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Advances in the Application of the Image Orthicon to Astronomy
Advances in the Application of the Image Orthicon to Astronomy J. A. H Y N E K , Q. BAKOS, J. DUNLAP and W. POWERS Northwestern University, U.S.A .
INTRODUCTION Northwestern University operates two modest observatories in New Mexico, separated by some 40 miles, housing 12-in. and 24-in. reflecting telescopes, respectively. Both have been used, largely on an experimental basis, in the application of the image orthicon tube to astroncmica1 problems, as part of a larger planned program of the use of various photoelectric image devices in astronomy. The work was begun under the sponsorship of the George C. Marshall Space Flight Center of the National Aeronautics and Space Administration (NASA), and continued through the support of the Goddard Space Flight Center of NASA, by the National Science Foundation, and by the Washington office of NASA.
IMAGE ORTHICONPERFORMANCE The usefulness of the image orthicon and related devices in astronomy lies particularly in those applications in which speed is essential, especially “real-time” speed, but fine detail resolution is not necessary. It is more a special-applications device rather than a substitute for photography, and at times it can do what photography cannot do, or would be hard pressed to do well. An excellent example to illustrate this is the photography of the field of faint stars immediately surrounding the totally eclipsed Moon (Fig. 1). The Moon and stars have a relative apparent motion of about O.B”/sec. Thus, to obtain both the Moon and the stars in sharp focus requires a very short exposure, far too short to record faint stars. The image orthicon, however, even with a modest 12-in. telescope, shows 14th magnitude stars immediately adjacent to the sharp limb of the totally eclipsed Moon in a 0.25-sec exposure. While this particular example is of limited value in astronomy, it and other examples given later, illustrates a capability which may find application in fields quite foreign to astronomy. A related example, and one of broader application, is the recording of images of faint moving objects; as for instance, asteroids and comets. Here again the problem is the differential motion of objects. To “stop” 713
23-2
714
J. A. HYNEK,
a. BAKOS, J. DUNLAP
AND
w. POWERS
the motion of an asteroid or a comet photographically, invariably involves “guiding” so that the image of the object moving relative to the stars remains sharply defined, thus causing the stars to appear as short tracks. With the image orthicon attached t o the 12-in. telescope, it
FIG.1. Faint stars, 14th magnitude, immediately adjacent to the sharp limb of totally eclipsed Moon (0.25 see exposure).
was possible to make a time-lapse motion picture of the rapidly moving faint asteroid, Betulia, against the star background, without the stars and asteroid appearing trailed. This would not have been possible by photography with the same telescope, since frames were taken every 4 sec. Figure 2 shows two representative frames from the Betulia film. A search program for faint short-period variable stars in galactic star clusters was conducted for some two years, during which about 25,000 individual photographs of the monitor screen were taken. Several new variables were discovered, but perhaps the aspect of the program of most value was the development of computer techniques for the detection of star variability which take into account the variation
APPLICATION OF THE IMAGE ORTHICON TO ASTRONOMY
715
FIG.2. Two frames of time-lapse film of asteroid Betulia.
of the star field as a whole on successive frames and especially on frames taken on different nights. The light curve of an eclipsing variable in NGC 1893 is shown in Fig 3. Photometric accuracy of a few hundredths of a stellar magnitude (a few per cent) can be attained with the image orthicon, although the
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(h) Fra. 3. Light curve of eclipsing variable in NGC 1893. Time
tube has little to recommend it as a basic photometric device at present. For ultra-violet photometry from an unmanned spacecraft in which the use of a signal generating tube is mandatory, such photometric accuracy is acceptable for very many purposes. But otherwise it should be used as such only where rapid variations in light make ordinary photographic methods impracticable. The overall sensitivity of the image orthicon, as used with the 12-in. telescope, is illustrated by the photograph (Fig. 4)of the monitor screen
716
J . A. IIYNEX, G . BAKOS, J . DUNLAP AND W. POWERS
a8 it displayed the open cluster NGC 6709. The sensitivity uf the system is such that 16ml can be reached a t a read-out rate of 1/30 sec and 17m4
in 8-sec read-outs. With respect to astrometry, the situation is not as good. While positional accuracies of 2" or 3" can be generally attained (angular coverage of monitor display: 18' x 22'), severe sporadic excursions of
FIG. 4. Image orthicon picture of open cluster NGC 6709 using the 12-in.telescope.
