Aperture shapes and the effectiveness of ground-based large and extremely large telescopes

ABSTRACT Aperture shapes in modern large and forthcoming extremely large telescopes (ELTs), with effective light-gathering sizes more than D ∼ 10 m, differ significantly from the desirable circular one. They deliver specific point spread functions, which may also differ notably from that of the fine structure of the classical Airy pattern. The optical power of such a telescope can be changed notably compared with a circular aperture with the same area. The presence of atmospheric optical turbulence complicates the effect additionally and makes it seeing- and wavelength-dependent. So, what is the impact of a non-circular pupil on telescope exploitation? It concerns the efficiency, which is an important point, especially for instruments of such a class. In this research an attempt is made to assess the values of these changes in the context of the Keck, HDRT, GMT, TMT and ELT telescopes. Relative performance characteristics (integral contrast and signal-to-noise ratio, S/N) of the telescopes, working in the seeing-limited regime, under a range of plausible turbulence conditions, for a wide (from UV to mid-IR) spectral region are obtained. The partial role of central obscuration is assessed. The effect of adaptive optics implementation in this context is also analysed. It is shown that, for instance, maximal S/N degradation due to the non-circularity of the pupil shape can be as much as $\sim 6~{{\ \rm per\ cent}}$ (TMT) to $30~{{\ \rm per\ cent}}$ (HDRT), depending on the telescope and observational mode. The numbers are comparable with or may even substantially exceed the losses that could be caused by the other parameters (e.g. residual wave-front error, optical transmittance) relevant to the quality of the optical system.

Towards a time-gated Raman spectrometer with VIS-NIR SPAD camera for stand-off planetary surface exploration

<p>Single-photon avalanche diode (SPAD) arrays are solid-state detectors that offer imaging capabilities at the level of individual photons. Very recently, time-gated cameras based on solid-state CMOS SPAD technology have been proposed for improving the performance and field applicability of Raman spectrometers for on-surface planetary geoscience through addressing the largely unmet challenge of suppression of fluorescence interference in highly fluorescent rocks (e.g. minerals containing phosphate, one of the chemical nutrients thought to be essential for life).</p><p>The effectiveness of Raman SPAD cameras currently proposed in the literature, however, is at present restricted to a small subset of samples and regimes of operations. This is largely owed to two main limitations. Firstly, their performance is optimised only for the VIS spectral region (typically around 532 nm), where the fluorescence issue tends to be exacerbated due to increased likelihood of electronic excitation for most molecular species compared to Raman excitation above 775 nm. Secondly, their 2D architecture is limited to few pixel rows, which reduces their light-gathering capability and consequently the detection performance of the Raman spectrometer.</p><p>We present the preliminary work towards the development of a novel time-gated Raman spectrometer that relies on a large format NIR-optimised SPAD camera prototype with time resolution better than 200 ps. This technology promises to deliver unsurpassed dual-wavelength Raman detection capabilities that would be transformative for stand-off sample analysis in surface exploration of Mars and Icy moons.</p><p>A performance analysis model for predicting the fluorescence and ambient light suppression performance levels in relation to the properties of various samples, environmental conditions and specifications of the laser and camera is presented, followed by the preliminary designs of the SPAD camera module and Raman spectrometer.</p>

LITE microscopy: Tilted light-sheet excitation of model organisms offers high resolution and low photobleaching

Fluorescence microscopy is a powerful approach for studying subcellular dynamics at high spatiotemporal resolution; however, conventional fluorescence microscopy techniques are light-intensive and introduce unnecessary photodamage. Light-sheet fluorescence microscopy (LSFM) mitigates these problems by selectively illuminating the focal plane of the detection objective by using orthogonal excitation. Orthogonal excitation requires geometries that physically limit the detection objective numerical aperture (NA), thereby limiting both light-gathering efficiency (brightness) and native spatial resolution. We present a novel live-cell LSFM method, lateral interference tilted excitation (LITE), in which a tilted light sheet illuminates the detection objective focal plane without a sterically limiting illumination scheme. LITE is thus compatible with any detection objective, including oil immersion, without an upper NA limit. LITE combines the low photodamage of LSFM with high resolution, high brightness, and coverslip-based objectives. We demonstrate the utility of LITE for imaging animal, fungal, and plant model organisms over many hours at high spatiotemporal resolution.

Mechanism of Sunlight Damage on Drip Tape by Pendant Droplets in Mulched Drip Irrigation

Drip irrigation under mulch has been applied in China for nearly 20 years, but sunlight damage from the lens effect through droplets beneath clear plastic mulch is always a problem that cannot be ignored. Droplet volume and mulch wettability affect the geometric parameters of the pendant droplets. Changes in geometric parameters were experimentally investigated by analyzing side-view images of droplets. Models were built to predict droplet focal length and light-gathering power based on the geometric parameters. A comparison between numerical and optical experimental results suggested that the focal length model was accurate and reliable. The effective incident area of the parallel light proposed in this study could also be used to represent the light-gathering power, which had a relationship with the drip tape burning rate. The increase in wettability of the clear mulch considerably increased the focal length of the pendant droplets, expanded the focal length range, enhanced the light-gathering power, and thus increased the risk of drip tape burning. In practice, pendant droplets with a wetting radius of 3 to 5 mm, with corresponding focal lengths of 5 to 12 mm, have a high probability of emergence. Therefore, the distance between the mulch and drip tape should be beyond this focal length range to reduce the risk of drip tape burning by pendant droplets. In addition, filming the mulch surface with hydrophobic materials to increase the contact angles of droplets can also protect the drip tape from sunlight damage. Keywords: Drip irrigation under mulch, Drip tape burning, Effective incident area of parallel light, Focal length, Pendant droplet.

LITE microscopy: a technique for high numerical aperture, low photobleaching fluorescence imaging

Fluorescence microscopy is a powerful approach for studying sub-cellular dynamics at high spatiotemporal resolution; however, conventional fluorescence microscopy techniques are light-intensive and introduce unnecessary photodamage. Light sheet fluorescence microscopy (LSFM) mitigates these problems by selectively illuminating the focal plane of the detection objective using orthogonal excitation. Orthogonal excitation requires geometries that physically limit the detection objective numerical aperture (NA), thereby limiting both light-gathering efficiency (brightness) and native spatial resolution. We present a novel LSFM method: Lateral Interference Tilted Excitation (LITE), in which a tilted light sheet illuminates the detection objective focal plane without a sterically-limiting illumination scheme. LITE is thus compatible with any detection objective, including oil immersion, without an upper NA limit. LITE combines the low photodamage of LSFM with high resolution, high brightness, coverslip-based objectives. We demonstrate the utility of LITE for imaging animal, fungal, and plant model organisms over many hours at high spatiotemporal resolution.

6. A mirror held up to nature

‘A mirror held up to nature’ looks at some of today’s remarkable optical and infrared large telescopes, highlighting the technologies that made them possible and the science results that have revealed galaxies at vast distances and, closer to home, exoplanets. The quest for telescopes with ever greater light-gathering power never stops. The convergence of several diverse technologies has, in the last few decades, produced the Very Large Telescopes. Three new mirror technologies in the 1970s and 1980s—segmented, honeycomb, and meniscus types—allied with active control of mirror shape (active optics), the use of altazimuth mounts, and computing technology transformed telescope production and their abilities. Adaptive optics is also described.