Prism Spectrograph Research Articles

Method for boosting dispersive spectrograph stability 1000× using interferometry with crossfaded pairs of delays

We demonstrate a key step along a technical route to achieving cm/s scale accuracy for astronomical spectrographs over long (multi-year) time scales, which is critical for the Doppler characterization of Earth sized exoplanets, and measurement of small cosmic redshift drift over many years. This same technique also enables searching exoplanet atmospheres for biosignificant molecules in direct planet imaging using, otherwise, insufficiently low resolution and drift prone dispersive (grating or prism) spectrographs. Using a method called crossfading for externally dispersed interferometers (EDIs) to get highly robust spectra, we recently demonstrated a factor of 1000 × reduction in the net shift of an EDI measured ThAr line to a deliberate simulated wavelength translation of the detector. This 1000 × gain in disperser stability can be combined with conventional stability gains afforded by fiber scramblers, vacuum tanks, and thermal control, to provide an additional 1 to 3 orders of magnitude reduction in the net point spread function shift drift. Crossfading combines high- and low-delay fringing signals that react oppositely in phase to cancel their net reaction to a detector wavelength drift. This can be implemented by an interferometer addition to a facility spectrograph.

Multichannel emission spectrometer for high dynamic range optical pyrometry of shock-driven materials

An emission spectrometer (450-850 nm) using a high-throughput, high numerical aperture (N.A. = 0.3) prism spectrograph with stepped fiberoptic coupling, 32 fast photomultipliers and thirty-two 1.25 GHz digitizers is described. The spectrometer can capture single-shot events with a high dynamic range in amplitude and time (nanoseconds to milliseconds or longer). Methods to calibrate the spectrometer and verify its performance and accuracy are described. When a reference thermal source is used for calibration, the spectrometer can function as a fast optical pyrometer. Applications of the spectrometer are illustrated by using it to capture single-shot emission transients from energetic materials or reactive materials initiated by km⋅s-1 impacts with laser-driven flyer plates. A log (time) data analysis method is used to visualize multiple kinetic processes resulting from impact initiation of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) or a Zr/CuO nanolaminate thermite. Using a gray body algorithm to interpret the spectral radiance from shocked HMX, a time history of temperature and emissivity was obtained, which could be used to investigate HMX hot spot dynamics. Finally, two examples are presented showing how the spectrometer can avoid temperature determination errors in systems where thermal emission is accompanied by atomic or molecular emission lines.

The Long wave (11–16 μm) spectrograph for the EChO M3 Mission Candidate study

The results for the design study of the Long Wave Infrared Module (LWIR), a goal spectroscopic channel for the EChO ESA medium class candidate mission, are presented. The requirements for the LWIR module were to provide coverage of the 11–16 μm spectral range at a moderate resolving power of at least R = 30, whilst minimising noise contributions above photon due to the thermal background of the EChO instrument and telescope, and astrophysical sources such as the zodiacal light. The study output module design is a KRS-6 prism spectrograph with aluminium mirror beam expander and coated germanium lenses for the final focusing elements. Thermal background considerations led to enclosing the beam in a baffle cooled to approximately 25–29 K. To minimise diffuse astrophysical background contributions due to the zodiacal light, anamorphic designs were considered in addition to the elliptical input beam provided by the EChO telescope. Given the requirement that measurements in this waveband place on the performance of the infrared detector array, an additional study on the likely scientific return with lower resolving power (R < 30) is included. If specific high priority molecules on moderately warm giant planets (e.g. CO2, H2O) are targeted, the LWIR channel can still provide improvements in determining the atmospheric temperature structure and molecular abundances. Thus, the inclusion of even a coarse-resolution (R≈10) LWIR module would still make an important contribution to measurements of exoplanet atmospheres made by EChO.

