J. J. Fernández-Valdivia

3D imaging and wavefront sensing with a plenoptic objective

Plenoptic cameras have been developed over the last years as a passive method for 3d scanning. Several superresolution algorithms have been proposed in order to increase the resolution decrease associated with lightfield acquisition with a microlenses array. A number of multiview stereo algorithms have also been applied in order to extract depth information from plenoptic frames. Real time systems have been implemented using specialized hardware as Graphical Processing Units (GPUs) and Field Programmable Gates Arrays (FPGAs). In this paper, we will present our own implementations related with the aforementioned aspects but also two new developments consisting of a portable plenoptic objective to transform every conventional 2d camera in a 3D CAFADIS plenoptic camera, and the novel use of a plenoptic camera as a wavefront phase sensor for adaptive optics (OA). The terrestrial atmosphere degrades the telescope images due to the diffraction index changes associated with the turbulence. These changes require a high speed processing that justify the use of GPUs and FPGAs. Na artificial Laser Guide Stars (Na-LGS, 90km high) must be used to obtain the reference wavefront phase and the Optical Transfer Function of the system, but they are affected by defocus because of the finite distance to the telescope. Using the telescope as a plenoptic camera allows us to correct the defocus and to recover the wavefront phase tomographically. These advances significantly increase the versatility of the plenoptic camera, and provides a new contribution to relate the wave optics and computer vision fields, as many authors claim.

Tip-tilt restoration of a segmented optical mirror using a geometric sensor

Abstract. We present a geometric sensor to restore the local tip-tilt in a segmented surface using the Van Dam and Lane algorithm [M. A. van Dam and R. G. Lane, Appl. Opt. 41(26), 5497–5502 (2002)]. The paper also presents an implementation of this algorithm using graphical processing units as specialized hardware. This compute unified device architecture implementation achieves real-time results inside the stability time of the atmosphere for resolutions of up to 1024×1024 pixels.

Concepts, laboratory, and telescope test results of the plenoptic camera as a wavefront sensor

The plenoptic camera has been proposed as an alternative wavefront sensor adequate for extended objects within the context of the design of the European Solar Telescope (EST), but it can also be used with point sources. Originated in the field of the Electronic Photography, the plenoptic camera directly samples the Light Field function, which is the four - dimensional representation of all the light entering a camera. Image formation can then be seen as the result of the photography operator applied to this function, and many other features of the light field can be exploited to extract information of the scene, like depths computation to extract 3D imaging or, as it will be specifically addressed in this paper, wavefront sensing. The underlying concept of the plenoptic camera can be adapted to the case of a telescope by using a lenslet array of the same f-number placed at the focal plane, thus obtaining at the detector a set of pupil images corresponding to every sampled point of view. This approach will generate a generalization of Shack-Hartmann, Curvature and Pyramid wavefront sensors in the sense that all those could be considered particular cases of the plenoptic wavefront sensor, because the information needed as the starting point for those sensors can be derived from the plenoptic image. Laboratory results obtained with extended objects, phase plates and commercial interferometers, and even telescope observations using stars and the Moon as an extended object are presented in the paper, clearly showing the capability of the plenoptic camera to behave as a wavefront sensor.

Laboratory and telescope demonstration of the TP3-WFS for the adaptive optics segment of AOLI

This work was supported by the Spanish Ministry of Economy under the projects AYA2011-29024, ESP2014-56869-C2-2-P, ESP2015-69020-C2-2-R and DPI2015-66458-C2-2-R, by project 15345/PI/10 from the Fundacion Seneca, by the Spanish Ministry of Education under the grant FPU12/05573, by project ST/K002368/1 from the Science and Technology Facilities Council and by ERDF funds from the European Commission. The results presented in this paper are based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Special thanks go to Lara Monteagudo and Marcos Pellejero for their timely contributions.

Atmospherical wavefront phases using the plenoptic sensor (real data)

Plenoptic cameras have been developed the last years as a passive method for 3d scanning, allowing focal stack capture from a single shot. But data recorded by this kind of sensors can also be used to extract the wavefront phases associated to the atmospheric turbulence in an astronomical observation. The terrestrial atmosphere degrades the telescope images due to the diffraction index changes associated to the turbulence. Na artificial Laser Guide Stars (Na-LGS, 90km high) must be used to obtain the reference wavefront phase and the Optical Transfer Function of the system, but they are affected by defocus because of the finite distance to the telescope. Using the telescope as a plenoptic camera allows us to correct the defocus and to recover the wavefront phase tomographically, taking advantage of the two principal characteristics of the plenoptic sensors at the same time: 3D scanning and wavefront sensing. Then, the plenoptic sensors can be studied and used as an alternative wavefront sensor for Adaptive Optics, particularly relevant when Extremely Large Telescopes projects are being undertaken. In this paper, we will present the first observational wavefront phases extracted from real astronomical observations, using punctual and extended objects, and we show that the restored wavefronts match the Kolmogorov atmospheric turbulence.

Plenoptic deconvolution in turbulent scenarios

The deconvolution applied to plenoptic sensors has only been studied in the area of light intensity, including no treatment for the possible path changes that optical rays suffer due to the refraction index changes in the medium. Using the plenoptic sensor not only will be possible to know the wavefront phase aberration induced by atmospheric turbulence but also, using the deconvolution presented here, it will be possible to restore the spatial structure of the observed source.

Depth map extraction from light field microscopes

The CAFADIS plenoptic camera was mounted onto a Leica M205A stereomicroscope and used as a prototype light field microscope. The ability to generate image stacks focused at different focal planes from a single plenoptic image was demonstrated. The focal stacks were used to create a composite extended focus image and depth map. The positions of the focal planes of the stack were measured experimentally and the data used to calibrate a computational model of the plane distribution. The resulting microscope images have an extended depth of field and a corresponding depth map with real distance estimates.

New developments at CAFADIS plenoptic camera

The CAFADIS camera project has consisted in building a camera to measure wave-front phases and distances under different scenarios (from microns to kilometres), using highly specialised electronic technology, namely Graphics Processing Units (GPUs) and Field Programmable Gate Arrays (FPGAs). It is a passive method of depth extraction, it uses incoherent light (natural light). In this paper we will present our new developments.

AOLI: near-diffraction limited imaging in the visible on large ground-based telescopes

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.

Privacy-enabled displays

In this work we have presented a brief insight into the capabilities of multilayer displays as to selectively display information in relation to the observers. We labeled the views of a light-field as blocked and non-blocked, and then a predefined text was assigned accordingly, modifying it to achieve a privacy criterion in the blocked case. Two ways to define the private views were presented. An evaluation of the output for both techniques was carried over in simulation, in both the spatial and frequency domain. Results showed that privacy was achievable and that each technique had an optimal operation point when taking into account the time-multiplexing capabilities of the multilayer display. Also, a trade-off between the quality of the blocked and non-blocked views was found.