We present integral field spectroscopy of galaxy cluster Abell 3827, using ALMA and VLT/MUSE. It reveals an unusual configuration of strong gravitational lensing in the cluster core, with at least seven lensed images of a single background spiral galaxy. Lens modelling based on HST imaging had suggested that the dark matter associated with one of the cluster's central galaxies may be offset. The new spectroscopic data enable better subtraction of foreground light, and better identification of multiple background images. The inferred distribution of dark matter is consistent with being centered on the galaxies, as expected by ΛCDM. Each galaxy's dark matter also appears to be symmetric. Whilst we do not find an offset between mass and light (suggestive of self-interacting dark matter) as previously reported, the numerical simulations that have been performed to calibrate Abell 3827 indicate that offsets and asymmetry are still worth looking for in collisions with particular geometries. Meanwhile, ALMA proves exceptionally useful for strong lens image identifications.

Balloon-borne astronomy offers an attractive option for experiments that require precise pointing and attitude stabilization, due to a large reduction in the atmospheric interference observed by ground-based systems as well as the low-cost and short development time-scale compared to space-borne systems. The Balloon-borne Imaging Testbed (BIT) is an instrument designed to meet the technological requirements of high-precision astronomical missions, and is a precursor to the development of a facility-class instrument with capabilities similar to the Hubble Space Telescope. The attitude determination and control systems (ADCS) for BIT, the design, implementation, and analysis of which are the focus of this paper, compensate for compound pendulation effects and other sub-orbital disturbances in the stratosphere to within 1-2 00 (rms), while back-end optics provide further image stabilization down to 0.05 00 (not discussed here). During the inaugural test flight from Timmins, Canada in September 2015, BIT ADCS pointing and stabilization performed exceptionally, with coarse pointing and target acquisition to within <0.1 and fine stabilization to 0.68 00 (rms) over long (10-30 min) integrations. This level of performance was maintained during flight for several tracking runs that demonstrated pointing stability on the sky for more than an hour at a time. To refurbish and improve the system for the threemonth flight from New Zealand in 2018, certain modifications to the ADCS need to be made to smooth pointing mode transitions and to correct for internal biases observed during the test flight. Furthermore, the level of autonomy must be increased for future missions to improve system reliability and robustness.

This paper presents optical night sky brightness measurements from the stratosphere using CCD images taken with the Super-pressure Balloon-borne Imaging Telescope (SuperBIT). The data used for estimating the backgrounds were obtained during three commissioning flights in 2016, 2018, and 2019 at altitudes ranging from 28 to 34 km above sea level. For a valid comparison of the brightness measurements from the stratosphere with measurements from mountain-top ground-based observatories (taken at zenith on the darkest moonless night at high Galactic and high ecliptic latitudes), the stratospheric brightness levels were zodiacal light and diffuse Galactic light subtracted, and the airglow brightness was projected to zenith. The stratospheric brightness was measured around 5.5 hr, 3 hr, and 2 hr before the local sunrise time in 2016, 2018, and 2019, respectively. The B, V, R, and I brightness levels in 2016 were 2.7, 1.0, 1.1, and 0.6 mag arcsec −2 darker than the darkest ground-based measurements. The B, V, and R brightness levels in 2018 were 1.3, 1.0, and 1.3 mag arcsec −2 darker than the darkest ground-based measurements. The U and I brightness levels in 2019 were 0.1 mag arcsec −2 brighter than the darkest ground-based measurements, whereas the B and V brightness levels were 0.8 and 0.6 mag arcsec −2 darker than the darkest ground-based measurements. The lower sky brightness levels, stable photometry, and lower atmospheric absorption make stratospheric observations from a balloon-borne platform a unique tool for astronomy. We plan to continue this work in a future midlatitude long duration balloon flight with SuperBIT.Unified Astronomy Thesaurus concepts: CCD photometry (208); Night sky brightness (1112); Sky brightness (1462); Stratosphere (1640); High altitude balloons (738); Optical observatories (1170); Diffuse radiation (383); Gegenschein (640)