Abstract

Sodium laser guide stars (LGSs) allow, in theory, Adaptive Optics (AO) systems to reach a full sky coverage, but they have their own limitations. The artificial star is elongated due to the sodium layer thickness, and the temporal and spatial variability of the sodium atom density induces changing errors on wavefront measurements, especially with Extremely Large Telescopes (ELTs) for which the LGS elongation is larger. In the framework of the Thirty–Meter–Telescope project (TMT), the AO-Lab of the University of Victoria (UVic) has built an LGS–simulator test bed in order to assess the performance of new centroiding algorithms for LGS Shack-Hartmann wavefront sensors (SH–WFS). The design of the LGS–bench is presented, as well as laboratory SH–WFS images featuring 29×29 radially elongated spots, simulated for a 30–m pupil. The errors induced by the LGS variations, such as focus and spherical aberrations, are characterized and discussed. This bench is not limited to SH–WFS and can serve as an LGS–simulator test bed to any other LGS–AO projects for which sodium layer fluctuations are an issue.

© 2008 Optical Society of America

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References

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  2. M. Talon and R. Foy, "Adaptive telescope with laser probe : Isoplanatism and cone effect," Astron. Astrophys.,  235, 549-557 (1990).
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    [CrossRef] [PubMed]
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    [CrossRef]
  5. M. A. van Dam, A. H. Bouchez, D. Le Mignant, E. M. Johansson, P. L. Wizinowich, R. D. Campbell, J. C. Chin, S. K. Hartman, R. E. Lafon, P. Jr. Stomski, and D. M. Summers, "TheW. M. Keck Observatory Laser Guide Star Adaptive Optics System: Performance Characterization," PASP 118, 310-318 (2006).
    [CrossRef]
  6. G. Herriot, P. Hickson, B. L. Ellerboek, D. A. Andersen, T. Davidge, D. A. Erickson, I. P. Powell, R. Clare, L. Gilles, C. Boyer, M. Smith, L. Saddlemyer, and J.-P. Veran, "NFIRAOS: TMT narrow field near-infrared facility adaptive optics," in Advances in Adaptive Optics II, B. Ellerbroek, D. Bonaccini Calia, eds., Proc. SPIE 6272, (2006).
    [CrossRef]
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    [CrossRef]
  11. J. W. Beletic, S. Adkins, B. Burke, R. Reich, B. Kosicki, V. Suntharalingham, Ch. Bleau, R. Duvarney, R. Stover, J. Nelson, and F. Rigaut, "The Ultimate CCD for Laser Guide Star Wavefront Sensing on Extremely Large Telescopes," Experimental Astronomy 19, 103-109 (2005).
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  14. L. Gilles, B. Ellerbroek, and J.-P. V´eran, "Laser guide star multi-conjugate adaptive optics performance of the Thirty Meter Telescope with elongated beacons and matched filtering," in Advances in Adaptive Optics II, B. Ellerbroek, D. Bonaccini Calia, eds., Proc. SPIE 6272, (2006).
    [CrossRef]

2007 (1)

2006 (3)

2005 (1)

J. W. Beletic, S. Adkins, B. Burke, R. Reich, B. Kosicki, V. Suntharalingham, Ch. Bleau, R. Duvarney, R. Stover, J. Nelson, and F. Rigaut, "The Ultimate CCD for Laser Guide Star Wavefront Sensing on Extremely Large Telescopes," Experimental Astronomy 19, 103-109 (2005).
[CrossRef]

2003 (1)

1992 (1)

F. Rigaut and E. Gendron, "Laser guide star in adaptive optics : The tilt determination problem," Astron. Astrophys. 261, 677-684 (1992).

1990 (1)

M. Talon and R. Foy, "Adaptive telescope with laser probe : Isoplanatism and cone effect," Astron. Astrophys.,  235, 549-557 (1990).

1976 (1)

Appl. Opt. (2)

Astron. Astrophys. (2)

F. Rigaut and E. Gendron, "Laser guide star in adaptive optics : The tilt determination problem," Astron. Astrophys. 261, 677-684 (1992).

M. Talon and R. Foy, "Adaptive telescope with laser probe : Isoplanatism and cone effect," Astron. Astrophys.,  235, 549-557 (1990).

