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The development and design of versatile, autonomous, fixed-focused deep-sea cameras capable of operation to depths of 6,000 m and 11,000 m, that have been deployed on numerous research submersibles and deep-sea platforms since 2016, is presented. The optical assembly of the cameras consists of two lens elements and a high pressure-corrected dome port, optimized to correct for image distortion, produce minimal vignetting, and yield a depth of field which extends from ~0.5 m to infinity within the subsea environment. Three configurations of deep-sea housing are integrated with these optics, such that the internal chassis designs permit GoPro HERO4™ and HERO11™ camera modules to be axially aligned with the corrector and dome optics. The GoPro cameras are fitted with a 5.4 mm non-distortion lens and 1TB microSD memory cards; and are connected to a high-capacity USB-C battery or custom Li-battery pack to provide selfcontained power. The supplemental power and recording media storage permit operations for >24 hours for 27MP still imaging at a high (~5 second) repetition rate, or ~18 hours for 4K or 5.3K cinematic video acquisition at 30 fps. The self-contained power and autonomous design of these cameras allow a wide range of installation options for deep-sea vehicles, towed systems, and seafloor sampling devices to document oceanographic processes. In addition to their use for highresolution documentation of Earth-ocean phenomena and life, they have been used in numerous outreach efforts to educate and engage students and the public about the importance of continued exploration and study of “inner space” – the global ocean and seafloor. 

ABSTRACT The black hole candidate and X-ray binary MAXI J1535−571 was discovered in 2017 September. During the decay of its discovery outburst, and before returning to quiescence, the source underwent at least four reflaring events, with peak luminosities of ∼1035–36 erg s−1 (d/4.1 kpc)2. To investigate the nature of these flares, we analysed a sample of NICER (Neutron star Interior Composition Explorer) observations taken with almost daily cadence. In this work, we present the detailed spectral and timing analysis of the evolution of the four reflares. The higher sensitivity of NICER at lower energies, in comparison with other X-ray detectors, allowed us to constrain the disc component of the spectrum at ∼0.5 keV. We found that during each reflare the source appears to trace out a q-shaped track in the hardness–intensity diagram similar to those observed in black hole binaries during full outbursts. MAXI J1535−571 transits between the hard state (valleys) and softer states (peaks) during these flares. Moreover, the Comptonized component is undetected at the peak of the first reflare, while the disc component is undetected during the valleys. Assuming the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions take place at the lowest luminosities ever observed in a black hole transient, while the soft-to-hard transitions occur at some of the lowest luminosities ever reported for such systems.