The limits to the angular resolution has, during the latest 70 years, been obtainable from the ground only through extremely expensive adaptive optics facilities at large telescopes, and covering extremely small spatial areas per exposure. Atmospheric turbulence therefore limits image quality to typically 1 arcsec in practice. We have developed a new concept of ground-based imaging instrument called GravityCam capable of delivering significantly sharper images from the ground than is normally possible without adaptive optics. The acquisition of visible images at high speed without significant noise penalty has been made possible by advances in optical and near IR imaging technologies. Images recorded at high speed can be aligned before combination and can yield a 3-5 fold improvement in image resolution, or be used separately for high-cadence photometry. Very wide survey fields are possible with widefield telescope optics. GravityCam is proposed to be installed at the 3.6m New Technology Telescope (NTT) at the ESO La Silla Observatory in Chile, where it will greatly accelerate the rate of detection of Earth sized planets by gravitational microlensing. GravityCam will also improve substantially the quality of weak shear studies of dark matter distribution in distant clusters of galaxies and provide a vast dataset for asteroseismology studies. In addition, GravityCam promises to generate a unique data set that will help us understand of the population of the Kuiper belt and possibly the Oort cloud.
Here we present the Adaptive Optics Lucky Imager (AOLI), a state-of-the-art instrument which makes use of two well proved techniques, Lucky Imaging (LI) and Adaptive Optics (AO), to deliver diffraction limited imaging at visible wavelengths, 20 mas, from ground-based telescopes. Thanks to its revolutionary TP3-WFS, AOLI shall have the capability of using faint reference stars. In the extremely-big telescopes era, the combination of techniques and the development of new WFS systems seems the clue key for success. We give details of the integration and verification phases explaining the defiance that we have faced and the innovative and versatile solutions for each of its subsystems that we have developed, providing also very fresh results after its first fully-working observing run at the William Herschel Telescope (WHT).
The limits to the angular resolution achievable with conventional ground-based telescopes are unchanged over 70 years. Atmospheric turbulence limits image quality to typically ~1 arcsec in practice. We have developed a new concept of ground-based imaging instrument called GravityCam capable of delivering significantly sharper images from the ground than is normally possible without adaptive optics. The acquisition of visible images at high speed without significant noise penalty has been made possible by advances in optical and near IR imaging technologies. Images are recorded at high speed and then aligned before combination and can yield a 3-5 fold improvement in image resolution. Very wide survey fields are possible with widefield telescope optics. We describe GravityCam and detail its application to accelerate greatly the rate of detection of Earth size planets by gravitational microlensing. GravityCam will also improve substantially the quality of weak shear studies of dark matter distribution in distant clusters of galaxies. The microlensing survey will also provide a vast dataset for asteroseismology studies. In addition, GravityCam promises to generate a unique data set that will help us understand of the population of the Kuiper belt and possibly the Oort cloud.
The Adaptive Optics Lucky Imager, AOLI, is an instrument developed to deliver the highest spatial resolution ever obtained in the visible, 20 mas, from ground-based telescopes. In AOLI a new philosophy of instrumental prototyping has been applied, based on the modularization of the subsystems. This modular concept offers maximum flexibility regarding the instrument, telescope or the addition of future developments.
The combination of Lucky Imaging with a low order adaptive optics system was demonstrated very successfully on the Palomar 5m telescope nearly 10 years ago. It is still the only system to give such high-resolution images in the visible or near infrared on ground-based telescope of faint astronomical targets. The development of AOLI for deployment initially on the WHT 4.2 m telescope in La Palma, Canary Islands, will be described in this paper. In particular, we will look at the design and status of our low order curvature wavefront sensor which has been somewhat simplified to make it more efficient, ensuring coverage over much of the sky with natural guide stars as reference object. AOLI uses optically butted electron multiplying CCDs to give an imaging array of 2000 x 2000 pixels.
