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Improving the Image Quality at the VTT/SP (1Sep95)
(from NSO, NOAO Newsletter No. 43, September 1995)

At the NSO/SP there is an ongoing effort to improve the optical
performance of the Vacuum Tower Telescope. During several observing
runs, an attempt was made to measure the optical aberrations present
at the VTT, their variation in time, and their source. The tools used
to measure the optical performance of the VTT are: (1) Laser
interferograms of the entrance window measure the aberrations
introduced by the entrance window. (2) A Dyson interferometer in
conjunction with a reference flat mounted in front of the entrance
window was used to determine the aberrations of the entire optical
system. (3) A Hartmann-Shack wavefront sensor was used to measure the
aberrations as a function of the telescope pointing.

Although the data reduction is still in progress it has been
determined that the optical aberrations are mainly introduced by the
VTT's entrance window. The window is 40mm thick, and is made of fused
silica. Aberrations are caused by stress due to atmospheric pressure
and by temperature variations across the window, which in turn cause
variations of refractive index and therefore aberrations in the
wavefront. It was determined that after pointing the telescope at the
Sun for about 20 minutes, the measured wavefront shows a "turned-up"
edge---in some cases by several waves, i.e., the entrance window
introduces spherical aberration. A change in focus is also observed.
In addition, we find a substantial amount of astigmatism, coma, and
some higher order Zernike terms. A new, more powerful window cooling
was installed in May 1995, which prevents the "turned-up" edge in the
wavefront to a large extent. However, the residual aberrations are
substantial. Therefore, it is planned to develop and install an active
optics system at the VTT, which will correct slowly varying, low-order
aberrations.

First experiments with a Hartmann-Shack wavefront sensor working on
granulation have been performed. The pupil of the VTT was sampled with
6, 8, and 12 subapertures across the diameter (76 cm). First results
are very promising. Even the images of granulation formed by the
subapertures of 6.3 cm diameter (12 subapertures) show sufficient
structure and the computed cross correlations show well-defined
correlation peaks. The time average of the computed wavefronts clearly
shows the telescope aberrations. These aberrations will be corrected
using a rubber mirror that presently is being purchased from the
XINETICS company. The mirror purchase is jointly funded by the Air
Force Phillips Lab and the NSO.

Thomas Rimmele, Richard Radick, Dick Dunn

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