A HARTMANN DIFFERENTIAL IMAGE MOTION MONITOR (H-DIMM) FOR ATMOSPHERIC-TURBULENCE CHARACTERIZATION

We describe the use of a multi-aperture Hartmann mask coupled to a sli ghtly out-of-focus focal plane array imager to monitor atmospheric tur bulence ('seeing') produced by refractive index fluctuations. The imag er (a CCD) is located inside or outside the focal surface of the imagi ng system so that each sub-aperture of the Hartmann mask produces an i mage well separated from all of the other images produced by the mask. Since the depth of focus of the sub-apertures is an order of magnitud e larger than that of the parent optical system, the individual images are still diffraction-limited, We obtain short (10 to 100 msec) expos ures and monitor the position fluctuations of the images. Analysis of the position and intensity fluctuations of the images can be used to d etermine the atmospheric parameter ro, the wind direction and velocity , and, under some circumstances, the distance of the turbulent layer f rom the observing site.