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The apollo telescope mount on skylab
Aeta Astronautica.
Vol. I, pp. 1315-1329. Pergamon Press 1974. Printed in the U.S.A.
The Apollo Telescope Mount on Skylab R E I N I S E t AND E U G E N E
H. C A G L E
N.A.S.A., George C. Marshall Space Flight Center, Ala. 35812, U.S.A.
(Received 25 February 1974) Abstract---The Apollo Telescope Mount on Skylab, launched into Earth orbit on May 14, 1973, as the first manned solar observatory in space, has provided a major advance in our knowledge about the Sun. In addition to solar physics research, it explored the advantages and limitations of man as an observer and operator of a major scientific spacecraft, and provided concepts for potential application to future space programs. The ATM contained eight solar telescopes and provided these instruments with the necessary pointing reference and control, electrical power, thermal control, and telemetry and command links. Additionally,the experiments and supporting systems were integrated with the necessary controls and displays to permit complete operation and control by the astronaut crew. The ATM, during the nearly nine-month mission, provided data from the experiments which exceeded the pre-mission expectations in terms of quality, quantity, and the spectrum of observed solar activity. Areas of potentially significant new scientific findings included rapid large scale changes in the corona, flare trigger mechanisms and energy relationships, coronal holes and their relationship to solar wind, and the existence of numerous bright spots in the X-ray corona. Thorough analysis of the vast amount of data returned will require several years, but the indications to date are that the mission has provided extremely valuable scientific information which will probably result in significant changes to the currently accepted solar model.
Introduction THE APOLLO T e l e s c o p e M o u n t ( A T M ) on S k y l a b (Fig. 1), o r b i t i n g 435 k m a b o v e t h e E a r t h , is p r o v i d i n g a n e w o p p o r t u n i t y f o r significant a d v a n c e s in s o l a r p h y s i c s f r o m s p a c e . In o r b i t s i n c e M a y 14, 1973, A T M is p r o v i d i n g d a t a t h a t i n d i c a t e t h e p e r f o r m a n c e o f A T M , its e x p e r i m e n t s , a n d its s y s t e m s h a s e i t h e r m e t or e x c e e d e d t h e p r e - m i s s i o n e x p e c t a t i o n s . A n d t h e p r e s e n c e o f m a n as a p a r t o f t h e e x p e r i m e n t p r o g r a m h a s a d d e d s i g n i f i c a n t l y t o t h e q u a l i t y a n d q u a n t i t y o f t h e scientific r e t u r n s . T h e c o n c e p t o f t h e A p o l l o T e l e s c o p e M o u n t w a s initially p r o p o s e d in e a r l y 1966 in r e s p o n s e t o a s t r o n g i n t e r e s t in a d v a n c e d s o l a r r e s e a r c h b y t h e U n i t e d S t a t e s National Academy of Sciences. A spacecraft design was prepared to accommodate larger solar telescopes and therefore higher resolutions than had been previously o b t a i n e d . In t h e d e v e l o p m e n t o f t h e U V a n d X - r a y i n s t r u m e n t s , it w a s c o n c l u d e d t h a t s p a t i a l r e s o l u t i o n s o f at l e a s t 2.5 a r c s e c o n d s a n d s p e c t r a l r e s o l u t i o n s o f b e t t e r t h a n 0.5 A w e r e a c h i e v a b l e . In a d d i t i o n t o t h e t e l e s c o p e size, this d i c t a t e d t h e pointing and thermal control requirements of the spacecraft. Several proposals for advanced telescopes and spectrographs were submitted b y s o m e o f t h e f o r e m o s t s o l a r p h y s i c i s t s a n d a s t r o n o m e r s in t h e U n i t e d S t a t e s . F i v e p r i n c i p a l i n v e s t i g a t o r s ( T a b l e l) w e r e s e l e c t e d t o d e v e l o p a c o m p l e m e n t a r y c o m b i n a t i o n o f i n s t r u m e n t s f o r flight t h a t c o u l d o b s e r v e s o l a r flares a n d a c t i v e tManager, Apollo Telescope Mount Project. 1315
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Fig. I. The Apollo Telescope Mount on Skylab.
regions as well as the quiet Sun. The resulting collection of eight telescopic instruments, comprise the ATM solar experiment program. The primary objective of the Apollo Telescope Mount is to obtain high resolution data over extended periods on solar features in wavelengths from 2 to 6563 ,~. Observing programs have also been established that complement ATM solar observations with ground based instruments and are performed simultaneously with ATM observations. A second objective is to explore advantages and limitations of man as an observer and operator of a major scientific spacecraft. The ATM is the first major scientific payload in space in which man is participating in an observational and operational role. The activities during the mission thoroughly exercise the capabilities of man to operate the complex scientific instruments. Man is enhancing experimentation by recovery of photographic film used on six of the instruments, selecting and acquiring targets of opportunity, and providing operations and maintenance of the solar observatory. The ATM is also providing much needed technological information on advanced concepts such as attitude and pointing control for potential application to future space programs. During its development, ATM was integrated into the Skytab manned space station cluster. The long duration operational requirements of Skylab dictated that
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Table 1. ATM principal investigators. Principal Investigator
Equipment
Objectives
Dr. Mac Queen High Altitude Observatory
Externally occulted Lyot coronagraph
Photograph in white light the solar corona to 6 solar radii over an extended period of time
Dr. Tousey Naval Research Laboratory
A--Extreme ultraviolet (XUV) spectroheliograph B--High resolution XUV chromospheric spectrograph
Photograph image of Sun in XUV wavelengths from 150 to 650 ,~, Photographically record line spectrograms of a 3 x 60 arc sec slit between 970 and 3940
Dr. Giacconi American Science and Engineering
Grazing incidence X-ray spectrographic telescope and flare detector
Photograph X-ray spectra of solar flares from 3 to 60 ,~
Dr. Reeves Harvard College Observatory
UV scanning polychromator spectroheliometer
Photoelectrically measure UV emissions from 5 arc min. aperture in wavelengths of 300-1350 A,with a resolution of 5 arc sec.
Mr. Milligan Marshall Space Flight Center
X-ray telescope and proportional counter
Obtain X-ray photographs of solar flares from 5 to 40
the critical ATM systems incorporate redundancy. The use of man in the ATM experiments required that the controls and displays be inside the habitable area of Skylab. The attitude and stability requirements for the solar experiments led to the design of an integrated total Skylab cluster attitude control system. Electrical power sharing between the Skylab A T M and w o r k s h o p systems was also incorporated. The achievement of high resolution spectrographic data from space, while operating as a part of a large manned space station, placed a number of stringent p e r f o r m a n c e requirements on the ATM. These requirements, and the actual p e r f o r m a n c e of the ATM systems and experiments, are contained in the following discussion.
