11 February 2013 Resolving the Southern African Large Telescope’s image quality problems
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Optical Engineering, 52(8), 081604 (2013). doi:10.1117/1.OE.52.8.081604
Abstract
Images obtained with the Southern African Large Telescope (SALT) during its commissioning phase in 2006 showed degradation due to a large focus gradient, astigmatism, and higher order optical aberrations. An extensive forensic investigation exonerated the primary mirror and the science instruments before pointing to the mechanical interface between the telescope and the spherical aberration corrector, the complex optical subassembly which corrects the spherical aberration introduced by the 11-m primary mirror. Having diagnosed the problem, a detailed repair plan was formulated and implemented when the corrector was removed from the telescope in April 2009. The problematic interface was replaced, and the four aspheric mirrors were optically tested and re-aligned. Individual mirror surface figures were confirmed to meet specification, and a full system test after the re-alignment yielded a root mean square wavefront error of 0.15 waves. The corrector was reinstalled in August 2010 and aligned with respect to the payload and primary mirror. Subsequent on-sky tests revealed spurious signals being sent to the tracker by the auto-collimator, the instrument that maintains the alignment of the corrector with respect to the primary mirror. After rectifying this minor issue, the telescope yielded uniform 1.1 arcsec star images over the full 10-arcmin field of view.
© 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)
Darragh E. O'Donoghue, Lisa A. Crause, James E. O'Connor, Francois Strumpfer, Ockert J. Strydom, Craig Sass, Janus D. Brink, Charl A. du Plessis, Eben Wiid, Jonathan Love, "Resolving the Southern African Large Telescope’s image quality problems," Optical Engineering 52(8), 081604 (11 February 2013). http://dx.doi.org/10.1117/1.OE.52.8.081604
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KEYWORDS
Mirrors

Telescopes

Monochromatic aberrations

Computer generated holography

Interfaces

Large telescopes

Image quality

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