accuracy of the order of a minute of arc frequently occur. For a fast moving, faint object which might escape direct photography, however, one must accept what accuracy one can get. The principal program for the 24-in. telescope, just begun, is that of constant lunar surveillance and monitoring of the lunar surface for possible color changes which might be indicative of volcanic activity. The lunar surface is systematically scanned through rotating color filters and a watch is kept for flickering spots which would signal such changes.
ELECTRONIC SYSTEM The electronic system for the lunar program was designed by Powers and constructed a t t,he Dearborn Observatory. It has several features of interest. The scanning system is different from traditional systems: the horizontal sweep waveform generator (Fig. 5 ) free-runs, and during
APPLICATION O F THE IMAGE ORTHICON TO ASTRONOMY
717
each retrace interval the vertical position is stepped a controllable amount; thus it is possible to control horizontal sweep speed and the number of scanning lines per frame independently. The scanning waveforms are generated by analog integrators: when the sweep potential reaches a fixed upper level, a flip-flop inverts and
Clamp input
Retrace pulse output
FIG.6. Horizontal sweep generator.
switches on the retrace current; when the output of the integrator reaches a fixed negative limit, the flip-flop and the input current to the integrator are restored to their original condition. The result is that a sweep waveform is generated with a linearity better than 0.25%, and an amplitude independent of speed. Special switching circuits make it possible to alternate read-out frames with exposures of the photocathode; while the photocathode potential is turned on, the sweep waveforms are clamped to zero, read-out taking place only after an exposure is complete. This procedure prevents the scanning fields from moving the image as it is being stored on the image orthicon target. The exposures may range between 1 msec and 100 sec. Horizontal speed is adjustable between 60 and 1000 psec per line, and the number of lines per frame may be set from 100 t o 9000. One sweep waveform generator drives a visual monitor, a photographic (precision) monitor, and the image orthicon. The deflexion currents required are generated by a wideband feedback circuit near each deflexion coil which makes the coil current, sampled by a small resistor, match the input sweep waveform potential a t all points during the scanning cycle. The deflexioii coil current is under control a t all times, even during fly-back, and there is no ringing or overshoot in either the horizontal or vertical drive circuits.
718
J . A. HYNEK, 0.BAKOS, J . DUNLAP AND W. POWERS
The pre-amplifier (Fig. 6) is of some special interest because of the use of capacitance cancellation as a means for improving the rise-time a t the output of the image orthicon. Dynodes 3, 4 and 6 are coupled together capacitively and are isolated from their potential supplies by resistors; positive feedback from the non-inverting first stage, with a gain of about 1.4, is connected to these dynodes through a capacitor. Since most of the output capacitance of an image orthicon is between the output anode and these three dynodes, this approach reduces the effective output capacitance from its normal value of 12 p F t o a smaller
2N 711 B
--Feedback
Dl
Dz
D3
D,
D,
0
V
To line driver
FIG.6. Pre-amplifier 1st stage.
