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J N CAR Conference Surveys Balloon Instrument Stabilization

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J N CAR Conference Surveys Balloon Instrument Stabilization NCAR Conference Surveys Balloon Instrument Stabilization If a telescope or other astronomical sensor can be lifted by a balloon above much of the earth’s atmos­ phere, more detailed planetary and stellar observations u»_o of various kinds are possible than can ever be achieved HO cd from the ground. In every balloon-borne experiment of this kind, however, the problems are the same: How can an astronomical sensor find and track a pinpoint target? How can the spatial orientation of the balloon platform be controlled sufficiently to allow the delicate E "e3 pointing apparatus to do its work? A considerable number of approaches have been or are being tried to solve these questions. A recent NCAR-sponsored conference brought together scien­ tists actively engaged in balloon-borne astronomical exploration, in order to sketch an over-all picture of the state of the art, and to accelerate progress by increasing communication among them. CONFERENCE IMPRESSIONS At the conference, held in Cambridge, Massachusetts r------i I " I in late October, some general impressions of the present state of the art emerged from descriptions of various stabilization systems: —Tracking stability to within one minute of arc is well within present capabilities. —The achievement of better-than-one-second-of-arc stability represents engineering of a high order of skill. J Nevertheless, theoretical limits of stability are much higher; even the .02 second of resolution hoped for the Stratoscope II system does not approach them. —A balloon vehicle provides a platform with excellent 0 / e ) P 5 ! stability. At float altitude, pendulum motion is usually five minutes of arc, and seldom exceeds thirty minutes. —In balloon flights involving precise tracking systems, most of which are relatively heavy, the reliability of the balloon vehicle is still the most serious problem. NATIONAL CENTER FOR Fundamental requirements can be stated that apply ATMOSPHERIC RESEARCH to all the projects discussed; for example, all must turn against some opposing force to compensate for the BOULDER, COLORADO balloon’s random rotation, and the sensors must find and hold to their targets as steadily as is appropriate to No. 8, March, 1963 the scientific task. But differences in requirements and Issued each March, May, July, September, and November. Second-class postage paid at Boulder, Colorado 2 SCIENTIFIC BALLOONING techniques to meet them vary widely. Here ficiently reduced to permit coronal observa­ is a brief rundown on the projects repre­ tions to several solar radii from the sun’s sented at the conference, and on their limb. The old workhorse, the Stratoscope I stabilization systems. gondola that has been flown successfully PROJECT BALAST nine times, will carry the coronagraph aloft. A coarse sun-seeker system is used for With the planet Venus as the observation initial orientation and a set of fine photo target for the Balast (for “balloon astron­ eyes holds the platform within one minute omy”) project, Dr. John Strong, of Johns of arc. In order to minimize vibration, Hopkins University, and his associates wish single-torque motors with gear coupling to redetermine spectral absorptions by have replaced the dual-torque motors and water vapor in the planet’s atmosphere. magnetic clutches used in the Stratoscope I Planned experiments include measurements system first flown by Dr. Martin Schwarzs­ of the planet’s near-infrared spectrum, its child of Princeton University in 1957. total albedo, and its total emission. Three flights of Coronascope II from Though Dr. Strong’s target is Venus, the the NCAR Scientific Balloon Flight Station, method for tracking it begins with the sun. Palestine, Texas are planned for late sum­ “Coarse eye” photoelectric sensors stabilize mer, 1963. the platform’s orientation to the sun while a torque motor driven against a reaction POLARISCOPE wheel turns the platform. A programmed In the Polariscope project of Dr. A. M. J. offset from the sun on two axes (Beta and Gehrels of the University of Arizona, a 28- rho) points the telescope within one degree inch telescope and a polarimeter are the of the Venus sight-line, and a vidicon basic instruments used to determine the tube on the telescope axis is then electron­ polarization and scattering of radiation from ically scanned in a spiral mode until the the planets. image of Venus is detected. Upon acqui­ Target acquisition is achieved by means sition of its target, the vidicon converts to of a television camera that has a relatively an x-y scanning pattern to provide the wide viewing angle, while a second tele­ error signals that are fed back to the track­ vision tube that pictures finer details is ing drive system to stabilize the telescope used for accurate alignment. on the planet’s image. Torquing azimuth and elevation gyro­ The telescope, which has a one-degree scopes connected