We present progress towards the development of a woofer-tweeter adaptive optics (AO) system using the first 37
actuators of a 91-actuator magnetic-liquid deformable mirror (MLDM) and a magnetic 97-actuator DM from ALPAO.
The MLDM, which has both very large single-actuator and inter-actuator strokes, but a low bandwidth, is used as
woofer, whereas the high bandwidth and lower stroke ALPAO DM is used as tweeter. The ALPAO DM should improve
the bandwidth of the MLDM while the MLDM will allow correction of strong aberrations.
With the growing number of complex-shaped lenses, aspheric and freeform surfaces, the demand for an appropriate and
cost effective measurement technique to test these high quality components is still very high. Ferrofluid deformable
mirrors (FDMs) offer a promising alternative. However, high accuracy profiles produced by FDMs have only been
demonstrated in a closed-loop system which is inappropriate for metrology applications as it requires an additional
measurement instrument and complicates the setup. Consequently, a FDM open-loop driving technique which maintains
good precision while being simple, robust and stable, is required. In the following paper, we present a new active null
test system based on a FDM for the testing of deep aspheric surfaces. We show a new driving method which provides an
accurate open-loop operation mode of a FDM. We demonstrate that the method gives a significant improvement in
comparison with the normalized influence function method. The results are promising enough to consider an active null
test configuration for measuring optical components having high sag departures or complicated continuous profiles.
The potential of a return of human presence to the Moon, raises the possibility of significant lunar
infrastructure and with it the possibility of astronomical installations which can make use of the lunar
surface as a stable platform and take advantage of the lack of atmosphere. Studies have been done in the
US and Canada on the feasibility of such installations, and in particular studies of large lunar liquid mirror
telescopes have been performed. We report here on the structural design concepts undertaken for one of
Many technical improvements have been made since we first introduced deformable mirrors that use magnetic liquids
(ferrofluids) whose surface are shaped by arrays of small electric coils. We present recent advances and experimental
results of a 91-actuator magnetic liquid deformable mirror that uses a novel technique that linearizes their response by
placing the array of actuators inside a strong and uniform magnetic field. We show that this improved ferrofluid
deformable mirror (FDM) can produce inter-actuator strokes of over 10 μm, is capable of generating wavefront having
peak-to-valley amplitudes of over 60 μm, and predict that amplitudes greater than 100 μm are achievable. We also
present experimental results showing that these improved FDMs are good candidates for astronomical, vision science,
and optical testing applications.
The new giant telescopes can be compared to space projects. They will require ground-based test
support equipment to fully characterize the optical sub-system functionalities and performances
before costly commissioning on the telescope. The support equipment must be designed to reproduce
the telescope or the front optical system's aberrated wavefront. We show that the aberrated
wavefront can be generated at low cost by a magnetic liquid deformable mirror. A prototype 91-
actuator liquid deformable mirror having a diameter of 33 mm was built and used to simulate the
off-axis aberrated CFHT's primary mirror up to 0.5 degrees FOV.
We present the research status of a deformable mirror made of a magnetic liquid whose surface is actuated by a
triangular array of small current carrying coils. We demonstrate that the mirror can correct a 11 μm low order aberrated
wavefront to a residual RMS wavefront error 0.05 μm. Recent developments show that these deformable mirrors can
reach a frequency response of several hundred hertz. A new method for linearizing the response of these mirrors is also
Ferrofluid mirrors have the potential to be an inexpensive adaptive optical element which can be used to improve images of structures at the rear of the eye. Their low cost could allow adaptive optics technology to find widespread use in clinical settings. As discussed elsewhere1, their stroke and speed are suitable for correcting the aberrations of the human eye. We present work on the static and dynamic responses of these mirrors using a Hartmann-Shack wavefront reconstruction technique. The displacement of the mirror versus the current in the magnetic field actuators has been measured, as well as actuator influence functions (including non-linearities). In addition, the real-time dynamics of the mirror have been characterized.
We give a progress report on an application of a new class of versatile optical elements pioneered by our
laboratory: By coating liquids we create reflective surfaces that can be shaped by rotation into a parabolic
mirror. Coated ferrofluids can also be shaped with magnetic fields.
