WHT vignetting by lower shutter

When observing at low elevation, the lower shutter of the WHT dome is usually folded out of the way to prevent it vignetting the telescope.

This shutter is currently (and probably until ~ July 2013) stuck in the raised position, and will vignette observations carried out at elevations < 25 deg, as shown below:

The dots in the above plot are derived from out-of-focus observations of stars, by Lilian Dominguez (31/12/2012) and Marie Hrudkova (27/3/2013) which show the vignetting directly:

The curve is a prediction, for a telescope of primary-mirror diameter 4.2 m, and an obscuring screen whose top edge lies at a horizontal distance of 11 m from the elevation axis, and at a height 2.9 m above it (this last number is a rough fit to the on-sky data).

Thanks to Berto Gonzalez for suggesting the on-sky test.

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