The AO Optical Laboratories

At Arecibo we have a variety of optical instruments that we use to study terrestrial airglow emissions. Presently, this set of equipment includes two Tilting-Filter Photometers, an Ebert-Fastie Spectrometer with a one meter focal length, and two pressure-scanned Fabry-Perot Interferometers each having six-inch diameter etalon plates. Four, roof-mounted, dual-axis mirror systems are also available, which can be programmed to direct light from different regions of the night sky into the instruments. This equipment is located in our Optical Laboratory, which is on site about 1000 feet from the center of the incoherent scatter radar. Airglow observations can be thought of as "passive" optical measurements in that we rely on the natural emissions from the atmosphere as the source of our signal.

As a complement to the airglow instrumentation we also maintain a set of optical instruments, called lidars, used for "active" remote sensing of the atmosphere. That is, we generate our own source signal using lasers and measure the characteristics of the light scattered back from the atmosphere. Housed in a separate building but near to the airglow instrumentation, in the Lidar Laboratory we operate a Nd:YAG-based Doppler Rayleigh lidar, and two tunable resonance fluorescence lidars, one utilizing a Dye laser, and the other based on an Alexandrite laser transmitter. Four 80 cm diameter telescopes are used as lidar receivers. Three of these are mounted on the roof of the lab and are fully pointable with azimuth and zenith axes under stepper motor control.

Typical Operation:

Photometers are easy to set up and operate. We use these instruments combined with a set of optical filters to measure the intensities of airglow emissions of the visible and near-infrared part of the optical spectrum. Specific filters are available at Arecibo to measure the airglow lines of various gases in the atmosphere, such as OH, O₂, O, O⁺, N, N₂⁺, H, He, and Na. Typical filter bandwidths range between 0.3 and 1.0 nm, which we consider as "coarse" spectral resolution. Wavelength positioning is accomplished by tilting the filters via stepping motors, and this can done in a continuous scan mode, or they can be programmed in a limited-wavelength step mode to improve the ratio of signal to noise. A combination of various filters can be used in the photometers to simultaneously study the intensity distribution of several emission features.

We use our Spectrometer to measure spectral blends of airglow emission bands at medium to high spectral resolution. We can program this instrument to sample either narrow or broad wavelength regions, or a combination of the two. The maximum (second order) scan range is about 100 nm. Practical limits of spectral resolution range between 0.02 and 1.0 nm.

The Interferometers are generally used to measure the Doppler temperature and winds that originate in the E and F-regions of the ionosphere, or to measure the spectral distribution and temporal variation of the hydrogen geocorona. Such studies are typically done at wavelengths of 557.7, 630.0, and 656.3 nm, respectively, but other choices of observable emissions are possible using the existing set of pre-filters to confine the wavelength range of the interferometers. The spectral resolution each of our Fabry-Perot Interferometers is typically of the order of 0.001 nm.

With the Rayleigh Lidar we measure the Doppler shifts and widths of the spectrum of the laser light that is broadened and backscattered from the atmosphere from about 15 to 70 km altitude. This lidar is based upon an injection-seeded, Nd:YAG laser, having light output at its fundamental and second harmonic wavelengths. An 80 cm Cassegrain telescope in the receiver is optically coupled to detectors in the lab using fiber optics. This lidar is fully pointable and is primarily used to measure the neutral winds and the temperatures of the middle atmosphere.

Our Resonance Fluorescence Lidars can measure various metallic species of the upper mesosphere and lower thermosphere between about 70 and 115 km altitude. We do this by tuning the laser transmission to the resonance line, or fingerprint, of the metallic species we wish to observe. As mentioned, two sub-systems are available -- a simple broadband dye-laser, which is pumped by the Nd:YAG laser described above, and a narrowband alexandrite ring laser. Both lasers are tunable and both also have pointable 80 cm diameter telescopes as collectors in their receivers. We generally observe the concentrations of metals or other similar species with the dye system, but these can often be measured with good temporal resolution. We have explored the dual-wavelength capability of the dye system for differential absorption studies of species such as H₂O and O₃, but such studies are not yet routine. In addition, our alexandrite-based lidar has a Doppler capability, which we use to measure temperatures and winds in the mesopause region of the upper atmosphere.

Each of the optical instruments function independently and can be programmed in a variety of observational modes. For example, an observer might choose to have an interferometer cycle through a sequence of 10 different mirror positions on the sky. While the interferometer is performing its own observation cycle, the spectrometer might be used to track the anti-sunward position of the sky, from dusk to dawn, as a function of time. The various observational modes that are available are described in supplementary documentation for the Arecibo Observatory Optical Laboratories.

Data acquisition and instrumental control for the airglow equipment (that is, the non-lidar instruments) is accomplished by micro-computer. Currently, we use a old, but reliable DEC Micro PDP-11/83 with a Camac interface to the instruments. This computer is being phased-out in favor of a PC (but, we will retain the Camac interface functions). Lidar operations utilize PCs and specialized interfaces. Collected data are generally recorded on local hard disk and can be displayed and partially reduced in real time. A series of real-time display tools are also accessible on our Web Pages. Data can also be copied to any of the Observatory's computers for off-line analysis. Software is available for acquisition, analysis and display of all airglow and lidar data that is acquired in routine fashion. However, any specialized observations or methods of analysis are the responsibility of the guest investigator, although staff members will provide assistance whenever possible.

Technical Data for the Arecibo Airglow and Optical Instrumentation

More specific information about the airglow and lidar instrumentation can be found at the above link. An operations manual for the Optical Laboratories is currently under major revision. It is recommended reading for those who anticipate conducting optical experiments at Arecibo and will be available later this year. In the meantime, simply contact us to discuss your observing requirements.


Craig Tepley -- ctepley@naic.edu
Revised -- 6 September 2000