanced Solar Coronal Explorer (ASCE) is a mid-explorer (MidEx) mission selected, together with other five, for the a Phase A Concept Study in the 1999 round of MidEX proposal. ASCEs spacecraft bus is a SPARTAN 400 reusable carrier deployed in low Earth orbit by the Space Shuttle. ASCEs payload comprises two instrument modules, the Spectroscopic and Polarimetric Coronagraph (SPC) and the Extreme Ultraviolet Imager (EUVI). The scientific objective of the mission is the investigation, through spectroscopic and polarimetric techniques, of the physics of the coronal heating and of the solar wind acceleration. A critical physical parameter of the corona is the magnetic field. Polarimetric measurements of UV coronal radiation and their interpretation through the Hanle effect can be used for coronal magnetic field diagnostics. One of the SPC spectrometers, the Spectroscopy/Polarimetry channel (SPCH), includes a reflecting Brewster-angle polarimeter for measurements of the linear polarization of the HI Lyman series lines (i.e., Ly-(alpha) , -(beta) , and (gamma) ) and of the O VI 1032 Angstrom line. In this paper, the optical design of the SPCH polarimeter is described. A relevant element of this design is the external occulter (EXO) that is supported on a boom, which is extended 10 m beyond the instrument aperture, once the instrument is in station. The analysis of the stray- light reduction provided by this occulting system is described in this paper. The principal source of stray light is solar disk light that is diffracted from the edge of the EXO and scattered from the telescope mirror. The analysis shows that the stray-light is less than 10-2 the coronal signal. This level of stray-light rejection minimizes the polarized stray light that may be introduced by the EXOs straight edge. The most appropriate material for the polarizer has been found to be CaF2. The material selection criteria are described. Finally, the paper illustrates with an example that if the linear polarization can be measured better than 1%, then the instrumental sensitivity to magnetic fields may reach a few gauss (greater than 2 gauss), in coronal active regions.

UV spectroscopy of the extended solar corona: Results from UVCS/Spartan

Remote sensing of the solar corona using ultraviolet spectroscopy can be used to characterize the physical state of the plasma along a wide region around the trajectory of a near-Sun flyby spacecraft (solar probe). Data from such an instrument can be used to separate spatial from temporal variations in the data received from in situ instruments aboard the probe. We present results from the UVCS/Spartan space shuttle missions in order to show examples of diagnostic techniques that can be used to determine velocity distributions and bulk outflow velocities of protons and minor ions in the solar wind.

Advanced spectroscopic and coronographic explorer: science payload design concept

The Advanced Spectroscopic and Coronagraphic Explorer (ASCE) was proposed in 2001 to NASAs Medium-Class Explorer (MIDEX) program by the Smithsonian Astrophysical Observatory in collaboration with the Naval Research Laboratory, Goddard Space Flight Center and the Italian Space Agency. It is one of four missions selected for Phase A study in 2002. ASCE is composed of three instrument units: an Advanced Ultraviolet Coronagraph Spectrometer (AUVCS), an Advanced Large Aperture visible light Spectroscopic Coronagraph (ALASCO), and an Advanced Solar Disk Spectrometer (ASDS). ASCE makes use of a 13 m long boom that is extended on orbit and positions the external occulters of AUVCS and ALASCO nearly 15 m in front of their respective telescope mirrors. The optical design concepts for the instruments will be discussed.

Absolute cross section for Si2+(3s3p 3p0→3s3p 1p0) electron-impact excitation

We have measured the absolute energy-averaged cross section for electron-impact excitation of Si 2 + (3s3p 3 P 0 →3s3p 1 P 0 ) from energies below threshold to the turn-on of the 3s3p 3 P 0 →3p 2 3 P transition. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.65 nm from the decay of the excited ions to the 3s 2 1 S ground state was detected using an absolutely calibrated optical system. The fractional population of metastable Si 2 + (3s3p 3 P 0 ) in the incident ion beam was determined to be 0.256′0.035(1.65σ). The experimental energy spread ranged from 0.85 eV (full width at half maximum) at the lowest energies to 0.56 eV at the highest. Resonance features consistent with 12-state close-coupling R-matrix calculations are seen.

Far-ultraviolet and visible light scatter measurements for CVD SiC mirrors for SOHO

Chemically-vapor-deposited (CVD) silicon carbide (SiC) has become a popular mirror material for spaceborne solar instrumentation for the vacuum ultraviolet wavelength range due to its appreciable broadband reflectance and favorable thermal and opto-mechanical properties. Scatter from surfaces of mirrors operating in this wavelength range can destroy otherwise good image contrast especially for extended targets such as the sun. While valid far ultraviolet (FUV) scatter measurements are entirely non-trivial to conduct and so are rarely performed, visible light scatter measurements are comparatively easy. Unfortunately, it is not straightforward to predict FUV scatter performance based on visible light scatter measurements for mirrors made of CVD SiC. It is hoped that by carrying out scatter measurements in both wavelength regimes for the same CVD SiC mirror, that the ability to make such predictions may be enhanced. Visible light (633 nm) scatter measurements were performed at Goddard Space Flight Center (GSFC) by two different means on CVD SiC telescope mirrors (from the same process and same vendor) for two instruments on the Solar and Heliospheric Observatory (SOHO) - - the Ultraviolet Coronagraph Spectrometer (UVCS) and Solar Ultraviolet Measurement of Emitted Radiation (SUMER). Additionally, extensive FUV scatter measurements were made for SUMER telescope mirrors. In this paper, we correlate the results for those FUV and visible light scatter measurements for this important material.