Learning Wavefront Coding for Extended Depth of Field Imaging

Depth of field is an important factor of imaging systems that highly affects the quality of the acquired spatial information. Extended depth of field (EDoF) imaging is a challenging ill-posed problem and has been extensively addressed in the literature. We propose a computational imaging approach for EDoF, where we employ wavefront coding via a diffractive optical element (DOE) and we achieve deblurring through a convolutional neural network. Thanks to the end-to-end differentiable modeling of optical image formation and computational post-processing, we jointly optimize the optical design, i.e., DOE, and the deblurring through standard gradient descent methods. Based on the properties of the underlying refractive lens and the desired EDoF range, we provide an analytical expression for the search space of the DOE, which is instrumental in the convergence of the end-to-end network. We achieve superior EDoF imaging performance compared to the state of the art, where we demonstrate results with minimal artifacts in various scenarios, including deep 3D scenes and broadband imaging

Experimental evaluation of signal-to-noise in spectro-holography via modified uniformly redundant arrays in the soft x-ray and extreme ultraviolet spectral regime

We present dichroic x-ray lensless magnetic imaging by Fourier transform holography with an extended reference scheme via a modified uniformly redundant array (mURA). Holographic images of magnetic domains simultaneously generated by a single pinhole reference as well as by a mURA reference are compared with respect to the signal-to-noise ratio (SNR) as a function of exposure time. We apply this approach for spectro-holographic imaging of ferromagnetic domain patterns in Co/Pt multilayer films. Soft x-rays with wavelengths of 1.59 nm (Co L 3 absorption edge) and 20.8 nm (Co M 2,3 absorption edges) are used for image formation and to generate contrast via x-ray magnetic circular dichroism. For a given exposure time, the mURA-based holography allows to decouple the reconstruction SNR from the spatial resolution. For 1.59 nm wavelength, the reconstruction via the extended reference scheme shows no significant loss of spatial resolution compared to the single pinhole reference. In contrast, at 20.8 nm wavelength the single pinhole reveals some very intricate features which are lost in the image generated by the mURA, although overall a high-quality image is generated. The SNR-advantage of the mURA scheme is most notable when the hologram has to be encoded with few photons, while errors associated with the increased complexity of the reconstruction process reduce the advantage for high-photon-number experiments.BMBF, 05K13KT3, Verbundprojekt 05K2013 - DynaMaX: Messplatz für ultraschnelle Dynamik bei BESSY II. Teilprojekt

EXIST: Mission Design Concept and Technology Program

The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te (CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600 keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab (10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable identification and study of sources ~10-20X fainter than with the ~15-100keV survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging) all-sky survey. EXIST will address a broad range of key science objectives: from obscured AGN and surveys for black holes on all scales, which constrain the accretion history of the universe, to the highest sensitivity and resolution studies of gamma-ray bursts it will conduct as the Next Generation Gamma-Ray Burst mission. We summarize the science objectives and mission drivers, and the results of a mission design study for implementation as a free flyer mission, with Delta IV launch. Key issues affecting the telescope and detector design are discussed, and a summary of some of the current design concepts being studied in support of EXIST is presented for the wide-field but high resolution coded aperture imaging and very large area array of imaging CZT detectors. Overall mission design is summarized, and technology development needs and a development program are outlined which would enable the launch of EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.Comment: 14 pages, 8 figures, 2 tables. PDF file only. Presented at SPIE (Aug. 2002) and to appear in Proc. SPIE, vol. 485

Studies of auroral X-ray imaging from high altitude spacecraft

Results of a study of techniques for imaging the aurora from a high altitude satellite at X-ray wavelengths are summarized. The X-ray observations allow the straightforward derivation of the primary auroral X-ray spectrum and can be made at all local times, day and night. Five candidate imaging systems are identified: X-ray telescope, multiple pinhole camera, coded aperture, rastered collimator, and imaging collimator. Examples of each are specified, subject to common weight and size limits which allow them to be intercompared. The imaging ability of each system is tested using a wide variety of sample spectra which are based on previous satellite observations. The study shows that the pinhole camera and coded aperture are both good auroral imaging systems. The two collimated detectors are significantly less sensitive. The X-ray telescope provides better image quality than the other systems in almost all cases, but a limitation to energies below about 4 keV prevents this system from providing the spectra data essential to deriving electron spectra, energy input to the atmosphere, and atmospheric densities and conductivities. The orbit selection requires a tradeoff between spatial resolution and duty cycle

Spectral Mapping Reconstruction of Extended Sources

Three dimensional spectroscopy of extended sources is typically performed with dedicated integral field spectrographs. We describe a method of reconstructing full spectral cubes, with two spatial and one spectral dimension, from rastered spectral mapping observations employing a single slit in a traditional slit spectrograph. When the background and image characteristics are stable, as is often achieved in space, the use of traditional long slits for integral field spectroscopy can substantially reduce instrument complexity over dedicated integral field designs, without loss of mapping efficiency -- particularly compelling when a long slit mode for single unresolved source followup is separately required. We detail a custom flux-conserving cube reconstruction algorithm, discuss issues of extended source flux calibration, and describe CUBISM, a tool which implements these methods for spectral maps obtained with ther Spitzer Space Telescope's Infrared Spectrograph.Comment: 11 pages, 8 figures, accepted by PAS

Non-invasive, near-field terahertz imaging of hidden objects using a single pixel detector

Terahertz (THz) imaging has the ability to see through otherwise opaque materials. However, due to the long wavelengths of THz radiation ({\lambda}=300{\mu}m at 1THz), far-field THz imaging techniques are heavily outperformed by optical imaging in regards to the obtained resolution. In this work we demonstrate near-field THz imaging with a single-pixel detector. We project a time-varying optical mask onto a silicon wafer which is used to spatially modulate a pulse of THz radiation. The far-field transmission corresponding to each mask is recorded by a single element detector and this data is used to reconstruct the image of an object placed on the far side of the silicon wafer. We demonstrate a proof of principal application where we image a printed circuit board on the underside of a 115{\mu}m thick silicon wafer with ~100{\mu}m ({\lambda}/4) resolution. With subwavelength resolution and the inherent sensitivity to local conductivity provided by the THz probe frequencies, we show that it is possible to detect fissures in the circuitry wiring of a few microns in size. Imaging systems of this type could have other uses where non-invasive measurement or imaging of concealed structures with high resolution is necessary, such as in semiconductor manufacturing or in bio-imaging