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Altair Laser Guide Star

The Gemini Mauna Kea Laser Guide Star System is designed to extend the use of Altair for targets for which a bright natural star is unavailable for adaptive optics correction.



The Laser projects a ~10 Watt beam of coherent radiation at 589 nm into the sky. This beam excites sodium atoms at ~ 90 km altitude produced from the disintegration of meteors in the Earth's mesosphere. These atoms re-emit at the 589 nm wavelength producing an artificial beacon (a Laser Guide Star, LGS) on which Altair can guide for high-order correction. A natural tip/tilt star is still needed for the low-order tip/tilt correction, but this star need not be very bright nor very close when compared to correcting all orders on a Natural Guide Star (NGS). We expect to be able to use stars down to R < 15 within 15 arcsec for high-Strehl LGS correction and R < 18.5 up to 25 arcseconds away from the science target for low-Strehl LGS correction. This is about a 3.5 magnitude increase in performance over NGS alone. The Laser will always be pointing at the center of the field of view, but will be invisible to the user as the Altair dichroic splits the correction and science beams at 1 micron.

Important Notes

The Altair Laser mode is now commissioned as operations are sufficient for useful scientific return. Updated performance information for LGS will be available on this webpage as the first science data is collected. In addition, some improvements are planned (as noted below) and will also be updated at this webpage as they take place.

Observing Constraints and Time-Critical Programs

Laser propagation can only be done under photometric conditions and the guide loops are only stable for decent seeing. Thus, only CC=50% and IQ=70% observations are offered for LGS. LGS runs will be generally be scheduled within 1 week of the full moon, and during this time both normal queue and LGS queue programs will be executed. In general, PIs of LGS programs should request SB=Any conditions so that full moon observations can be conducted. However, for guide stars between 18 < R < 18.5, a dark time request (SB=50%) is required.

U.S. Air Force Space Command clearance is required for all LGS targets one week prior to a scheduled LGS run. In addition, the Federal Aviation Administration requires the use of laser spotters to ensure that the Laser does not pose a distraction for pilots. This additional operational overhead means that in general, LGS observations will only occur one week of the month, and that a complete target list must be cleared with Space Command one week before the run begins. Thus, rapid ToOs (such as immediate observation of Gamma-Ray Bursts) cannot be accepted as LGS observations, although of course they will be accepted for other instruments on the telescope during LGS runs. "Slow" ToOs can use LGS provided that the targets can be made active eight days before the LGS run begins. Beginning in 2008A Gemini can offer limited LGS observations of Band 1 and 2 ToOs that are triggered less than a week before, or during, an LGS run. The observations must be made during a planned LGS run at the telescope. Also, only two such targets (for all programs) can be observed during any typically week-long LGS run, and only one such target (for all programs) can be observed on any given night. All effort will be made to approve and observe a target within 24 hours, however this cannot be guaranteed, and the observation may occur two or three nights after the trigger is made. For a detailed list of when LGS runs are scheduled, PIs should consult the current semester schedule for Gemini North (GN).

Elevation Limit

The use of LGS is currently limited to targets with elevation of 40 degrees or greater (the previous 49 degree limit has been improved due to a change in the optical bench). The current 40 degree limit is a hard limit due to the optomechanical design of the instrument. Futher improvements will be very difficult to make and would only take place on timescales of months or greater. These improvements are being considered. The largest science impact of the 40 degree elevation limit is that the center of the galaxy (Dec ~ -30 degrees) cannot be reached with LGS. For more detail, see this plot of target availability for Semester A. The plot assumes that a target must be available for at least 3 hours per night for a month to be considered "observable" by LGS, since LGS runs occur no more frequently than once per month. Targets near the RA boundary may be observable if they are short duration observations. Targets below the Dec boundary cannot be observed under any circumstances due to the elevation limit.


High Strehl Correction in H, K and L (1.6 to 2.5 microns) requires a bright (8.5 < R < 15) tip/tilt star within 15 arcseconds of the science target. Delivered Strehls are currently about 50% of that seen for NGS: 10% in H and up to 20% in K. L-band LGS has not been tested. The background in L will be considerably higher with Altair in the beam. We believe that some signal-to-noise improvement is likely at L and angular resolution will be close to the diffraction limit. Observations shortward of H (such as z and J) should not be considered High Strehl even for bright tip/tilt stars.