System description and requirements The Skylab is an assembly of several modules (Fig. 2), the largest of which is the w o r k s h o p containing the crew living quarters. At the front end of the workshop, an airiock and a docking module provide the Skylab with a controlled atmosphere, electrical power, additional crew working space, and the control console for the ATM. The airlock provides capability for the crew to leave the laboratory for maintenance and A T M film retrieval. The A T M was launched in front of the docking module within a protective shroud. After orbit was reached the shroud was released, the A T M was rotated to the side, and the A T M solar arrays were extended. The A T M consists of two major elements, the cylindrical experiments package
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Fig. 2. Skylab orbital configuration. containing all of the e x p e r i m e n t s (mounted in the center of the A T M module), and the rack or surrounding structure. The rack supports the experiment package through a two axis of f r e e d o m gimbal ring. The rack also supports the four solar arrays and most of the electronic equipment of the ATM. The A T M Control and Display panel is located inside the Skylab docking module. It is f r o m here that the telescopes are pointed toward the targets of interest and the television displays of the solar observations are presented. A T M provides the primary attitude control for the total Skylab using a system of three large constant speed g y r o s c o p e s mounted orthogonally within the A T M rack. This s y s t e m is able to control the massive Skylab which is affected by both external and internal disturbances. The external disturbances consist mainly of gravity gradient and a e r o d y n a m i c torques while internal disturbances are caused by the astronauts' m o v e m e n t within the workshop. Substantial counter m o m e n t s are available to offset these disturbances using the m o m e n t u m exchange devices called control m o m e n t gyroscopes. Each of the g y r o s c o p e s has a stored m o m e n t u m capacity of 3117 newton meter seconds. This method of controlling the attitude provides a c c u r a c y and stability m e a s u r e d in a few arc minutes and is limited only by the capability of the g y r o s c o p e s to store angular m o m e n t u m . The angular m o m e n t u m stored in the g y r o s c o p e s is neutralized during the dark part of the orbit by the action of the gravity gradient force acting on Skylab. A Sun sensor is used for position reference during the daylight portion of each orbit, and rate gyros provide the necessary stability during the dark side. The concept of control m o m e n t g y r o s c o p e s for primary attitude control and stabilization was selected instead of a mass expulsion system since the latter could contaminate the highly sensitive optical m e a s u r e m e n t s and would b e c o m e very h e a v y for long duration missions.
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A T M is capable of providing over half of the Skylab electrical p o w e r needs from its four solar wings. The A T M electrical p o w e r system generates, conditions, stores, controls and distributes an average of 4500 watts at 28 volts DC to the A T M systems and experiments. The system also incorporates a power sharing network capable of parallel operation with the other power system on Skylab. It was this power sharing capability that supplied Skylab with the needed power during the first three weeks of its orbital operation before the one remaining solar array of the w o r k s h o p could be deployed. Total peak power capability of the A T M solar array is 12 kilowatts, however, average power requirements of the A T M systems are in the order of about 3500watts. The A T M uses rechargeable nickel cadmium batteries for power storage. A s u m m a r y of A T M systems p e r f o r m a n c e requirements is shown in Table 2. Table 2. ATM systems requirements. Mission Parameters
435 km circular orbit, 50° inclination 8 months lifetime (5 manned) Experiments
1coronagraph, 2 XUV spectrographs, 1 UV spectrometer, 2 X-ray telescopes, 2 H-alpha telescopes Pointing and Control
2.5 arc second accuracy 2.5 arc second stability (15 minutes) pitch and yaw. 10arc minutes roll Automatic solar disc centering; target selection by astronaut manual offset control and visual verification Thermal
Thermal stability of the instruments--+ 0.6°C Electrical Power
Generation and storage of 4500watts average power Scientific Data
500 to 600 hours data time 4 sets of 6 photographic cameras retrieved and replaced by astronaut EVA Continuous telemetered data from one experiment (7 photomultiplier detectors)
A semi-passive thermal control system is used for the c o m p o n e n t s mounted on the structural rack. Thermal balance is maintained by a carefully tailored combination of insulations, radiative surfaces, and electrical heaters. Monitoring and receiving data on the p e r f o r m a n c e of the A T M systems are provided by conventional telemetry. The A T M contains eight telescopes covering wavelengths ranging from 2 to 6563 A. They are mounted in the experiments package and coaligned on an aluminum cruciform optical bench. Of the six photographic instruments, four have cameras located at the rear, and two are located at the front because of optical requirements. Primary p e r f o r m a n c e requirements are contained in Table 3. A detailed discussion of each experiment follows. The white light coronagraph (SO52) obtains high resolution photographs of the c o r o n a from 1.5 to 6 solar radii to study brightness, form, polarization, and
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Table 3. ATM experiments performance requirements.
Experiment
White Light Coronagraph (SO52) XUV
Spectral Range
Spatial Resolution
Spectral Resolution
3700-6500 A
8 arc sec
--
150-650 A
5 arc sec
0.13 ~,
Spectroheliograph (SO82A) XUV Spectrograph (SO82B)
970-3940 A
3 × 60 arc sec as defined by
Data 8000 exposures per camera 200 exposures per camera
0-08-0-16 ,~
1600 exposures per camera 7000 exposures per camera
the entrance slit X-ray Spectrographic Telescope (SO54) UV Scanning Polychromator Spectroheliometer (SO55) X-ray Telescope (SO56) H-alpha # 1 H-alpha # 2
3-60 A
3 arc sec
variable
300-1350 A
5 arc sec
1.6 ,~
Telemetry
540 A
5 arc sec
5A
6563 A
1.0 arc sec (film)
--
6563 A
2.5 arc sec
--
6000 exposures per camera 1600 exposures per camera TV only
evolution of coronal activity, and its correlation with surface events. The instrument is a Lyot coronagraph consisting of three fixed external and one automatically centered internal occulting discs located along a 315 cm optical path. A four position polaroid wheel is used to identify polarization characteristics of the corona. The image is focused on a reel film camera and also is displayed to the astronaut through a low light level television camera. The X U V coronal spectroheliograph (SO82A) photographically records images of the entire Sun in the extreme ultraviolet wavelengths to study the composition, density, and temperature of the chromosphere and inner corona. The instrument is a two meter slitless spectrograph, using a 3600 line per millimeter concave, normal incidence, two position grating, to focus the dispersed solar image onto a 35 x 248 mm film strip. Either of the two wavelength bands and three ranges of exposure times may be selected by the astronaut according to the solar activity existing at the time. The X U V spectrograph (SO82B) photographically records line spectra of small selected areas on and off the solar disc and across the limb and observes an X U V image of the full solar disc. The instrument consists of a one meter focal length mirror, a two position predisperser grating, and a 600 line per millimeter, two meter focal length main grating. A pointing reference system provides a visual display of the slit plate to the astronaut and holds the instrument automatically on the limb by automated control of the main mirror. A monitor displays an X U V image of the solar disc to the astronaut and the principal investigator to assist in identifying emitting regions of special interest. The X-ray spectrographic telescope (SO54) obtains X-ray emissions of flares and other sources of solar X-rays. It follows the evolution of both spatial image and spectrum throughout the lifetime of a flare, evolution of nonflaring active regions, and correlation of coronal X-ray structure with surface events. The instrument