servomechanically to the field of view, has performed well in labora­ gondola provide the rotation. The tendency tory tests, achieving a tracking accuracy of of the gyros to drift after alignment on two seconds. However, the flight program target is overcome by fine tracking eyes has been delayed both by failure of the that feed back to the gyros. Tracking ac­ system during final ground checks to ac­ curacy of the system is expected to be on quire its Venusian target through the hazy the order of one minute of arc. daytime atmosphere, and by a series of First flight of the telescope and stabili­ balloon failures. The tracking system is zation system is scheduled for October, being “tightened up” and tougher balloons, 1963 from the canyon of the Colorado made from Schjeldahl GT-12 Mylar-and- River just below Glen Canyon Dam, Ari­ Dacron scrim, will be used for future flights. zona, CORONASCOPE II SKYTOP The plan of Dr. Gordon Newkirk, of the A north-seeking gyro provides the basic High Altitude Observatory, NCAR, for azimuthal orientation of Dr. Alvin H. How­ Coronascope II is to photograph the solar ell’s Skytop telescope platform. The 12- corona from above 100,000 feet, where inch telescope, coupled to an interferometer, atmospheric light scattering should be suf­ will be used for observations of Venus. SCIENTIFIC BALLOONING 3 Two motor-driven wheels spinning at a component that results in a precession constant 12,000 r.p.m. provide the torque. torque. The energy consumption per radian In the neutral configuration, the two spin­ of payload rotation is a modest one-eighth ning wheels have a common axis of rotation watt-second. that lies in a hinged horizontal shaft. An To prevent excessive winding of the 100- amplified signal from the north-seeking foot parachute train as the result of balloon gyro causes the hinge to bend, providing rotation, the parachute and balloon have the vertical with an angular momentum a motor-driven coupling that is kept rough­ SOME BALLOON ASTRONOMY STABILIZATION SYSTEMS Balost Coronascope II Polariscope Skytop Star Gazer Stratoscope II Principal J ohn Strong, Gordon New­ A.M.J. Gehrels, John Salisbury, J. Allen Hynek, Martin scientist Johns Hop­ kirk, High Al­ Univ. of Air Force Northwestern Schwarzschild, kins Univ. titude Obser- Arizona Cambridge Re- Univ. Princeton Univ. atory, NCAR search Labs. Stabilization James L. Robert Lee, Ernest Morri­ Alvin H. M. Winston Harold Hem- system Pritchard, HAO; son, Univ. of Howell, Markey, street, Perlan- principals Idea Lab.; ( Stratoscope Arizona Tufts Univ. M.I.T. Elmer Co. Morris I, Russell Birnbaum, Nidey, et al.) Labrascope Scientific Measurement Study of the Polarization Infrared Variation of High-resolution missions of the near- solar corona of planetary measurements, stellar scintil­ observations of infrared to several radiation 30-micron lation with planets and spectrum and solar radii band height. Feasi­ other astrono­ albedo of bility of man­ mical objects. Venus ned astronom­ ( Infrared meas­ ical flight. urements of Infrared Mars, Feb. ’63) measurements Principal Telescope Corona- 28-inch 12-inch 12-inch 36-inch scientific and infrared graph telescope and telescope and telescope (200- telescope measurements spectrometer polarimeter infrared inch focal interferometer length Casse- grain) Acquisition Sun-seeker Automatic Ground control North-seeking Manual Ground control mode with pro­ sun-seeker through tele­ gyro and pro­ through grammed off­ vision link grammed off­ television link. set to Venus. set to planet Spiral scan on vidicon. Tracking X and Y Photo eyes Gyro-stabilized Photo eyes Gyro-stabilized Photo eyes mode scan on photo eyes photo eyes vidicon. Stabilization 2 sec. 20 sec. 1 min. 6 sec. 1 min. .02 sec. accuracy (Root Mean Squared) Weight of 2,000 1,200 1,500 450 pounds 405 pounds for 6,300 pounds telescope and pounds pounds jiounds telescope and stabilization stabilization; system (4,000 pounds including two- man gondola) Number of 0 0 (6 for orig. 0 3 1 0 completed Stratoscope I flights and 3 for Co­ ronascope I ) SCIENTIFIC BALLOONING ly oriented to magnetic north by a magnetic on the ground in the Polariscope system. sensor. One flight of Star Gazer was made in Target acquisition data is programmed December, 1962. The balloon vehicle func­ onto two recording tapes, the incremental tioned completely successfully. However, signals of which are amplified and received apparently low battery voltages caused by the telescope drive motors. One of the faulty operation of the stabilization gear tapes gives the azimuth coordinate relative and the system tended to drift from its to north as a function of time and the other targets, so that many of the scientific ob­ tape does likewise for the elevation co­ jectives were not met. ordinate, the assumption being made that Two additional flights are now planned the gondola remains in the vertical. Ground for the Star Gazer system. commands can correct the program to com­ STRATOSCOPE II pensate for such conditions as a large error Dr.
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