Many adaptive optics (AO) systems in use today require the use of bright reference objects to determine the effects of atmospheric distortions. Typically these systems use Shack-Hartmann Wavefront sensors (SHWFS) to distribute incoming light from a reference object between a large number of sub-apertures. Guyon et al. evaluated the sensitivity of several different wavefront sensing techniques and proposed the non-linear Curvature Wavefront Sensor (nlCWFS) offering improved sensitivity across a range of orders of distortion. On large ground-based telescopes this can provide nearly 100% sky coverage using natural guide stars. We present work being undertaken on the nlCWFS development for the Adaptive Optics Lucky Imager (AOLI) project. The wavefront sensor is being developed as part of a low-order adaptive optics system for use in a dedicated instrument providing an AO corrected beam to a Lucky Imaging based science detector. The nlCWFS provides a total of four reference images on two photon-counting EMCCDs for use in the wavefront reconstruction process. We present results from both laboratory work using a calibration system and the first on-sky data obtained with the nlCWFS at the 4.2 metre William Herschel Telescope, La Palma. In addition, we describe the updated optical design of the wavefront sensor, strategies for minimising intrinsic effects and methods to maximise sensitivity using photon-counting detectors. We discuss on-going work to develop the high speed reconstruction algorithm required for the nlCWFS technique. This includes strategies to implement the technique on graphics processing units (GPUs) and to minimise computing overheads to obtain a prior for a rapid convergence of the wavefront reconstruction. Finally we evaluate the sensitivity of the wavefront sensor based upon both data and low-photon count strategies.
Lucky Imaging combined with a low order adaptive optics system has given the highest resolution images ever taken in
the visible or near infrared of faint astronomical objects. This paper describes a new instrument that has already been
deployed on the WHT 4.2m telescope on La Palma, with particular emphasis on the optical design and the predicted
system performance. A new design of low order wavefront sensor using photon counting CCD detectors and multi-plane
curvature wavefront sensor will allow virtually full sky coverage with faint natural guide stars. With a 2 x 2 array of
1024 x 1024 photon counting EMCCDs, AOLI is the first of the new class of high sensitivity, near diffraction limited
imaging systems giving higher resolution in the visible from the ground than hitherto been possible from space.
Electron Multiplying CCDs (EMCCDs) are used much less often than they might be because of the challenges they offer
camera designers more comfortable with the design of slow-scan detector systems. However they offer an entirely new
range of opportunities in astrophysical instrumentation. This paper will show some of the exciting new results obtained
with these remarkable devices and talk about their potential in other areas of astrophysical application. We will then
describe how they may be operated to give the very best performance at the lowest possible light levels. We will show
that clock induced charge may be reduced to negligible levels and that, with care, devices may be clocked at significantly
higher speeds than usually achieved. As an example of the advantages offered by these detectors we will show how a
multi-detector EMCCD curvature wavefront sensor will revolutionise the sensitivity of adaptive optics instruments and
been able to deliver the highest resolution images ever taken in the visible or the near infrared.
The highest resolution images ever taken in the visible were obtained by combining Lucky Imaging and low order
adaptive optics. This paper describes a new instrument to be deployed on the WHT 4.2m and GTC 10.4 m telescopes on
La Palma, with particular emphasis on the optical design and the expected system performance. A new design of low
order wavefront sensor using photon counting CCD detectors and multi-plane curvature wavefront sensor will allow
dramatically fainter reference stars to be used, allowing virtually full sky coverage with a natural guide star. This paper
also describes a significant improvements in the efficiency of Lucky Imaging, important advances in wavefront
reconstruction with curvature sensors and the results of simulations and sensitivity limits. With a 2 x 2 array of 1024 x
1024 photon counting EMCCDs, AOLI is likely to be the first of the new class of high sensitivity, near diffraction
limited imaging systems giving higher resolution in the visible from the ground than hitherto been possible from space.
The Adaptive Optics Lucky Imager (AOLI) is a new instrument under development to demonstrate near diffraction
limited imaging in the visible on large ground-based telescopes. We present the adaptive optics system being designed
for the instrument comprising a large stroke deformable mirror, fixed component non-linear curvature wavefront sensor
and photon-counting EMCCD detectors. We describe the optical design of the wavefront sensor where two photoncounting
CCDs provide a total of four reference images. Simulations of the optical characteristics of the system are
discussed, with their relevance to low and high order AO systems. The development and optimisation of high-speed
wavefront reconstruction algorithms are presented. Finally we discuss the results of simulations to demonstrate the
sensitivity of the system.
In this paper we present a preliminary analysis of variation in the isoplanatic patch size over short timescales and
wide angular separations. We tested a visible band photon counting camera running with four 1K2 detectors
to provide a contiguous field of view of 1000 × 4000 pixels. Resolution was 35-100 mas per pixel at frame rates
from 20-111hz, providing data on atmospheric turbulence at angular separations of up to 400 arcseconds. We
discuss the potential of such cameras to perform high resolution optical surveys using developments of standard
lucky imaging techniques, and the implications of our results for adaptive optics systems design.