Low Strehl Correction will occur with a faint (15 < R < 18.5) tip/tilt star within 25 arcseconds of the science target. Delivered Strehls should be a few percent in J, up to 10% in H and K. The 25 arcsecond limit is a hard limit based on the Altair optics, i.e. guide stars further than this cannot be observed with LGS. Note that for guide stars between 18 < R < 18.5, a dark time request is required. All observations shortward of H (1.6 microns) should be considered Low Strehl. Note that if your Guide Star is brighter than R < 10, you should use NGS instead of LGS due to the danger of damaging the tip/tilt wavefront sensor. A future LGS upgrade may allow the placement of an optional neutral density (ND) filter into the beam which will allow the use of LGS on bright targets (such as calibration stars).

Both high-Strehl and low-Strehl correction is possible for any combination of non-sidereal and sidereal science targets and tip/tilt stars. Note that galaxies with stellar-like cores may also be used for guiding. We have only sparse data on which cores should work and which will fail at this point. We currently expect guiding to work if the full width at half-maximum of the core is smaller than about 1 arcsec. (This corresponds to about a 1.5 magnitude drop in surface brightness per arcsec of distance from the core.) More galaxy cores near this performance limit need to be tested to refine this number.

Altair/LGS is formally a very high performance guider, and thus its observing modes are primarily due to the science instrument. There are two basic Instrument Modes with LGS:

  1. LGS+NIRI Imaging and Spectroscopy:
      LGS+NIRI Imaging is available at J, H or K with f/14 and at J, H, K or L with f/32. LGS+NIRI Spectroscopy can be performed in J, H or K at f/32 only.
  2. LGS+NIFS IFU Spectroscopy:
      LGS+NIFS IFU Spectroscopy is available in the z, J, H and K band.

Below is an analysis of a R=13.3 magnitude star compared to diffraction limited observations (Kprime, 0.5 arc V-band uncorrected seeing, elevation 76 deg, 5 sec x 2 coadds). In the radial plots (top panels), the LGS PSF is denoted by x's and the diffraction limit is shown as a solid line. An image of the LGS PSF is shown in the bottom left panel compared to the diffraction limit in the bottom right panel (images span 1.8 arcseconds).

Exposure Times and Overheads

See the NIRI or NIFS webpages for more detailed science instrument information. We are currently modifying the NIRI and NIFS ITCs for LGS support as of this writing. Once the modifications are finished (mid-March 2007), you can compute exposure times directly using the NIRI ITC or the NIFS ITC. To compute exposure times and sensitivity using the old NIRI+Altair or NIFS Integration Time Calculators (ITCs) make the following modifications: (1) all LGS observations are conducted with the Field Lens, so divide the effective guide star separation by 4 before input into the ITC and (2) all LGS observations should increase the effective brightness of the tip/tilt star by 3 magnitudes. Thus, if using a 15 arcsec separation R = 14 magnitude star, one would input 3.75 arcsec separation and R = 11 into the ITC, yielding a 33% Strehl in the K-band. If using a 25 arcsec separation R = 18 magnitude star (the current limit of our correction ability), one would input 6.25 arcsec separation and R = 15, giving a 11% Strehl in the K-band. To compute observing overheads, add 10 minutes of extra setup per pointing (a pointing is defined as a unique tip/tilt star). Note that dithers using the same tip/tilt star do not require a separate setup, but will incur an additional ~3 seconds per dither position above NGS overheads. These numbers may be reduced as we gain more experience with the system, but likely not by more than a factor 2.

OT Notes

Performance is in general a strong function of guide star brightness. As the LGS system performance encompasses a factor 1000 range in flux, it is important that the guide star brightness be estimated accurately. We have found that the best catalog for guide star brightness is the UCAC2 catalog U magnitude, which roughly approximates the R-band observations of the tip/tilt star. The UCAC2 catalog limit is about 16th magnitude. Below this limit the USNO-B1.0 catalog is the most accurate. Note however, that the catalog has uncertainties of order 0.5 to 0.75 magnitudes so backup targets should be provided for the faintest stars.

As described above, stars brighter than R=10 should not be observed in LGS mode until an ND filter for LGS is available. This means that most telluric standards and many photometric standards should be observed in NGS mode.

Page Compiled by Chad Trujillo. Last update September 17 2007, by Sandy Leggett.