Experimental Astronomy (1)

J. W. Beletic, S. Adkins, B. Burke, R. Reich, B. Kosicki, V. Suntharalingham, Ch. Bleau, R. Duvarney, R. Stover, J. Nelson, and F. Rigaut, "The Ultimate CCD for Laser Guide Star Wavefront Sensing on Extremely Large Telescopes," Experimental Astronomy 19, 103-109 (2005).
[CrossRef]

J. Opt. Soc. Am. (1)

Mon. Not. R. Astron. Soc. (1)

S. Thomas, T. Fusco, A. Tokovinin, M. Nicolle, V. Michau, and G. Rousset, "Comparison of centroid computation algorithms in a ShackHartmann sensor," Mon. Not. R. Astron. Soc. 371, 323-336 (2006).
[CrossRef]

Opt. Express (1)

Opt. Lett. (1)

Other (5)

G. Herriot, P. Hickson, B. Ellerbroek, J.-P. Veran, C. S. Ye, R. Clare, and D. Looze, "Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the thirty meter telescope," in Advances in Adaptive Optics II, B. Ellerbroek, D. Bonaccini Calia, eds., Proc. SPIE 6272, (2006).
[CrossRef]

M. A. van Dam, A. H. Bouchez, D. Le Mignant, E. M. Johansson, P. L. Wizinowich, R. D. Campbell, J. C. Chin, S. K. Hartman, R. E. Lafon, P. Jr. Stomski, and D. M. Summers, "TheW. M. Keck Observatory Laser Guide Star Adaptive Optics System: Performance Characterization," PASP 118, 310-318 (2006).
[CrossRef]

G. Herriot, P. Hickson, B. L. Ellerboek, D. A. Andersen, T. Davidge, D. A. Erickson, I. P. Powell, R. Clare, L. Gilles, C. Boyer, M. Smith, L. Saddlemyer, and J.-P. Veran, "NFIRAOS: TMT narrow field near-infrared facility adaptive optics," in Advances in Adaptive Optics II, B. Ellerbroek, D. Bonaccini Calia, eds., Proc. SPIE 6272, (2006).
[CrossRef]

G. Rousset, "Wave-front sensors," in Adaptive optics in astronomy, F. Roddier, eds., (Cambridge University Press, 1999), p. 91.

L. Gilles, B. Ellerbroek, and J.-P. V´eran, "Laser guide star multi-conjugate adaptive optics performance of the Thirty Meter Telescope with elongated beacons and matched filtering," in Advances in Adaptive Optics II, B. Ellerbroek, D. Bonaccini Calia, eds., Proc. SPIE 6272, (2006).
[CrossRef]

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Figures (11)

Fig. 1.
Fig. 1.

(Color online) (a) SH-WFS spots geometry for the TMT pupil pointing an LGS at zenith. (b) The radial elongation of the spots can be reproduced in the laboratory with a DM generating an alternating defocus during the integration time. The source intensity is modulated synchronously with the DM to also reproduce sodium profile variations.

Fig. 2.
Fig. 2.

(Color online) Control sequence of the DM, the source and the camera generating elongated spots for a given sodium profile. The DM generates a focus staircase from +18.4 to -18.4µm ptv with 41 discrete steps. The camera exposure begins with the first focus position. The source intensity is updated for each focus position to reproduce the sodium profile. The source is turned off between two steps to prevent the DM transient response from blurring the spot (Sec. 5.2). The camera exposure ends with the last focus position, and the DM goes back to the first focus position during the image acquisition in order to be ready for the next image.

Fig. 3.
Fig. 3.

(Color online) Optical design of the LGS SH-WFS simulator bench, and key-parameters (pupil size and image sampling) used for generating 29×29 elongated spot SH-WFS images (Fig. 8).

Fig. 4.
Fig. 4.

(Color online) Picture of the LGS-bench.

Fig. 5.
Fig. 5.