KEYWORDS: Signal to noise ratio, Stars, Point spread functions, Interference (communication), Sensors, Photon counting, Image processing, Data acquisition, Monte Carlo methods, Stochastic processes
Lucky imaging is a proven technique for near diffraction limited imaging in the visible; however, data reduction
and analysis techniques are relatively unexplored in the literature. In this paper we use both simulated and real
data to test and calibrate improved guide star registration methods and noise reduction techniques. In doing
so we have produced a set of "best practice" recommendations. We show a predicted relative increase in Strehl
ratio of ~ 50% compared to previous methods when using faint guide stars of ~17th magnitude in I band, and
demonstrate an increase of 33% in a real data test case. We also demonstrate excellent signal to noise in real
data at flux rates less than 0.01 photons per pixel per frame above the background flux level.
Near-diffraction limited imaging and spectroscopy in the visible on large (8-10 meter) class telescopes has proved to be
beyond the capabilities of current adaptive optics technologies, even when using laser guide stars. The need for high
resolution visible imaging in any part of the sky suggests that a rather different approach is needed. This paper describes
the results of simulations, experiments and astronomical observations that show that a combination of low order adaptive
optic correction using a 4-field curvature sensor and fast Lucky Imaging strategies with a photon counting CCD camera
systems should deliver 20-25 milliarcsecond resolution in the visible with reference stars as faint as 18.5 magnitude in I
band on large telescopes. Such an instrument may be used to feed an integral field spectrograph efficiently using
configurations that will also be described.
The design of electron multiplying CCD cameras require a very different approach from that appropriate for slow scan
CCD operation. This paper describes the main problems in using electron multiplying CCDs for high-speed, photon
counting applications in astronomy and how these may be substantially overcome. With careful design it is possible to
operate the E2V Technologies L3CCDs at rates well in excess of that claimed by the manufacturer, and that levels of
clock induced charge dramatically lower than those experienced with commercial cameras that need to operate at unity
gain. Measurements of the performance of the E2V Technologies CCD201 operating at 26 MHz will be presented
together with a guide to the effective reduction of clock induced charge levels. Examples of astronomical results
obtained with our cameras are presented.
We have recently demonstrated diffraction-limited resolution imaging in the visible on the 5m Palomar Hale telescope.
The new LAMP instrument is a Lucky Imaging backend camera for the Palomar AO system. Typical resolutions of
35-40 mas with Strehls of 10-20% were achieved at 700nm, and at 500nm the FWHM resolution was as small as 42
milliarcseconds. In this paper we discuss the capabilities and design challenges of such a system used with current and
near future AO systems on a variety of telescopes. In particular, we describe the designs of two planned Lucky Imaging
+ AO instruments: a facility instrument for the Palomar 200" AO system and its PALM3K upgrade, and a visible-light
imager for the CAMERA low-cost LGS AO system planned for the Palomar 60" telescope. We introduce a Monte Carlo
simulation setup that reproduces the observed PSF variability behind an adaptive optics system, and apply it to predict the
performance of 888Cam and CAMERA. CAMERA is predicted to achieve diffraction-limited resolution at wavelengths as
short as 350 nm. In addition to on-axis resolution improvements we discuss the results of frame selection with the aim of
improving other image parameters such as isoplanatic patch sizes, showing that useful improvements in image quality can
be made by Lucky+AO even with very temporally and spatially undersampled data.
We have analyzed the application of frame selection ("lucky imaging") to adaptive optics (AO), short-exposure
observations of faint companions. We have used the instantaneous Strehl ratio as an image quality metric. The
probability density function (PDF) of this quantity can be used to determine the outcome of frame selection in terms of
optimizing the Strehl ratio and the peak-signal-to-noise-ratio of the shift-and-add image. In the presence of static
speckles, frame selection can lead to both: improvement in resolution--as quantified by the Strehl ratio, as well as faint
signal detectability--given by the peak-signal-to-noise-ratio. This theoretical prediction is confirmed with real data from
AO observations using Lick Observatory's 3m Shane telescope, and the Palomar Observatory's 5m Hale telescope. In
addition, we propose a novel statistics-based technique for the detection of faint companions from a sequence of AO-corrected
exposures. The algorithm, which we call stochastic speckle discrimination, utilizes the "statistical signature" of
the centre of the point spread function (PSF) to discriminate between faint companions and static speckles. The
technique yields excellent results even for signals invisible in the shift-and-add images.
