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LGS commissioned for science with NIRI and NIFS

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22 Feb 2007


Following two successful LGS commissioning runs, the Altair Laser Guide Star (LGS) system is now commissioned for use in science applications using NIRI and NIFS. The system is still relatively new, so full performance under all conditions is not known at this time. However, performance is sufficient for science use and operations are now smooth enough that LGS operation can be integrated into the queue. Below we summarize the main engineering details of the commissioning run. Current science-relevant performance details are on the LGS Component webpage. 2006B-SV, 2006B-Q and 2007A-Q programs have been conducted with the LGS system.

Engineering and Operations Details:

  1. The distance limit for tip/tilt stars has been mapped. It turns out to be 25 arcseconds at the current time, which is below the advertised 30 arcseconds we initially expected it to be. A possible optics replacement is being considered which could boost the tip/tilt distance limit to 30 arcseconds.
  2. The final version of the field lens has been installed. Transmission and flat field effects have not yet been measured, but it should be considerably better than the 92% transmission of the old lens.
  3. Laser output has been within 9 - 11 Watts output from the laser bench. After transmission to the top end, output on the sky is about 8 Watts, which is sufficient to run at the fastest correction rate (1 kHz) during the winter months when Sodium abundance is high and half rate (500 Hz) in the summer months when Sodium abundance is low. Laser performance is now stable in terms of power output, wavelength lock, and beam quality. Beam quality is within specs at about 1.2 arcseconds FHWM.
  4. The high-order LGS loop can be closed on the laser in a stable and repeatable manner to correct for all aberrations besides tip and tilt, as per original specs.
  5. The tip/tilt (STRAP) loop can be closed on natural guide stars for tip/tilt correction at up to 1 kHz. The STRAP device has been photometrically calibrated in the R band. Guide stars as faint as R=18.5 (dark time) or R=18.0 (bright time) can be used for tip/tilt correction, although for full correction R=15 or brighter is recommended.
  6. The use of the slow focus sensor (SFO) loop, which corrects for the change in altitude of the Sodium layer (typically no more than 1 m/s) is now fully integrated into the system. Automatic dark frame lookup has been implemented for the SFO. Observation sequences can now be run with transparent interaction of the SFO loop.
  7. Focus offsets for both NIRI and NIFS have been measured for the SFO and have proven stable on the sky for the duration of an LGS run.
  8. Flexure between Altair LGS and NIFS/NIRI has been measured and compensation models have been implemented.
  9. The Field Steering Mirror (FSM) has been calibrated, which steers the tip/tilt sensor around the field and allows for offsets of the science target.
  10. Use of the Sequence Executor (seqexec) has been fully integrated into LGS operations, allowing full sequences with offsets, sky images, filter changes, etc. to be run during operations.
  11. Switching between Altair Natural Guide Star (NGS) and Laser Guide Star (LGS) modes is now robust, allowing full integration of LGS into the science queue.
  12. Some improvements have been made to the Laser Traffic Control System (LTCS), which coordinates Gemini observations with other observatories. This will allow simpler operation of LGS during the nighttime.
  13. Significant upgrades to the Telescope Control Console (TCC), which is used to control the telescope have been made which have streamlined LGS operations to an acceptable level.
  14. The LGS Zoom model has been updated, which allows short-term (i.e. less than 30 minutes) basic operations without the SFO loop (for example, when offsetting to sky).
  15. Basic steering of the Laser on the sky has been characterized sufficiently for science operations. More data on this model will be collected during acquisitions which will allow blind acquisition (i.e. no pointing correction) of the laser on the LGS wavefront sensor when finished.

Last update Feb 22, 2007; Chad Trujillo.

Gemini Observatory Participants