Ermanno F. Borra

Deposition of metal films on an ionic liquid as a basis for a lunar telescope.

An optical/infrared telescope of 20–100 m aperture located on the Moon would be able to observe objects 100 to 1,000 times fainter than the proposed next generation of space telescopes. The infrared region of the spectrum is particularly important for observations of objects at redshifts z > 7. The apparent simplicity and low mass of a liquid mirror telescope, compared with a traditional pointable glass mirror, suggest that the concept should be considered further. A previously proposed liquid mirror telescope, based upon a spinning liquid metallic alloy, is not appropriate for infrared applications, which will require a liquid below 130 K. Here we report the successful coating of an ionic liquid with silver. The surface is smooth and the silver coating is stable on a timescale of months. The underlying ionic liquid does not evaporate in a vacuum and remains liquid down to a temperature of 175 K. Given that there are ∼106 simple and ∼1018 ternary ionic liquids, it should be possible to synthesize liquids with even lower melting temperatures.

Liquid mirrors - Optical shop tests and contributions to the technology

We have undertaken work to determine whether optical quality liquid mirrors are technologically feasible. We have built a testing facility, equipped with a scatterplate interferometer interfaced with a CCD for data acquisition and a microcomputer for data analysis. Extensive interferometric tests of a 1.5 m diameter f/2 liquid mirror show that it is diffraction limited as they give Strehl ratios of order 0.8 and rms surface deviations of order λ/20. Direct observations of the point spread function show that the mirror does not suffer an excessive amount of scattered light

Optical tests with Bessel beam interferometry

In this paper we demonstrate how Bessel beam interferometry can be used to characterize the curvature of a reflecting surface. The approach is based on the fact that the intensity distribution produced by the coherent superposition of Bessel beams is a sensitive function of the relative phases between the constituting beams. We show how this phase sensitivity can translate into accurate measurements of the curvature of a wavefront. Experimental tests were made with a liquid mirror. We have also used Bessel beams to measure the precession angle of the liquid mirror. Our results show that Bessel beam interferometry is a very accurate tool for the optical testing of non-stationary surfaces and that it could be used as a general method of real-time, non-contact sensing. Bessel beam interferometry has the advantage of not requiring any reference arm that needs to be stabilized.

Wavefront correction with a 37-actuator ferrofluid deformable mirror

This paper discusses an innovative low-cost deformable mirror made of a magnetic liquid (ferrofluid) whose surface is actuated by an hexagonal array of small current carrying coils. Predicted and experimental performances of a 37-actuator ferrofluid deformable mirror are presented along with wavefront correction examples. We show the validity of the model used to compute the actuators currents to obtain a desired wavefront shape. We demonstrate that the ferrofluid deformable mirror can correct a 11 microm low order aberrated wavefront to a residual RMS error of 0.05 microm corresponding to a Strehl ratio of 0.82.

A Search for Peculiar Objects with the NASA Orbital Debris Observatory 3 Meter Liquid Mirror Telescope

The NASA Orbital Debris Observatory (NODO) astronomical survey uses a transit 3 m liquid mirror telescope to observe a strip of sky in 20 narrowband filters. In this article, we analyze a subset of data from the 1996 observing season. The catalog consists of 18,000 objects with 10 < V < 19 observed in 10 narrowband filters ranging from 500 to 950 nm. We first demonstrate the reliability of the data by fitting the Bahcall-Soneira model of the Galaxy to the NODO magnitude counts and color counts at various Galactic latitudes. We then perform a hierarchical clustering analysis on the sample to extract 206 objects, out of a total of 18,000, showing peculiar spectral energy distributions. It is a measure of the reliability of the instrument that we extract so few peculiar objects. Although the data and results, per se, may not seem otherwise particularly remarkable, this work constitutes a milestone in optical astronomy, since this is the first article that demonstrates astronomical research with a radically new type of mirror.

