Seeing and Image Quality at the William Herschel Telescope

Results Summary
  • The median intrinsic seeing at the WHT site is 0.69 arcsec, determined from measurements with the ING DIMM seeing monitor between 1995 and 1998  (histogram).  This is comparable with statistics for the Mauna Kea and Paranal observatories.
  • The contribution from dome seeing  is negligible under normal observing conditions.
  • The optical quality and tracking accuracy of the telescope are good, so that image widths close to the limit imposed by the site seeing can be expected at the telescope focus.
  • Accurate focus of the telescope is crucial if optimum image widths are to be obtained. A new method for optimising the focus is under test.
  • The seeing estimate provided from the WHT autoguiders tend to over-estimate the true seeing. This is because the autoguider cameras have an independent focus adjustment which is not always set optimally for seeing measurements.
  • The median turbulence outer scale length is 15m (approx.).  This means that the seeing FWHM in the red and IR for WHT is likely to be less than predicted by the DIMM.

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