A blazed grating – also called echelette grating (from French échelle = ladder) – is a special type of diffraction grating . It is optimized to achieve maximum grating efficiency in a given diffraction order . For this purpose, maximum optical power is concentrated in the desired diffraction order while the residual power in the other orders (particularly the zeroth) is minimized. Since this condition can only exactly be achieved for one wavelength, it is specified for which blaze wavelength the grating is optimized (or blazed ). The direction in which maximum efficiency is achieved is called the blaze angle and is the third crucial characteristic of a blazed grating directly depending on blaze wavelength and diffraction order.
18-627: The High Accuracy Radial Velocity Planet Searcher ( HARPS ) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile . The first light was achieved in February 2003. HARPS has discovered over 130 exoplanets to date, with the first one in 2004, making it the most successful planet finder behind the Kepler space telescope . It
36-431: A higher order to overlap with the next order(s) of a shorter wavelength, which is usually an unwanted side effect. In echelle gratings, however, this behavior is deliberately used and the blaze is optimized for multiple overlapping higher orders. Since this overlap is not directly useful, a second, perpendicularly mounted dispersive element ( grating or prism ) is inserted as an "order separator" or "cross disperser" into
54-400: A step structure. The steps are tilted at the so-called blaze angle θ B {\displaystyle \theta _{B}} with respect to the grating surface. Accordingly, the angle between step normal and grating normal is θ B {\displaystyle \theta _{B}} . The blaze angle is optimized to maximize efficiency for the wavelength of
72-523: Is a second-generation radial-velocity spectrograph, based on experience with the ELODIE and CORALIE instruments. The HARPS can attain a precision of 0.97 m/s (3.5 km/h), making it one of only two instruments worldwide with such accuracy. This is due to a design in which the target star and a reference spectrum from a thorium lamp are observed simultaneously using two identical optic fibre feeds, and to careful attention to mechanical stability:
90-603: Is chosen such that diffraction angle and incidence angle are identical. For a reflection grating , this means that the diffracted beam is back-reflected into the direction of the incident beam (blue beam in picture). The beams are perpendicular to the step and therefore parallel to the step normal. Hence it holds in Littrow configuration α = β = θ B {\displaystyle \alpha =\beta =\theta _{B}} . All other geometries yield anamorphic Littrow expansion or compression of
108-511: The HARPS is Michel Mayor who, along with Didier Queloz and Stéphane Udry , have used the instrument to characterize the Gliese 581 planetary system , home to one of the smallest known exoplanets orbiting a normal star, and two super-Earths whose orbits lie in the star's habitable zone . It was initially used for a survey of one-thousand stars. Since October 2012 the HARPS spectrograph has
126-574: The HARPS. The list is sorted by the date of the discovery's announcement. As of December 2017, the list contains 134 exoplanets. Similar instruments: Space based detectors : Echelle grating An echelle grating (from French échelle , meaning "ladder") is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at high incidence angles and therefore in high diffraction orders . Higher diffraction orders allow for increased dispersion (spacing) of spectral features at
144-413: The beam path. Hence the spectrum consists of stripes with different, but slightly overlapping, wavelength ranges that run across the imaging plane in an oblique pattern. It is exactly this behavior that helps to overcome imaging problems with broadband, high-resolution spectroscopic devices, as in the utilisation of extremely long, linear detection arrays, or strong defocus or other aberrations , and makes
162-503: The beam. Diffraction angles at the grating are not influenced by the step structure. They are determined by the line spacing and can be calculated according to the in-plane version of the grating equation : where: For the Littrow configuration, this becomes 2 d sin θ B = m λ {\displaystyle 2d\sin {\theta _{B}}=m\lambda } . By solving for θ B {\displaystyle \theta _{B}}
180-411: The blaze angle can be calculated for arbitrary combinations of diffraction order, wavelength and line spacing: Blazed gratings can also be realized as transmission gratings . In this case the blaze angle is chosen such that the angle of the desired diffraction order coincides with the angle of the beam refracted at the grating material. A special form of a blazed grating is the echelle grating . It
198-487: The detector, enabling increased differentiation of these features. Echelle gratings are, like other types of diffraction gratings, used in spectrometers and similar instruments. They are most useful in cross-dispersed high resolution spectrographs, such as HARPS , PARAS , and numerous other astronomical instruments. The concept of a coarsely-ruled grating used at grazing angles was discovered by Albert Michelson in 1898, where he referred to it as an "echelon". However, it
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#1732773225842216-423: The diffraction pattern can be altered by tilting the grating. With reflective gratings (where the holes are replaced by a highly reflective surface), the reflective portion can be tilted (blazed) to scatter a majority of the light into the preferred direction of interest (and into a specific diffraction order). For multiple wavelengths the same is true; however, in that case it is possible for longer wavelengths of
234-405: The instrument sits in a vacuum vessel which is temperature-controlled to within 0.01 kelvins. The precision and sensitivity of the instrument is such that it incidentally produced the best available measurement of the thorium spectrum. Planet-detection is in some cases limited by the seismic pulsations of the star observed rather than by limitations of the instrument. The principal investigator on
252-445: The precision to detect a new category of planets: habitable super-Earths. This sensitivity was expected from simulations of stellar intrinsic signals, and actual observations of planetary systems. Currently, the HARPS can detect habitable super-Earth only around low-mass stars as these are more affected by gravitational tug from planets and have habitable zones close to the host star. This is an incomplete list of exoplanets discovered by
270-419: The use of readily available 2D-detection arrays feasible, which reduces measurement times and improves efficiency. Blazed grating Like every optical grating, a blazed grating has a constant line spacing d {\displaystyle d} , determining the magnitude of the wavelength splitting caused by the grating. The grating lines possess a triangular, sawtooth-shaped cross section, forming
288-412: The used light. Descriptively, this means θ B {\displaystyle \theta _{B}} is chosen such that the beam diffracted at the grating and the beam reflected at the steps are both deflected into the same direction. Commonly blazed gratings are manufactured in the so-called Littrow configuration . The Littrow configuration is a special geometry in which the blaze angle
306-450: The wavelength of the diffracted light. The light of a single wavelength in a standard grating at normal incidence is diffracted to the central zero order and successive higher orders at specific angles, defined by the grating density/wavelength ratio and the selected order. The angular spacing between higher orders monotonically decreases and higher orders can get very close to each other, while lower ones are well separated. The intensity of
324-411: Was not until 1923 that echelle spectrometers began to take on their characteristic form, in which the high-resolution grating is used in tandem with a crossed low-dispersion grating. This configuration was invented by Nagaoka and Mishima and has been used in a similar layout ever since. As with other diffraction gratings, the echelle grating conceptually consists of a number of slits with widths close to
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