Large-amplitude anomalous events have been observed in CCDs due to radioactive emission from high-index optical glasses, some producing charge-trapping like artefacts. We have identified the source of these events from one glass (Ohara S-YGH51) as α-particles from trace amounts of nuclides in the actinium decay series, parent <sup>235</sup>U. We present measurements of the anomalous event rate for samples of 15 separate optical glasses with n<sub>d</sub> ≥ 1.6. There is a variation in anomalous count rates of orders of magnitude range over these materials. Care should be taken in the selection of optical glasses to be located in close proximity to detectors.
GHOST is a high resolution spectrograph system currently being built for the Gemini South Observatory in Chile. In the Cassegrain unit, the observational targets are acquired on integral field units and guided during science exposures, feeding the fiber cable to the temperature-stabilized echelle spectrograph. The Cassegrain unit is mounted on the Gemini telescope, and consists of a main structural plate, the two object positioners and ballast frame. The image from each of the two science beams passes through a field lens and a mini-atmospheric dispersion corrector and is then captured by the integral field unit. The positioner moves each corrector-integral field unit assembly across the focal surface of the telescope. The main structural plate provides the interface for the positioner and ballast frame to the telescope structure. In this paper we describe the final design and assembly of the GHOST Cassegrain unit and report on the outcome of on-sky testing at the telescope in Chile.
The Gemini High-Resolution Optical SpecTrograph (GHOST) is the newest instrument being developed for the Gemini telescopes, in a collaboration between the Australian Astronomical Observatory (AAO), the Herzberg Institute for Astrophysics, National Research Council (HIA-NRC) in Canada, and the Australian National University. This paper describes the design of the fiber optic system, developed by AAO. This system links the GHOST multi-object positioner, mounted on Gemini's Cassegrain focus, with the HIA-NRC developed spectrograph, located in the pier lab, 20 meters below the main observatory floor. The GHOST optical cable consists of 62 fibers, Polymicro FBP53/74/94P (53 μm core, 94 μm polyimide buffer), packed into 8 furcation tubes. The optical fibers are held inside the furcation tubes by friction, with between one and twelve fibers in each of the individual tubes. The furcation tubes are mechanically secured to manifold and anchor assemblies by bonding to integral Kevlar yarn within the tubing. The cable includes an interlock switch, linked to the telescope control system, to halt all telescope motions if the cable becomes overstressed. Fibers are terminated by two integral field units (IFU1 and IFU2), guiding and science slits and a calibration light entry port. Mode scrambling is achieved by mechanical agitation in two orthogonal directions, with adjustable frequency and amplitude of up to 10 Hz and 50 mm, respectively.
VELOCE is an IFU fibre feed and spectrograph for the AAT that is replacing CYCLOPS2. It is being constructed by the AAO and ANU. In this paper we discuss the design and engineering of the IFU/fibre feed components of the cable. We discuss the mode scrambling gain obtained with octagonal core fibres and how these octagonal core fibres should be spliced to regular circular core fibres to ensure maximum throughput for the cable using specialised splicing techniques. In addition we also describe a new approach to manufacturing a precision 1D/2D array of optical fibres for some applications in IFU manufacture and slit manufacture using 3D printed fused silica substrates, allowing for a cheap substitute to expensive lithographic etching in silicon at the expense of positional accuracy. We also discuss the Menlo Systems laser comb which employs endlessly-singlemode fibre to eliminate modal noise associated with multimode fibre transmission to provide the VELOCE spectrograph with a stable and repeatable source of wavelength calibration lines.
The Gemini High-Resolution Optical SpecTrograph (GHOST) is the newest instrument chosen for the Gemini South telescope. It is being developed by a collaboration between the Australian Astronomical Observatory (AAO), the NRC - Herzberg in Canada and the Australian National University (ANU). Using recent technological advances and several novel concepts it will deliver spectroscopy with R>50,000 for up to 2 objects simultaneously or R>75,000 for a single object. GHOST uses a fiber-image-slicer to allow use of a much smaller spectrograph than that nominally required by the resolution-slit–width product. With its fiber feed, we expect GHOST to have a sensitivity in the wavelength range between 363-950 nm that equals or exceeds that of similar directly-fed instruments on world-class facilities. GHOST has entered the build phase. We report the status of the instrument and describe the technical advances and the novel aspects, such as the lenslet-based slit reformatting. Finally, we describe the unique scientific role this instrument will have in an international context, from exoplanets through stellar elemental abundances to the distant Universe. Keywords: Gemini, spectrograph, spectroscopy, ́echelle, high resolution, radial velocity, fiber image slicer, integral field unit.
