Measuring astronomical seeing: The DA/IAC DIMM

Abstract

Accurate, absolute, and reproducible measurements are the expectedqualities of any instrument. A differential image motion monitor (DIMM)is presented which assesses astronomical seeing with those requirements.It has been designed (with collaboration of the staff of the Departmentd'Astrophysique de l'Universite de Nice and the Instituto de Astrofisicade Canarias: DA/IAC) also to be reliable, simple, transportable, andeasy to set up, operate, and maintain. Its accuracy has been studied,both from theoretical and experimental points of view, mainly withrespect to CCD sampling, photon noise, and threshold influence oncentroid determination. The DA/IAC DIMM has been cross calibrated withthe existing ESO DIMM. It monitors the seeing with a temporal resolutionbetter than half a minute. This type of instrument is essential fortesting new and already existing sites and to ascertain that thesurroundings of telescopes do not generate additional turbulence. It isalso indispensable to optimize astronomical instrument selection in'flexible scheduling.'