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consists of a double-reflecting grazing incidence mirror which focuses the solar image through an objective grating used as a slitless spectrograph to a 70 mm reel camera. The camera can be operated in manual, automatic, or rapid automatic modes. A flare detector provides a coarse X-ray image of the Sun to the astronaut for monitoring of solar activity. The UV scanning polychromator/spectroheliometer (SO55A) obtains photometric ultraviolet observations of the solar atmosphere near or above active regions on the solar disc. The instrument consists of an off-axis parabolic mirror which images the solar disc on the 5 × 5 arc second entrance slit of the spectrometer. The mirror is capable of motion in two axes to provide either a 5 x 5 arc minute raster scan or a five arc second single line scan. A rotating grating mount in the spectrometer provides wavelength scan with a fixed mirror position or selected wavelength operation with the mirror in motion. The data are obtained through a photometric conversion system and are transmitted to the ground through telemetry. The X-ray telescope (SO56) photographs X-ray emitting solar regions during both active and quiescent periods with high spatial and temporal resolution. It monitors and records total solar X-ray flux measurements in specific spectral regions. The instrument also uses a grazing incidence mirror to focus the solar X-ray image through a six position filter wheel on a reel camera. Two proportional counters with a narrow band pass filter detect total X-ray flux emitted from the solar disc. Counter outputs are displayed to the astronauts and are also recorded. Two H-alpha telescopes observe the Sun in the light of the H-alpha spectral line at 6563 ,~. The intensity of the emission varies with the structural features on the solar disc and displays many fine scale features in the solar atmosphere. By tuning the filter slightly off center of the line, a clearer display of solar flares is obtained. The H-alpha telescopes are the principal aiming device the astronauts use to point the other instruments. They also give him a common reference with ground based observers during the mission. Associated with certain telescopic instruments are television cameras. The astronaut may select an image from five different instruments (two H-alpha telescopes, SO82A and SO82B, and SO52), and display this image on one of two television monitors on the control panel in the docking module. Capitalizing on the opportunity of being able to return photographic recordings from space, four of the principal investigators chose to use film to collect the data. The advantages of film are the higher spatial resolution which can be obtained and the total density of data recorded, as opposed to photoelectric devices which provide more accurate flux measurements. These films, however, are sensitive to the storage and operational environment of Skylab. Degradation of film sensitivity, latent images, and background fogging could result from high temperatures and radiation existing in the near-Earth space, primarily the South Atlantic anomaly region of the van Allen radiation belt. To protect from this environment, the film is stored in thick-walled aluminum film vaults in the docking module. During extravehicular activities crewmen retrieve the camera assemblies and film from the experiments through access doors at work stations. Exposed film is returned with each crew. To permit the spatial resolutions demanded by the experiments, high pointing
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accuracies are provided by the ATM. The pointing of the experiments package containing the telescopes is accomplished by the experiments pointing control system which acts as a vernier of the attitude control system. This system, capable of precise pointing to within 2.5 arc seconds, isolates the experiments from the ATM rack by a two axis (pitch and yaw) gimbai ring flex pivot. This flex pivot mounting allows virtually frictionless, ultra-low breaking torque performance of the gimbals about the pitch and yaw axes. The experiment package may be rotated __+120° about the axis pointed at the Sun and -+ 2° about its pitch and yaw axes. A high precision Sun sensor is used for this fine pointing reference. The two axis gimbal system was designed to provide a pointing stability of 0.63 arc seconds for 15 minutes in both pitch and yaw axes. Pointing uncertainty is limited only by the 2.5 arc second resolution of the video displays of the solar target. A tightly controlled thermal control system was developed to assure the stability of the instruments optical axes and focal plane. The experiments package thermal control system is completely separate from the structural rack. A mixture of methanol and water circulates through the walls of the experiments package since highly stable wall temperatures, 10-2°C, are required. The experiments are thermally isolated from the optical bench with low conductance mountings. Essentially, all of the internally generated heat is transferred from the experiments to the canister wall through radiation. To meet the thermal stability requirements of ---0.6°C for the instruments, the thermal design is biased to the cold side and the instruments are brought up to the exact operating temperature by use of electrical heater panels. The heater panels cover essentially all of the telescope surfaces and cycle within a temperature band that is controlled to a very accurate average effective temperature. An overall consideration in the Skylab/ATM Program was the effect of contamination on the optics of the precision telescopes due to contamination from venting of the workshop waste tank and other vents. Outgassing was also important to avoid corona problems associated with operation of the SO54 and SO56 experiments. Pressures less than 10 ~tort are required to assure that corona problems would not be experienced. To meet these requirements programmed minimal venting was incorporated, and an extensive materials control program was implemented. In addition, several sounding rockets are launched containing subscale models of ATM's ultraviolet spectrographs to calibrate any residual contamination or other degradation effects on ATM solar observations. The sounding instruments acquire data simultaneously with ATM instruments resulting in calibration data for post-mission analysis of ATM's solar data.
Performance and preliminary findings Observations by the Apollo Telescope Mount telescopes to date have provided the ATM scientists with data which equals or exceeds their expectations. The data have revealed features in the corona, the transition region, the chromosphere, and the photosphere never before observed or recorded with such accuracy.
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The experiments and supporting subsystems have demonstrated excellent performance, and none of the few problems experienced were of sufficient consequence to cause an impact on the success of the observational program. In many cases, the systems have surpassed their design requirements. One of the more notable examples of this was the ability of the ATM electrical power system to provide power for the entire Skylab beginning shortly after launch and lasting for three weeks. Also, the canister fluid thermal control system has exceeded performance specifications maintaining wall temperatures to l0 -+0.6°C and holding the optical bench temperatures essentially constant at 17.7°C, changing only 0.12°C with operational cycling of the experiments. Crew performance to date clearly demonstrates man's ability to function in the space environment as a scientist, technician, and operator of a complex orbiting solar observatory.
Experiment performance The solar experiments have performed exceptionally well during the Skylab mission. Preliminary results of the solar observations from the first manned period indicate high quality data. The second manned period has more than tripled the total quantity of data obtained because of the longer mission duration (59 days), an increase in solar activity, and the improvement of observing programs as the result of experience from the first mission. The time of ATM experiment manned operation now totals over 500 hours. In addition, over 600 hours have been spent in ground controlled observation. In total, during the first two manned missions, over 105,000 frames of film have been exposed of an available 115,000 frames, or about 92 percent. During the last manned mission another 53,000 frames will be available for solar observations.
Pointing system performance The ATM Attitude and Pointing Control System performance has exceeded the requirements and provided a stable base for ATM viewing. The observed pointing accuracies, or the ability to point to a selected target, during the manned and unmanned phases were better than 0.4 arc minute for the entire Skylab assembly. Stability, or the ability to hold Skylab to a target during the manned phase was better than 1.4 arc minutes and during the unmanned phase was 0.4 arc minute for 15 minutes. The Experiment Pointing Control System which provides the precise pointing for the solar telescopes has performed flawlessly. Pointing accuracies of better than one arc second were achieved. Stability, except for a few brief moments when the crew were exercising inside the workshop, also was better than the predicted 2.5 arc seconds for 15 minutes.
Crew performance During the EVA's, the astronauts performed the scheduled film replacement and retrieval tasks without problems and more rapidly than expected. Early in the mission, an unscheduled extravehicular activity was performed by the astronauts. They retrieved and replaced a failed SO82A camera; unpinned and
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permanently latched open the SO54 Sun-end aperture door which had malfunctioned; and r e m o v e d lint from the forward external occulting disc of the coronagraph. Activities of subsequent EVA's included removal and replacement of a failed SO52 camera and repair of a charger-battery-regulator-module. During the first mission, the crew demonstrated their ability to refine, in real time, the targets selected by the ground, thereby enhancing scientific data. On June 15, 1973, an M-4 class flare and on September 7, 1973, a very large X-I.5 class flare were observed, demonstrating the crew's ability to locate the flare, point the ATM canister quickly to the flare and begin taking data at a high rate to maximize the data return. The crew has also successfully performed other unplanned activities such as troubleshooting problems and making equipment adjustments or corrections on a real time basis. Without this capability valuable data would have been lost. Based upon the favorable performance during the early phases of Skylab, the ATM Principal Investigators have assigned greater freedom to the astronauts for selecting targets and observing programs during the mission.
