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We present and discuss the design and expected performances of the SCAO-WFS module of SIMPLE, the highsd
resolution near-infrared spectrometer for the E-ELT which is designed to operate in the wavelengths range 0.84-2.5μm
with an entrance slit width of 27mas which yields a spectral resolving power of R=130,000. We analyze both pyramid
and SH wave-front sensors operating at near-infrared wavelengths (1-2 μm). The main results can be summarized as
follows.
- Good levels of AO correction can only be achieved within a few arc-sec of the reference star. The primary
applications of SCAO correction are therefore observations where the science object is compact and bright enough
to be used as reference star, feeding the WFS via a beam-splitter. We concentrate on a WFS operating at nearinfrared
wavelengths because many of the scientific targets of SIMPLE are intrinsically very red.
- The pyramid WFS provides better performances than the SH, because it is less affected by aliasing errors.
- The quality of AO correction is almost independent on the wavelength at which the wave-front is sensed. Therefore,
we simplify the design the WFS module by not including the K-band, i.e. operating in the 1.0-2.1μm range.
Extension to shorter wavelengths is also possible but requires exchanging the atmospheric dispersion compensator.
- Using a pyramid WFS with 84x84 sub-apertures one can achieve remarkably high values of light-concentration in
the slit, i.e. ~70% in K and ~40% at 0.9μm.
- The limiting magnitude for the 84×84 WFS is about 13.5+2.5•log(η), where η is the fraction of stellar light sent to
the WFS by the beam-splitter.
- Somewhat fainter limits, i.e. magnitudes ~14.0+2.5•log(η), can be achieved by changing the camera optics of the
WFS and sampling 42×42 sub-apertures.
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