On the optimisation of the spectral resolution in spectrographs for cold neutrons based on refraction at grazing incidence

Recently the wavelength dispersion of cold neutrons in the refraction process at inclined interfaces was identified as an efficient tool for neutron spectrographs, in which a larger wavelength band can be registered simultaneously. This registration mode reduces the data acquisition time significantly as no need to monochromatise the incident neutron beam by use of inefficient choppers exists. In the related studies the spectrograph performance is treated with rather complex equations. This study instead provides a theoretical treatment of the dispersion properties with simpler analytical equations, which were previously used in connection with X-rays. It can be shown, that the spectral resolution in the original spectrographs is mostly limited by the finite size of the refracted beam, which is inconveniently increasing upon refraction at grazing internal incidence onto an inclined refracting interface. The blurring of the beam size of a monochromatic beam at the detector due to the angular spread of the incident beam is mostly negligible. It is thus proposed that a significant improvement in the spectral resolution of such a spectrograph can be achieved, when the beam size at the detector is reduced by introducing focusing in the refraction process. It is shown, that the spectral resolution can then ultimately be limited by the smaller size of the blurred image caused by the angular spread of the beam. Then the improvement in this beam divergence limit can be by an order of magnitude and it is achieved by refraction upon internal incidence onto a concave interface. It is found that such a configuration will focus appropriately in a larger wavelength interval. By this means for wavelengths between 5Å and 12Å spectral resolutions of below 1% are feasible, which are not yet reported for such prism spectrographs.

Measuring the wavelength of light wave with four-sides prism spectrograph

The angular dispersion of four-sides prism is presented. A more accurate amended expression of measuring the wavelength of light wave with the spacing between two spectral lines is presented. The spacing between two spectral lines is measured with measuring microscope, the wavelength of light wave is calculated with the amended expression, the experimental values of wavelengths agree well with the reference value.

Crowded-Field Astrometry with SIM PlanetQuest. II. An Improved Instrument Model

ABSTRACTIn a previous paper we described a method of estimating the single-measurement bias to be expected in astrometric observations of targets in crowded fields with the future Space Interferometry Mission PlanetQuest. That study was based on a simplified model of the instrument and the measurement process involving a single-pixel focal plane detector, an idealized spectrometer, and continuous sampling of the fringes during the delay scanning. In this paper we elaborate on this “instrument model” to include the following additional complications: spectral dispersion of the light with a thin prism, which turns the instrument camera into an objective prism spectrograph; a multiple-pixel detector in the camera focal plane; and binning of the fringe signal during scanning of the delay. The results obtained with this improved model differ in small but systematic ways from those obtained with the earlier simplified model. We conclude that it is the pixelation of the dispersed fringes on the focal plane detector that is responsible for the differences. The improved instrument model described here suggests additional ways of reducing certain kinds of confusion, and provides a better basis for the evaluation of instrumental effects in the future.

Design and implementation of a sensitive high-resolution nonlinear spectral imaging microscope

Live tissue nonlinear microscopy based on multiphoton autofluorescence and second harmonic emission originating from endogenous fluorophores and noncentrosymmetric-structured proteins is rapidly gaining interest in biomedical applications. The advantage of this technique includes high imaging penetration depth and minimal phototoxic effects on tissues. Because fluorescent dyes are not used, discrimination between different components within the tissue is challenging. We have developed a nonlinear spectral imaging microscope based on a home-built multiphoton microscope, a prism spectrograph, and a high-sensitivity CCD camera for detection. The sensitivity of the microscope was optimized for autofluorescence and second harmonic imaging over a broad wavelength range. Importantly, the spectrograph lacks an entrance aperture; this improves the detection efficiency at deeper lying layers in the specimen. Application to the imaging of ex vivo and in vivo mouse skin tissues showed clear differences in spectral emission between skin tissue layers as well as biochemically different tissue components. Acceptable spectral images could be recorded up to an imaging depth of approximately 100 microm.

