Mauna Kea has multiple operating LGS AO facilities. In addition to
the W. M. Keck Observatory (Keck2), Gemini has recently
begun operating their LGS AO facility. Subaru and Keck1 will begin
LGS operations in the future. The operating procedures for laser
equipped telescopes on Mauna Kea have been established by a working
group composed of representatives from several institutions. The
rule imposed on Mauna Kea LGS facilities is that a laser equipped
telescope must yield (shutter) for a non laser equipped telescope
when the non-lasing telescope indicates a sensitivity to laser
emissions and a crossing geometry occurs. Each participating telescope
is responsible for providing its position, FOV, and a laser sensitivity
indicator. A safety system installed at the lasing facility performs
necessary calculations using the provided information to predict
collisions and shutter as needed to prevent contamination of science
data with unwanted emission. The safety system responsible
for these calculations is the Laser Traffic Control System (LTCS).
The LTCS has been implemented in collaboration with several Mauna Kea
institutions, and operates at a lasing facility whenever LGS observing
is occurring on Mauna Kea. The LTCS calculates crossings of the
telescope FOV for both Rayleigh scatter and the LGS itself.
Predictions are continuously updated, and system status is visible
to approved facilities via a web-page hosted by the lasing facility.
An example of the LTCS status web page is shown here:
LTCS Status page
Keck1 and Keck2 both participate in LTCS calculations. By agreement
of all Mauna Kea directors, pointing data will not to be used for
any purpose other than to satisfy the necessary LTCS safety function.
Other participating institutions include CFHT, Gemini, IRTF, Subaru,
and UH. A version of LTCS also operates at La Palma Spain, and is
being coordinated for use at some Chilean sites.
Laser Emission Background.
In the upper part of the Earth's mesosphere (90±10 km altitude),
there is a 5-10 km thick layer rich in Na atoms, deposited by the
ablation of micrometeorites.
These atoms can be excited and caused to radiate by sponateous emission
by projecting a laser tuned to the Na D atomic
transition (589 nm) in the direction of the science target. For example,
the magnitude of the Keck artificial guide star roughly corresponds to
9.5<V<11.0 and varies with laser power, beam collimation, and
Na column density. More information on the Keck LGS facility can be
obtained at
Keck2 LGS AO
The laser guide star is not point-like. This is because it is
being produced in an atmospheric Na layer 5-10 km thick.
The apparent size of the LGS will vary as a function of the distance
from the lasing telescope.
1. Keck LTCS System Data
Keck has implemented software to automate processing of telescope
control system state data. It uses this data to determine transitions
of laser sensitivity. Included is state information for the dome shutter
position, dome tracking condition, telescope tracking condition, and
guiding state. When the dome is open, telescope and dome are
tracking, and the telescope is guiding, the program generates a laser
sensitive indicator. When any of these conditions are false, the
program issues a laser insensitive indicator. In this way, Keck
telescopes will not cause unnecessary shuttering of another facility
due to telescope moves (slewing), and only indicates sensitivity during
science operations. Also generated by the program is the instrument FOV
value (based upon instrument lookup) and pointing.
2. Operating Considerations for Keck observers on LGS nights
On a night of LGS operations on Mauna Kea, Keck support astronomers are
made aware of lasing activity via email. They will inform the visiting
astronomers of this condition. Observers need not take any action other
than to be aware. During nighttime operations, it is possible that a
lasing facility may encounter a "collision". The site could potentially call
our facility to ask for information. Typically, this might
occur when the collision is of long duration. Of interest to the lasing
facility would be whether the observer's program is truely laser sensitive,
and how long the observer intends to remain on target. Based upon the
response, the LGS operators may either move to another target, wait for
the involved telescope to complete its observation, or ask for a LTCS
facility override.
3. LTCS facility Overrides
In the special case where a collision occurs, the lasing facility calls
to confirm laser sensitivity, and an observer indicates a willingness to
allow the lasing facility to "override" the LTCS calculations (i.e.
allowing the laser to continue observations using laser emission), sodium
emission in the astronomer's field can be expected. To date, no
guiding functions have been observed to be adversely affected due to
sodium emission, but spectra have clearly shown the presence of sodium
(at least in the case of Keck2, Keck1, and Subaru. Observers have the
right (at any time) to allow or dis-allow permission for LTCS overrides.
Observers should indicate at the time of request how long a permitted
override will be allowed. The time can be any duration (to include all
night). The LGS operators at the laser site will implement and remove
the override as directed. No action is required on the part of an observer
other than awareness and to provide/deny permission.