Faint object diffraction limited imaging in the visible from the ground has recently been demonstrated on a 5 m
telescope with more than twice the resolution of Hubble for the first time. It has shown the way towards diffraction
limited imaging in the visible with the next generation of large telescopes. This paper describes the results of
experiments to show how this is achieved and what is needed to work well with faint natural guide stars. The
importance of a large isoplanatic patch size is also emphasised. In particular, we will describe a new approach to the
design of high efficiency, low order adaptive curvature sensors which use photon counting CCD detectors. Such
systems used on larger telescopes together with image segmentation and resynthesis techniques using closure phase
techniques are shown to have an important place in achieving these goals. The optimum combination of these different
techniques will be explained for a variety of different applications.
We present a summary of activity at the Cambridge Optical Aperture Synthesis Telescope (COAST) group
during the period 2004-2006. Our main program has focused on technical design and prototyping for future
facility arrays such as the VLTI and Magdalena Ridge Observatory Interferometer, but with a small parallel
effort of focused astronomical observations with COAST, in particular multi-wavelength studies of supergiants.
We report on progress on these and other technical areas over the past 2 years.
Electron multiplying CCDs, e.g. as delivered by E2V Technologies and
Texas Instruments, have the potential to become the detectors of
choice for all future optical interferometers, replacing expensive
fibre-fed arrays of APDs. We report here on the development of a new
500-channel spectroscopic back end for the COAST interferometer that
exploits such a device. An E2V Technologies CCD97 (back illuminated)
EMCCD with sub-electron readout noise is used as the detector, and can
be read out at pixel rates of up to 30~MHz. We present software and
hardware approach used to integrate a new CCD controller with the
COAST as well as results from lab tests of the detector and
controller.
We present a summary of the activity of the Cambridge Optical Aperture
Synthesis Telescope (COAST) team and review progress on the
astronomical and technical projects we have been working on in the
period 2002--2004. Our current focus has now moved from operating
COAST as an astronomical instrument towards its use as a test-bed for
strategic technical development for future facility arrays. We have
continued to develop a collaboration with the Magdalena Ridge
Observatory Interferometer, and we summarise the programmes we expect
to be working on over the next few years for that ambitious
project. In parallel, we are investigating a number of areas for the
European Very Large Telescope Interferometer and these are outlined
briefly.
Many astronomical imaging studies, such as those of weak gravitational lensing, call for better angular resolution than is normally possible from the ground over wide fields of view . For many of these studies astronomers need images which show a consistent point spread function across the field even if this comes at the expense of the ultimate in angular resolution at the centre of the field. Adaptive Optics does not show any prospect of being able to achieve fields of view as large as are needed at visible wavelengths and therefore a new technique of delivering high resolution images from the ground must be developed. Electron multiplying CCDs are available that allow images to be taken at high speed without the usual penalty of read noise. We have developed a new technique called Lucky Imaging which achieves high resolution by selecting the better images from a sequence of images, then shifting and adding each to give a much higher resolution output image. Resolutions in the range 0.1-0.2 arc seconds can be obtained routinely under relatively good conditions on a 2.5 metre telescope working in I band (850 nanometres) and using as much as 30% of the images taken. Even under poorer conditions we find that image selection allows the final resolution to be better than the traditional seeing value by a factor of as much as three. This paper describes the technique and some of the results obtained using this method.
The Cambridge Infra-red Panoramic Survey Spectrograph (CIRPASS) is described. This near-infrared (NIR) spectrograph has been used on the 8m Gemini-South Telescope, the 3.9m Anglo-Australian Telescope (AAT) and the 4.2m William Herschel Telescope (WHT). Its performance in both integral field mode and multi-object mode is discussed and some scientific highlights are presented. A multi-IFU system, which is currently under construction, is also described.