Low cost is what makes rotating mercury LM Telescopes interesting. However, they are limited by the
fact that they cannot be tilted. We are now working on a new generation of LMs that can be tilted. The goal is
to produce large inexpensive LMTs that can be tilted by at least twenty degrees. Early work demonstrated a
tilted LM that used a high viscosity liquid. An extrapolation law, confirmed by our experiments, shows that it
should be possible to tilt LMs by twenty degrees, assuming a liquid having a few times the viscosity of
glycerin. Rotating nanoengineered LMTs are interesting even without tilting, since their lower weight would
make then less costly than Hg mirrors and high viscosity makes them less sensitive to winds.
We have made two major recent technological breakthroughs: We have made a robotic machine which
is capable of producing the large quantities of coating material required for large mirrors. We have also
developed a technique that allows us to coat the appropriate class of liquids by simply spraying the
nanoengineered coating on them. In this contribution, we present optical tests of our liquids as well as optical
shop tests of rotating mirrors.
We have studied the feasibility and scientific potential of a 20 - 100 m aperture astronomical telescope at the lunar pole,
with its primary mirror made of spinning liquid at less than 100K. Such a telescope, equipped with imaging and
multiplexed spectroscopic instruments for a deep infrared survey, would be revolutionary in its power to study the
distant universe, including the formation of the first stars and their assembly into galaxies. The LLMT could be used to
follow up discoveries made with the 6 m James Webb Space Telescope, with more detailed images and spectroscopic
studies, as well as to detect objects 100 times fainter, such as the first, high-red shift stars in the early universe. Our
preliminary analysis based on SMART-1 AMIE images shows ridges and crater rims within 0.5° of the North Pole are
illuminated for at least some sun angles during lunar winter. Locations near these points may prove to be ideal for the
LLMT. Lunar dust deposited on the optics or in a thin atmosphere could be problematic. An in-situ site survey appears
necessary to resolve the dust questions.
We give a progress report on a new class of versatile optical elements pioneered by our laboratory. By coating ferromagnetic liquids we create reflective surfaces that can be shaped with magnetic fields, allowing us to make complex wavefronts that can vary rapidly in time. This new technology is capable of achieving complex surfaces that cannot be obtained with existing technology. The short-term objective is to perfect the technology for adaptive optics for both astronomical and ophthalmology applications. We have made a functional 112 actuator deformable mirror and characterized the ferrohydrodynamic response of the actuators. We have used high speed sensors to analyze the mirror surface subject to transient and periodic driving forces. We have developed algorithms to shape the surfaces. We have made new types of ferrofluids that are easier to coat with our nanoengineered layers. Theoretical model shown how the mirror parameters can be tuned as function of the applications. Challenges in design are outlined, as are advantages over traditional deformable mirrors.
The entire funding has recently been obtained in Belgium for the construction of a 4m Liquid Mirror Telescope. Its prime focus will be equipped with a semi-conventional glass corrector allowing to correct for the TDI effect and a thinned, high quantum efficiency, 4K × 4K pixel equivalent CCD camera. It will be capable of subarcsecond imaging in the i'(760 nm) and possibly r', g' band(s) over a field of ~ 30' in diameter. This facility will be entirely dedicated to a deep photometric and astrometric variability survey over a period of ~ 5 years. In this paper, the working principle of liquid mirror telescopes is first recalled, along with the advantages and disadvantages of the latter over classical telescopes. Several science cases are described. For a good access to one of the galactic poles, the best image quality sites for the ILMT are located either in Northern Chile (latitude near -29°30') or in North-East India (Nainital Hills, latitude near +29°30'). At those geographic latitudes, a deep (i' = 22.5 mag.) survey will approximately cover 90 square degrees at high galactic latitude, which is very useful for gravitational lensing studies as well as for the identification of various classes of interesting galactic and extragalactic objects (cf. microlensed stars, supernovae, clusters, etc.). A description of the telescope, its instrumentation and the handling of the data is also presented.
Optical aberrations reduce the imaging quality of the human eye. In addition to degrading vision, this limits our ability to illuminate small points of the retina for therapeutic, surgical or diagnostic purposes. When viewing the rear of the eye, aberrations cause structures in the fundus to appear blurred, limiting the resolution of ophthalmoscopes (diagnostic instruments used to image the eye). Adaptive optics, such as deformable mirrors may be used to compensate for aberrations, allowing the eye to work as a diffraction-limited optical element. Unfortunately, this type of correction has not been widely available for ophthalmic applications because of the expense and technical limitations of current deformable mirrors. We present preliminary design and characterisation of a deformable mirror suitable for ophthalmology. In this ferrofluidic mirror, wavefronts are reflected from a fluid whose surface shape is controlled by a magnetic field. Challenges in design are outlined, as are advantages over traditional deformable mirrors.
