Wavefront Sensing with WFCAM

The following links contain instructions for initial alignment of WFCAM during the first nights of a WFCAM block and how to carry out, analyse and implement WFCAM wavefront sensing data. On the first night of a block, the order should be 1) tilt correction, 2) wavefront sensing and 3) M2 translation correction. On a typical QA night, only the wavefront sensing is necessary.

Wavefront Sensing with WFCAM
WFCAM Secondary Alignment

Wavefront Sensing with Cassegrain Instruments

Latest Wavefront Characteristics
Background
Zernike Polynomials
“Typical” Wavefront images and errors
Gemini Comparison Test
Presentation given at the 1998 SPIE meeting in Kona, Hawai’i 
by Ant Chrysotomou
JOSE results
How to conduct the WFS observations
How to reduce the WFS observations

Latest Wavefront Results

The 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;

• Z5, Z6 — Astigmatism
• Z7, Z8 — Coma
• Z9, Z10 — Trefoil (triangular coma)
• Z11 — Spherical

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.:

Background

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. 

ernike-polynomialsernike-polynomialsWavefront 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.

Zernike Polynomials

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): 
 

Astigmatism
Coma
Trefoil
Spherical

  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

ernike-polynomialsClick 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.

JOSE Results

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/ ).