The Gemini High-resolution Optical SpecTrograph (GHOST) is a fiber fed spectrograph primarily designed for high efficiency and broad wavelength coverage (363 -1000nm), with an anticipated commissioning early in 2018. The primary scientific goal of the Precision Radial Velocity (PRV) mode will be follow-up of relatively faint (R>12) transiting exoplanet targets, especially from the TESS mission. In the PRV mode, the 1.2 arcsec diameter stellar image will be split 19 ways, combined in a single slit with a simultaneous Th/Xe reference source, dispersed at a resolving power of 80,000 and imaged onto two detectors. The spectrograph will be thermally stabilized in the Gemini pier laboratory, and modal noise will be reduced below other sources through the use of a fiber agitator. Unlike other precision high resolution spectrographs, GHOST will not be pressure controlled (although pressure will be monitored precisely), and there will be no double scrambler or shaped (e.g. octagonal) fibers. Instead, GHOST will have to rely on simultaneous two-color imaging of the slit and the simultaneous Th/Xe fiber to correct for variable fiber illumination and focal-ratio degradation. This configuration presents unique challenges in estimating a PRV error budget.
In the wavelength regime between 60 and 300 microns there are a number of atomic and molecular emission lines that
are key diagnostic probes of the interstellar medium. These include transitions of [CII], [NII], [OI], HD, H<sub>2</sub>D+, OH, CO,
and H<sub>2</sub>O, some of which are among the brightest global and local far-infrared lines in the Galaxy. In Giant Molecular
Clouds (GMCs), evolved star envelopes, and planetary nebulae, these emission lines can be extended over many arc
minutes and possess complicated, often self absorbed, line profiles. High spectral resolution (R> 10<sup>5</sup>) observations of
these lines at sub-arcminute angular resolution are crucial to understanding the complicated interplay between the
interstellar medium and the stars that form from it. This feedback is central to all theories of galactic evolution. Large
format heterodyne array receivers can provide the spectral resolution and spatial coverage to probe these lines over
The advent of large format (~100 pixel) spectroscopic imaging cameras in the far-infrared (FIR) will fundamentally
change the way astronomy is performed in this important wavelength regime. While the possibility of such instruments
has been discussed for more than two decades, only recently have advances in mixer and local oscillator technology,
device fabrication, micromachining, and digital signal processing made the construction of such instruments tractable.
These technologies can be implemented to construct a sensitive, flexible, heterodyne array facility instrument for
SOFIA. The instrument concept for StratoSTAR: Stratospheric Submm/THz Array Receiver includes a common user
mounting, control system, IF processor, spectrometer, and cryogenic system. The cryogenic system will be designed to
accept a frontend insert. The frontend insert and associated local oscillator system/relay optics would be provided by
individual user groups and reflect their scientific interests. Rapid technology development in this field makes SOFIA the
ideal platform to operate such a modular, continuously evolving instrument.
CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver, is a far-infrared
and submillimeter heterodyne spectrometer, being developed for the Stratospheric Observatory For Infrared Astronomy,
SOFIA. CASIMIR will use newly developed superconducting-insulating-superconducting (SIS) mixers.
Combined with the 2.5 m mirror of SOFIA, these detectors will allow observations with high sensitivity to be
made in the frequency range from 500 GHz up to 1.4 THz. Initially, at least 5 frequency bands in this range
are planned, each with a 4-8 GHz IF passband. Up to 4 frequency bands will be available on each flight and
bands may be swapped readily between flights. The local oscillators for all bands are synthesized and tuner-less,
using solid state multipliers. CASIMIR also uses a novel, commercial, field-programmable gate array (FPGA)
based, fast Fourier transform spectrometer, with extremely high resolution, 22000 (268 kHz at 6 GHz), yielding
a system resolution > 10<sup>6</sup>. CASIMIR is extremely well suited to observe the warm, ≈ 100K, interstellar medium,
particularly hydrides and water lines, in both galactic and extragalactic sources. We present an overview of the
instrument, its capabilities and systems. We also describe recent progress in development of the local oscillators
and present our first astronomical observations obtained with the new type of spectrometer.
CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver is a multiband, far infrared
and submillimeter, high resolution, heterodyne spectrometer under development for SOFIA. It is a first generation, PI
class instrument. CASIMIR is designed for detailed, high sensitivity observations of warm (100 K) interstellar gas both
in external galaxies and Galactic sources, including molecular clouds, circumstellar envelopes, and protostellar cores.
Combining the 2.5 m SOFIA mirror with state of the art superconducting mixers, will give CASIMIR unprecedented
sensitivity. Initially, CASIMIR will have two bands, at 1000 and 1250 GHz, and a further three bands, 550, 750, 1400
GHz, will be added soon after. Any four bands will be available on each flight. The availability of multiple bands during
each flight will allow for efficient use of flight time. For example, searches for weak lines from rare species in bright
sources can be carried out on the same flight with observations of abundant species in faint or distant objects.