Significant anomalies N o permanent failures in any of the ATM experiment equipment have been encountered to date. The most serious anomalies occurred with the SO52 and SO82A cameras which jammed during the first mission. During an unscheduled EVA, the astronauts exchanged the SO82A camera with another unit, thus eliminating a potentially serious loss of data. The SO52 camera was replaced later during a scheduled EVA. As a result of an SO54 experiment aperture door anomaly which required astronaut deactivation of the door in the open position, the automated instrument operation logic was interrupted so as to lose an indication to the crew of the completion of an exposure sequence. This necessitated cumbersome manual timing of exposures. The problem was remedied by the design and fabrication of an auxiliary timer which the second crew carried up and installed on the control console, virtually restoring normal operation. A serious concern with the rate gyro sensors which had reduced system redundancy was also resolved with replacement hardware which was carried aloft, installed, checked out, and activated by the second crew.
Preliminary scientific findings During the available observing time in the first manned mission, primary emphasis was placed on achieving broad coverage of all prominent solar features using the various pre-planned observing programs. The second manned mission then concentrated on more specific investigations based upon the previous data acquired. The total volume of data already obtained is enormous, challenging the principal investigator's capacity for detailed analysis. Consequently, the scientific findings to date reflect only a general assessment. Detailed analysis will require months and even years of work before firm conclusions and possible changes to the solar model can be established. The results of this work will be presented by the principal investigators in various journals and symposia as they become
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Table 4. Scientific findings. Rapid large scale changes in the corona Coronal voids Coronal holes Velocity changes on surface Rapid flare development Higher flare frequency Possible magnetic coupling of flares New UV structures
available. This summary (Table 4) is a very early and, consequently, qualitative assessment of the findings indicated by the data so far. The first of the reportable findings is that the changes observed in the corona occur much more rapidly than was previously thought. The changes are dramatic in scale and sometimes occur even in short spans of one or two hours. The changes occur in the structure of the overall magnetic field of the outer solar atmosphere. The SO52 coronagraph has given us an extremely detailed examination of the outer corona and its interactions of electrons and magnetic fields (Fig. 3).
Fig. 3. Photograph of the solar corona made on June 10, 1973 by the High Altitude Observatory white light coronagraph on Skylab (Courtesy of Dr. R. MacQueen, High Altitude Observatory).
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A new p h e n o m e n o n has been discovered which has been called a coronal void and is thought to be a region void of free electrons. The material that remains is that of the F-Corona or the Zodiacal light. This discovery lends itself to further study and understanding of the content of the background corona. An explosion in the Sun's corona occurred on August 10, 1973. Enormous loops of structure were expelled from the corona at velocities which exceeded a million kilometers per hour and to distances of three and one-half solar radii and possibly as far as six solar radii. This occurrence, called a coronal transient, was captured on film by the ATM instruments. The preliminary assessments indicate a restructuring of the magnetic field lines took place after the event. Possibly the explosion involved two filaments or prominences combining in an active region. An interesting question which remains to be answered is why there were no radio waves emitted when this enormous ejection from the corona occurred. The dark spots previously observed at the North and South poles (Fig. 4) have been referred to as corona holes. Observations in the X-ray and ultraviolet spectra have shown that they may be much more than coronal holes. They may extend down, well into the lower outer atmosphere and have temperatures as low as 50,000°C. This leads to the possibility that there are holes that extend to the bottom of the outer atmosphere and perhaps reflect the Sun's minimum temperature. The ever-present velocity changes of matter on the Sun's surface represents another significant finding. This change in velocity was seen while observing two limbs of the Sun simultaneously. A difference in velocity was noted from one side
Fig. 4. Photograph of a large flare made on June 15, 1973in two contrasting emissions by the Naval Research Laboratory XUV spectroheliograph on Skylab (Courtesy of Dr. R. Tousey. Nawd Research Laboratory).
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of the Sun to the other. T o further substantiate this, the A T M was pointed at active regions and o b s e r v e d actual shifts in large areas of the Sun in lines of the transition region and the corona. T h e stages of a flare for n u m e r o u s small flares and two very large flares including the initial indications in an active region, the first disturbance of the surface, the flare activity, the flare decay, and the bright point where the flare originated h a v e b e e n o b s e r v e d b y the A T M (Fig. 5).
Fig. 5. X-ray photograph of the solar corona obtained May 28, 1973 by the American Science and Engineering X-ray telescope on Skylab (Courtesy of Dr. G. Vaiana, Solar Physics Group, American Science and Engineering, Inc.). The bright points, which h a v e been observed, seem to be the remains of a network that is present all o v e r the Sun. T h e s e bright points m a y be small active regions, capable of flaring or they m a y represent a new kind of solar activity. T h e flare activity that has been o b s e r v e d has pointed out the benefits realized f r o m the orbiting laboratory. A flare was o b s e r v e d f r o m the ground and thought to be a very small flare, or sub-flare. O b s e r v e d in the X-ray region f r o m ATM, the s a m e flare was quite prominent and exhibited very rapid changes in its development. T h e findings show that a flare can grow in a v e r y short time, in the order of seconds. Also flare activity n o w appears to be m o r e frequent. T h e A T M principal investigators are currently analyzing the data available, with the newly discovered flare characteristics in mind, in an effort to establish relationships b e t w e e n large flare activity and the smaller flares that m a y be reactions to the more prominent
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flare activity. It is not known currently if this is the case, but the data available from the ATM missions is believed to hold the key to the mechanisms that generate flares. The SO55 experiment electronically recorded an image showing clear, detailed loops of ionized particles with temperatures of 2 million degrees Celsius streaming away from the Sun's surface more than 41,000 km in space (Fig. 6). This was one of the most exciting products of the ultraviolet spectroheliometer. Some details of structure, composition, and active processes revealed in the ultraviolet photographs are the first ever seen by man.
Fig. 6. Coronal loop prominence at the solar limb in the 417 ,~ emission line of Fe XV, recorded photoelectrically by the Harvard College Observatory scanning UV spectroheliometer on Skylab, June 1973 (Courtesy of Dr. E. Reeves, Harvard College Observatory}.
Summary It can be concluded from the successful ATM operations to date that more high quality solar data has been recorded in this one mission than all the previous solar research efforts added together. This was achieved with the long duration flight of high resolution instruments on a single platform with a wide range of spectral coverage and pointed simultaneously at specific targets. Additionally,
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man was integrated as a scientific observer, operator and repairman to assure m a x i m u m return of data. As in the early phases of any research, the A T M scientists at this m o m e n t may well h a v e found more questions than answers, but the volumes of data waiting to be analyzed and understood should soon unlock some of the previously well kept secrets of the Sun. The total scientific value of the first manned solar o b s e r v a t o r y in space will then be fully realized. Addendum
Since preparation of this paper, the Skylab mission has been successfully completed. The A T M instruments continued to provide high quality data throughout the remainder of the extended Skylab program. As a result of the extension to the Skylab mission, additional film was resupplied by the third crew and more time was allocated to solar observation. T h e resulting time of A T M instrument operation during manned phases totaled o v e r 1600 hours, and more than 700hours were spent in ground controlled instrument operation. More than 175,000 f r a m e s of film were e x p o s e d out of an available 182,842 frames. The A T M project has now m o v e d f r o m the operations phase into the data analysis phase. Each of the principal investigators will publish a preliminary scientific report for his respective e x p e r i m e n t about March 1975. H o w e v e r , it is e x p e c t e d that a complete analysis of the data returned f r o m A T M will take m a n y years to perform.
