The Ultimate CCD for Laser guide star wavefront sensing on Extremely Large Telescopes

Beletic, James W.; Adkins, Sean M.; Burke, Barry E.; Reich, Robert B.; Kosicki, Bernie; Suntharalingham, Vyshnavi; Bleau, Charlie; DuVarney, Ray; Stover, R. J.; Nelson, Jerry; Rigaut, François

doi:10.1007/1-4020-4330-9_36

Cited by 2 publications

(3 citation statements)

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“…For the former, the idea was to exploit the specific geometry of the LGS spots, and a radial charge-coupled device (CCD) has been proposed to fit the spot elongations. 17 Indeed, due to the central launch configuration, the spots seen by each of the LGSWFS Shack-Hartmann will be radially oriented. The proposed design implemented sub-apertures of about 4×16 pixels, with a radial geometry adapted to the spot elongation.…”

Section: Requirements For Eltsmentioning

confidence: 99%

Performance of a complementary metal-oxide-semiconductor sensor for laser guide star wavefront sensing

Bustos

Atwood

et al. 2022

J. Astron. Telesc. Instrum. Syst.

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The adaptive optics (AO) systems of future Extremely Large Telescopes (ELTs) will be assisted with laser guide stars (LGS) which will be created in the sodium layer at a height of ≈ 90 km above the telescopes. In a Shack-Hartmann wavefront sensor, the long elongation of LGS spots on the sub-pupils far apart from the laser beam axis constraints the design of the wavefront sensor (WFS) which must be able to fully sample the elongated spots without undersampling the non-elongated spots. To fulfill these requirements, a newly released large complementary metal oxide semiconductor (CMOS) sensor with 1100×1600 pixels and 9 µm pixel pitch could be employed. Here, we report on the characterization of such a sensor in terms of noise and linearity, and we evaluate its performance for wavefront sensing based on the spot centroid variations. We then illustrate how this new detector can be integrated into a full LGS WFS for both the ESO ELT and the TMT.

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Section: Requirements For Eltsmentioning

confidence: 99%

Performance of a complementary metal-oxide-semiconductor sensor for laser guide star wavefront sensing

Bustos

Atwood

et al. 2022

J. Astron. Telesc. Instrum. Syst.

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“…Given the non-zero read-noise from the CCD pixels, together with background from Rayleigh scattering and sky brightness, spreading out the signal from the laser beacon across multiple pixels reduces the signal to noise ratio when measuring the spot positions in the radial direction, and requires more laser power. A polar coordinate CCD 13 is under development to improve the situation. It has individual rectangular regions, one for each lenslet of the LGS WFS; each is oriented radially to minimize the number of pixels readout.…”

Section: Dynamic Refocussingmentioning

confidence: 99%

“…The authors of Ref. 13 propose a further refinement when using this CCD with a pulsed laser, illustrated in the right panel of Figure 7. If the laser was pulsed briefly, say for 3 µs, the shot of photons would be 1 km long.…”

Section: Dynamic Refocussingmentioning

confidence: 99%

Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope

et al. 2006

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Laser guide star (LGS) adaptive optics systems for extremely large telescopes must handle an important effect that is negligible for current generation telescopes. Wavefront errors, due to improperly focusing laser wavefront sensors (WFS) on the mesospheric sodium layer, are proportional to the square of the telescope diameter. The sodium layer, whose mean altitude is approximately 90 km, can move vertically at rates of up to a few metres per second; a few seconds lag in refocusing can substantially degrade delivered image quality (15 m of defocus can cause 120 nm residual wavefront error on a 30-m telescope.) As well, the range of temporal frequencies of sodium altitude focus, overlaps the temporal frequencies of focus caused by atmospheric turbulence. Only natural star wavefront sensors can disentangle this degeneracy. However, applying corrections with representative focus mechanisms having modest control bandwidths, causes appreciable tracking errors. In principle, electronic offsets measured by natural guide star detectors could be rapidly applied to laser WFS measurements, but to provide useable sky coverage, integrating sufficient photons causes an unavoidable time delay, again resulting in potentially serious focus tracking errors. However, our analysis depends on extrapolating to temporal frequencies greater than 1 Hz from power spectra of sodium profile time series taken at 1-2 minute intervals. In principle, with a pulsed laser, (e.g. 3-µs pulses) and dynamic refocusing on a polar-coordinate CCD, this focus tracking error may be eliminated. This result is an additional benefit of dynamic refocusing beyond the commonly recognized amelioration of LGS WFS spot elongation.

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The Ultimate CCD for Laser guide star wavefront sensing on Extremely Large Telescopes

Cited by 2 publications

References 0 publications

Performance of a complementary metal-oxide-semiconductor sensor for laser guide star wavefront sensing

Performance of a complementary metal-oxide-semiconductor sensor for laser guide star wavefront sensing

Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope

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