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. |