Vol. I, pp. 1315-1329. Pergamon Press 1974. Printed in the U.S.A.
The Apollo Telescope Mount on Skylab R E I N I S E t AND E U G E N E
H. C A G L E
N.A.S.A., George C. Marshall Space Flight Center, Ala. 35812, U.S.A.
(Received 25 February 1974) Abstract---The Apollo Telescope Mount on Skylab, launched into Earth orbit on May 14, 1973, as the first manned solar observatory in space, has provided a major advance in our knowledge about the Sun. In addition to solar physics research, it explored the advantages and limitations of man as an observer and operator of a major scientific spacecraft, and provided concepts for potential application to future space programs. The ATM contained eight solar telescopes and provided these instruments with the necessary pointing reference and control, electrical power, thermal control, and telemetry and command links. Additionally,the experiments and supporting systems were integrated with the necessary controls and displays to permit complete operation and control by the astronaut crew. The ATM, during the nearly nine-month mission, provided data from the experiments which exceeded the pre-mission expectations in terms of quality, quantity, and the spectrum of observed solar activity. Areas of potentially significant new scientific findings included rapid large scale changes in the corona, flare trigger mechanisms and energy relationships, coronal holes and their relationship to solar wind, and the existence of numerous bright spots in the X-ray corona. Thorough analysis of the vast amount of data returned will require several years, but the indications to date are that the mission has provided extremely valuable scientific information which will probably result in significant changes to the currently accepted solar model.
Introduction THE APOLLO T e l e s c o p e M o u n t ( A T M ) on S k y l a b (Fig. 1), o r b i t i n g 435 k m a b o v e t h e E a r t h , is p r o v i d i n g a n e w o p p o r t u n i t y f o r significant a d v a n c e s in s o l a r p h y s i c s f r o m s p a c e . In o r b i t s i n c e M a y 14, 1973, A T M is p r o v i d i n g d a t a t h a t i n d i c a t e t h e p e r f o r m a n c e o f A T M , its e x p e r i m e n t s , a n d its s y s t e m s h a s e i t h e r m e t or e x c e e d e d t h e p r e - m i s s i o n e x p e c t a t i o n s . A n d t h e p r e s e n c e o f m a n as a p a r t o f t h e e x p e r i m e n t p r o g r a m h a s a d d e d s i g n i f i c a n t l y t o t h e q u a l i t y a n d q u a n t i t y o f t h e scientific r e t u r n s . T h e c o n c e p t o f t h e A p o l l o T e l e s c o p e M o u n t w a s initially p r o p o s e d in e a r l y 1966 in r e s p o n s e t o a s t r o n g i n t e r e s t in a d v a n c e d s o l a r r e s e a r c h b y t h e U n i t e d S t a t e s National Academy of Sciences. A spacecraft design was prepared to accommodate larger solar telescopes and therefore higher resolutions than had been previously o b t a i n e d . In t h e d e v e l o p m e n t o f t h e U V a n d X - r a y i n s t r u m e n t s , it w a s c o n c l u d e d t h a t s p a t i a l r e s o l u t i o n s o f at l e a s t 2.5 a r c s e c o n d s a n d s p e c t r a l r e s o l u t i o n s o f b e t t e r t h a n 0.5 A w e r e a c h i e v a b l e . In a d d i t i o n t o t h e t e l e s c o p e size, this d i c t a t e d t h e pointing and thermal control requirements of the spacecraft. Several proposals for advanced telescopes and spectrographs were submitted b y s o m e o f t h e f o r e m o s t s o l a r p h y s i c i s t s a n d a s t r o n o m e r s in t h e U n i t e d S t a t e s . F i v e p r i n c i p a l i n v e s t i g a t o r s ( T a b l e l) w e r e s e l e c t e d t o d e v e l o p a c o m p l e m e n t a r y c o m b i n a t i o n o f i n s t r u m e n t s f o r flight t h a t c o u l d o b s e r v e s o l a r flares a n d a c t i v e tManager, Apollo Telescope Mount Project. 1315
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Fig. I. The Apollo Telescope Mount on Skylab.
regions as well as the quiet Sun. The resulting collection of eight telescopic instruments, comprise the ATM solar experiment program. The primary objective of the Apollo Telescope Mount is to obtain high resolution data over extended periods on solar features in wavelengths from 2 to 6563 ,~. Observing programs have also been established that complement ATM solar observations with ground based instruments and are performed simultaneously with ATM observations. A second objective is to explore advantages and limitations of man as an observer and operator of a major scientific spacecraft. The ATM is the first major scientific payload in space in which man is participating in an observational and operational role. The activities during the mission thoroughly exercise the capabilities of man to operate the complex scientific instruments. Man is enhancing experimentation by recovery of photographic film used on six of the instruments, selecting and acquiring targets of opportunity, and providing operations and maintenance of the solar observatory. The ATM is also providing much needed technological information on advanced concepts such as attitude and pointing control for potential application to future space programs. During its development, ATM was integrated into the Skytab manned space station cluster. The long duration operational requirements of Skylab dictated that
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Table 1. ATM principal investigators. Principal Investigator
Equipment
Objectives
Dr. Mac Queen High Altitude Observatory
Externally occulted Lyot coronagraph
Photograph in white light the solar corona to 6 solar radii over an extended period of time
Dr. Tousey Naval Research Laboratory
A--Extreme ultraviolet (XUV) spectroheliograph B--High resolution XUV chromospheric spectrograph
Photograph image of Sun in XUV wavelengths from 150 to 650 ,~, Photographically record line spectrograms of a 3 x 60 arc sec slit between 970 and 3940
Dr. Giacconi American Science and Engineering
Grazing incidence X-ray spectrographic telescope and flare detector
Photograph X-ray spectra of solar flares from 3 to 60 ,~
Dr. Reeves Harvard College Observatory
UV scanning polychromator spectroheliometer
Photoelectrically measure UV emissions from 5 arc min. aperture in wavelengths of 300-1350 A,with a resolution of 5 arc sec.
Mr. Milligan Marshall Space Flight Center
X-ray telescope and proportional counter
Obtain X-ray photographs of solar flares from 5 to 40
the critical ATM systems incorporate redundancy. The use of man in the ATM experiments required that the controls and displays be inside the habitable area of Skylab. The attitude and stability requirements for the solar experiments led to the design of an integrated total Skylab cluster attitude control system. Electrical power sharing between the Skylab A T M and w o r k s h o p systems was also incorporated. The achievement of high resolution spectrographic data from space, while operating as a part of a large manned space station, placed a number of stringent p e r f o r m a n c e requirements on the ATM. These requirements, and the actual p e r f o r m a n c e of the ATM systems and experiments, are contained in the following discussion.
