Afocal Telescope Research Articles

Optical design of a 5× zoom afocal telescope with deformable mirrors

All-reflective zoom afocal telescopes can be integrated with existing imaging systems as foreoptics, enabling flexible imaging with adjustable resolution. This paper proposes an approach to designing a 5× zoom afocal telescope using deformable mirrors. Based on matrix optics, the first-order parameters for a coaxial reflective 5× zoom afocal telescope with a fixed primary mirror and three deformable mirrors are determined. To prevent obscuration, the mirrors are tilted at their vertices, which introduces asymmetric aberrations. To correct these aberrations, XY polynomial surfaces are employed on the primary mirror and an additional flat fold mirror. The final design and image simulations indicate that the system achieves good image quality across all zoom positions, enabling 5× zoom imaging with a maximum root mean square wavefront error of less than 0.125λ (λ = 632.8 nm), thereby validating the efficacy of the proposed design methodology.

Multiconfiguration afocal freeform telescopes.

An approach to designing multiconfiguration afocal telescopes is developed and demonstrated. Freeform surfaces are used to maximize the achievable diffraction-limited zoom ratio while staying in a compact volume for a two-position multiconfiguration afocal optical system. The limitations of these systems with three-mirror beam paths are discussed and subsequently overcome by introducing an additional degree of freedom. In a four-mirror beam path system, the goal of a 5x zoom ratio is achieved with a compensated exit pupil and diffraction-limited performance. A significant benefit in optical performance when using freeform surfaces is shown compared to more conventional surface types.

Analysis and Design of Infrared Search and Track System with Afocal Zoom Telescope

The infrared search and track system (IRST) is a type of special electrical optical (EO) system that can be used in various scenarios to fulfill situation awareness, reconnaissance, and tracking of targets. We proposed a homogeneous coordinate transformation method to analyze the residual image wandering induced by the rotation of the scanning platform and the compensation fast steering mirror and help with the commonly trivial selection of the telescope magnification and the objective focal length. The analysis and simulation are carried out with specified IRST optics, which adopt a 640 × 512 array and 15 μm pitch detector, in a focal range of 60 mm~360 mm, and a scan speed of 360°/s to 60 °/s at 50 fps, and optical specifications are determined further. The presented optical system, with only three kinds of common infrared materials, works at 3.7 μm~4.8 μm, demonstrates good image performance and tolerance characteristics, and shows potential in manufacturing. Also, the resulting image wandering of 8 μm, less than a 0.6-pixel size, at an integral time of 16 ms, proves the correctness of the method and makes the scheme of considerable interest for electrical optical systems.

Pupil aberrations correction of the afocal telescope for the TianQin project

TianQin is a planned Chinese space-based gravitational wave (GW) observatory with a frequency band of 10−4–1 Hz. Optical telescopes are essential for the delivery of the measurement beam to support a precise distance measurement between pairs of proof masses. As the design is driven by the interferometric displacement sensitivity requirements, the stability control of optical path length (OPL) is extremely important beyond the traditional requirement of diffraction-limited imaging quality. The recurring tilt-to-length (TTL) coupling noise arises from the OPL variation due to the wavefront deformation and angular misalignment. Reducing the residual chief ray aberration in the optical design helps suppress TTL coupling noise. To correct the pupil aberrations, we derive primary pupil aberrations in a series expansion form, and then refine the formulation of merit function by combining the pupil aberration theory and traditional image aberration theory. The automatic correction of pupil aberrations is carried out by using the macro programming in the commercial optical software Zemax, leading to a high performance telescope design. The design results show that on one side the pupil aberrations have been corrected, and on the other side, its optical performance meets the requirements for TianQin project. The RMS wavefront error over the science field of view (FOV) is less than λ/200 and the maximum TTL coupling noise over the entire ±300 μrad FOV is 0.0152 nm µrad−1 . We believe that our design approach can be a good guide for the space telescope design in any other space-based GW detection project, as well as other similar optical systems.