L3CCDs represent a major step in CCD performance with great potential for astronomical applications because of their
ability to work at very high pixel rates with negligible readout noise. This paper describes the results of tests on some of
the L3CCDs now available and discusses how the operating conditions may be optimised for a variety of different applications. In particular, at high gain they can be used for photon counting work at photon rates well in excess of one photon per pixel per second. Readout rates which can be as high as 35MHz are entirely practical for a number of astronomical applications. This paper describes some of the compromises and trade-offs that have to be made in designing high-speed controllers to work effectively with these devices. The importance of integrating high-speed controllers for astronomy with significant amount of real-time processing power is also discussed.
In 1996, it was proposed to build a near-infrared imager for the 3.8-m UK Infrared Telescope in Hawaii, to exploit the 1024 pixel format detectors that were then becoming available. In order to achieve a fast delivery, the instrument was kept simple and existing designs were reused or modified where possible. UFTI was delivered within 2.5 years of the project start. The instrument is based around a 1k Rockwell Hawaii detector and a LSR Astrocam controller and uses the new Mauna Kea optimized J,H,K filter set along with I and Z broad-band filters and several narrow-band line filters. The instrument is cooled by a CTI cry-cooler, while the mechanisms are operated by cold, internal, Bergelahr stepping motors. On UKIRT it can be coupled to a Fabry-Perot etalon for tunable narrow-band imaging at K, or a waveplate for imaging polarimetry through 1-2.5 μm; the cold analyzer is a Barium Borate Wollaston prism. UFTI was designed to take full advantage of the good image quality delivered by UKIRT on conclusion of the upgrades program, and has a fine scale of 0.09 arcsec/pixel. It is used within the UKIRT observatory environment and was the first instrument integrated into ORAC, the Observatory Reduction and Acquisition Control System. Results obtained during instrument characterization in the lab and over the last 3 years on UKIRT are presented, along with performance figures. UFTI has now been used on UKIRT for several hundred nights, and aspects of instrument performance are discussed.
KEYWORDS: Visibility, Charge-coupled devices, Signal detection, Signal to noise ratio, Interference (communication), Sensors, Electrons, Digital signal processing, Interferometry, Signal processing
Most current CCDs cannot be used as optical interferometric sensors because the high readout noise disguises the small signal. However, new low light level charge coupled devices (L3CCD) have a large on-chip gain which can allow a signal to be detected above the noisy readout amplifier. This gain has a statistical nature, meaning that the photon input cannot be predicted exactly. We investigate several techniques for photon prediction at different light levels, and demonstrate how this affects the noise on the signal. Accurate signal estimation can be achieved with very faint signals, up to about one photon per pixel per read. Above this, accuracy gradually decreases, though our signal-to-noise ratio is never worse than square root(2n). Optical interferometry requires detection of very faint signals, and the use of an L3CCD is found to allow reproduction of interferometric visibilities to high precision. Custom instrumentation used for control is also detailed.
We present a summary of the status of the Cambridge Optical Aperture
Synthesis Telescope, and review developments at the array through the
period 2000-2002. Summaries of the astronomical and technical
programmes completed, together with an outline of those that are
currently in progress are presented. Since our last report two years
ago in 2000, there have been significant changes in the context for
astronomical interferometry in the UK. We review these developments,
and describe our plans for the near and intermediate term at COAST,
and with colleagues in Europe at the VLTI and in the USA at the
Magdalena Ridge Observatory in New Mexico.
The use of faint reference stars for the selection of good short exposure images has recently been demonstrated as a technique which can provide essentially diffraction-limited I band imaging from well-figured ground-based telescopes as large as 2.5 m diameter. The faint limiting magnitude and enhanced isoplanatic patch size for the selected exposures technique means that 20% of the night sky is within range of a suitable reference star for I-band imaging. Typically the 1%-10% of exposures with the highest Strehl ratios are selected. When these exposures are shifted and added together, field stars in the resulting images have Strehl ratios as high as 0.26 and FWHM as small as 90 milliarcseconds. Within the selected exposures the isoplanatic patch is found to be up to 50 arcseconds in diameter at 810 nm wavelength. Images within globular clusters and of multiple stars from the Nordic Optical Telescope using reference stars as faint as I~16 are presented. The technique relies on a new generation of CCDs which provide sub-electron readout noise at very fast readout rates. The performance of the selection technique for various astronomical programs is discussed in comparison with natural guide star Adaptive Optics (AO).
