Wynne correctors

For conventional speckle interferometry the detected bandwidth can be increased by transforming the partially proportional wavelength dependence of the speckle scale. Here, the proportionality is more accurate since the residual phase on the wave is made small by the adaptive optics. The photon rate being critical in ground-based dark-speckle imaging, there is much to gain in increasing the usable spectral bandwidth.
One of us (DK) has re-calculated the corrector solution obtained by Wynne (1979) (Fig. 2). The pair of null triplets shrinks the blue pupil, while enlarging the red pupil, so that both Airy patterns, or speckle patterns, are of nearly identical size. With respect to the original Wynne design, we have suppressed the power in the second triplet, in order to keep more flexibility in the final magnification and we have replaced the SF8 glass by SFL5 for optimal correction in the red range (600-850nm).

• At $\lambda_0 = 635 nm$, the glasses have the same refractive index and the two triplets behave as two plane-parallel plates.
• System performance is limited by the non-linearity of the glass dispersion. In the range 600-850nm, the final radial chromatism, including diffraction, is lower than 1.7% of the axial distance (Fig. 6).
• The corrected field is quite small owing to the strong curvatures which limit the diameter of the exit triplet.
• The star image must be kept on axis, and any companion has its Airy peak dispersed, although not very much with the small relative spectral bandwidth utilized. Field spectrocopy techniques ([Bacon et al. 1995]) or wavelength sensitive detectors such as the Super-conductive Tunnel Junction camera ([Peacok et al. 1998]) could solve this residual problem.
• Although it would be nice to insert the occulting mask in the achromatised image, the following pupil would become chromatic, which would complicate the apodization masking.

The analytical derivation is given in Appendix.