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Wavefront
Sensing on UKIRT
Latest Wavefront ResultsThe following plots illustrate the magnitudes of Zernikes applied to UKIRT to correct for low-order aberrations. These are derived from analysis of monthly wavefront-sensing observations, which allow us to monitor, on a month-by-month basis, the accuracy of the primary figure and the alignment of the secondary. The plots show the Peak-to-Valley Zernike values derived that have been entered each month into the telescope model; In all cases units are in nanometers. The sine (S) or cosine (C) dependence of each Zernike with respect to the telescope hour angle (H) and declination (D) are also modelled and included in these plots.Following Noll (1976, J.Opt.Soc.A., 66, 207; see also the discussion below), the RMS variation across the wavefront associated with each Zernike (aberration) is given by dividing by the appropriate scaling constant, e.g.:
Coma; Z7, Z8 / (8)0.5 Trefoil; Z9, Z10 / (8)0.5 Spherical; Z11 / (5)0.5 UKIRT's optical performance is measured by Curvature Sensing (Roddier & Roddier, 1993. J.Opt. Soc., America A, 10, 2278) in which images from either side of focus are subtracted and normalised to yield a Laplacian of the wavefront which is then obtained by integration. The wavefront sensor (Roddier et al. 1994. Proc. SPIE, 2199, 1172) was constructed at the JAC and normally resides on the east port of the Instrument Support Unit. It comprises an optical bench enclosed in a rigid box and supporting a commercial filter wheel located at the telescope focus, a camera lens of 250mm focal length and an Astrocam CCD on which the lens forms an image of the secondary. (A 50mm lens may be installed instead, to re-image the focal plane on the CCD for study of in-focus stellar images.) The Astrocam CCD has a pixel scale of 0.314 arcsec/pixel, normally binned to 3x3 to give 0.942 arcsec/pixel. The filter wheel carries two pairs of AR-coated positive and negative lenses, one of ±1m and one of ±2m focus, a knife-edge and a clear aperture. In use, after alignment on a star and focussing the image of the illuminated secondary mirror on the CCD, images are taken in sequential pairs defocused by means of the lenses (almost all data has been taken with the ±1m lens pair). The two defocused images are then spatially aligned, subtracted, normalised and iteratively analysed for the resulting Zernike polynomials. Wavefront sensing on UKIRT began in earnest in August 1996 after the installation of the new tip-tilt secondary mirror. This campaign is aimed at characterising the telescope optical system and its performance as a function of Hour Angle and Declination, as well as to discern any seasonal variations which may be present throughout the year. Thus the campaign took the form of measuring the wavefronts from approx. 60 stars spread across the full sky field of view of UKIRT at regular monthly intervals. The information gained from this campaign allowed us to develop a telescope lookup model which could be used to actively figure the telescope during a nights observing to correct for astigmatism, trefoil and spherical aberrations. Coma is corrected for by secondary mirror alignment. The orthogonal Zernike polynomials (in polar
coordinates) used in our analysis of the measured wavefronts are
adapted from Noll (1976, J. Opt. Soc. Amer., 66, p. 207):
Here, rho and theta represent the radius and rotation angle in the circular wavefront (with respect to the axial centre of the optical system), with rho normalised to unity at the edge of the wavefront image. Note that these polynomials are equivalent to Peak-to-Valley variations across the wavefront (see above). "Typical" Wavefront images and errors Click on the following link to see a typical wavefront image, before and after subtraction of the Zernike polynomials. Gemini Comparison Tests In January 1998, Gemini staff brought their Shack-Hartmann wavefront sensing equipment to UKIRT to run some comparison tests with a view to not only test their system, but for UKIRT staff to simultaneously evaluate the use of a Shack-Hartmann system over the present curvature sensing technique. The results were very favourable for both camps with no compelling evidence which points to a preference of one system over another. UKIRT periodically takes data for the Joint Observatory Seeing Experiment. The results are analysed by staff at Imperial College (follow the link to http://op.ph.ic.ac.uk/ukirt/ ).
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