Arecibo Optical Facility (AOF)

The optical observations at Arecibo Observatory (AO) initiated shortly after the Gordon Telescope was in full operation. In 1967 Dr. Vincent Wickwar observed the 630.00-nm airglow using a tilting filter photometer. (Wickwar, 1971).

The latter years of the 1970 decade brought new data taking capability to the AOL, and with it greatly improved duty cycles for the airglow instruments. The custom data logger was replaced after nearly a decade of service by a HP 9825A computer in 1978. The associated High-Level Programming (HPL) capability permitted simultaneous operation and data acquisition from four tilting filter photometers, the 1m Ebert-Fastie spectrometer, and a Fabry-Perot interferometer (FPI).

The Ebert-Fastie spectrometer wavelength scan and photomultiplier tube detector were automated with HP 9825 control and a variable speed stepping motor [Meriwether, 1979]. A series of subsequent publications used the spectrometer and the tilting filter photometers to explore the nature of the OI 7774Å , OII 7320Å, O2 atmospheric 8645Å, N2 1st negative 4278Å, and OH Meinel airglow emissions. Observations of the feeble HI Balmer-beta emission at 4861Å and the N2 4278Å emissions following magnetic storm onset demonstrated that energetic neutral H following ring current charge exchange was the collisional excitation source of the low latitude aurora [Tinsley and Burnside, 1980]. By the end of the 1970 decade, the observational reach of the Optical Laboratory was expanded from the mesosphere to the exosphere above Arecibo, and across the visible spectrum. [e.g. Meriwether et al., 1977, 1978; Burnside et al., 1980; Tepley et al., 1981].
The FPI was established in the late 1970s and is of University of Pittsburgh FPI heritage. The 6.25-inch aperture remains the largest aeronomical FPI in service. The original automated mirror system designed by L.M. LaLonde allowed sky mapping Doppler resolution measurements of the OI redline and greenline emissions that continue today [Burnside et al., 1981; Burnside and Tepley, 1989]. By quantifying the O+ continuity equation with ISR data and comparing the derived neutral transport terms to the FPI neutral wind measurements, Burnside et al. [1987] advocated a factor of 1.7 correction to the classical Banks [1966] O-O+ collision cross section. In late 1979 through 1981, interferometric measurements of the HI Balmer-alpha emission revealed a narrowing of the emission line due to the loss of energetic hydrogen atoms from the velocity distribution above 500 km. This initial detection of “gravitational cooling” with increasing altitude above the exobase was described by Meriwether et al. [1981].

In the early 1980s, Dr. Craig Tepley brought a vigorous new age to Arecibo optics. The original optical laboratory was razed, and replaced by a metal building in the summer and fall of 1984. The venerable HP 9825A model data acquisition computer and system was replaced by a DEC PDP-11 computer with associated instrument controlling modules placed in a Computer Aided Measurement and Control rack (CAMAC crate). Each instrument was placed beneath a scanning mirror system which operated beneath large Plexiglas domes. For the first time, observations could be made without direct exposure to the nighttime weather, and a background transition from primarily campaign-based use to regular new moon observations transpired. It is generally held that at this time, the colloquial “Photometer Shack” became the “Arecibo Optical Laboratory”. Remodeling added a room to the structure in the early 1990s.

Arecibo airglow measurements at AOL are calibrated for absolute brightness using a 14C excited phosphor screen that was designed to fit precisely within the photometer telescope tube. The acquisition and calibration of that device was led by Dr. Herb Carlson, who cataloged the first brightness calibration of that constant brightness source (traceable to NIST standards) from 3800Å to 8000Å in November, 1968, in collaboration with Dr. M. Gadsden [Blacker and Gadsden, 1966]. That low brightness source was calibrated against dozens of other calibrations sources at Intercalibration Workshops sponsored by Dr. Marcia Torr of Utah State University. The first of these workshops that established international airglow brightness measurement consistency was convened in Seattle WA in 1979, and a second was convened at Aberdeen University in August, 1981. Results from these workshops, including calibration of the Arecibo 14C low brightness source (LBS) appear in Torr et al., 1981, and Torr, 1983. It is important to note that these meetings continue on an annual basis. Current meetings and calibration archives are maintained by Urban Brändström at the Swedish Institute of Space Physics, Kiruna, Sweden [Brändström et al., 2012]. All archival data, including that for the Arecibo LBS, may be retrieved from the website http://alis.irf.se/ewoc. The Arecibo 14C LBS remains a powerful resource in the AOL toolbox.

The Arecibo LBS allows brightness comparisons of airglow data across the entire 50-year history of AOL airglow measurements, and of for data subsets within. After using observation campaign data to make the first determinations of H density from the ground [Kerr and Tepley, 1987; He et al., 1993 ], Arecibo 6563Å brightness data were compared across two decades, 1981-2001, to evaluate model predictions of secular increase in H density at the exobase in response to increasing methane deposition in the troposphere.

In late 1985 Arecibo purchased a commercial 16” reflecting telescope and drive to allow optical measurements of the 1986 Comet Halley apparition. The telescope was fiber-optic coupled to the Fabry Perot Interferometer (FPI) to sample cometary OI and H emissions, and photochemistry dependent water production rates from the comet were published [Kerr et al., 1988].

The first community-access all-sky imager was placed at the site for multi-instrument campaigns in 1990 and 1995 by Dr. Michael Mendillo and Dr. Jeff Baumgardner. The Boston University all sky imager (ASI) became a permanent guest instrument at Arecibo in 2002, and still operates today [Mendillo et al., 1997]. In May of 2003 a second ASI from Penn State University was positioned at Arecibo [Seker et al., 2007], joining a narrow field PSU imager in use in some configuration since the AIDA-’89 campaign. The PSU ASI offers 6300Å, 5577Å and 7774Å filters. ASIs at Arecibo have shown that the signatures of wave activity below the F2 region thermosphere are common, with gravity wave, Mesoscale Ionospheric Disturbances (MSTIDs) or some other wave event occurring on the majority of nights. The climatology of MSTID apparitions above Arecibo sampled by the BU ASI is described by Martinis et al., [2010].

During the 2000 – 2010 decade the PDP-11 computer and CAMAC crate began to fail, and the optical instruments at the AOL began a transition to individual, PC control. In 2012 a two-year conversion of the pressure scanned, single channel PMT detection FPIs to CCD array detection was completed. In November 2015, the optical measurements at AOL expanded to the island of Culebra and the Remote Optical Facility (ROF) was established. The ROF promises expanded sampling of upper atmospheric dynamics as a second potentially clear-sky site near AO, in addition to the purpose of novel common volume experiments between the AOL and the ROF. A redline all-sky imager has been operating intermittently at that site since then. A 6300Å FPI has also recently been established at that site for the purpose of common volume experiments. The bi-static data taking and analysis activities between AOL and ROF hold great promise for this new optical expansion for the Arecibo Observatory.

The AOL attracted broad scientific and instrumental contributions by researchers external to the Observatory. Together with the new ROF, remain a tremendous data resource for the Space Physics community – and a rich career opportunity for young researchers.