LGSAO Basics and System Description

Astronomical observations using natural guide star (NGS) adaptive optics are limited to areas of the sky within ~30 arcseconds of a fairly bright star (R<13.5 using the Keck system). This leaves 99% of the sky beyond the reach of AO. The W. M. Keck Observatory provides a sodium (Na) laser guide star on the Keck I and II telescope to dramatically expand the sky area accessible by AO observations.

In the upper part of the Earth's mesosphere (90±10 km altitude), there is a 5-20 km thick layer rich in Na atoms, deposited by the ablation of micrometeorites. These atoms can be excited and caused to radiate by spontaneous emission by projecting a laser tuned to the Na D atomic transition (589 nm) in the direction of the science target. The magnitude of this artificial guide star corresponds to 9<R<11 on Keck I and 7<R<8 on Keck II, and varies with laser power, beam collimation, and Na column density.

There are four fundamental differences between the operation of the Keck AO system using the laser guide star (LGS) with respect to that using an equivalent-magnitude NGS:

1. The tip-tilt (TT) of the science object cannot be derived from the LGS since the TT of the LGS includes TT on the upward path to the sodium layer. A separate natural TT reference star near the science object must therefore be used. This object can be fainter than that required for full NGS AO correction, and may also be further from the science target. A visible TT sensor (STRAP) with 4 avalanche photodiodes in a quad cell arrangement is used to measure the atmospheric TT on Keck II. Keck I observers can use STRAP or TRICK, a near-infrared camera with a Teledyne H2RG detector (see Fig. 1). Both sensors allow the use of TT reference stars within 60 arcsec of the science target. STRAP is mounted on a X,Y translation stage to move around the field, while small regions of interest around the TT star are read out from the fixed TRICK field. A choice of H or Ks reflective dichroic beamsplitter is inserted in front of OSIRIS to feed light to TRICK.

Fig. 1: Keck LGSAO control loops
2. The LGS is not located at infinity, but at a finite and slowly varying altitude. Though it can be used to measure the rapid atmospheric focus variations due to turbulence, it cannot be used to measure the absolute focus of the telescope. On Keck II, the TT reference is therefore also used to measure the telescope focus, by diverting a small fraction of its light to the low-bandwidth wavefront sensor. On Keck I when using TRICK the TT and LBWFS reference stars need not be the same object. (LBWFS, see Fig. 1). The focus of the TT reference recorded by the LBWFS is then used to drive the position of the WFS focus stage which tracks the altitude of the sodium layer.

3. Another consequence of the finite altitude of the LGS is reduced performance due to the cone effect, also known as focal anisoplanatism. The atmospheric volume in which a star's wavefront is distorted by turbulence is cylindrical, while that over which the wavefront of the LGS is sampled is conical (with the apex at the altitude of the Na layer). Some fraction of the turbulence affecting the science target will therefore not be sensed by the wavefront measurement of the LGS, and the AO-corrected image quality will consequently be reduced.

Fig. 2: Image of LGS with primary unstacked for side projection of the laser
4. The LGS images are not point-like since they are being produced in an atmospheric Na layer 5-20 km thick. The apparent size of the LGS will therefore vary across the telescope pupil as a function of the distance from the laser launch telescope. This effect can be mitigated by applying a deformable mirror (DM) loop gain which varies across the telescope pupil. Fig. 2 shows the LGS images from the unstacked Keck primary for side launch of the laser; both Keck I and II now launch the laser from behind the Keck telescope secondary mirror which halves the amount of LGS elongation.

Factors limiting system performance

1. Seeing
The AO correction achievable with LGSAO is somewhat more sensitive to seeing than that of NGSAO, due to the blurring of the upward-propagating laser beam under turbulent conditions. The signal-to-noise ratio (SNR) of the wavefront measurement on the LGS will be proportional to the reciprocal of its apparent size, which will be dramatically increased under poor seeing conditions. The Keck LGSAO system has been successfully operated in visible (0.5 micron) seeing between 0.3" and 0.9". Performance estimates given in this document apply to visible seeing of 0.6", approximately the median for the Keck 2 telescope.

2. Laser beam quality
The full-width-at-half-maximum of the LGS on both telescopes is < 1.5" under excellent seeing conditions. The spot size depends on laser beam quality and alignment, and the seeing conditions (sqrt(2) times the seeing due to the upward and downward path of the laser). The wavefront measurement error will be linearly proportional to the LGS spot size.

3. Returned power
The power returned from the sodium layer is a function of the projected laser power, the frequency tuning and pulse format of the laser, and the density of the sodium layer. The average power output of the Keck I and II lasers are generally 15-20 W and 20 W, respectively. On Keck I the LGS equivalent R magnitude is 9 to 11 and on Keck II it is 7 to 9 magnitude. The return has been observed to vary by over a factor of 4 from run to run. The SNR of the wavefront measurement is proportional to the counts per subaperture.

4. TT reference magnitude
The magnitude of the TT reference determines the quality of the tip-tilt control, but not the control of higher order modes. Thus the image degradation suffered with a faint TT reference is a broadening of the PSF core, rather than a reduction in the ratio of the power in the core to that in the halo, as is the case with a faint NGS. The TT bandwidth and measurement noise both increase for fainter stars, and begin to dominate the error budget for TT references with R>15. The LGSAO system has been operated for science with TT reference stars as faint as R=19. Note that the PSF when using faint TT reference stars has a unique structure, since high-order aberations are still well corrected. This results in a PSF with little power in the seeing-limited halo, but no clear diffraction-limited core. The TRICK requirement is to use stars as faint as 16th magnitude in H or K-band.

5. TT reference separation
We have measured the isokinetic angle, the angle at which the phase becomes decorrelated by 1 radian rms due only to TT, to be 72" at 2.1 microns on one occasion (07/27/04). This may not be typical. Above a separation of ~40", tilt anisoplanatism becomes a dominant term in the LGSAO error budget. Under most circumstances, the science object will be kept near the telescope optical axis (or science detector center). The TSS stage translates the STRAP TT sensor and LBWFS across the focal plane to reach the TT reference; for TRICK use the region of interest is moved to acquite the TT reference. Certain regions of the TSS focal plane and TRICK detector are currently inaccessible due to vignetting or bad pixels, respectively (see TSS view), thus the PA of the science instrument may be constrained for TT references >10" from the science target.

6. Elevation
As with seeing, the corrected image quality is more strongly dependent on elevation in LGSAO than in NGSAO. Under median seeing conditions on 02/04/04, LGSAO corrected images with a V=15 TT reference at 49° elevation had a Strehl of only 0.06, while that near zenith with a similar TT reference was 0.15. We are prohibited from projecting the laser below 20° elevation.

7. Pointing accuracy
Since TT control is performed by STRAP or TRICK placement of the science object on the science instrument is determined by the positioning accuracy of these devices rather than by the less accurate field steering mirrors (FSMs) as is the case for NGSAO. The TSS stage which positions STRAP and the LBWFS has proven to be repeatable to ~4 microns rms, corresponding to 3 mas on sky.

For more information please contact Jim Lyke or R. Campbell: (randyc or jlyke @keck.hawaii.edu).