Determination of the Intrinsic Site Seeing for the Hobby-Eberly Telescope

A long term program to quantify the intrinsic site seeing at McDonald Observatory, using two differential image motion monitors (DIMMs) has been initiated on Mt. Fowlkes where the Hobby-Eberly Telescope (HET) is located. Raw DIMM data are corrected to the zenith and to a uniform 10msec integration time. Nightly median seeing measurements (FWHM) along with the max/min range are presented for 186 nights over the 13 month period between July 2001 and July 2002. A definite seasonal effect is present in the dataset with the median seeing in the spring-summer-fall months (0.93+/-0.18 arcsec) being significantly better than the winter months (1.24+/-0.33 arcsec). The measured seeing was better than 0.70 arcsec about 9% of the time. Since the DIMM units were operated at ground level these data are not quite lower limits to the site seeing performance. Even so, the seeing of this West Texas continental site at 6,650ft (2,027m) elevation in the Davis Mountains is superior to what has been assumed in the past, based on less direct seeing measurements. Future plans are described for moving a DIMM telescope to a tower mounted, semi-automated observatory to sample the site seeing at an elevation above the ground similar to the HET mirror. Analysis remains to be carried out to determine the frequency distributions for the Mt. Fowlkes site seeing as a function time of night and meteorological conditions.