The trend towards ever larger telescopes and more advanced adaptive optics systems such as multi-conjugate adaptive optics is driving the need for deformable mirrors with a large number of low cost actuators. Other applications require strokes larger than those readily available from conventional mirrors. Magnetically deformable liquid mirrors are a potential solution to both these problems. Depositing a thin silver colloid known as a metal liquid-like film (MELLF) on the ferrofluid surface solves the problem of low reflectivity of pure ferrofluids. This combination provides a liquid optical surface that can be precisely shaped in a magnetic field. We have demonstrated a reflective coating that is stable for more than 30 days with a reflectivity of 50% in the near infrared. Additional experiments indicate that MELLF coatings can provide near infrared reflectivity values in excess of 80%. We also report on recent response time measurements of liquid deformable mirrors. We have demonstrated liquid mirror actuators with slew rates of 800 μm/s, corresponding to an actuator bandwidth of approximately 40 Hz and 80 Hz for strokes of 10 μm and 5 μm respectively.
In this paper we present preliminary results on a new type of optical material. By combining a thin reflective colloidal film with a superparamagnetic liquid known as a ferrofluid, it is possible to produce an optical quality surface that can be shaped by the application of a magnetic field. Ferrofluids are colloidal suspensions of nanometer-sized magnetic particles and are considered a well established, low-risk technology. We have demonstrated deformations of several microns at frequencies exceeding 100 Hz, making the material useful as a deformable mirror for adaptive optics and also of potential interest in numerous other optical devices. Liquid optics are relatively inexpensive when compared to conventional glass surfaces of similar quality and are free of mechanical constraints such as resonance and limits on the displacement of adjacent actuators. We present results to date and discuss some of the potential applications of liquid optics as well as the challenges remaining in realising practical devices based on this technology.
The trend towards ever larger telescopes and more advanced adaptive optics systems is driving the need for deformable mirrors with a large number of low cost actuators. Liquid mirrors have long been recognized a potential low cost alternative to conventional solid mirrors. By using a water or oil based ferrofluid we are able to benefit from a stronger magnetic response than is found in magnetic liquid metal amalgams and avoid the difficulty of passing a uniform current through a liquid. Depositing a thin silver colloid known as a metal liquid like film (MELLF) on the ferrofluid surface solves the problem of low reflectivity of pure ferrofluids. This combination provides a liquid optical surface that can be precisely shaped in a magnetic field. We present experimental results obtained with a prototype deformable liquid mirror based on this combination.
Mercury liquid mirror telescopes work. This ia by now a well-established fact. However they suffer from a major limitation: they cannot be tilted. We have recently proposed that liquid mirrors can be tilted by several tens of degrees, provided one can develop a high-viscosity liquid having a high coefficient of reflectivity. The technology promises inexpensive large telescopes. We present the concept and some of the ongoing work.
The surface of a spinning liquid takes the shape of a paraboloid that can be used as a reflecting mirror. Liquid mirrors have many characteristics that make them useful for optical applications: low costs, large sizes, excellent optical qualities, possibility of very high or very low numerical apertures, low scattered light, etc... The largest mirror built so far has a diameter of 3.7 meters. The largest mirror that has been extensively tested has a diameter of 2.5 meters. Interferometric tests show that it is diffraction limited. We discuss several technical issues related to liquid mirrors. A handful of liquid mirrors have now been built that are used for scientific work. We briefly discuss a practical application of liquid mirrors: We built and tested a telecentric f-θ 3D scanner that uses a liquid mirror as its objective. The prototype has a stand- off distance of 1.5 meters, a scan length up to 1 meter (telecentric), a depth of view of 1 meter and a relative depth resolution of 1 mm or less. The design is based on the auto-synchronized scanner and is f-(theta) corrected for field scanning distortion. We therefore claim that the liquid mirror technology gives a new tool to the optical designer.
The Large Zenith Telescope is a zenith-pointing telescope with a 6-meter diameter rotating mercury mirror. Located in mountains near Vancouver, Canada, it is expected to see first light in 1998. Equipped with a low-noise drift- scanning CCD camera, the telescope will survey a 17-arcmin- wide strip of sky using a set of medium-band filters. The data are expected to provide spectral energy distributions and photometric redshifts for over a million galaxies which will form a base for studies of galaxy evolution and large- scale structure.