Exit pupil quality analysis and optimization in freeform afocal telescope systems.

Afocal telescopes are often used as foreoptics to existing imaging systems to allow for application flexibility. To properly combine an afocal telescope with an existing imaging system, the exit pupil of the afocal telescope and the entrance pupil of the imaging system must be coincident. Additionally, the exit pupil of the afocal telescope must be well-formed; that is, it must be the correct size and shape to mitigate pupil-matching challenges. This work introduces processes for designing freeform afocal telescopes with an emphasis on understanding how to analyze and control the exit pupil quality of such systems. The included 3-mirror design examples demonstrate the advantages of using freeform surfaces in afocal systems and quantify the tradeoffs required to improve the exit pupil quality.

Reflective mirror-based line-scan adaptive optics OCT for imaging retinal structure and function.

Line-scan OCT incorporated with adaptive optics (AO) offers high resolution, speed, and sensitivity for imaging retinal structure and function in vivo. Here, we introduce its implementation with reflective mirror-based afocal telescopes, optimized for imaging light-induced retinal activity (optoretinography) and weak retinal reflections at the cellular scale. A non-planar optical design was followed based on previous recommendations with key differences specific to a line-scan geometry. The three beam paths fundamental to an OCT system -illumination/sample, detection, and reference- were modeled in Zemax optical design software to yield theoretically diffraction-limited performance over a 2.2 deg. field-of-view and 1.5 D vergence range at the eye's pupil. The performance for imaging retinal structure was exemplified by cellular-scale visualization of retinal ganglion cells, macrophages, foveal cones, and rods in human observers. The performance for functional imaging was exemplified by resolving the light-evoked optical changes in foveal cone photoreceptors where the spatial resolution was sufficient for cone spectral classification at an eccentricity 0.3 deg. from the foveal center. This enabled the first in vivo demonstration of reduced S-cone (short-wavelength cone) density in the human foveola, thus far observed only in ex vivo histological preparations. Together, the feasibility for high resolution imaging of retinal structure and function demonstrated here holds significant potential for basic science and translational applications.

Generating starting points for designing freeform imaging optical systems based on deep learning.

Deep learning is an important aspect of artificial intelligence and has been applied successfully in many optics-related fields. This paper proposes a generalized framework for generation of starting points for freeform imaging optical design based on deep learning. Compared with our previous work, this framework can be used for highly nonrotationally symmetric freeform refractive, reflective, and catadioptric systems. The system parameters can be advanced and the ranges of these system parameters can be wide. Using a special system evolution method and a K-nearest neighbor method, a full dataset consisting of the primary and secondary parts can be generated automatically. The deep neural network can then be trained in a supervised manner and can be used to generate good starting points directly. The convenience and feasibility of the proposed framework are demonstrated by designing a freeform off-axis three-mirror imaging system, a freeform off-axis four-mirror afocal telescope, and a freeform prism for an augmented reality near-eye display. The design framework reduces the designer's time and effort significantly and their dependence on advanced design skills. The framework can also be integrated into optical design software and cloud servers for the convenience of more designers.

Some aspects of scaling the orbital angular momentum of light with conical diffraction

The mechanisms leading to scaling the orbital angular momentum (OAM) from a Gaussian beam with no OAM up to integers 1, 2… have been pointed out in the cascaded conical diffraction process. The study is illustrated with two different biaxial crystals: KGd(WO2)4 and Bi2ZnOB2O6 (non-degenerate cascade). Scaling the OAM is the repetition of the following basic stage: the polarization of the highest OAM state generated by a previous crystal is changed with a quarter-wave plate in order that the OAM increment through the next biaxial crystal is maximized under a projection operator. This latter is constituted from a quarter-wave plate and a linear polarizer. The image of the field at the output of a crystal is repeated through the following one with a two-lenses afocal telescope, preventing its natural spread. The OAM is visualized by interference patterns with a reference beam and by a cylindrical lens. All the experimental patterns (intensity, phase, OAM) are quite well described by a full numerical model. The role of each crystal of the cascade in the OAM increment is simply exhibited in the Fourier space.