System description and requirements The Skylab is an assembly of several modules (Fig. 2), the largest of which is the w o r k s h o p containing the crew living quarters. At the front end of the workshop, an airiock and a docking module provide the Skylab with a controlled atmosphere, electrical power, additional crew working space, and the control console for the ATM. The airlock provides capability for the crew to leave the laboratory for maintenance and A T M film retrieval. The A T M was launched in front of the docking module within a protective shroud. After orbit was reached the shroud was released, the A T M was rotated to the side, and the A T M solar arrays were extended. The A T M consists of two major elements, the cylindrical experiments package
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Fig. 2. Skylab orbital configuration. containing all of the e x p e r i m e n t s (mounted in the center of the A T M module), and the rack or surrounding structure. The rack supports the experiment package through a two axis of f r e e d o m gimbal ring. The rack also supports the four solar arrays and most of the electronic equipment of the ATM. The A T M Control and Display panel is located inside the Skylab docking module. It is f r o m here that the telescopes are pointed toward the targets of interest and the television displays of the solar observations are presented. A T M provides the primary attitude control for the total Skylab using a system of three large constant speed g y r o s c o p e s mounted orthogonally within the A T M rack. This s y s t e m is able to control the massive Skylab which is affected by both external and internal disturbances. The external disturbances consist mainly of gravity gradient and a e r o d y n a m i c torques while internal disturbances are caused by the astronauts' m o v e m e n t within the workshop. Substantial counter m o m e n t s are available to offset these disturbances using the m o m e n t u m exchange devices called control m o m e n t gyroscopes. Each of the g y r o s c o p e s has a stored m o m e n t u m capacity of 3117 newton meter seconds. This method of controlling the attitude provides a c c u r a c y and stability m e a s u r e d in a few arc minutes and is limited only by the capability of the g y r o s c o p e s to store angular m o m e n t u m . The angular m o m e n t u m stored in the g y r o s c o p e s is neutralized during the dark part of the orbit by the action of the gravity gradient force acting on Skylab. A Sun sensor is used for position reference during the daylight portion of each orbit, and rate gyros provide the necessary stability during the dark side. The concept of control m o m e n t g y r o s c o p e s for primary attitude control and stabilization was selected instead of a mass expulsion system since the latter could contaminate the highly sensitive optical m e a s u r e m e n t s and would b e c o m e very h e a v y for long duration missions.
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A T M is capable of providing over half of the Skylab electrical p o w e r needs from its four solar wings. The A T M electrical p o w e r system generates, conditions, stores, controls and distributes an average of 4500 watts at 28 volts DC to the A T M systems and experiments. The system also incorporates a power sharing network capable of parallel operation with the other power system on Skylab. It was this power sharing capability that supplied Skylab with the needed power during the first three weeks of its orbital operation before the one remaining solar array of the w o r k s h o p could be deployed. Total peak power capability of the A T M solar array is 12 kilowatts, however, average power requirements of the A T M systems are in the order of about 3500watts. The A T M uses rechargeable nickel cadmium batteries for power storage. A s u m m a r y of A T M systems p e r f o r m a n c e requirements is shown in Table 2. Table 2. ATM systems requirements. Mission Parameters
435 km circular orbit, 50° inclination 8 months lifetime (5 manned) Experiments
1coronagraph, 2 XUV spectrographs, 1 UV spectrometer, 2 X-ray telescopes, 2 H-alpha telescopes Pointing and Control
2.5 arc second accuracy 2.5 arc second stability (15 minutes) pitch and yaw. 10arc minutes roll Automatic solar disc centering; target selection by astronaut manual offset control and visual verification Thermal
Thermal stability of the instruments--+ 0.6°C Electrical Power
Generation and storage of 4500watts average power Scientific Data
500 to 600 hours data time 4 sets of 6 photographic cameras retrieved and replaced by astronaut EVA Continuous telemetered data from one experiment (7 photomultiplier detectors)
A semi-passive thermal control system is used for the c o m p o n e n t s mounted on the structural rack. Thermal balance is maintained by a carefully tailored combination of insulations, radiative surfaces, and electrical heaters. Monitoring and receiving data on the p e r f o r m a n c e of the A T M systems are provided by conventional telemetry. The A T M contains eight telescopes covering wavelengths ranging from 2 to 6563 A. They are mounted in the experiments package and coaligned on an aluminum cruciform optical bench. Of the six photographic instruments, four have cameras located at the rear, and two are located at the front because of optical requirements. Primary p e r f o r m a n c e requirements are contained in Table 3. A detailed discussion of each experiment follows. The white light coronagraph (SO52) obtains high resolution photographs of the c o r o n a from 1.5 to 6 solar radii to study brightness, form, polarization, and
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Table 3. ATM experiments performance requirements.
Experiment
White Light Coronagraph (SO52) XUV
Spectral Range
Spatial Resolution
Spectral Resolution
3700-6500 A
8 arc sec
--
150-650 A
5 arc sec
0.13 ~,
Spectroheliograph (SO82A) XUV Spectrograph (SO82B)
970-3940 A
3 × 60 arc sec as defined by
Data 8000 exposures per camera 200 exposures per camera
0-08-0-16 ,~
1600 exposures per camera 7000 exposures per camera
the entrance slit X-ray Spectrographic Telescope (SO54) UV Scanning Polychromator Spectroheliometer (SO55) X-ray Telescope (SO56) H-alpha # 1 H-alpha # 2
3-60 A
3 arc sec
variable
300-1350 A
5 arc sec
1.6 ,~
Telemetry
540 A
5 arc sec
5A
6563 A
1.0 arc sec (film)
--
6563 A
2.5 arc sec
--
6000 exposures per camera 1600 exposures per camera TV only
evolution of coronal activity, and its correlation with surface events. The instrument is a Lyot coronagraph consisting of three fixed external and one automatically centered internal occulting discs located along a 315 cm optical path. A four position polaroid wheel is used to identify polarization characteristics of the corona. The image is focused on a reel film camera and also is displayed to the astronaut through a low light level television camera. The X U V coronal spectroheliograph (SO82A) photographically records images of the entire Sun in the extreme ultraviolet wavelengths to study the composition, density, and temperature of the chromosphere and inner corona. The instrument is a two meter slitless spectrograph, using a 3600 line per millimeter concave, normal incidence, two position grating, to focus the dispersed solar image onto a 35 x 248 mm film strip. Either of the two wavelength bands and three ranges of exposure times may be selected by the astronaut according to the solar activity existing at the time. The X U V spectrograph (SO82B) photographically records line spectra of small selected areas on and off the solar disc and across the limb and observes an X U V image of the full solar disc. The instrument consists of a one meter focal length mirror, a two position predisperser grating, and a 600 line per millimeter, two meter focal length main grating. A pointing reference system provides a visual display of the slit plate to the astronaut and holds the instrument automatically on the limb by automated control of the main mirror. A monitor displays an X U V image of the solar disc to the astronaut and the principal investigator to assist in identifying emitting regions of special interest. The X-ray spectrographic telescope (SO54) obtains X-ray emissions of flares and other sources of solar X-rays. It follows the evolution of both spatial image and spectrum throughout the lifetime of a flare, evolution of nonflaring active regions, and correlation of coronal X-ray structure with surface events. The instrument
