IRIS (InfraRed Imaging Spectrograph) is one of the first light instruments for the Thirty Meter Telescope which, with the help of adaptive optics, offers diffraction-limited near-infrared(0.84 to 2.4 μm) imaging and integral field spectroscopic capabilities. The imager optics design was based on four Teledyne H4RG-10 detectors with a pixel size of 10 microns. However, due to the cost and schedule uncertainty related to acquiring 64-channel H4RG-10 detectors which arose in the final design phase, a re-design of the optics to work with H4RG-15 detectors was initiated to investigate a possible optical design baseline. The major challenge of the optical design is a limited space available due to maturity of other IRIS subsystems in the final design phase while the focal length of the camera optics becomes 1.5 times longer. After extensively exploring the design parameter and trade space, we manage to introduce the new design which is based on two-mirror telephoto camera optics. The new design not only meets all requirements but also offers advantages, including two less aspheric mirrors and less detector tilt angle. Further analyses associated with the alignment strategy confirm that the new design is robust and implementable.
Adaptive optics (AO) is a powerful tool for correcting wavefront errors induced by complex structures of biological samples which significantly causes image degradation. A scene-based sensing technique is being popular in microscopic AO systems with Shack-Hartmann (SH) wavefront sensors. A problem in application of the technique is that the shapes of images observed on SAs vary dependently to their positions on the aperture, especially when using microscopic objectives with higher NAs. To mitigate this problem, a differential sensing technique is used that enables measuring image shifts with high correlations over the aperture. Experiments using an artificial testing target including fluorescent beads, which simulates the leaf of moss, were conducted to investigate imaging performances of the present AO system. Unbiased maximum ratios were measured from blurred and AO-corrected images, and then the Strehl ratios were derived from them. Resultant Strehl ratios were around 0.58.
Significance: A scene-based adaptive-optics (AO) system is developed and a method for investigating its imaging performance is proposed. The system enables derivation of Strehl ratios from observed images via collaboration with computer simulations. The resultant Strehl ratios are comparable with those of other current AO systems.
Aim: For versatile and noninvasive AO microscopy, a scene-based wavefront-sensing technique working on a Shack–Hartmann wavefront sensor is developed in a modal control system. The purpose of the research is to clarify the imaging performance of the AO system via the derivation of Strehl ratios from observed images toward applications in microscopy of living cells and tissues.
Approach: Two imaging metrics that can be directly measured from observed images (i.e., an energy concentration ratio and unbiased maximum ratio) are defined and related to the Strehl ratio via computer simulations. Experiments are conducted using artificial targets to measure the imaging metrics, which are then converted to Strehl ratios.
Results: The resultant Strehl ratios are >0.7 and 0.5 in the cases of defocus and higher aberrations, respectively. The half-widths at half-maximum of the AO-corrected bead images are favorably comparable to those of on-focus images under simple defocus aberration, and the AO system works both under bright-field illumination and on fluorescent bead images.
Conclusions: The proposed scene-based AO system is expected to work with a Strehl ratio of more than 0.5 when applied to high-resolution live imaging of cells and tissues under bright-field and fluorescence microscopies.
We have developed a microscopic adaptive optics (AO) system that corrects wavefront phase errors induced by complex structures of biological samples. The technique of correlation-based Shack-Hartmann (SH) sensing used in the AO system enables wavefront measurement using complex structures in a target as the reference. However, sub-images in the SH sensor become deformed dependently on the positions of sub-apertures as the NA of the microscopic objective is higher. This often deteriorates the accuracy of wavefront sensing. To mitigate the undesirable effect, we here propose a differential wavefront sensing technique with a mathematical formula, which is expected to measure wavefront at a better precision. Because differences in image shapes are less significant between nearby SAs, correlations between adjacent SAs are measured in the proposed method. We confirmed that the AO system worked as designed by experiments.
Adaptive optics (AO) is a promising technique for correcting wavefront errors induced by complex structures of biological samples which significantly causes image degradation. We develop a microscopic AO system with a Shack-Hartmann wavefront sensor based on image correlation. The correlation-based wavefront sensing is feasible using an extended object under bright-field illumination, as well as spot fluorescence. To make the correlation-based sensing more reliable, we newly introduce a technique of excluding sub-images with insufficient quality. We show experimental results under a variety of conditions for objects, light sources, and wavefront error sources. In any cases, we confirmed that the AO system effectively worked so as to improve image qualities.
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