Prism-Based Infrared Spectrographs Using Modern-Day Detectors

A comparison of prism-based spectrographs to grating-based spectrographs is made when each of the systems is coupled to a modern-day liquid-nitrogen-cooled photovoltaic array detector. A comparison of the systems is also made using a room-temperature microbolometer array detector. Finally, infrared microspectroscopy of samples whose size is approximately 10 micrometers will be demonstrated using a prism spectrograph outfitted with both types of detectors. The results of the study show that prism-based spectrographs offer an economical alternative to grating-based systems when spectral coverage is more critical than spectral resolution. The results also demonstrate that spectra with good signal-to-noise ratios can be collected on any of the systems with a total integration time of 10 seconds or less.

Evolution of Instrumentation for Detection of the Raman Effect as Driven by Available Technologies and by Developing Applications

The evolution of Raman instrumentation from the time of the initial report of the phenomenon in 1928 to the present will be reviewed. The earliest systems were prism spectrographs with photographic plates, and the spectrum was excited with a mercury arc lamp. Because samples were synthesized and then extensively purified to guarantee that the spectrum was representative of the sample and not impurities, problems of Rayleigh scattering and fluorescence that became important in the period between the 1960s and 1990s were not present. During the period between the mid-1950s to the late-1970s most systems were double grating monochromators, scanned with photomulplier detectors. During the mid-1970s the first microprobes were introduced on scanning instruments, but were then adapted to spectrographs after the multichannel detectors became the method of choice for detection. Initially these were triple spectrographs where the first two stages were used in subtractive mode to filter the laser line, but after the introduction of the holographic notch filters in 1990, a new generation of truly benchtop Raman systems were developed and saw increasing popularity.

Jesse Leonard Greenstein (1909–2002)

Jesse Leonard Greenstein (1909–2002)

Early days of biochemical spectroscopy at Liverpool

From 1924 to 1940 R.A. Morton1 was Special Lecturer in Spectroscopy in the Chemistry Department of the University of Liverpool. He used a Hilger E3 prism spectrograph (Figure 1) attached to a rotating sector photometer (Figure 2) and illuminated by two types of light source (see later). Before considering how Morton's activities moved from pure chemistry into biological chemistry, it is interesting to know how this antediluvian apparatus functioned, because it is highly unlikely that any reader under the age of 70 years will have seen it, let alone used it.

Survey of Neon and Argon Emission Between 3800 Å and 6000 Å in the EBIT-II Electron Beam Ion Trap

Using the low-energy electron beam ion trap EBIT-II at the Lawrence Livermore National Laboratory, we surveyed neon and argon spectra throughout the wavelength range from 3800 to 6000 Å employing a prism spectrograph and a scientific-grade CCD camera. By varying the electron beam energy, spectra from singly ionized to highly ionized neon and argon ions were recorded. In total, about 130 lines were observed from ions in low charge states. About half of the lines have not been reported before. From high charge state ions, several lines of forbidden transitions were observed, among which are the prominent M1 transitions from Ar XIV at 4412 Å and Ar X at 5534 Å.

Optical Spectra from Highly Charged Ions

Optical spectra of Ne, Ar and Kr have been excited in an electron beam ion trap (EBIT). A prism spectrograph was used for broad band (3700–6000 Å) spectral surveys with moderate resolution to establish the excitation thresholds and thus the charge states of transitions in highly charged ions. A high-resolution transmission grating spectrometer for high precision measurements yielded detailed spectra. The “inverted trap” technique was employed for in situ wavelength calibrations.

