Combined MASS–DIMM instruments for atmospheric turbulence studies

Several site-testing programs and observatories currently use combined MASS-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 elec- tronics 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 instru- mental 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 applica- tions.