This paper is intended to discuss the impact of noiseless CCD detectors in three significant areas of the development of large telescopes and the instruments that go on them. These are (1) CCDs that have all the characteristics that we are used to seeing in CCDs can now be made with negligible readout noise even at higher pixel rates (> 10 MHz) and will allow rather different approaches to the design of instrument is generally, (2) the technique of achieving diffraction limited imaging in ground-based telescopes known as Lucky Astronomy in which images are taken at high speed has been demonstrated to work under a variety of different conditions and (3) some suggestions as to how these methods may be applied directly to much larger diameter telescopes in order to achieve high resolution imaging and spectroscopy without the expense of laser guide stars or multi-conjugate adaptive optics.
A radically new CCD development by Marconi Applied Technology has enabled substantial internal gain within the CCD before the signal reaches the output amplifier. With reasonably high gain, sub-electron readout noise levels are achieved even at MHz pixel rates. This paper reports a detailed assessment of these devices, including novel methods of measuring their properties when operated at peak mean signal levels well below one electron per pixel. The devices are shown to be photon shot noise limited at essentially all light levels below saturation. Even at the lowest signal levels the charge transfer efficiency is good. The conclusion is that these new deices have radically changed the balance in the perpetual trade-off between read out noise and the speed of readout. They will force a re- evaluation of camera technologies and imaging strategies to enable the maximum benefit to be gained form these high- speed, essentially noiseless readout devices. This new LLLCCD technology, in conjunction with thinning should provide detectors which will be very close indeed to being theoretically perfect.
Craig Mackay, Richard McMahon, Martin Beckett, Meghan Gray, Richard Ellis, Andrew Firth, M. Hoenig, James Lewis, Steve Medlen, Ian Parry, Jim Pritchard, Chris Sabbey
The search for galaxies at redshifts > becomes increasingly difficult in the visible since most of the light emitted by these objects is redshifted into the near IR. The recent development of high-performance near IR arrays has made it practical to built a wide field survey instrument for operation in the near IR part of the spectrum. CIRSI, the Cambridge IR Survey Instrument, uses four of the Hawaii-1 MCT arrays each of which has 1024 by 1024 pixels. This paper describes a number of the novel feature of CIRSI and summarizes the present performance achieved by CIRSI and the scientific programs it is principally engaged in.
Ian Parry, Craig Mackay, Rachel Johnson, Richard McMahon, Andrew Dean, Anamparambu Ramaprakash, David King, Jim Pritchard, Steve Medlen, Chris Sabbey, Richard Ellis, Alfonso Aragon-Salamanca
We describe CIRPASS which is currently being completed at the Institute of Astronomy in Cambridge. The optical design is presented and contrasted with more conventional software- suppression designs. The limiting magnitudes on Gemini are expected to be J 22.6 and H 21.7 in a 3 hour exposure.
We present the latest astronomical results from the Cambridge Optical Aperture Synthesis Telescope (COAST). COAST is a first-generation stellar interferometer, which uses an array of small (40 cm) separated telescopes to perform high-resolution imaging at visible and near-infrared wavelengths. The new science results from COAST exploit two recently-added capabilities of the COAST array, namely the ability to observe in any over the infrared J, H and K bands as well as at visible wavelengths, plus operation with five telescopes. We present contemporaneous observations of the red supergiant Betelgeuse at three wavelengths in the red and near-infrared. These data show that the apparent symmetry of the stellar disk is a strong function of wavelength, but that the bright spots seen in visible light are consistent with a convective origin. Data obtained using all five array elements on the symbiotic star CH Cygni reveal an elliptical distortion of the disk of the red giant, possibly related to mass transfer of a compact companion.
We present a summary of the status of the Cambridge Optical Aperture Synthesis Telescope (COAST). Since our last report we have concentrated on improving both the efficiency of use of the array and its astrophysical capabilities. In particular we have achieved useful improvements in throughput, detector sensitivity and the efficiency of securing measurements of visibility amplitudes and closure phases. With five telescopes fully operational, COAST is now being used routinely for parallel programs of astrophysics and as a technical test-bed for its proposed successor, the Large Optical Array--LOA.
At the Cambridge Optical Aperture Synthesis Telescope (COAST), first-generation photon counting avalanche photodiodes (APDs) have been used as the pupil-plane fringe detectors in the optical regime. These are being replaced with EG&G's super-low k (`SliK') APDs, which have an exceptionally low dark count (fewer than 100 counts per second) and high detection efficiency (up to 70% at 700 nm). The new detectors have increased the limiting magnitude of the telescope, enabling the observation of targets previously too faint to be seen. We shall discuss the operation of these devices at COAST and present new interferometric observations of stellar objects at visible magnitudes of eight and fainter.