In this communication we propose a design combining the advantages of the space invariance of telecentric triangulation with high relative lateral resolution and large measuring volume at the same time. Because the scan motion of the laser beam is decoupled from physical transport of the sensor head, this enables the fast scan in large volume. However we need a large aperture optics as large as the scan areas. We used a liquid mirror as aperture for this scanner. The surface of a spinning reflecting liquid takes the shape of a paraboloid that can be used as a reflecting mirror. This very old and nearly forgotten concept as recently been revived, with success. Low costs, large sizes, high optical qualities are the main advantages of liquid mirrors. The main limitation of liquid mirrors come from the fact that the optical axis must be aligned vertically and cannot be tilted. The prototype involves a stand-off distance of 1.5 meters, a scan length up to 1 meter (telecentric), a depth of view of 1 meter and a relative depth resolution of 1 mm (can be less). The design is based on the auto-synchronized scanner and is well corrected for field scanning distortion (f-0).
Triangulation systems that are based on an autosynchronized scanning principle to provide accurate and fast acquisition of 3D shapes are able to scan large fields. It is done generally by a coordinate measuring machine (CMM) carrying a small-volume 3D camera. However the acquisition speed is limited by the CMM movement and also by the image fusion time required to get the complete 3D shape. This paper describes some practical consideration for large volume 3D inspections with emphasis on telecentric scanning. We present the analytical and the optical design of a large telecentric scanner using a large reflective surface. Some results of the laboratory prototype will be presented. We also discuss applications and the viability of this new approach.
We review the present status of liquid mirror telescopes. Interferometric tests of liquid mirrors (the largest one having a diameter of 2.5 meters) show excellent optical qualities. The basic technology is now sufficiently reliable that it can be put to work. Indeed, a handful of liquid mirrors have now been built that are used for scientific work. A 3.7-m diameter LMT is presently being built in the new Laval upgraded testing facilities. Construction of the mirror can be followed on the Web site: http://astrosun.phy.ulaval.ca/lmt/lmt-home.html. Finally we address the issue of the field accessible to LMTs equipped with novel optical correctors. Optical design work, and some exploratory laboratory work, indicate that a single LMT should be able to access, with excellent images, small regions anywhere inside fields as large as 45 degrees.
An astronomical telescope employing a 2.7-meter diameter rotating liquid mercury mirror has recently begun operation at a site near Vancouver. The telescope achieves seeing-limited performance, and can detect galaxies as faint as 21st magnitude. Equipped with 2048 X 2048 pixel low-noise CCD detector, the telescope is now surveying a 20 arcmin wide strip of sky centered at +49 degree(s) declination. The CCD is operated in TDI mode, providing continuous imaging with a resolution of 0.6'/pixel and an integration time of 129 seconds. The primary scientific program of this instrument is to obtain spectral energy distributions of all objects in the survey area, by means of imaging through a series of 40 interference filters spanning the wavelength range 0.4 - 1.0 um. These data will then be used to identify and estimate redshifts for order X105 galaxies and X103 quasars.
We review the status of the liquid mirror project. Interferometric tests of a f/1.2 2.5-m diameter liquid mirror carried out with a scatterplate interferometer show Strehl ratios of order 0.6, close to the value of 0.8 usually taken to signify that diffraction limit has been reached. The mirror is certainly better than implied by the data because the interferograms were taken with 1/500 second exposures and the wavefronts therefore include the effects of seeing in the testing tower. Correctable small variations of the rotational velocity account for another substantial fraction of the deviations from a parabola. We have videotaped hours of interferogram and PSF observations that show that those we analyze are representative.
The advantages of using a liquid mirror in a lunar telescope are summarized. Interferometric measurements of a 1.5-m diameter liquid mirror are presented which show that liquid mirrors can have the very high optical quality required of a lunar telescope. Some of the liquid metals that could be used are discussed.
Since liquid mirrors are potentially useful in science (e.g., astronomy, atmospheric sciences, and optical testing), work has been undertaken to determine whether they are technologically feasible. A testing tower has been equipped with a scatterplate interferometer interfaced with a CCD for data acquisition and a microcomputer for data analysis. This equipment was used to test a 1.5-m-diameter f/2 liquid mirror, showing that it is diffraction limited; interferometric measurements give Strehl ratios of order 0.8. A 2.7-m-diameter liquid mirror and astronomical observatory presently under construction is briefly described.