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.
KEYWORDS: Zone plates, Opacity, Solar processes, Photoresist materials, Extreme ultraviolet, Photography, Objectives, Space telescopes, Telescopes, Sun
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.
Present projects of space interferometers dedicated to the detection and analysis of extrasolar planets (DARWIN in Europe, TPF in the United States) are based on the nulling interferometry concept. This concept has been proposed by Bracewell in 1978 but has never been demonstrated with high values of rejection, in the thermal infrared range, where the planet detection should be performed (6 - 18 micrometers ). We have thus built a two-beam laboratory interferometer to validate this concept in a monochromatic case (at 10 micrometers ). The keypoint of our interferometer is the use of optical filtering by pinhole and optical fibers to clean the interfering beams. We present in this paper the principle of the experimental setup, its realization, and the first measurements of rejection it allowed. We also present the future developments of this interferometer.
Converting linear coordinates in the plane of the detector of an astronomical instrument to celestial coordinates involves in principle the equivalent focal length of the instrument. However, most methods in astrometry manage to reduce observations in a global manner without actually measuring a focal length. We point out a case for solar space observations where the long term stability of angular distance measurements is better than the ground calibration of the angular value of a pixel. We report and discuss this ground calibration.
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.
Microlens arrays play an important role in atmospheric wavefront sensors used by ground based astronomers in order to obtain diffraction limited images of stars in the presence of wavefront disturbances introduced by the earth's atmosphere. One technique for making these arrays is to use a two axes rastering process for engraving in photoresist coatings, producing square lenses with no room left between them. Obviously the efficiency of the arrays depends on the quality of the lenses. The surface shape is normally checked qualitatively with Nomarski and quantitatively using stylus profiling. Phase stepping microscopy has recently been applied to the problem, for measuring the shape of individual lenses and comparing the results with spherical profiles. The initial results have been very successful. Some typical examples are given and the results discussed to show the potential of such work in improving the quality of microlens arrays.
We have developed a technique for fabricating microlens arrays by engraving photoresist coatings. These microlens arrays are designed for astronomical applications for atmospheric wavefront sensors. First, we describe the apparatus and the manufacturing process. Second, we review the characteristics of the different photoresist types used in this process. Third, we report on the different optical testing methods to measure the microlens' performances. Then we deduce the several inherent advantages and limitations of this method. Fourth, we show how to produce monolithic arrays using ion beam milling with photoresist microlens arrays as a pattern and we demonstrate how these arrays can improve upon the performances of photoresist arrays.
Microlens arrays are a component of atmospheric wavefront sensors used recently with success by ground-based astronomers to obtain diffraction-limited images of stars in spite of wavefront disturbances introduced by the earth's atmosphere. Typical requirements for individual lenses are a size measured in tenths of millimeters, a sag measured in micrometers, and no room left between lenses, i.e., square or hexagonal lenses rather than circular lenses. Actual monolithic square microlens arrays up to 20 x 20 mm in size for 0.18- to 1.5-mm individual contiguous lenses of 0.0035-mm maximum sag are engraved in photoresist coatings by a two-axes rastering process. These arrays have been used for diurnal and nocturnal atmospheric wavefront measurements.
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