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consists of a double-reflecting grazing incidence mirror which focuses the solar image through an objective grating used as a slitless spectrograph to a 70 mm reel camera. The camera can be operated in manual, automatic, or rapid automatic modes. A flare detector provides a coarse X-ray image of the Sun to the astronaut for monitoring of solar activity. The UV scanning polychromator/spectroheliometer (SO55A) obtains photometric ultraviolet observations of the solar atmosphere near or above active regions on the solar disc. The instrument consists of an off-axis parabolic mirror which images the solar disc on the 5 × 5 arc second entrance slit of the spectrometer. The mirror is capable of motion in two axes to provide either a 5 x 5 arc minute raster scan or a five arc second single line scan. A rotating grating mount in the spectrometer provides wavelength scan with a fixed mirror position or selected wavelength operation with the mirror in motion. The data are obtained through a photometric conversion system and are transmitted to the ground through telemetry. The X-ray telescope (SO56) photographs X-ray emitting solar regions during both active and quiescent periods with high spatial and temporal resolution. It monitors and records total solar X-ray flux measurements in specific spectral regions. The instrument also uses a grazing incidence mirror to focus the solar X-ray image through a six position filter wheel on a reel camera. Two proportional counters with a narrow band pass filter detect total X-ray flux emitted from the solar disc. Counter outputs are displayed to the astronauts and are also recorded. Two H-alpha telescopes observe the Sun in the light of the H-alpha spectral line at 6563 ,~. The intensity of the emission varies with the structural features on the solar disc and displays many fine scale features in the solar atmosphere. By tuning the filter slightly off center of the line, a clearer display of solar flares is obtained. The H-alpha telescopes are the principal aiming device the astronauts use to point the other instruments. They also give him a common reference with ground based observers during the mission. Associated with certain telescopic instruments are television cameras. The astronaut may select an image from five different instruments (two H-alpha telescopes, SO82A and SO82B, and SO52), and display this image on one of two television monitors on the control panel in the docking module. Capitalizing on the opportunity of being able to return photographic recordings from space, four of the principal investigators chose to use film to collect the data. The advantages of film are the higher spatial resolution which can be obtained and the total density of data recorded, as opposed to photoelectric devices which provide more accurate flux measurements. These films, however, are sensitive to the storage and operational environment of Skylab. Degradation of film sensitivity, latent images, and background fogging could result from high temperatures and radiation existing in the near-Earth space, primarily the South Atlantic anomaly region of the van Allen radiation belt. To protect from this environment, the film is stored in thick-walled aluminum film vaults in the docking module. During extravehicular activities crewmen retrieve the camera assemblies and film from the experiments through access doors at work stations. Exposed film is returned with each crew. To permit the spatial resolutions demanded by the experiments, high pointing
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accuracies are provided by the ATM. The pointing of the experiments package containing the telescopes is accomplished by the experiments pointing control system which acts as a vernier of the attitude control system. This system, capable of precise pointing to within 2.5 arc seconds, isolates the experiments from the ATM rack by a two axis (pitch and yaw) gimbai ring flex pivot. This flex pivot mounting allows virtually frictionless, ultra-low breaking torque performance of the gimbals about the pitch and yaw axes. The experiment package may be rotated __+120° about the axis pointed at the Sun and -+ 2° about its pitch and yaw axes. A high precision Sun sensor is used for this fine pointing reference. The two axis gimbal system was designed to provide a pointing stability of 0.63 arc seconds for 15 minutes in both pitch and yaw axes. Pointing uncertainty is limited only by the 2.5 arc second resolution of the video displays of the solar target. A tightly controlled thermal control system was developed to assure the stability of the instruments optical axes and focal plane. The experiments package thermal control system is completely separate from the structural rack. A mixture of methanol and water circulates through the walls of the experiments package since highly stable wall temperatures, 10-2°C, are required. The experiments are thermally isolated from the optical bench with low conductance mountings. Essentially, all of the internally generated heat is transferred from the experiments to the canister wall through radiation. To meet the thermal stability requirements of ---0.6°C for the instruments, the thermal design is biased to the cold side and the instruments are brought up to the exact operating temperature by use of electrical heater panels. The heater panels cover essentially all of the telescope surfaces and cycle within a temperature band that is controlled to a very accurate average effective temperature. An overall consideration in the Skylab/ATM Program was the effect of contamination on the optics of the precision telescopes due to contamination from venting of the workshop waste tank and other vents. Outgassing was also important to avoid corona problems associated with operation of the SO54 and SO56 experiments. Pressures less than 10 ~tort are required to assure that corona problems would not be experienced. To meet these requirements programmed minimal venting was incorporated, and an extensive materials control program was implemented. In addition, several sounding rockets are launched containing subscale models of ATM's ultraviolet spectrographs to calibrate any residual contamination or other degradation effects on ATM solar observations. The sounding instruments acquire data simultaneously with ATM instruments resulting in calibration data for post-mission analysis of ATM's solar data.
Performance and preliminary findings Observations by the Apollo Telescope Mount telescopes to date have provided the ATM scientists with data which equals or exceeds their expectations. The data have revealed features in the corona, the transition region, the chromosphere, and the photosphere never before observed or recorded with such accuracy.
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The experiments and supporting subsystems have demonstrated excellent performance, and none of the few problems experienced were of sufficient consequence to cause an impact on the success of the observational program. In many cases, the systems have surpassed their design requirements. One of the more notable examples of this was the ability of the ATM electrical power system to provide power for the entire Skylab beginning shortly after launch and lasting for three weeks. Also, the canister fluid thermal control system has exceeded performance specifications maintaining wall temperatures to l0 -+0.6°C and holding the optical bench temperatures essentially constant at 17.7°C, changing only 0.12°C with operational cycling of the experiments. Crew performance to date clearly demonstrates man's ability to function in the space environment as a scientist, technician, and operator of a complex orbiting solar observatory.
Experiment performance The solar experiments have performed exceptionally well during the Skylab mission. Preliminary results of the solar observations from the first manned period indicate high quality data. The second manned period has more than tripled the total quantity of data obtained because of the longer mission duration (59 days), an increase in solar activity, and the improvement of observing programs as the result of experience from the first mission. The time of ATM experiment manned operation now totals over 500 hours. In addition, over 600 hours have been spent in ground controlled observation. In total, during the first two manned missions, over 105,000 frames of film have been exposed of an available 115,000 frames, or about 92 percent. During the last manned mission another 53,000 frames will be available for solar observations.
Pointing system performance The ATM Attitude and Pointing Control System performance has exceeded the requirements and provided a stable base for ATM viewing. The observed pointing accuracies, or the ability to point to a selected target, during the manned and unmanned phases were better than 0.4 arc minute for the entire Skylab assembly. Stability, or the ability to hold Skylab to a target during the manned phase was better than 1.4 arc minutes and during the unmanned phase was 0.4 arc minute for 15 minutes. The Experiment Pointing Control System which provides the precise pointing for the solar telescopes has performed flawlessly. Pointing accuracies of better than one arc second were achieved. Stability, except for a few brief moments when the crew were exercising inside the workshop, also was better than the predicted 2.5 arc seconds for 15 minutes.
Crew performance During the EVA's, the astronauts performed the scheduled film replacement and retrieval tasks without problems and more rapidly than expected. Early in the mission, an unscheduled extravehicular activity was performed by the astronauts. They retrieved and replaced a failed SO82A camera; unpinned and
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permanently latched open the SO54 Sun-end aperture door which had malfunctioned; and r e m o v e d lint from the forward external occulting disc of the coronagraph. Activities of subsequent EVA's included removal and replacement of a failed SO52 camera and repair of a charger-battery-regulator-module. During the first mission, the crew demonstrated their ability to refine, in real time, the targets selected by the ground, thereby enhancing scientific data. On June 15, 1973, an M-4 class flare and on September 7, 1973, a very large X-I.5 class flare were observed, demonstrating the crew's ability to locate the flare, point the ATM canister quickly to the flare and begin taking data at a high rate to maximize the data return. The crew has also successfully performed other unplanned activities such as troubleshooting problems and making equipment adjustments or corrections on a real time basis. Without this capability valuable data would have been lost. Based upon the favorable performance during the early phases of Skylab, the ATM Principal Investigators have assigned greater freedom to the astronauts for selecting targets and observing programs during the mission.