Turbulent UV Lidar BSE-5

An eye-safe turbulent UV (355 nm) lidar BSE-5 designed for the study of atmospheric turbulence is described. Lidar works on the basis of the backscatter enhancement effect, which occurs when a light wave propagates twice in a random medium. The design of the lidar is based on a transceiving afocal Mersen telescope, which supports thermomechanical stability during long-term operation of the device. To reduce the telescope size, the edges of the primary mirror have been cut off, because they are not used during the lidar operation. The lidar was tested at Tolmachevo airport, Novosibirsk. During the tests, the turbulence was continuously monitored over a runway and over the aircraft parking. The lidar confidently recorded a turbulent wake of any aircraft during takeoff and landing. The lifetime of a strong artificial turbulence over the airfield was found to be 2–3 min.

Non-Rotationally Symmetric Field Mapping for Back-Scanned Step/Stare Imaging System

Step/stare imaging with focal plane arrays (FPAs) has become the main approach to achieve wide area coverage and high resolution imaging for long range targets. A fast steering mirror (FSM) is usually utilized to provide back-scanned motion to compensate for the image motion. However, the traditional optical design can just hold one field point relatively stable, typically the central or on-axis field point, on the FPA during back-scanning; all other field points may wander during the exposure due to imaging distortion characteristics of the optical system, which reduces the signal to noise ratio (SNR) of the target. Aiming toward this problem, this paper proposes a non-rotationally symmetric field mapping method for the back-scanned step/stare imaging system, which can make all field points stable on the FPA during back-scanning. First of all, the mathematical model of non-rotationally symmetric field mapping between object space and image space is established. Then, a back-scanned step/stare imaging system based on the model is designed, in which this non-rotationally symmetric mapping can be implemented with an afocal telescope including freeform lenses. Freeform lenses can produce an anamorphic aberration to adjust distortion characteristics of the optical system to control image wander on an FPA. Furthermore, the simulations verify the effectiveness of the method.

Spatial coherence mapping of structured sources: a flexible instrument for solar studies.

We conceptually describe and design, to first order, an instrument to locally map the spatial coherence of extended and structured sources, such as fiber bundles or the Sun, considered as a mosaic of individual solar cells, which is our main motivation. Our solar coherence instrument (SCI) is an instrument for an afocal solar space telescope; the light from its exit pupil dynamically selects one individual solar cell at a time and performs a series of Young-like experiments with different baselines in order to measure the spectral degree of coherence and therefore the effective correlation length that can be assigned for that solar cell. SCI needs flexibility in terms of selective imaging and Young experiments, which is provided by two digital micromirror devices (DMDs), a technology currently under space qualification. SCI is a compact instrument based on retroreflections, and it generates all data required to image the source, to select the cells, and to implement sequentially a series of Young apertures on a re-imaged pupil. It was designed using the (already launched) Hinode solar optical telescope as a baseline, and the first estimation of the SNR, using commercial DMDs and array sensors, while measuring the modulation of Young interference fringes validates our first-order design.

Turbulent Lidar: I−Design

Two designs of a laser radar system based on the backscatter amplification effect (BSA) are suggested. The system is a micropulse aerosol lidar with two receiving channels, one of which records an increase in the lidar returns at the laser beam axis as the atmospheric turbulence intensifies. The second channel does not respond to the BSA and is required for calibration. The BSA effect manifests itself in a narrow spatial region around the laser beam axis; so, the receiver aperture should be small enough and comparable with the Fresnel zone. The creation of the turbulent lidar became possible with the advent of compact diode-pumped micropulsed lasers with high pulse repetition rates. The lidar is intended for continuous long-term unattended operation. It is eye-safe. Two designs of the turbulent lidar based on an afocal Mersenne telescope (mirror collimator) are suggested. BSA-2 and BSA-3 turbulent lidars are described. An algorithm is suggested for retrieval of the structure parameter of optical turbulence C 2 from lidar data based on the Vorob’ev’s approximation for a statistically homogeneous turbulent medium.

