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WHT vignetting by lower shutter
When observing at low elevation, the lower shutter of the WHT dome is usually folded down out of the way to prevent it vignetting the telescope.
Currently (as of 15 March 2017) the shutter is usable, but will not routinely be lowered at the start of the night. Rather, it wil be lowered whenever the observer needs to observe at elevation < 25 deg, and will then be left in the lowered position for the rest of the night. Shutter movement is a manual operation and takes a few minutes.
From mid-Dec 2016 to mid-Mar 2017, the shutter was stuck in its raised (park) position (as it was in January - July 2013), and observations carried out at elevations < 25 deg were vignetted, 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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