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WHT optical aberrations
The goal is to maintain image FWHM, due to the telescope optics alone, < 0.25 arcsec (i.e. << typical seeing). The quality of the telescope optics is checked regularly by inspecting the symmetry of out-of-focus images (recipe here), and occasionally (every few years, e.g. when the primary mirror is replaced after realuminising) using a Shack-Hartmann camera (recipe here).
The results of past Shack-Hartmann tests at Cassegrain are shown below, and a similar plot for prime focus can be found here.
More details can be found in the WHT optics log.
In the Shack-Hartmann camera, a lenslet array focuses collimated light from a star onto the CCD. Each image on the CCD is formed by light passing through one lenslet i.e. by light from a given area of primary mirror. Aberrations in the telescope optics shift the images on the CCD relative to those formed by light from an illuminated pinhole in the camera. Example star and lamp exposures are shown below.
In the left-hand image, the shadows cast by the four vanes supporting the secondary mirror can just be seen.
For comparison, the diffraction limit of a 4.2-m aperture, at a wavelength of 5000 A, is ~ 0.03 arcsec.
The true position of the focus drive is the sum of the nominal position, a correction for changing temperature, a correction for elevation (Cassegrain focus only) and a correction for the filter being used. The nominal position, and the temperature and filter corrections are reported at the bottom of the telescope-control system information display.
The nominal position can be requested from the instrument-control prompt,
For the Nasmyth foci, there is no correction of focus for elevation. Focus vs elevation at the GRACE Nasmyth focus was checked by Marie Hrudkova in 1-arcsec seeing on 10/2/2012, and any dependence was too small to measure (<~ 0.02 mm for elevations 23 - 80 deg).
The WHT focus database lists past values of WHT best focus for each of the common-user instruments currently in use.
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