Four-mirror flat-field anastigmat aplanat telescopes with simple closed-form solutions

A four-mirror anastigmat aplanat telescope can be constructed using a Gregorian-form Mersenne afocal telescope as a feeder for a concentric spherical Schwarzschild objective. A flat image field can be obtained by adjusting the ratio of the radii of the mirrors. With the tertiary at the image of the aperture stop (the primary) the primary's conic K1 can be varied, using the tertiary's conic K3 to cancel the primary's spherical aberration without affecting the system's coma or astigmatism. For a spherical primary, the tertiary is an oblate ellipsoid (K3>0). The minimum obscuration ratio is 44.7% (20% by area).

An afocal telescope configuration for the ESA ARIEL mission

ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is one of the three candidates for the next ESA medium-class science mission (M4) expected to be launched in 2026. This mission will be devoted to observing spectroscopically in the infrared (IR) a large population of known transiting planets in the neighborhood of the Solar System, opening a new discovery space in the field of extrasolar planets and enabling the understanding of the physics and chemistry of these far away worlds. ARIEL is based on a 1-m class telescope ahead of two spectrometer channels covering the band 1.95 to 7.8 microns. In addition there are four photometric channels: two wide band, also used as fine guidance sensors, and two narrow band. During its 3.5 years of operations from L2 orbit, ARIEL will continuously observe exoplanets transiting their host star. The ARIEL optical design is conceived as a fore-module common afocal telescope that will feed the spectrometer and photometric channels. The telescope optical design is composed of an off-axis portion of a two-mirror classic Cassegrain coupled to a tertiary off-axis paraboloidal mirror. The telescope and optical bench operating temperatures, as well as those of some subsystems, will be monitored and fine tuned/stabilised mainly by means of a thermal control subsystem (TCU-Telescope Control Unit) working in closed-loop feedback and hosted by the main Payload electronics unit, the Instrument Control Unit (ICU). Another important function of the TCU will be to monitor the telescope and optical bench thermistors when the Payload decontamination heaters will be switched on (when operating the instrument in Decontamination Mode) during the Commissioning Phase and cyclically, if required. Then the thermistors data will be sent by the ICU to the On Board Computer by means of a proper formatted telemetry. The latter (OBC) will be in charge of switching on and off the decontamination heaters on the basis of the thermistors readout values.

MWIR thermal imaging spectrometer based on the acousto-optic tunable filter.

Mid-wavelength IR (MWIR) thermal imaging spectrometers are widely used in remote sensing, industrial detection, and military applications. The acousto-optic tunable filter (AOTF)-based spectrometer has the advantages of fast tuning, light weight, and no moving parts, which make it ideally suited for MWIR applications. However, when designing an AOTF imaging spectrometer, the traditional method uses a refractive grating or parallel glass model in optical design software to simulate the AOTF, lowering the imaging performance of the optical system. In this paper, an accurate simulating model for an actual MWIR AOTF using the user-defined surface function in ZEMAX is presented, and an AOTF-based MWIR thermal imaging spectrometer is designed and tested successfully. It is based on a MWIR tellurium dioxide (TeO2) AOTF with an operational spectral range from 3.0 to 5.0μm and a spectral resolution of 30.8nm at 3.392μm. The optical system employs a three-mirror off-axis afocal telescope with a 2.4°×2.0° field of view. The operation of the MWIR thermal imaging spectrometer and its image acquisition are computer controlled. Furthermore, the imaging spectrometer is tested in the laboratory, and several experiments are also presented. The experimental results indicate that the proposed AOTF model is efficient, and also show that the imaging spectrometer has the ability to distinguish the real hot target from the interfering target effectively.

