Several site‐testing programmes and observatories currently use combined Multi‐Aperture Scintillation Sensor (MASS)–Differential Image Motion Monitor (DIMM) instruments for monitoring parameters of optical turbulence. The instrument is described here. After a short recall of the measured quantities and operational principles, the optics and electronics of MASS–DIMM, interfacing to telescopes and detectors, and operation are covered in some detail. Particular attention is given to the correct measurement and control of instrumental parameters to ensure valid and well‐calibrated data, to the data quality and filtering. Examples of MASS–DIMM data are given, followed by the list of present and future applications.

An algorithm that permits one to measure atmospheric turbulence by statistical analysis of light flux fluctuations in four concentric‐ring apertures is described in detail. It consists of computing the scintillation indices for each aperture and pairwise aperture combination and in fitting the set of measured indices to a model with a small number of turbulent layers. The performance of this method is analysed by means of simulations and using the real data from a multi‐aperture scintillation sensor. It is shown that a turbulence profile with a vertical resolution of Δh/h∼ 0.5 can be reconstructed and that the errors of the measured intensities of turbulent layers are typically around 10 per cent of the integrated intensity. The integral parameters such as the seeing and the isoplanatic angle are measured with few per cent accuracy.

We report the discovery of a nearby massive white dwarf with He-H atmosphere. The white dwarf is located at a distance of 74.5 ± 0.9 pc. Its radius, mass, effective temperature, H/He ratio and age are R = 2500 ± 100 km, M = 1.33 ± 0.01 $\rm M_{\odot }$, Teff = 31200 ± 1200 K, H/He ∼ 0.1 and 330 ± 40 Myr, respectively. The observed spectrum is redshifted by Vr = +240 ± 15 km s−1, which is mostly attributed to the gravitational redshift. The white dwarf shows a regular stable photometric variability with amplitude Δg ≈ 0.06m and period P = 353.456 s suggesting rapid rotation. This massive, hot and rapidly rotating white dwarf is likely to originate from the merging of close binary white dwarf system that avoided explosion in a thermonuclear type Ia supernova.