. In the physics of the upper atmosphere the solar extreme ultraviolet (EUV) radiation plays a dominant role controlling most of the thermospheric/ionospheric (T/I) processes. Since this part of the solar spectrum is absorbed in the thermosphere, platforms to measure the EUV fluxes became only available with the development of rockets reaching altitude levels exceeding 80 km. With the availability of V2 rockets used in space research, recording of EUV spectra started in 1946 using photographic films. The development of pointing devices to accurately orient the spectrographs toward the sun initiated intense activities in solar–terrestrial research. The application of photoelectric recording technology enabled the scientists placing EUV spectrometers aboard satellites observing qualitatively strong variability of the solar EUV irradiance on short-, medium-, and long-term scales. However, as more measurements were performed more radiometric EUV data diverged due to the inherent degradation of the EUV instruments with time. Also, continuous recording of the EUV energy input to the T/I system was not achieved. It is only at the end of the last century that there was progress made in solving the serious problem of degradation enabling to monitore solar EUV fluxes with sufficient radiometric accuracy. The data sets available allow composing the data available to the first set of EUV data covering a period of 11 years for the first time. Based on the sophisticated instrumentation verified in space, future EUV measurements of the solar spectral irradiance (SSI) are promising accuracy levels of about 5% and less. With added low-cost equipment, real-time measurements will allow providing data needed in ionospheric modeling, e.g., for correcting propagation delays of navigation signals from space to earth. Adding EUV airglow and auroral emission monitoring by airglow cameras, the impact of space weather on the terrestrial T/I system can be studied with a spectral terrestrial irradiance camera (STI-Cam) and also be used investigating real-time space weather effects and deriving more detailed correction procedures for the evaluation of Global Navigation Satellite System (GNSS) signals. Progress in physics goes with achieving higher accuracy in measurements. This review historically guides the reader on the ways of exploring the impact of the variable solar radiation in the extreme ultraviolet spectral region on our upper atmosphere in the altitude regime from 80 to 1000 km.

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Extreme ultraviolet spectral irradiance measurements since 1946

A compact, lightweight, yet sturdy imaging spectrometer, especially for use in satellites, flying bodies and the like, has elliptical mirrors for its objective or lens, for its collimator, and for its imager The collimator and imager are formed by the same mirrors which reflect the beam twice The optical arrangement of the objective or lens, the collimator, and the imager is such that any bending of an object strip by a diffraction lattice or grid is compensated Such a spectrometer is usable for a wide field and wide spectral range, including the non-visible range

Imaging spectrometer having a wide spectral range

Off-axis parabolic telescopes, widely used in astronomy and laser optics, if perfectly tuned, are virtually free from aberrations along the parabola's axis direction, but their alignment is very critical. We present a detailed method to align a high magnification off-axis afocal parabolic telescope. The method is composed of two steps: an initial pre-alignment using autocollimators, followed by a fine tuning with a collimated laser beam. Due to the large telescope magnification, the outcoming beam cannot be measured without being refocused. The beam is therefore reflected on a flat mirror and sent back through the telescope. This double-pass configuration allows the measurement of the beam quality without the need for large additional optics. In the fine-tuning step, a numerical simulation is also used to identify the degrees of freedom to be adjusted. The experimental results presented are obtained with one of the mode-matching parabolic telescopes of the gravitational wave interferometric detector Advanced Virgo.

Tuning of a high magnification compact parabolic telescope for centimeter-scale laser beams.

The Sun was one of the central points for observations as long as human cultures are known. After researchers observed the extension of the solar rainbow spectrum towards shorter wavelengths with increasing altitude they started measurements of the solar ultraviolet spectrum from balloons in 1934. The acquisition of spectra in the extreme ultraviolet and soft X-ray spectral regions was beginning after 1946 using photographic recordings aboard rockets. However, it took six more years to achieve good spectral resolution below about 220 nm because servomechanisms had to be developed to fairly accurately point the spectrographs to the Sun during the rocket flight. Another big step forward was achieved by the application of photoelectric recording technology and by using satellites as a platform to observe qualitatively the strong variability of the solar ultraviolet irradiance on short-, medium- and long-term scales. Until the end of the past century these measurements have been strongly impeded by the inherent degradation of the EUV instruments with time. Since there is progress solving this serious problem, measurements with sufficient radiometric accuracy allow composing the data available to the first set of ultraviolet data covering a period of eleven years. Based on the sophisticated instrumentation verified in space, future EUV measurements of the solar spectral irradiance on the International Space Station are promising accuracy levels of about 5%. Added by low-cost equipment on-line measurements will allow providing data needed in ionospheric modeling for correcting propagation delays of navigation signals from space to Earth. Adding ultraviolet airglow and auroral emission measurements the impact of space weather on the ionosphere can be studied and also be used deriving more detailed correction procedures for the evaluation of Global Navigation Satellite System (GNSS) signals.

History of extreme ultraviolet solar measurements in XX–XXI centuries: from balloons to the International Space Station as instrumental platforms

Metasurface, fluorescent microscopy and scanning near-field optical microscopy can improve the resolution of microscopes remarkably, while the resolution of the telescope remains unimproved constrained by its giant objective lenses and distant targets. Here we put forward a way to raise the resolution of telescopes simply by adding a binary optical thin surface around its focal plane. Simulation results show that the surface can raise the image quality in the Cassegrain and Kepler telescope. By nano-lathe, we fabricated a designed binary surface and experiment it in the Kepler telescope. The results are consistent with those of simulation results. More details of the calibrated target are resolvable on the image plane after applying the binary optical surface. It proves that the binary optic surface can make contribute to the resolution of the telescope, thus is beneficial in astronomy, military surveillance field.

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Improvement of telescope resolution using a diffractive phase modulater

The properties of a nonfocusing collimating parabolic telescope are studied in detail by a ray-tracing method. The best optical quality with respect to coma and astigmatism is calculated for rays close to the optical axis of the telescope. For rectangular fields of view the borders are not sharp because of the aberration. A combination of a parabolic telescope and an echelle-type spectrometer gives very high spectral resolution. Wadsworth, Ebert-Fastie, and other types of spectrometers are equally well suited to be combined with this telescope. For some cases spot diagrams demonstrate the optical performance. In addition, diffraction by the aperture and its implications are discussed.

K. H. Hager

Papers

Parabolic telescope and spectrometer combination