Variation of Methane Absorption over the Jovian Disk from the Data of Zonal CCD Spectrophotometry

A comprehensive zonal spectrophotometry of Jupiter's disk was performed with a prism spectrograph and an ST-6V CCD camera mounted on the 1-m telescope at the Assy observatory in 1997 and 1998. In addition to the spectra of the central meridian and the equatorial belt, 10 sets of spectrograms of 37 to 39 of Jupiter's zones were recorded with a step of about 1.2″ (each set contained the measurements of absorption bands for approximately 3200 points of Jupiter's disk). Based on these data, an atlas of zonal variations in the central depths Rνof the methane absorption bands in the wavelength range 560–950 nm is compiled and numerical-contour and 3D half-tone maps representing the distributions of Rνover the disk are constructed. For most zones, methane absorption changes slightly from the central meridian to the limb. The intensities of strong and moderate methane bands exhibit pronounced zonal variations, whereas the intensity of a relatively weak band at 619 nm varies more chaotically. This effect is likely due to the fact that, at the effective depth where the weaker bands are formed, the medium is more homogeneous, since the convective and turbulent mixing is more intense at this depth.

Spatially-encoded fourier spectrometer: Prospects for plasma studies

Abstract Analysis of the main characteristics (geometrie factor, resolving power, signal-to-noise ratio) of a spatially-encoded Fourier spectrometer (SFS) with recording by a position-sensitive photodetector shows that it can be widely used for piasma studies. A sum of advantages of the SFS appears to be the best in comparison both with grating (prism) spectrographs and a classic “dynamie” Fourier spectrometer. A technique, suggested for numerical correction of imperfection of optical elements of the SFS, enables to reach a resolution close to the theoretical one. Examples of measurements of different spectra are given.

Dispersion measurement in doublet-line wings by a double-layer interferometer crossed with a prism spectrograph

A double-layer interferometer containing a flame-excited vapour of sodium atoms is irradiated by a continuous-spectrum source. Light transmitted by this interferometer is resolved by a prism spectrograph of moderate resolution where a set of concentric rings of equal chromatic order (RECO) replaces the continuous spectrum of the background source. The elliptic RECOs are perturbed as they are being crossed by the absorption D-line of Na. The RECO perturbation is measured so as to construct the dispersion curve in the extended wings of the absorption line. The obtained results are fitted to a modified Sellmeier dispersion function to deduce the level population due to the flame excitation of Na atoms.

Compact prism spectrographs based on aplanatic principles

Compact prism spectrographs based on aplanatic principles

Accurate measurement of refraction and dispersion of a solid by a double-layer interferometer

A silica plate of plane-parallel faces is inserted into one gap of a double-layer interferometer that transmits white light to a prism spectrograph in order to produce elliptic rings of equal chromatic order (RECO's). The silica plate is rotated and the expanding RECO's are counted at their center while this center is coincident with a standard wavelength. An analytic formula that relates the fringe count to the rotated angle enables the refractive index of the rotated plate to be accurately determined for different wavelengths. The results are fitted to a single-term Sellmeier dispersion function to find the peak wavelength of the ultraviolet absorption band and the atomic number density for such a transition. The variation of either the dispersion coefficient or the group-velocity factor with wavelength is determined from either the displacement of the RECO center across the visible spectrum if one of the double-layer interferometer's mirrors is displaced parallel to itself or from measurements on the RECO diameters.

Combined infrared emission spectra and radar reflectivity studies of cirrus clouds

Thermal-infrared emission spectra from cirrus clouds taken with a unique prism spectrograph are combined with K/sub a/-band radar reflectivity measurements to obtain mass concentration and size information on the cirrus ice crystals. A two-stream radiation transfer model utilizing Mie scattering theory was used to infer the averaged effective ice sphere diameter which, for the cirrus studied, was near 50 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spectroscopic study on the crystal growth in gas evaporation: emission and absorption spectra from Cu smoke

Optical emission and absorption spectra from a growth zone in Cu smoke were measured by using a quartz prism spectrograph and a single-grating spectrophotometer. Atomic lines of Cu, band systems of Cu2 were observed. Two new band systems with 0–0 band heads at 252.88 and 238.79 nm were found. The continuous spectra due to Rayleigh scattering by fine particles were also observed. Variations of density of various species as a function of vertical position were revealed.