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Completion of Altair/LGS technical commissioning

17 May 2006


We have completed the Technical Commissioning of the Altair Laser Guide Star system. Technical Commissioning includes all tasks related to laser propagation, closing the three main guiding loops simultaneously: (1) the high-order Laser loop (LGS), (2) the low-order tip/tilt loop (aka STRAP), and (3) the slow focus sensor (SFO). It also includes basic functionality of the operations including basic dithering, delivery of the beam to the science instrument and tests of non-sidereal modes common to all instruments that will use the Altair-corrected beam (NIRI and NIFS).

Science Commissioning is still underway in June and July. This is basically optimizing the performance and operations at a higher level than is required for the Technical Commissioning. In addition, commissioning of LGS + NIFS will take place in July. Below, we summarize what we know about the performance of the LGS system so far.

Engineering and Operations Details:

  1. Laser power has been around 12 Watts at the output of the laser system, with about 9 Watts actually projected in the sky.
  2. Laser beam quality is stable and within specs at about 1.3 arcseconds FWHM. Recent upgrades to the Laser Launch Telescope have been successful.
  3. The LGS loop has been successfully closed on the Laser. Current performance is sufficient to get 200 Hz corrections during the spring season (average sodium abundances). This is sufficient for science work. The corrections appear very stable after the application of a software patch to the secondary mirror.
  4. The tip/tilt loop (STRAP) performance has been crudely characterized. We expect to be able to ready R=18 with moderate moon.
  5. The slow focus sensor (SFO), which corrects for changes in the height of the sodium layer with time as well as elevation works. However, improvements still need to be made to the software and hardware. The SFO is currently the limiting factor in faint work as we think it can only go to R=18 during dark time. We are looking at some improvements in this device in June.
  6. Various software gain levels have been tested and implemented. These are needed for the basic operation of LGS in the science mode.

Science Details:

  1. Overheads have been crudely tested. We believe that the LGS setup time will be an additional 5 minutes per target in most cases over Altair NGS. There is currently a somewhat high overhead for dithers (about 7 seconds) we are working on reducing this time.
  2. Image quality has not been fully optimized. We have seen a few images where the LGS performance equaled that of the NGS performance in K-band, but on a fainter star. However, we still need to fully optimize the Non-Common Path Aberrations (NCPA), which will allow better PSFs to be delivered consistently. Despite this, we believe that performance will be good as we have seen that the PSFs are generally quite stable for a given target even over the course of an hour. Additionally, there is very little anisoplanatism seen even in across the f/14 images. It appears that the LGS system should work very much like the NGS system + field lens, but with fainter guide stars.
  3. Three basic modes of Altair/LGS have been tested: (1) Guiding on a single tip/tilt star and dithering (only one setup). (2) Guiding on a different tip/tilt star at each dither position (separate setup for each dither position). (3) Guiding on a non-sidereal target as the science target and tip/tilt star. Additional tests are need to ensure that we can guide on a sidereal tip/tilt star with the non-sidereal target, however, we estimate that there should not be a problem with this as the software for this scenario is largely transparent to Altair/LGS and is handled by the telescope software. We already know that the telescope software works with the other Gemini facility guiders.
  4. Please see the LGS System Verification webpage for details about proposing to use Altair in SV time, including overheads.

Some images:

Altair/LGS Eagle Nebula HST Eagle Nebula
Above: Altair/LGS Eagle Nebula image. Above: Comparison with rotated HST Eagle Nebula image.
NIRI/LGS f/14 image of the Eagle Nebula. This image was obtained in typical LGS mode, with a single tip/tilt star (slightly above and to the left of center). Field of view is about 40 arcsec per side (click image for full size). Compare to HST image of the same region. The Gemini image has comparable resolution but many more stars as it was taken in the near-infrared where extinction is lower. Note that the PSF is fairly stable across the entire f/14 field.

Above: NIRI/LGS f/14 image of M13. This image was collected with several different pointings, each using a different tip/tilt star centrally located. Field of view is about 100 arcsec horizontal (click image for full size). The PSF is again fairly stable across the entire dithered field. This mode requires additional setup time as each pointing must be treated as a separate observation.

Note that in the above images, the PSF has still not been optimized. This is particularly apparent in the images of M13, as stars on the periphery show a large amount of astigmatism (+ or X shaped wings). We expect to be able to eliminate these aberrations in the upcoming June commissioning run.

Last update May 17, 2006; Chad Trujillo.