Laser Guide Star Light Impact for Maunakea Observatories

Light from a laser guide star (LGS) may contaminate some astronomical data. Contamination may occur when the beams of a lasing telescope and another telescope intersect. This is defined as a laser collision.

The lasers used on Maunakea are sodium lasers that emit near 5890 Å (589 nm).

Contents


Impacted Data

Optical imaging in V- or R-band
Backgrounds (R. Wainscoat priv. comm.)
  • May be higher by 1.5 mag/arcsec^2 in V-band
  • May be higher by 2.2 mag/arcsec^2 in R-band
Out of focus artificial star (Hayano et al. 2003)
Depending on telescope separation
  • 14.4 mag/arcsec^2 at 80 m
  • 18.8 mag/arcsec^2 at 1000 m
Optical spectroscopy at 5890 ± 2 Å
See example spectra
Guiding in V- or R-band
Guide stars fainter than V=12 mag may get lost in the elevated background

Non-impacted Data

Infrared imaging and spectroscopy
Infrared guiding
Optical imaging outside V- or R-band
Optical spectroscopy shortward or longward of 5890 ± 2 Å
Guiding outside V- or R-band

Predicting, Detecting, and Avoiding Collisions

Laser Traffic Control System (LTCS) software has been deployed among the Maunakea observatories to predict, detect, and avoid laser collisions. This software calculates a cone above each telescope that represents its instrument field of view. There is a safety margin of 3 in diameter for each cone. For example, for a 6-arcminute science field of view, LTCS calculates an 18-arcminute cone. If a lasing telescope cone is predicted to intersect (or collide) with another telescope cone, LTCS issues a collision warning and lists which telescope has priority. If the lasing telescope does not have priority, its laser will shutter automatically when the telescope cones intersect.

First on Target Priority

Telescope priority is determined by which telescope was on its target first. If the lasing telescope was on its target first, it is allowed to continue lasing. The other telescope will see a warning that there may be laser light in its data. If the non-lasing telescope was on its target first, the lasing telescope will shutter its laser.

Overrides

An impacted telescope may grant an override to a lasing telescope at their discretion.

Examples of Contamination

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.