Seeing and Image Quality at the William Herschel Telescope

Introduction The Half Arcsecond Programme (HAP) was set up by the ING in 1994, to investigate the image quality of the WHT.  Its aims were to determine the statistical properties of the seeing at the site, and to quantify and minimise any other  (local) sources of image degradation so that the best (i.e. ~ half arcsecond) image widths would be achieved as frequently as possible.

All observations with ground-based astronomical telescopes are limited by the effects of  seeing.  Seeing is the image distortion which results when starlight passes through turbulence in the atmosphere above the observatory. The seeing limited FWHM of the point-spread function (PSF) for long exposure images at optical wavelengths is rarely less than 0.5 arcseconds even at the best observing sites -  which is an order of magnitude poorer than the diffraction limited resolution of a 4 metre telescope.

In reality, imperfections of the telescope itself may also contribute to the width of the PSF.  These include tracking and focus errors, and any other aberrations of the telescope optics or their alignment.  Aberrations resulting from turbulence of the air within (or around) the telescope dome may also be important, and are referred to as dome seeing.   These telescope based contributions are all within our control, and we seek to minimise their effects.

The term seeing is often used to refer to the measured width of the PSF, even though atmospheric turbulence may be only one of several contributions to the image spread.  Hence we refer to the atmospheric contribution as the intrinsic seeing or free atmosphere seeing. The intrinsic seeing represents the limiting resolution of the telescope  unless adaptive optical correction or interferometric methods are used.

The various possible contributions to image width listed above cannot  be assessed directly from stellar images, so that other diagnostic tools are required to isolate and quantify their effects. We have used a differential image motion monitor (DIMM) mounted close to the WHT to measure the intrinsic seeing. In addition, a seeing monitor based on a Shack-Hartmann wavefront sensor, mounted at the GHRIL focal station of the WHT has provided data on dome seeing, telescope tracking errors, and on the effects of the outer scale of turbulence.

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