Submillimeter Heterodyne Astronomy from Mauna Kea

Abstract

This paper reports on the results of a collaboration between NASA Goddard Space Flight Center and MIT Lincoln Laboratory in which a laser heterodyne receiver was developed and used at the NASA infrared telescope on Mauna Kea. The NASA IRTF is located on the island of Hawaii at an elevation of 14000 ft (4200 m). The atmosphere from the summit is just barely adequate for submm observations and only days with extremely low water vapor are usable (<1 precipitable mm H2O). This is due to very strong lines of water vapor in the band. Fig. 1 illustrates the atmospheric transmission spectrum at 0.5 mm of H2O. At our operating frequency of 691 GHz the atmospheric transmission is about 50%. Observations made during February 1981 showed an atmospheric transmission in this range on three out of the four days on the telescope. However, an earlier observing period in May 1980 produced measurable transmission on two days and the maximum transmission was 20%. The transmission spectrum shown here is an example of what can be expected on a very good day in winter from Mauna Kea.