UBC/Laval 2.7 meter liquid mirror telescope

We report the successful development and operation of a large astronomical liquid-mirror telescope. Employing a rotating 2.7-meter diameter mirror with a surface of liquid mercury, the telescope images a half-degree diameter field centered at the zenith. Located near Vancouver, British Columbia, it is equipped with a low-noise 2048x2048-pixel CCD detector, operating in TDI mode, which produces continuous imaging of a 20 arcminute-widestrip of sky with 2 minute integration time. Images with FWHM of 2 arcseconds or less are regularly obtained. This image quality is limited only by atmospheric seeing and star-trail curvature. The telescope is equipped with a series of narrow-band filters, designed to produce 40-point spectral energy distributions from 0.4 to 1.0 microns of all detected objects. These will allow classification and redshift estimation of approximately 25,000 galaxies and 1,000 quasars to a limiting magnitude of R~21.We report the successful development and operation of a large astronomical liquid-mirror telescope. Employing a rotating 2.7-meter diameter mirror with a surface of liquid mercury, the telescope images a half-degree diameter field centered at the zenith. Located near Vancouver, British Columbia, it is equipped with a low-noise 2048x2048-pixel CCD detector, operating in TDI mode, which produces continuous imaging of a 20 arcminute-widestrip of sky with 2 minute integration time. Images with FWHM of 2 arcseconds or less are regularly obtained. This image quality is limited only by atmospheric seeing and star-trail curvature. The telescope is equipped with a series of narrow-band filters, designed to produce 40-point spectral energy distributions from 0.4 to 1.0 microns of all detected objects. These will allow classification and redshift estimation of approximately 25,000 galaxies and 1,000 quasars to a limiting magnitude of R~21.

Optical tests of a 2.5-m-diameter liquid mirror: behavior under external perturbations and scattered-light measurements

Interferometric tests of an f/1.2, 2.5-m-diameter liquid mirror show rms surface deviations of ~lambda/20 and Strehl ratios of the order of 0.6, which show that it is diffraction limited. The mirror is certainly better than what is implied by the data because of aberrations introduced by the auxiliary testing optics. We made detailed studies of the scattered light of the mirror. We studied the behavior of the mirror under external perturbations. We reached an important milestone because we now have a good understanding of liquid mirrors.

Optical tests of nanoengineered liquid mirrors

We describe a new technology for the fabrication of inexpensive high-quality mirrors. We begin by chemically producing a large number of metallic nanoparticles coated with organic ligands. The partides are then spread on a liquid substrate where they self-assemble to give optical quality reflective surfaces. Since liquid surface can be modified by a variety of means (e.g., rotation, electromagnetic fields), this opens the possibility of making a new class of versatile and inexpensive optical elements that can have complex shapes and that can be modified within short time scales. Interferommetric measurements show optical quality surfaces. We have obtained reflectivity curves that show 80% peak reflectivities. We are confident that we can improve the reflectivity curves because theoretical models predict higher values. We expect nanoengineered liquid mirrors to be useful for scientific and engineering applications. The technology is interesting for large optics, such as large rotating parabolic mirrors, because of its low cost. Furthermore, because the surfaces of of ferrofluids can be shaped with magnetic fields, one can generate complex, time-varying surfaces that are difficult to make with conventional techniques.

Linearization of the response of a 91-actuator magnetic liquid deformable mirror

We present the experimental performance of a 91-actuator deformable mirror made of a magnetic liquid (ferrofluid) using a new technique that linearizes the response of the mirror by superposing a uniform magnetic field to the one produced by the actuators. We demonstrate linear driving of the mirror using influence functions, measured with a Fizeau interferometer, by producing the first 36 Zernikes polynomials. Based on our measurements, we predict achievable mean PV wavefront amplitudes of up to 30 microm having RMS residuals of lambda/10 at 632.8 nm. Linear combination of Zernikes and over-time repeatability are also demonstrated.

A magnetic liquid deformable mirror for high stroke and low order axially symmetrical aberrations.

We present a new class of magnetically shaped deformable liquid mirrors made of a magnetic liquid (ferrofluid). Deformable liquid mirrors offer advantages with respect to deformable solid mirrors: large deformations, low costs and the possibility of very large mirrors with added aberration control. They have some disadvantages (e.g. slower response time). We made and tested a deformable mirror, producing axially symmetrical wavefront aberrations by applying electric currents to 5 concentric coils made of copper wire wound on aluminum cylinders. Each of these coils generates a magnetic field which combines to deform the surface of a ferrofluid to the desired shape. We have carried out laboratory tests on a 5 cm diameter prototype mirror and demonstrated defocus as well as Seidel and Zernike spherical aberrations having amplitudes up to 20 microm, which was the limiting measurable amplitude of our equipment.