(Color online) Control architecture of the LGS-bench. The bench control system can be subdivided into 4 tasks: (i) Source intensity modulation: A 16 bit analog output PCI DAQ-card (UEI PowerDAQ PD2-AO-8-16) drives the current of the source. (ii) DM control: A second PCI DAQ-card (Adlink PCI-7200) drives the DM actuators via the ALPAO-DE64 controller. (iii) Camera control: The camera must begin and end integration at varying time intervals, depending on the DM state, therefore it has been configured to be triggered externally. The UEI PowerDAQ PD2-AO-8-16 is also used for sending the trigger signal toward the camera, which sends back a strobe signal to confirm that the trigger has been successfully received. (iv) Image acquisition: The image readout is performed via a 9 pin Firewire cable and a fast PCI frame grabber (IEEE-1394b OHCI PCI Host Adapter 3-port 800Mb/s card).

Fig. 6.
Fig. 6.

(Color online) Peak-to-valley residual wavefront errors (in tip-tilt, residual focus, astigmatism, coma and spherical) of closed-loop calibrations of the DM for 41 discrete focus amplitudes.

Fig. 7.
Fig. 7.

(Color online) Sequence of 88 real sodium profiles used for generating LGS-like SH-WFS images on the bench. Time resolution is 70 s, altitude resolution is 25 m (file TMT.AOS.TEC.07.005_Nap).

Fig. 8.
Fig. 8.

(Color online) 29×29 elongated spot SH-WFS image obtained in laboratory. Spot sampling is 2×8 pixels on the edge of the pupil (for a 10 km-thick sodium profile) as for NFIRAOS (see enlarged spots). The sodium profile used for this image is the 87 th profile of the LIDAR sequence (Fig. 7). The solid curve shows the intensity profile sent to the source (i.e. the projected sodium profile sampled over 41 altitude bins). The dashed curve shows the theoretical profile for a spot located on the edge of the pupil (i.e. the source intensity profile convolved by the PSF and resampled over 16 pixels). The measured spot profile (filled circles) perfectly matches the theoretical spot profile, except for the pixel 16 where the spot overlaps with another.

Fig. 9.
Fig. 9.

(Color online) Wavefront analysis made with the 29×29 spot SH-WFS of the bench, using the CoG centroiding algorithm for 4 different configurations: (I) with non-elongated spot images to assess the bench stability, (II) with static uniform 10 km-thick sodium profile to check the DM repeatability, (III) with variable sodium profiles without focus tracking to highlight the focus error induced by sodium layer variation, (IV) and with focus tracking to characterize the higher-order LGS aberrations. The temporal mean values and standard deviations of centroids and Zernike modes (ptv amplitude) were computed from a time sequence of 700 SH-WFS images for cases I/II, and from a sequence of 88 real sodium profiles for cases III/IV. The phase reference is a flat wavefront for cases I/II and a uniform 10 km-thick sodium profile SH-WFS image for cases III/IV.

Fig. 10.
Fig. 10.

(Color online) Global tip versus global tilt for a time series of 352 (88×4) SH images with a 1/100 th pixel dither signal (4 discrete positions) for non-elongated spots and for elongated spots (i.e. for sodium profiles). A similar dispersion pattern (inside each discrete position) is also obtained without dither signal.

Fig. 11.
Fig. 11.

(Color online) (a) Correlation between the mean altitude of 88 sodium profiles and the focus error measured on the bench for each profile, without focus tracking (config. III). (b) Correlation between the sodium profile asymmetry (Eq. (7)) and the spherical aberration (Z11) measured on the bench, with or without focus tracking (config. III or IV).

Tables (1)

Tables Icon

Table 1. Typical temporal RMS fluctuations of the phase measured on the bench for the 4 configurations described in Fig. 9. σϕ is the ptv amplitude noise of the wavefront for all modes. σTT , σ Z4 and σHO are respectively the ptv amplitude noise for tip-tilt, focus and higher order modes. All values are in nm ptv rms.

Equations (7)

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θ r σ Na h 0 2 cos ( z ) ,
ξ = ± E 2 λ d ,
ϕ ( ρ ) = A pt v ( ρ ρ 0 ) 2 ,
d ϕ d ρ ρ = ρ 0 = 2 A pt v ρ 0 .
A pt v = ± E λ 8 N spot ,
h ( p ) h 0 1 r 2 h 0 3 p Δ e 1 + 1 h 0 p Δ e ,
Asym = ( h 2 h G ) ( h G h 1 ) h 2 h 1 P ( h ) dh ,

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