Orbiting around the Sun on an inclined orbit with a 0.2 UA perihelion, the Solar Orbiter probe will provide high resolution views of the Sun from various angles unattainable from Earth. Together with a set of high resolution imagers, the Full Sun Imager is part of the EUV Imaging suite of the Solar Orbiter mission. The mission's ambitious characteristics draw severe constraints on the design of these instruments. We present a photometrically efficient, compact, and lightweight design for the Full Sun Imager. With a 5 degrees field of view, this telescope will be able to see the global solar coronal structure from high viewing angles. Thermal solutions reducing the maximum power trapped in the High Resolution Imagers are also proposed.
The Extreme Ultraviolet Imager (EUVI) is part of the SECCHI instrument suite currently being developed for the NASA STEREO mission. Identical EUVI telescopes on the two STEREO spacecraft will study the structure and evolution of the solar corona in three dimensions, and specifically focus on the initiation and early evolution of coronal mass ejections (CMEs). The EUVI telescope is being developed at the Lockheed Martin Solar and Astrophysics Lab. The SECCHI investigation is led by the Naval Research Lab. The EUVI’s 2048 x 2048 pixel detectors have a field of view out to 1.7 solar radii, and observe in four spectral channels that span the 0.1 to 20 MK temperature range. In addition to its view from two vantage points, the EUVI will provide a substantial improvement in image resolution and image cadence over its predecessor SOHO-EIT, while complying with the more restricted mass, power, and volume allocations on the STEREO mission.
We elaborate about obtaining images of the solar disc and of the solar corona at discrete wavelengths along the EUV emission solar spectrum on board the Solar Orbiter spacecraft. Refractive optics cannot be used. The thermal load is twenty five times higher than on a near Earth orbit. As on one side the efficiency of a stenopeic device is too low, and as on the other side mirrors exposed directly to the light and to the particles emitted by the Sun may severely degrade during time, we investigated using the EUV analog of a Fresnel lens, i.e. a photon sieve.
An opaque self supporting flat piece of heat resistant metal let the solar light shine through a large numbers of a few thousand holes properly designed in positions and diameters in order to obtain constructive interferences at some focus.
We report about practical experiments in the visible.
The LYOT (LYman Orbiting Telescope) solar mission is proposed to be implemented on a micro-satellite of CNES (France) under phase A study. It includes two main instruments, which image the solar disk and the low corona up to 2.5 R<sub>o</sub> in the H I Lyman-α line at 121.6 nm. The spatial resolution is about 1” for the disk and 2.5” for corona. It also carries an EIT-type telescope in the He II (30.4 nm) line. The coronagraph needs a super polished mirror at the entrance pupil to minimize the light scattering. Gratings and optical filters are used to select the Lyman-α wavelength. VUV cameras with 2048×2048 pixels record solar images up to every 10 seconds.
The satellite operates at a high telemetry rate (more then 100 kb/s, after onboard data compression). The envisaged orbits are either geostationary or heliosynchronous. Possible launch dates could be end of 2006 - beginning of 2007.
The ability to derive physical parameters of the Sun from observations by the Solar and Heliospheric Observatory (SOHO) Extreme Ultraviolet Imaging Telescope (EIT) greatly increases the scientific return of the mission. The absolute and time variable calibration of EIT therefore is of extreme interest. The NRL EIT Calibration Sounding Rocket (CalRoc) program was initiated to provide well calibrated, contemporaneous observations in support of SOHO EIT. These observations provide three benefits to the SOHO EIT data, absolute calibration points, temporal and spatial information of the EIT EUV response variability in flight via flat field information and clues to the physics of the degradation. Details of the bandpasses of the multilayered optics and the total telescope photometry are presented. Comparisons are shown with the contemporaneous images from SOHO EIT. Plans for the second CalRoc flight are discussed. Loss of reflectivity in the multilayer mirrors has been identified as a new component to the SOHO EIT and CalRoc degradation.
Two beam lines have been built at the Institute d'Astrophysique Spatiale (IAS) d'Orsay to perform absolute calibration of the EPIC (European photon imaging camera). EPIC consists of three x-ray charge coupled device (CCD) cameras having imaging and spectroscopic performances set at the Wolter telescope focal planes on board the x-ray multi mirror mission (XMM) planned to be launched by ESA in August 1999. To cover the desired 0.1 - 15 keV range a dedicated beam line has been built on each synchrotron sources of the Laboratoire pour l'Utilisation du Rayonnement Synchrotron (LURE): SACO (0.8 GeV) and DCI (1.5 GeV). Both beam lines are merging in a clean 23 m<SUP>3</SUP> vacuum tank containing the camera to calibrate. (1) The SACO windowless beam line is equipped with a grating monochromator. Four plane VLS gratings are used to cover the low energy range (0.1 - 1.2 keV). A triple grazing incidence mirror system set in front of the entrance slit removes the overlapping orders. (2) The high energy beam line on DCI has a 50 micrometer beryllium window and a double flat crystals monochromator equipped with four different crystal pairs. A double grazing incidence mirror system set close to the source absorbs the high energy photon spectra. CCD calibrations will be performed during 1997 second semester and years 1998.
Optical characteristics in the wavelength range 15 - 75 nm of the EUV imaging telescope to be launched soon on the SOHO mission are discussed. Bandpasses and photometric sensitivity of the multilayered optics telescope have been measured by a dedicated synchrotron light source at Orsay, France.