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Ferrofluids can be used to make deformable mirrors having highly interesting characteristics (e.g., extremely large strokes and low costs). Until recently, such mirrors were thought to be restricted to corrections of frequencies lower than 10 Hz, thus limiting their usefulness. We present counterintuitive results that demonstrate that the limiting operational frequency can be increased by increasing the viscosity of the ferrofluid. We tested the response of ferrofluids having viscosities as high as 494 cP, finding that they could allow an adaptive optics correction frequency as high as 900 Hz. We also demonstrate that we can counter the amplitude loss due to the high viscosity by overdriving the actuators. The overdriving technique combines high current, short duration pulses with ordinary driving step functions to deform the mirror. The integration of a FDM in a complete closed-loop adaptive optics system running at about 500 Hz thus appears to be a realistic goal in the near future. OCIS codes: 220.1000, 220.1080, 230.3810.

We discuss a new type of deformable mirror made from nanoengineered reflective layers deposited onto liquids. The surfaces are shaped by heating with a laser. The response times of the deformed surfaces are slow (>1 s). Simplicity and low cost appear to be the main advantages of thermally deformable liquid mirrors. OCIS codes: 230.0230, 230.4040, 350.1260.

The experimental results obtained with a ferrofluidic deformable mirror controlled by electro-magnet actuators are presented here. Using a step input through a single actuator, we obtained a steady-state settling time of 100 ms; however, different combinations of overdrive inputs can be used to decrease it to 25 ms. A new technique which consists of laying down an elastomer membrane, coated with an aluminum film, on the ferrofluid is also discussed. By adding the membrane on the ferrofluid, it further decreases the time response by a factor of 2. Furthermore, the thin aluminum layer improves the reflectivity of the mirror. Finally, using the membrane and the overdrive techniques combined, the time response is improved by a factor of 20. Numerical simulations show that ferrofluidic mirrors using membranes and improved electronics should reach settling times of the order of a millisecond. Presumably, even lower settling times could be possible. [ABSTRACT FROM AUTHOR]

ABSTRACT A procedure for the fabrication of aluminum-coated reflective PDMS thin films having a low surface roughness and high reflectance is presented. Elastomer films are prepared by spin-coating on a sacrificial layer that permits subsequent release from the substrate. Appropriate surface treatments are established for the deposition of a thin, highly reflective metal coating. The composite membranes are deposited on an ethylene glycol based ferrofluid to fabricate a new ferrofluidic deformable mirror. Surface deformations of a few micrometers can be produced with low strength magnetic fields. The reflective elastomer membranes do not alter the rheological behavior of the ferrofluid in magnetic fields and thus offer new perspectives for the use of ferrofluidic mirrors in adaptive optics. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44542. [ABSTRACT FROM AUTHOR]

A Fourier transform analysis of 2.5 million spectra in the SDSS survey was carried out to detect periodic modulations contained in the intensity versus frequency spectrum. A statistically significant signal was found for 223 galaxies, while the spectra of 0.9 million galaxies were observed. A plot of the periods as a function of redshift clearly shows that the effect is real without any doubt, because the modulations are quantized at two base periods that increase with redshift in two very tight parallel linear relations. We suggest that this result could be caused by light bursts separated by times on the order of 10-13 s, but other causes may be possible. We investigate the hypothesis that the modulation is generated by the Fourier transform of spectral lines, but conclude that this hypothesis is not valid. Although the light burst suggestion implies absurdly high temperatures, it is supported by the fact that the Crab pulsar also has extremely short unresolved pulses (<0.5 ns) that imply similarly high temperatures. Furthermore, the radio spectrum of the Crab pulsar also has spectral bands similar to those that have been detected. Finally, decreasing the signal-to-noise threshold of detection gives results consistent with beamed signals having a small beam divergence, as expected from non-thermal sources that send a jet, like those seen in pulsars. Considering that galaxy centers contain massive black holes, exotic black hole physics may be responsible for the spectral modulation. However, at this stage, this idea is only a hypothesis to be confirmed with further work. [ABSTRACT FROM AUTHOR]

It has been proposed to study gravitational lenses from measurements of the spectrum of the fluctuations of the output current of a quadratic detector. The spatial correlation coefficient of the source is the fundamental parameter of the technique. The experimental work discussed in this article confirms that the correlation coefficient must be evaluated at the frequencies of the spectrum of the current fluctuations. This validates a powerful yet simple technique to find unresolved gravitational lenses and to study the lensing event and the source. The validation is needed before starting the extensive theoretical and observational work that must now follow. [ABSTRACT FROM AUTHOR]

Measurements of the spectrum of the fluctuations of the output current of the quadratic detector of a telescope can be used to find unresolved astronomical gravitational lenses and determine time delays between their image components. These time delays can be used for astronomical studies. The spatial correlation coefficient of a source is an important parameter that quantifies the loss of contrast, caused by the extendedness of the source, in the spectral modulation of the intensity fluctuations. This work shows that the correlation coefficient must not be evaluated at the frequency of observation, but must instead be evaluated at the much lower beat frequencies of the spectrum of the fluctuations. This opens up a powerful, novel technique to find unresolved gravitational lenses and to study the lensing event and the source. [ABSTRACT FROM AUTHOR]

Presents the results of interferometric tests of well-tuned mirrors in different conditions, as well as mirrors that have been perturbed with the express purpose of studying their behaviors under external perturbations and misalignments. Defects caused by leveling errors; Spiral-shaped defects; Conclusion of the interferometry; Scattered light caused by vibrations; Concluding remarks to study.

Examines a family of two-mirror correctors to compensate for the aberrations of a parabolic mirror observing at a large angle from the zenith. Aspherization of an active vase mirror for wave-front corrections up to third- and fifth-order aberrations; Corrector designs for a four-m-diameter fixed primary mirror observing off axis.