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:
- Laser contamination in data.
Spectra may be contaminated by sodium emission over a range of
about 2 Å centered at 5890 Å. The example images below
are both from HIRES. Figure 1 provides two sample
HIRES sky spectra, one with and one without the laser.
Preliminary experience indicates that laser-contamination, like
sky emission, can be removed from spectra via subtraction.
We belive that images taken in passbands that include the
sodium emission will suffer the effects of background levels
which are higher and non-uniform (as the collimated beam is
approximately 0.4m in diameter), but this
effect has not yet been quantified.
- Guiding problems. If the beam crosses the
field-of-view of the guider it may be difficult or impossible to
continue guiding. For some instruments, observers may be able to
change the filter in the guider to a passband which excludes the
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.
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:
- MAGIQ Tool
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
- 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