This paper describes the results of a test program to evaluate four Rockwell HAWAII and two PICNIC near IR array detectors with a view to their application in imaging, spectroscopy and in fast telescope tracking and interferometer fringe detection. Results of the laboratory test of the arrays are presented, together with a guide for their general operation.
We have built a panoramic wide field near infrared imaging camera based on 4 Rockwell HAWAII 1024 X 1024 detectors. The baseline survey instrument operates in the region 0.8 to 1.8 micrometers on non-IR optimized telescopes with an upgrade at K band in 1999. The instrument was commission on the 2.5m INT and 4.2m WHT telescopes in December 1997 and January 1998. The main design goals in this project were to produce a highly productive astronomical instrument in a very short space of time and for low cost. Survey instruments are by their nature very versatile and CIRSI will support the wide range of astronomical interests at the Institute of Astronomy. Furthermore, since CIRSI is a traveling instrument and we are able to operate at a number of different telescopes to take opportunity of a range of image sizes and scales.
COHSI was successfully commissioned at the United Kingdom IR Telescope on Mauna Kea during a seven night observing run which coincided with this conference. Here we briefly describe the instrument and give a preliminary report on its performance at this time. The suppression optics and masks worked extremely well and the instrument background was found to be very low.
This paper reviews the current performance of the Cambridge Optical Aperture Synthesis Telescope as an imaging array. Tests of the hardware and methods of measuring fringe visibility and closure phase are described in the context of prospects for a Large Optical Array.
A low-resolution CCD spectrometer has been installed at COAST to provide multi-wavelength fringe measurements across the band 650-950 nm. The measurements are based on the analysis of time-series of channeled spectra. Laboratory tests and stellar observations are presented. The advantages and limitations of the system are discussed.
There are difficult problems involved in building a near-infrared interferometer which uses more than two elements simultaneously. These problems have been overcome at the Cambridge Optical Aperture Synthesis Telescope(COAST). This has allowed us to make the first closure phase measurements on an astronomical source in the infrared.
We describe a scheme for fast, low noise readout of an infrared focal-plane array detector, capable of adequately sampling pupil plane fringes on three simultaneous baselines, as well as a procedure for aligning a many-component beam-combiner in the infrared. Finally, the performance of the working COAST infrared system is discussed.
In September 1995 the Cambridge optical aperture synthesis telescope (COAST) became the first optical interferometer to produce an image of a stellar source from phase-closure and visibility amplitude measurements. These observations demonstrated for the first time the feasibility of operating long-baseline optical/near-infrared interferometers for high dynamic range high-resolution imaging. Here we present these and subsequent observations made with COAST and describe the methods used to analyze such data.
The Cambridge optical aperture synthesis telescope (COAST) is a four element interferometer which measures visibility amplitudes and closure-phases. It produced its first images in 1995 and is now in a complete form, very similar to the original conception. In this paper we discuss the design and current status of the interferometer.
One of the main limitations to the study of faint objects in the near-infrared (1 to 2 micrometer) is the luminous, varying sky background from very narrow OH emission lines originating in the Earth's upper atmosphere. This source of background contributes 95% to 98% of the total sky counts in the J & H atmospheric windows. We present the optical layout of the Cambridge OH suppression instrument. COHSI is designed to deliver OH suppressed, R equals 500, spectroscopy for both J & H spectral bands simultaneously providing an integral field mode and a multi-object mode. COHSI also has an OH suppression imaging mode. A modular approach has been selected for COHSI with the instrument consisting of three components. The first section consists of simple re-imaging lenses and a lens array interfacing the telescope to a set of optical fibers. This decouples the design of COHSI's main components from the telescope allowing COHSI to be easily used with different telescopes and making it free from flexure problems. The second section of COHSI is the OH suppression 'filter' itself. The size of this section is significantly smaller than in other similarly planned instruments. The third and final module of COHSI is the cryogenic low-resolution imaging spectrograph.
