  ULTRASPEC is a high-speed imaging photometer mounted permanently at one of
the Nasmyth focii of the 2.4-m Thai National Telescope (TNT) on Doi Inthanon,
Thailand's highest mountain. ULTRASPEC employs a 1024x1024 pixel
frame-transfer, electron-multiplying CCD (EMCCD) in conjunction with re-imaging
optics to image a field of 7.7'x7.7' at (windowed) frame rates of up to ~200
Hz. The EMCCD has two outputs - a normal output that provides a readout noise
of 2.3 e- and an avalanche output that can provide essentially zero readout
noise. A six-position filter wheel enables narrow-band and broad-band imaging
over the wavelength range 330-1000 nm. The instrument saw first light on the
TNT in November 2013 and will be used to study rapid variability in the
Universe. In this paper we describe the scientific motivation behind ULTRASPEC,
present an outline of its design and report on its measured performance on the
TNT.