Balanced receivers are under development at the Caltech Submillimeter Observatory (CSO) for the 230/460 GHz and 345/660 GHz atmospheric windows. The mixers are tunerless, implemented in a balanced configuration, have a 4-8 GHz IF, and can be used in dual frequency observation mode. As shall be seen, the balanced arrangement provides a high level of amplitude noise immunity and allows all of the available LO power to be used. In turn, this permits complete automation of the receivers by means of synthesized LO source(s).
A disadvantage of balanced mixers is, perhaps, that the sidebands at the IF remain convolved (DSB), unlike sideband separating (2SB) receivers. The latter, however are unbalanced and do not have the noise and LO injection advantages of balanced mixers. For the CSO, balanced mixers covering the range 180-720 GHz were judged most promising to facilitate many of the astrophysical science goals in the years to come.
In parallel, a dual polarization 280-420~GHz continuous comparison (correlation) receiver is in an advanced state of development. The instrument has two beams on the sky; a reference and a signal beam. Using only cooled reflecting optics, two polarizing grids, and a quadrature hybrid coupler, the sky beams are coupled to four tunerless SIS mixers (both polarizations). The 4-12 GHz mixer IF outputs are, after amplification, correlated against each other. In principle, this technique results in flat baselines with very low RMS noise, and is especially well suited for high redshift Galaxy work.
Not only do these changes greatly enhance the spectroscopic capabilities of the CSO, they will also enable the observatory to be integrated into the Harvard-Smithsonian Submillimeter Array (SMA), as an additional telescope.
Under development at the Caltech Submillimeter Observatory is a dual polarization, continuous comparison (correlation) receiver. The instrument has two beams on the sky; a reference and a signal beam. Using only cooled reflecting optics, two polarizing grids, and a quadrature hybrid coupler, the sky beams are coupled to four tunerless SIS mixers (both polarizations). The 4-8 GHz mixer IF outputs are, after amplification, correlated against each other. In principle, this technique results in flat baselines with very low RMS noise and is especially well suited for high redshift Galaxy work. At the same time an upgrade is planned to the existing facility heterodyne instrumentation. Dual frequency mode receivers are under development for the 230/460 GHz and 345/660 GHz atmospheric windows. The higher frequency receivers are implemented in a balanced configuration, which reduces both the LO power requirement and noise. Each mixer has 4 GHz of IF bandwidth and can be controled remotely.
Not only do these changes greatly enhance the spectroscopic capabilities of the CSO, they also enable the observatory to be integrated into the Harvard-Smithsonian Submillimeter Array (SMA) as an additional baseline.
The CAltech Submillimeter Interstellar Medium Investigations Receiver (CASIMIR) is a multichannel, heterodyne spectrometer being developed for the Stratospheric Observatory for Infrared Astronomy (SOFIA). It has a very high resolution, up to a million, over the submillimeter and far-infrared wavelength range of 150 to 600 micrometers , or 2.0 to 0.5 THz. CASIMIR is extremely well suited to the investigation of both the galactic and extragalactic warm, approximately 100 K, interstellar medium. A combination of advanced SIS and Hot Electron Bolometers receivers will be used to cover this frequency range with very high sensitivity. CASIMIR will use only solid state local oscillators, with quasioptical coupling to the mixers. We present a description of the instrument and its capabilities, including detailed discussions of the receivers, local oscillators and IF systems.
On future astronomical instruments for the soft x-ray to FUV, stray light may be a significant cause of background events. Currently, we are engaged in an ongoing program to identify materials that are suitable for use as low- reflectance surfaces in space based instruments. As a result, we have measured the scattering performance in this spectral region, of wide a selection of low-reflectivity materials, produced with a range of processes. We present preliminary measurements of the absolute bidirectional reflectance distribution function (BRDF) for a selection of seven of these materials. Measurements were obtained at a five spectral lines, including strong geocoronal lines, over the wavelength range 44 to 1216 angstrom at near grazing incidence. We find that in most cases for constant incident and scatter angles, the total variation of BRDF with wavelength over this range is only a factor of order ten. We also find that although we have identified materials which in many instances have lower reflectances than bead blasted aluminum, it is still a good choice for most applications given its low cost and convenience.
Small pore size microchannel plates (MCPs) are needed to satisfy the requirements for future high resolution small and large format detectors for astronomy. MCPs with pore sizes in the range 5 micrometer to 8 micrometer are now being manufactured, but they are of limited availability and are of small size. We have obtained sets of Galileo 8 micrometer and 6.5 micrometer MCPs, and Philips 6 micrometer and 7 micrometer pore MCPs, and compared them to our larger pore MCP Z stacks. We have tested back to back MCP stacks of four of these MCPs and achieved gains greater than 2 multiplied by 10<SUP>7</SUP> with pulse height distributions of less than 40 percent FWHM, and background rates of less than 0.3 events sec<SUP>-1</SUP> cm<SUP>-2</SUP>. Local counting rates up to approximately equals 100 events/pore/sec have been attained with little drop of the MCP gain. The bare MCP quantum efficiencies are somewhat lower than those expected, however. Flat field images are characterized by an absence of MCP fixed pattern noise.
