Photograph by John McDonald
Looking just west of south from the CFHT dome, the view is dominated by the domes of (left to right) Gemini, University of Hawaii and UKIRT observatories. Aside from occasional and momentary light from the domes themselves, this direction is otherwise devoid of light pollution and contamination.
Just above and to the left of the UKIRT dome, one may occasionally see the greenish beam of the Lidar experiment at the Mauna Loa Observatory, approximately 34 km (21 miles) away. On hazy nights with low thin clouds, if the experiment is operating, the beam is on the edge of visible with the naked eye (this section of the image was underexposed and has been greatly enhanced, thus overexposing faint light within the UKIRT dome, but bringing out the Lidar beam). Unless, under such conditions, a telescope is pointed in that particular direction (accounting for parallax from different telescopes), this light poses no threat to astronomy on Mauna Kea.
In the far lower right corner, the long bright streak of amber light is a flare launched over the Pohakuloa Training Area, which is located in the saddle between Mauna Loa and Mauna Kea. During night training exercises, flares are occasionally launched which cast fairly bright light upward upon the domes at the summit. The light is not as bright as the full moon, but is brighter than any other artificial light source seen from the mountain. The flares each last typically several minutes. There may be several in succession, depending on the exercises at PTA. While we do not have a spectrum of this light, the color suggests these are sodium flares.
Also visible, of course, is the trail of automobile tail lights traveling up the road toward the UKIRT dome. Drivers familiar with the summit typically drive by parking lights alone. Occasional drivers will use bright headlights. All automotive lights do cast light upon the domes. During the night, this is usually only momentary and occasional. On an average night, only a handful of vehicles come and go at the summit.