Significant anomalies N o permanent failures in any of the ATM experiment equipment have been encountered to date. The most serious anomalies occurred with the SO52 and SO82A cameras which jammed during the first mission. During an unscheduled EVA, the astronauts exchanged the SO82A camera with another unit, thus eliminating a potentially serious loss of data. The SO52 camera was replaced later during a scheduled EVA. As a result of an SO54 experiment aperture door anomaly which required astronaut deactivation of the door in the open position, the automated instrument operation logic was interrupted so as to lose an indication to the crew of the completion of an exposure sequence. This necessitated cumbersome manual timing of exposures. The problem was remedied by the design and fabrication of an auxiliary timer which the second crew carried up and installed on the control console, virtually restoring normal operation. A serious concern with the rate gyro sensors which had reduced system redundancy was also resolved with replacement hardware which was carried aloft, installed, checked out, and activated by the second crew.
Preliminary scientific findings During the available observing time in the first manned mission, primary emphasis was placed on achieving broad coverage of all prominent solar features using the various pre-planned observing programs. The second manned mission then concentrated on more specific investigations based upon the previous data acquired. The total volume of data already obtained is enormous, challenging the principal investigator's capacity for detailed analysis. Consequently, the scientific findings to date reflect only a general assessment. Detailed analysis will require months and even years of work before firm conclusions and possible changes to the solar model can be established. The results of this work will be presented by the principal investigators in various journals and symposia as they become
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Table 4. Scientific findings. Rapid large scale changes in the corona Coronal voids Coronal holes Velocity changes on surface Rapid flare development Higher flare frequency Possible magnetic coupling of flares New UV structures
available. This summary (Table 4) is a very early and, consequently, qualitative assessment of the findings indicated by the data so far. The first of the reportable findings is that the changes observed in the corona occur much more rapidly than was previously thought. The changes are dramatic in scale and sometimes occur even in short spans of one or two hours. The changes occur in the structure of the overall magnetic field of the outer solar atmosphere. The SO52 coronagraph has given us an extremely detailed examination of the outer corona and its interactions of electrons and magnetic fields (Fig. 3).
Fig. 3. Photograph of the solar corona made on June 10, 1973 by the High Altitude Observatory white light coronagraph on Skylab (Courtesy of Dr. R. MacQueen, High Altitude Observatory).
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A new p h e n o m e n o n has been discovered which has been called a coronal void and is thought to be a region void of free electrons. The material that remains is that of the F-Corona or the Zodiacal light. This discovery lends itself to further study and understanding of the content of the background corona. An explosion in the Sun's corona occurred on August 10, 1973. Enormous loops of structure were expelled from the corona at velocities which exceeded a million kilometers per hour and to distances of three and one-half solar radii and possibly as far as six solar radii. This occurrence, called a coronal transient, was captured on film by the ATM instruments. The preliminary assessments indicate a restructuring of the magnetic field lines took place after the event. Possibly the explosion involved two filaments or prominences combining in an active region. An interesting question which remains to be answered is why there were no radio waves emitted when this enormous ejection from the corona occurred. The dark spots previously observed at the North and South poles (Fig. 4) have been referred to as corona holes. Observations in the X-ray and ultraviolet spectra have shown that they may be much more than coronal holes. They may extend down, well into the lower outer atmosphere and have temperatures as low as 50,000°C. This leads to the possibility that there are holes that extend to the bottom of the outer atmosphere and perhaps reflect the Sun's minimum temperature. The ever-present velocity changes of matter on the Sun's surface represents another significant finding. This change in velocity was seen while observing two limbs of the Sun simultaneously. A difference in velocity was noted from one side
Fig. 4. Photograph of a large flare made on June 15, 1973in two contrasting emissions by the Naval Research Laboratory XUV spectroheliograph on Skylab (Courtesy of Dr. R. Tousey. Nawd Research Laboratory).
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of the Sun to the other. T o further substantiate this, the A T M was pointed at active regions and o b s e r v e d actual shifts in large areas of the Sun in lines of the transition region and the corona. T h e stages of a flare for n u m e r o u s small flares and two very large flares including the initial indications in an active region, the first disturbance of the surface, the flare activity, the flare decay, and the bright point where the flare originated h a v e b e e n o b s e r v e d b y the A T M (Fig. 5).
Fig. 5. X-ray photograph of the solar corona obtained May 28, 1973 by the American Science and Engineering X-ray telescope on Skylab (Courtesy of Dr. G. Vaiana, Solar Physics Group, American Science and Engineering, Inc.). The bright points, which h a v e been observed, seem to be the remains of a network that is present all o v e r the Sun. T h e s e bright points m a y be small active regions, capable of flaring or they m a y represent a new kind of solar activity. T h e flare activity that has been o b s e r v e d has pointed out the benefits realized f r o m the orbiting laboratory. A flare was o b s e r v e d f r o m the ground and thought to be a very small flare, or sub-flare. O b s e r v e d in the X-ray region f r o m ATM, the s a m e flare was quite prominent and exhibited very rapid changes in its development. T h e findings show that a flare can grow in a v e r y short time, in the order of seconds. Also flare activity n o w appears to be m o r e frequent. T h e A T M principal investigators are currently analyzing the data available, with the newly discovered flare characteristics in mind, in an effort to establish relationships b e t w e e n large flare activity and the smaller flares that m a y be reactions to the more prominent
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flare activity. It is not known currently if this is the case, but the data available from the ATM missions is believed to hold the key to the mechanisms that generate flares. The SO55 experiment electronically recorded an image showing clear, detailed loops of ionized particles with temperatures of 2 million degrees Celsius streaming away from the Sun's surface more than 41,000 km in space (Fig. 6). This was one of the most exciting products of the ultraviolet spectroheliometer. Some details of structure, composition, and active processes revealed in the ultraviolet photographs are the first ever seen by man.
Fig. 6. Coronal loop prominence at the solar limb in the 417 ,~ emission line of Fe XV, recorded photoelectrically by the Harvard College Observatory scanning UV spectroheliometer on Skylab, June 1973 (Courtesy of Dr. E. Reeves, Harvard College Observatory}.
Summary It can be concluded from the successful ATM operations to date that more high quality solar data has been recorded in this one mission than all the previous solar research efforts added together. This was achieved with the long duration flight of high resolution instruments on a single platform with a wide range of spectral coverage and pointed simultaneously at specific targets. Additionally,
The Apollo Telescope Mount on Skylab
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man was integrated as a scientific observer, operator and repairman to assure m a x i m u m return of data. As in the early phases of any research, the A T M scientists at this m o m e n t may well h a v e found more questions than answers, but the volumes of data waiting to be analyzed and understood should soon unlock some of the previously well kept secrets of the Sun. The total scientific value of the first manned solar o b s e r v a t o r y in space will then be fully realized. Addendum
Since preparation of this paper, the Skylab mission has been successfully completed. The A T M instruments continued to provide high quality data throughout the remainder of the extended Skylab program. As a result of the extension to the Skylab mission, additional film was resupplied by the third crew and more time was allocated to solar observation. T h e resulting time of A T M instrument operation during manned phases totaled o v e r 1600 hours, and more than 700hours were spent in ground controlled instrument operation. More than 175,000 f r a m e s of film were e x p o s e d out of an available 182,842 frames. The A T M project has now m o v e d f r o m the operations phase into the data analysis phase. Each of the principal investigators will publish a preliminary scientific report for his respective e x p e r i m e n t about March 1975. H o w e v e r , it is e x p e c t e d that a complete analysis of the data returned f r o m A T M will take m a n y years to perform.