150 m/280 Gbps WDM/SDM FSO link based on OEO-based BLS and afocal telescopes

A 150 m/280 Gbps free-space optical (FSO) link based on an optoelectronic oscillator (OEO)-based broadband light source (BLS), afocal telescopes, and wavelength-division-multiplexing (WDM)/space-division-multiplexing (SDM) convergence is proposed. Experimental results show that the transmission distance of FSO links is significantly increased by afocal telescopes, and the transmission rate of FSO links is greatly enhanced by WDM and SDM convergence. With the aid of a low noise amplifier and clock/data recovery, good bit error rate performance and a clear eye diagram are achieved at 150 m/280 Gbps operation. This proposed 150 m/280 Gbps WDM/SDM FSO link is shown to be a prominent alternative not only because of its advancement of indoor FSO communications but also because of the advantages of optical wireless communications for a long transmission distance and high transmission rate.

Tuning of a high magnification compact parabolic telescope for centimeter-scale laser beams.

Off-axis parabolic telescopes, widely used in astronomy and laser optics, if perfectly tuned, are virtually free from aberrations along the parabola's axis direction, but their alignment is very critical. We present a detailed method to align a high magnification off-axis afocal parabolic telescope. The method is composed of two steps: an initial pre-alignment using autocollimators, followed by a fine tuning with a collimated laser beam. Due to the large telescope magnification, the outcoming beam cannot be measured without being refocused. The beam is therefore reflected on a flat mirror and sent back through the telescope. This double-pass configuration allows the measurement of the beam quality without the need for large additional optics. In the fine-tuning step, a numerical simulation is also used to identify the degrees of freedom to be adjusted. The experimental results presented are obtained with one of the mode-matching parabolic telescopes of the gravitational wave interferometric detector Advanced Virgo.

Starting configuration design method of freeform imaging and afocal systems with a real exit pupil.

Optical system configurations with a real exit pupil have important applications. However, there are few effective design methods of these systems for choice, especially for the systems using freeform surfaces. In this paper, we propose a novel starting configuration design method of freeform optical systems with a real exit pupil before the image plane. This method works for both the cases of imaging systems with optical power and afocal systems. Each single freeform surface in the starting configuration is generated directly using the light rays of multiple fields and different pupil coordinates. With a proposed multi-step design strategy, not only the given system specifications and the desired object-image relationships (or magnification for the afocal system) are achieved, the generation of a real, small-distorted exit pupil with a given size and shape (which means the imaging relationships of the pupils) can be also considered. The system generated by this method can be taken as a good starting configuration for further optimization. The benefits and feasibility of this design method are demonstrated by two design examples. One example is a freeform off-axis three-mirror imaging system. The other example is a freeform off-axis three-mirror afocal telescope. Both of the systems have a modulation transfer function (MTF) that is closed to the diffraction limit.

星载激光测距仪扩束系统的高精度装调

The beam expender of a space-borne laser range finder was an afocal telescope, the alignment requirement of which was rigorous. The alignment accuracy was the key factor of the capability of the laser range finder. The sensitivity of optical element and the compensator were obtained by means of computer aided alignment. High precision coordinate measuring and dual-path centering technology were presented, being aimed at the characteristic of performance parameter determined by distance of elements. Finally the wavefront of the system was tested by the 1 064 nm interferometer, the divergence angle was tested using a pin-hole. The result demonstrates that the RMS of the system is better than 0.225 and the divergence angle is 0.41 mrad. The results meet the requirement of design.

First-order design of a reflective viewfinder for adaptive optics ophthalmoscopy

Adaptive optics (AO) ophthalmoscopes with small fields of view have limited clinical utility. We propose to address this problem in reflective instruments by incorporating a viewfinder pupil relay designed by considering pupil and image centering and conjugation. Diverting light from an existing pupil optical relay to the viewfinder relay allows switching field of view size. Design methods that meet all four centering and conjugation conditions using either a single concave mirror or with two concave mirrors forming an off-axis afocal telescope are presented. Two different methods for calculating the focal length and orientation of the concave mirrors in the afocal viewfinder relay are introduced. Finally, a 2.2 × viewfinder mode is demonstrated in an AO scanning light ophthalmoscope.