V2 NRL Spectrograph

After World War II, captured V2 rockets provided a means for sending scientific instruments above the bulk of the earth's atmosphere, which
absorbed ultraviolet (UV) radiation. To study the nature of that absorption, and to examine the ultraviolet portion of the solar spectrum, a group at
the Naval Research Laboratory (NRL) in Washington D.C. led by physicist Richard Tousey designed a rugged solar spectrograph to fly in the V2
warhead. About one dozen spectrographs similar in design to the one in the NASM collection were manufactured by Baird Associates of
Cambridge MA under contract to NRL. The first successful flight came on 10 October 1946, which returned a series of solar spectra that revealed
the nature of ozone absorption in the Earth's atmosphere. The sample (likely unflown) in the NASM collection was found in Tousey's office at NRL
and was lent to the Smithsonian in August 1982 and transferred permanently in 1983, after it was placed on exhibit in the Stars Gallery where it
remained until October 1997. A similar spectrograph, with protective shroud, is in the collection of the National Museum of American History.

Dimensions

H: 60 cm (2 ft. aprox)
D: 30 cm (1 ft. aprox)

Description

Roughly conical structure constructed of thick welded steel parts. Two parallel circular metal platforms are mounted perpendicularly on the lower
extremity of a rectangular box housing the optics. These support the electromechanical gear for the spectrograph and the camera. Small
electrical motor with gearing mechanisms and electrical wiring are visible, as are the heavily armored film cassette and the two tiny lithium
fluoride entrance apertures. Sunlight from a wide angle entered via one of the two beads, reflected off folding mirrors to a single concave
Rowland grating, which dispersed the light and focussed it onto a film strip at the base of the instrument. The clock and gear mechanism
advanced the film so that multiple exposures could be taken as the missile moved through and beyond the atmosphere.

History

It was well known by the 1940s on theoretical grounds that a major portion of the energy radiated by the sun lies in the ultraviolet (UV) range. A
study of the spectra of this radiation would supply a wealth of information about the elemental composition of the sun as well as the nuclear
transitions that those elements undergo. The absorption of UV radiation by elements in the earth's atmosphere, in particular ozone, was known
independently from laboratory experiments. This effectively precluded observation of UV spectra of celestial objects even from high-level balloon
flights.

The availability of a supply of captured operational German V2 rockets after World War II provided a means for sending scientific instruments
above the atmosphere. Successive spectra taken at different altitudes as the rocket rose would also provide information on the earth's
atmosphere, revealing changes in UV absorption as a function of altitude.

In the spring of 1946, a group at the Naval Research Laboratory led by Richard Tousey designed a simple but heavy armored Rowland
spectrograph design that was then constructed (about a dozen were built) by Baird Associates of Cambridge, MA. The spectrograph contains a
Rowland grating, two entrance apertures made of lithium fluoride, and two sets of folding mirrors. Either aperture captured sunlight, which was
then reflected by one of the mirrors onto the curved Rowland grating, which then dispersed and focused a tiny spectrum onto an exposed portion
of photographic film. In the absence of a pointing mechanism in the early versions, the entrance apertures had to have wide-angle optical
characteristics. Tiny 3mm beads proved to be quite effective, though they could not produce high quality line spectra. John Strong at the Johns
Hopkins University manufactured the Rowland diffraction gratings for the spectrographs.

The first spectrograph was placed in the warhead of the missile for a flight in June 1946 and confirmed that recovery was going to be a major
problem. The spectrographs were then placed in the tail fins, and explosive bolts were added break the vehicle into two pieces on descent,
destroying is aerodynamic form.

The first successful flight of the NRL UV spectrograph was on October 10, 1946. The missile reached an altitude of 173 km and the series of
spectra obtained during ascent showed the decrease in UV absorption with altitude and helped set the upper limit to the Earth's ozone layer. The
instrument provided the first UV spectrum of the sun between 210 and 300 nm, the nominal atmospheric cutoff. The relatively poor resolution of
the spectrum with the consequent line blending made detailed interpretation difficult. The V2 program laid the groundwork for the extensive work
carried out by Tousey and NRL on solar spectroscopy using Aerobee rockets.

Reference

DeVorkin, D. H., Science With a Vengeance, Springer, NY, NY, 1992, 135-146, 197-220.

DL/DD January 1998