We are currently building a panoramic wide field near infrared imaging camera based on 4 Rockwell Hawaii HgCdTe 10242 detectors. The survey instrument will operate in the J and H bands and will be as scientifically versatile and as easy to use as a large format CCD camera. It is expected to be ready for astronomical use by late 1997. It will be particularly well-suited for surveys of star-forming regions, low mass stars, distant galaxies, clusters and QSOs. The camera will be commissioned at the prime focus of the 2.5 m Isaac Newton telescope, where the image scale is 0.45'/pixel, giving an effective field of view of 14.6 by 14.6 arc minutes. The field of view of this camera with 0.15' pixels is 5.1 by 5.1 arc minutes and is thus approximately 60 times larger than the current near-infrared imager on Keck (NIRC). When combined with a 4.0 m class telescope, the combination is approximately 10 times as powerful as the Keck 10.0 m, when the apertures are taken into account. The options for upgrading the camera into a wide field spectroscopic survey instrument are currently being investigated.
The Cambridge Optical Aperture Synthesis Telescope, COAST, is a four-telescope array for high resolution imaging using measurements of complex visibilities and closure phases. This paper describes what its component parts are and why.
The Cambridge Optical Aperture Synthesis Telescope, COAST, now has the capacity to measure visibility amplitudes and closure phase for stellar sources. This paper summarizes the current status of the instrument and how the data is analyzed.
In this paper we describe an instrument to obtain diffraction limited images on a large telescope using aperture masking. Detailed images of giant stars have been routinely obtained on the 4.2-m William Herschel Telescope (WHT) in La Palma with this technique. When using this method one of the major causes of data loss is the difficulty in detecting the secondary mirror support spiders crossing, and thus obscuring part of the aperture mask. To overcome this problem a system for continuously monitoring the front face of the aperture mask has been devised. Exposure times have been significantly reduced by placing the image in a corner of the CCD chip, the size of which is determined by the seeing, and by compressing the image to a single row of pixels. In order to perform this technique the fringes must be accurately aligned with the columns of the CCD chip. To eliminate the extra losses in throughput due to an optical image rotator the CCD camera, rather than the image, is rotated using a motorized mount. The aperture mask can be rotated independently to various position angles with respect to the object so that full UV coverage can be obtained.
The COAST group in the University of Cambridge have a lot of operational experience of using fast guiding systems to control the pointing of up to four separate telescopes simultaneously as well as a lot of operational experience of using array CCD systems and avalanche photodiode photon counting systems for signal detection. This paper looks at the advantages and disadvantages of these systems for fast guiding and wavefront sensing applications.
Cooled, slow-scan CCD cameras are the systems of choice when there is a requirement for systems to work at the lowest light levels or with the most extreme dynamic range. Developments in the design of these systems have led to new architectures that allow very wide dynamic range imaging (up to 20-bits) or optionally full 16-bits at up to 400 Khz pixel rates or multiple CCD output channels at even faster rates. Additionally they allow very complicated read-out patterns to be programmed for fast multiple sub-array read-out rates for applications in many different areas. The controllers allow extensive on-board image processing and analysis to improve system throughput in time-critical applications. For efficient use such systems need to be closely integrated with software systems that allow the user to get the very best from the data produced by the cameras.
Cooled, slow-scan CCD cameras are the systems of choice when there is a requirement for systems to work at the lowest light levels or with the most extreme dynamic range. Systems are available now with as much as 20-bit dynamic range and intelligent read-out (multiple sub- arrays, on chip binning, on board processing power). The principal draw-back in such systems is the relatively slow read-out rate that makes such systems inappropriate for the measurement of fast changing scenes that require real-time response and often makes the setting-up of a slow-scan system especially tedious (alignment and focusing). A new CCD system has been developed by Astromed that allows the best of both worlds. It provides a fast read-out rate for alignment, focusing or set-up, and a selection of slower rates for data taking. Full on-chip binning, and sub-array read-out are allowed with a wide dynamic range, fast analog-to-digital converter. A sophisticated software package allows an extensive range of data taking, reduction, display and analysis facilities.
A large telescope spends over 70 per cent of its time observing isolated objects on the telescope axis -an excessive waste of the available field of view. This paper describes a CCD camera which images the off-axis light on the William Herschel Telescope whilst an on-axis observer uses the telescope as normal. This enables a major background survey to be performed at minimal cost, and with no additional observing time. The cost of the system is about $150,000; by doubling the use of a telescope which costs $10,000 per night to run it pays for itself within a matter of weeks. Implementing similar systems on the new generation of large telescopes would ensure that the quality of background surveys will automatically keep pace with the advancing telescope technology.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.