|Image Spread||Shack-Hartmann wavefront sensor measurements of the optical aberrations
of the WHT optics predict a PSF with a FWHM of approximately 0.25 arcsec
in the absence of tracking and focus errors, and seeing.
This image spread results mainly from small-scale figure errors of the primary and secondary mirrors, and is within the original design specification for the telescope. When added in quadrature to the image spread due to seeing, the contribution from this term is relatively small. For example the median intrinsic seeing will be increased from 0.69 to 0.73 arcseconds FWHM.
Errors in the telescope focus setting potentially contribute more significantly to image spread. Focus is controlled by the position of the secondary mirror. The surface of the mirror is positioned by the focus drive and encoder mechanisms to a demanded setting with an uncertainty of less than 10 microns. A focus error of this size would increase the PSF width by less than 0.1 arcsec added in quadrature to the FWHM - an insignificant contribution even under the best seeing conditions.
However much larger errors can occur in determining the correct focus position in the first instance. Rapid changes in the PSF width, due to seeing variations, limit the accuracy with which the optimum focus position can be estimated. The optimum focus for a given instrument is typically estimated at the beginning of each night by measuring the FWHM for short exposure images of a bright star for a range of focus positions. The focus is set to the location of the minimum of a curve fitted to the measured set of FWHM values.
The accuracy of this method is limited by changes in the intrinsic seeing
during the focus run. Substantial seeing fluctuations may be expected on
short timescales. The typical rms FWHM fluctuations for sequential
Clearly the accuracy of the focus estimate can be improved by repeating the measurement in order to average out the effects of seeing fluctuations, but this is time consuming. Focus routines which are independent of changes in the intrinsic seeing are preferable. For example, the telescope pupil may be masked to form two subapertures. Telescope focus errors cause the images of a star produced by each aperture to separate at the image plane. The optimum focus position can then be determined by measuring the image separation as a function of focus position. This method is currently being tested for use with the ISIS spectrograph at the WHT.
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