First-on-Target Laser Policy

March 10, 2008



Effective October 31, 2007, WMKO has implemented a new LGS-AO policy, called first on target. Until now, Keck II has had to defer to Keck I when using the laser guide-star system: if the Keck I observers wanted to point across the beam of the Keck II laser, the Keck II side would have to shutter their laser, and usually move to some other target, or wait out the "collision."

The new policy, soon to extend to all telescopes on Mauna Kea, gives equal priority to all telescopes whether they are lasing or not. Whichever telescope is in position first gets priority. So in the above example, if Keck II is observing a field and Keck I wants to point across the Keck II beam, Keck II will not be required to stop lasing. The Keck I observer then has the choice of observing across the beam and possibly suffering the effects of laser light in the resulting data, or observing a different target which is not be affected by the laser.

This new protocol will require Keck I observers and operators to be aware of Keck II's position on Keck II LGS-AO nights. As described below, Keck's LGS-AO team has created tools to allow observers and OAs to determine when collisions may occur. Eventually the MAGIQ software will perform the same function, through both the OA GUI and the observer's eavesdrop. The GUI will allow the observer or the OA to check the next target to see whether there will be a laser collision if Keck I points there. If your science would be affected by laser light contamination, then you should ask your OA to check the laser impact status of your next target before slewing, as described below. If a collision is indicated, you might want to choose another target.

Eventually, all of the Mauna Kea telescopes will operate under this new policy, but they will be brought on line one at a time.

Laser Impact on Keck I

Observers are naturally wondering about the possible impact of the laser on their observing programs. The Keck II laser is a sodium laser and thus the emission is entirely in the visible regime near 5890 Å. Two effects are possible if the Keck II laser should cross the Keck I beam:
Figure 1: A comparison of two HIRES spectra illustrating the effects of K2 laser contamination. Both are sky spectra. The red line is from a spectrum taken at a time when the laser was crossing the HIRES field of view, while the green line was taken (with the same spectrograph settings) a short time later when the laser was no longer nearby. Y-axis units are data numbers. The intensity at the 5890 Å peak is a factor of 10 greater in the laser-impacted spectrum.
Figure 2: Difference of spectra with and without the laser, showing the excess light attributable to the laser.
Figure 3: Same as figure 2, but with modified vertical scale to show low-level laser contamination.

Detecting and Predicting Collisions

The Keck I observing assistant is primarily responsible for monitoring the status of the Keck II laser's impact on Keck I observing and will notify the observer when a laser collision is occuring or predicted. Software will generate an audio alarm on the OA's computer, and that information will be relayed to the observer.

Additional software tools are available to help observers determine when laser light will affect their observations:

With the new MAGIQ observer GUI you can indicate a selected target to the OA. To do this, right-click on the target and then select Highlight As Next Target on the pull down that appears. The OA can then determine the laser impact status for your target.
LTCS Site Status Tool for Keck I
This page indicates whether the current target being observed by Keck I is currently affected by the laser on Keck II. The information on this page will only be valid on nights when Keck II is lasing. On nights when Gemini is lasing, a different URL will need to be referenced. However, given that we always win collisions with Gemini (for now), this won't be needed immediately. The data on this page are only valid when the 3 heartbeat indicators at bottom are all green.
LTCS Query Tool for Keck I
This page allows the Keck I observer or OA to determine whether any target, not only the one currently being observed, is currently or will in the future be affected by the Keck II laser. The user must manually enter the target coordinates and the equinox and must change the FOV to the appropriate value (0.17 deg for LRIS), then click Submit. The LTCS (laser traffic control system) will check for current or future conflicts with the lasers on Keck I, Gemini, and Subaru (if they are using their lasers) and after a short delay will report any upcoming events, or display the message NO COLLISIONS PREDICTED.

Allowing Lasing Telescopes to Override Keck I

It is possible that the observers on Keck II may want to obtain data on a target which requires them to cross the Keck I beam. If you were on your target before they start observing theirs, they will be prevented from using the laser unless you choose to grant them an override. The override allows the Keck II laser to cross your beam even if you were first on target. An important side effect of granting an override to another telescope is that the LTCS software will no longer report when a laser crosses your beam, so you will have no way of knowing whether you can expect to see laser light in a particular exposure.

Last modified: Fri May 9 16:15:52 HST 2008