The microchannel plates for the detectors in the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in late 1995 are described. A low resistance Z stack of microchannel plates (MCPs) is employed in a detector format of 27 mm multiplied by 10 mm using a multilayer cross delay line anode (XDL) with 1024 by 360 digitized pixels. The MCP stacks provide gains of greater than 2 multiplied by 10<SUP>7</SUP> with good pulse height distributions (as low as 25% FWHM) under uniform flood illumination. Background rates of approximately equals 0.6 event cm<SUP>-2</SUP> sec<SUP>-1</SUP> are obtained for this configuration. Local counting rates up to approximately equals 800 events/pixel/sec have been achieved with little drop of the MCP gain. MCP preconditioning results are discussed, showing that some MCP stacks fail to have gain decreases when subjected to a high flux UV scrub. Also, although the bare MCP quantum efficiencies are close to those expected (approximately equals 10%), we found that the long wavelength response of KBr photocathodes could be substantially enhanced by the MCP scrubbing process. Flat field images are characterized by a low level of MCP fixed pattern noise and are stable. Preliminary calibration results for the instruments are shown.
It has been found that the gain depression in MCP's operated at high gains is a relatively long range phenomenon. Active pores can significantly depress the gain in the surrounding quiescent pores at distances of the order of millimeters. This is of fundamental importance for detectors in which high point source count rates are encountered. We have measured this effect for a variety of plate operating conditions and point source count rates and find that in all cases there is a constant limiting radius. We have also determined that the gain depression has a long term effect on the MCP.
Microchannel plate (MCP) detectors are often used with charge division anode readouts, such as the SPAN anode, to provide high position resolution. This paper discusses the effect on image quality, of digitization (causing fixed patterning), electronic noise, pulse height distribution (PHD) and charge cloud size. The discussion is supported by experimental data obtained from a one dimensional SPAN anode, developed for the SOHO Coronal Diagnostic Spectrometer (CDS) Grazing Incidence Spectrometer (GIS). Results from a computer model of this detector, and from a charge cloud simulation model, are also included. The SPAN anode normally has three sinusoidal electrodes with phase differences of 120 degree(s)C. An alternative configuration is to use a phase difference of 90 degree(s)C. This paper compares the advantages and disadvantages of these arrangements.
The SPAN position readout device uses a charge division and measurement method to encode the coordinates of the centroid of a charge cloud and thus provides a technique for imaging with photon counting detectors of various formats; for example, microchannel plate intensifiers and gas proportional counters. Its principle of operation causes the position resolution to substantially exceed the charge measurement precision. The reduced signal to noise requirement compared with the competitive devices of comparable imaging format size enables the SPAN readout system to operate at higher input count rates. We present imaging performance results from SPAN readout systems incorporated in several detector formats. The dependence of the physical parameters of the SPAN pattern design on the detector type and geometry together with the performance trade-offs between speed and resolution for these particular detectors are discussed. The practical implementation of the SPAN readout decoding algorithm is outlined. We describe the experimental applications for which the SPAN readout system has been proposed.
The compact photon-counting detector SPAN is described which offers 25-micron spatial resolution and a 25-mm imaging diam. The SPAN detector incorporates position readout within a vacuum-sealed optical-intensifier tube, and a photocathode is used to sense the images with high-blue and near-UV sensitivity. A microchannel-plate intensifier generates an electron cloud that is measured with a position-sensitive readout developed for this application. The position-sensitive readout is a conductive device that, in the context of the SPAN, permits a high count rate and spatial resolution greater than the charge-measurement precision of each electrode. Preliminary photon counting is demonstrated, and the results suggest that the SPAN has a resolution of better than 1/1000 and effective linearity with 8-bit digitization.
The concept for the 2D position-readout device for the SPAN photon-counting detector is presented with attention to the count rates, spatial resolution, and charge-measurement precision. The electrodes which are deposited on the planar substrate result from charge division induced by a charge cloud, the centroid position of which is encoded by the ratio of charge magnitudes. The SPAN electrode design is analyzed and theorized to permit 1000 x 1000-pixel resolution at 1 MHz. The SPAN spiral-anode six-electrode design is compared to the Vernier-anode twelve-electrode structure for encoding 2D position, and digital precision is analyzed at count rates up to 1 MHz. The SPAN readout affords resolution levels of up to 1/1000 across the entire active area at 8-bit digitization.