STRATOSCOPE II - SCIENTIFIC FLIGHT # 6

The Stratoscope II system was one of the most sophisticated balloon-borne telescope systems ever flown. It's goal was to photograph planets and stellar nebulae from Earth's stratosphere with previously unseen precision in an effort that can be viewed in perspective as a distant precursor to the Hubble Space Telescope. The project was under the direction of Princeton University, and was sponsored by the National Science Foundation (NSF), the Office of Naval Research (ONR) and counted with additional support from the National Aeronautics and Space Administration (NASA). The main contract for the construction of the telescope was awarded to the Perkin-Elmer Corporation from Norwalk, Connecticut while Vitro Laboratories from Silver Spring, Maryland served as program manager and supervised the development of the balloon system, the launching and recovery techniques, and planned and conducted the flights.

THE TELESCOPE

At left we can see a basic scheme of the telescope as deployed in-flight (click to enlarge) and the configuration of the double-balloon tandem system (more details below). The assembly consisted of an 18-foot main tube and an 11-foot side arm which together formed a fixed L-shaped structure. The basic telescope was housed in the main tube with the objective mirror, the secondary mirror, which was also the focusing element, and a television camera with a 10º field, used to locate the general section of the sky to be studied. The side arm assembly housed the guidance package, camera equipment, and another television camera, this one with a 1º field, which looked through the reflecting telescope and fixed the specific celestial object to be photographed. At it's core was a 36-inch diameter optical mirror fabricated by the Corning Glass Works from New York. Fused silica was choosen as material for the mirror because it was unaffected by the sharp changes in temperature the telescope experienced in flight. That was the first time a fused-silica blank of that size was successfully formed and with high accuracy to a true parabolic shape.

The L-section, weighing 5.500 pounds, was supported by the "knuckle" which contained the drives and the axes that allowed the telescope to point higher or lower in the sky (elevation) and to rotate along the axis of its tube (roll). The L-section and knuckle were supported by the azimuth bearing, which allowed freedom to turn around the vertical axis (azimuth). The telescope was moved to any required elevation angle by a motor-gear drive, but during tracking operated within the freedom of +50 flexure bearings in both roll and elevation. The azimuth bearing was a mercury-floated type wherein the telescope was floated by the buoyant force of a pool of mercury. The telescope tube was locked vertically against the azimuth frame during launch. After deployment, it had a viewing range extending from 70º down to 20º above the horizontal over an azimuth of 360º. A 14-foot diameter inertia ring was fastened to the top half of the azimuth frame. That ring supported the batteries which powered the instrument, ballast containers, provided the counter-inertia in azimuth and finally served to absorb part of the shock of landing at the end of the flight.

A fully instrumented mobile ground control van was located strategically in advance of each mission to remain within the coverage range across the balloon's flight path. From that van astronomers operated the telescope almost in real time through a specially built command and telemetry system with 40 and 64 channels respectively which permited many functions of the telescope and related equipment to be controlled remotely. Two complete radio links were employed to carry out command activities, the channels being divided equally between them. If one link failed, commands normally carried by it could be switched to the other link. The ''eyes" of the ground team were a remote-control television system, and a high-resolution image orthicon tube developed by the RCA Laboratories, Princeton, New Jersey. To acquire the target image, the coarse and fine television screens were used to find a pre-selected guide star. The fine guidance system consisted of lenses within the telescope which moved the image for precision tracking. The image was split between the recording camera and the four tracking eyes. The eyes provided lock-on to two guide stars to provide stabilization in two axes. All exposures obtained during the observational run were recorded on 70-mm film and stored in the camera magazine which was recovered after the telescope returned to the ground.

THE TANDEM BALLOON SYSTEM

In planning Stratoscope II, one of the prime considerations was to construct a balloon strong enough to lift such a heavy weight to the required altitude of 80,000 feet. Polyethylene film, commonly used for balloons, was then limited to a load-carrying capacity of about 4,000 pounds, and thus was unsatisfactory for the project. To meet this problem, the G.T. Schjeldahl Company of Northfield, Minnesota under contract to ONR, developed a reinforced laminated plastic film having high tensile strength and tear resistance. Schjeldahl also developed, a new balloon system capable of lifting the heavy telescope payload off the ground quickly while remaining unbuffeted by ground winds as high as 15 knots. The system employed two balloons -a small one and a large one, with the former mounted on top of the latter. The small balloon was composed of a 0.5 mil Mylar and dacron laminate called S-12 with a volume of 250,000 cubic feet. The main balloon consisted of a 0.3 mil Mylar and dacron material known as S-11 with a volume of 5,250,000 cubic feet. The two balloons were connected by a 2-1/2 foot metal tube, 3 feet in diameter, attached to a 3-foot hole in each balloon.

LAUNCH AND FLIGHT PROFILE

The launch balloon was designed to hold enough helium to lift the payload to altitude. In preparation for the flight the launch balloon was inflated while connected to the main balloon which remained in its shipping box. Once the smaller balloon was full, it was attached to a cable and allowed to ascend slowly, lifting the main balloon (which was snugly wrapped in a plastic sheath), until the entire system stood erect above the telescope. Wrapping the uninflated main balloon prevented its ''sailing'' and sweeping the payload out horizontally. Immediately before launch the entire system towered over 600-feet tall. At launch an explosive charge released the restraining cable and the balloon took-off gently, without sway or rotation. Once the balloon left the ground, the protective sheath was ripped from the main balloon. As the balloon ascended the change in atmospheric pressure with altitude induced a flow of gas from the launch balloon into the main balloon. At 30.000 feet both were fully inflated.

Althought Stratoscope II carried two cargo parachutes for emergency landing if necessary, the common procedure once the mission was completed was to lower the telescope gently to earth with the help of the balloon itself. At dawn, an aircraft approached the balloon and sent a command to valve-off helium establishing a rate of descent of about 600 feet per minute. The balloon was carefully ''stepped down" to around 5000 feet, and held at this altitude until rendezvous was established with an helicopter to which command was transferred; it then eased the balloon down close to the ground (avoiding cities, electric lines, railways. etc.), until it drifted over an open area, whereupon the telescope was landed. Upon cantact with the ground an explosive bolt was fired, releasing the balloon to prevent it dragging the instrument. Freed of its load, the balloon ascended, to be immediately deflated by the ignition of a large squib at its apex, thus preventing it from continuing an uncontrolled free flight.

Balloon launched on: 5/18/1968 at 19:55 local
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: Vitro Laboratories
Balloon manufacturer/size/composition: Zero Pressure Balloon  
End of flight (L for landing time, W for last contact, otherwise termination time): 5/19/1968
Landing site: Near Cushing, Texas, US

The balloon of the sixth scientific flight of Stratoscope II was launched from NASA balloon base in Palestine Texas at 19:55 local time on May 18, 1968. The flight ended the next day and the telescope was succesfuly recovered near Cushing, Texas.

The telescope operated reliably during the flight but a thermal gradient across the telescope tube, due to a radiation imbalance ofearth versus space, degraded the performance of the diffraction-limited optical system. During the flight, telescope balance was readily controlled, command and telemetry were strong, and the telescope was easily moved about and pointed at various objects. The telescope pointed as low as 30º above the horizontal (lower limit is 20°) and acquisition/tracking was accomplished on nine different stars, varying between 5 and 7.5 magnitudes.

Useful photographs of the nucleus of the Seyfert galaxy NGC4151 were obtained.

• An Upper Limit to the Angular Diameter of the Nucleus of NGC 4151 Astrophysical Journal, vol. 154, p.L117