value; 0.5 p P is achievable with careful adjustment, and the rise-time with a 40-kQ load resistor is actually about 0.2 p e c with an amount of feedback that is considerably less than critical. It seems to be the case that this method prevents the marked emphasis of high-frequency noise from the first stage that usually appears with ordinary high-peaking methods. The rise-time of the first pre-amplifier stage must be very fast: the present one shows a rise-time equal t o 22 nsec, which is the responsetime of the Tektronix oscilloscope used to measure it. The equivalent input noise current is about 20 nA. Blanking of retrace lines in the image orthicon is achieved by switching off the scanning beam during each horizontal and vertical retrace rather than repelling it from the target with a large negative target potential, as is customary. This method of blanking introduces the inconvenience of a white, rather than a black reference-level but two considerable advantages are obtained. First, the image section is not thrown out of focus during the blanking periods by a change in photocathode-to-target potential. Second, the clamping of the video signal to a reference potential, which must take place during retrace to
APPLICATION OF THE IMAGE ORTHICON TO ASTRONOMY
719
prevent shifts in d.c. signal level, is done during a time when no beam is present, and hence the clamp level is not disturbed by beam noise, and there is no line-to-line wandering of the d.c. picture-level. Clamping is done only at one stage in the main video amplifier; from there on to the monitor cathode-ray tube grid, coupling is direct. When an observer wishes to photograph the screen, the press of a button causes the recording camera shutter to open, and transfers control of sweep speed, lines per frame, beam current, and target potential to a second set of controls, which may be pre-set for maximum resolution. Maximum resolution requires sweep rates which are too slow for visual observation over long periods, and it was therefore decided to have a visual mode and a photographic mode of operation t o permit optimum resolution in both cases. After the “take” button is depressed, the camera remains open for the subsequent read-out frame, which is scanned at the rate determined by the second set of controls, and then normal read-out is automatically restored and the film is advanced by a motor. Pictures may be taken as fast as 2 frameslsec, or automatic time-lapse photography at rates from two pictures every second to one picture every five seconds may be instituted by throwing one switch. I n the lunar surveillance program, the virtue of the image orthicon is not its speed (although this does allow very short exposures and thus works to alleviate the effects of atmospheric turbulence, the bane of ground-based astronomy, and allows short exposures through narrow band filters) but its convenience: operation in “real time” so that suspected ephemeral changes can be verified a t once, a virtual impossibility with photography; the ability to alter contrast electronically; and most important, the ability to intercompare in rapid succession the lunar surface as viewed in different colors. To do the latter directly at the eye-piece of a telescope without the interposition of a television system monitor would prove fatiguing in the extreme and would surely lead to many “false alarms” which could not be checked by simultaneous photography, as is done in the present instance.
SUPERNOVA DETECTION An astronomical program of considerable significance is the search for supernovae by a systematic monitoring of distant galaxies; it is a program for which the image orthicon is well suited but one in which the use of ordinary photography would be extremely awkward. A supernova is a major occurrence in a galaxy, and becomes so bright that fine photographic resolution is not necessary for its detection; a supernova stands out like the proverbial sore thumb. At present, photographic searches for supernovae are tedious, and occasionally weeks pass before a supernova that was there when the plate was taken
720
J . A. HYNE K,
a. BAKOS,
J . DUNLAP AND
W. POWERS
is discovered. By then it is too late to get a spectrogram of the supernova near maximum, or to get its light curve. Thus the problem is the detection in “real time” of supernova explosions in galaxies whose distances are already reasonably well known. If supernovae in these galaxies can be detected near and especially before maximum, the true brightnesses of both type I and type I1 supernovae can be calibrated. Then the discovery of supernovae in very distant galaxies whose distances are only approximately known, will enable the extra-galactic distance scale t o be recalibrated which, coupled with observation of the red-shift, will give a better value of the Hubble parameter .which expresses the rate of the expansion of the universe. Statistics of supernovae are not well known, but it now appears probable that in the more open spiral galaxies a supernova occurs about once every 50 years. A supernova is visible as a bright object for 2 weeks or more. If one galaxy were to be observed once every 2 weeks for 50 years it could be expected on the average that one supernova would be discovered. Thus, if a galaxy is observed only once in a 2-week period, there appears to be one chance in 1300 of discovering a supernova. This means that if it is wished that 10 supernovae should be discovered each year, it would be necessary to observe 13,000 galaxies, i.e. 1000 during each dark of the Moon period. Allowing that one half of the nights will be cloudy, 1000 would have to be observed each week, or some 150 per night, which would be by no means impossible with a computer directed telescope. The feasibility of such a program has been established by trial runs with the 12-in. telescope. From photographs of representative galaxies it is apparent that with exposures as short as 4 sec, a supernova, if present, would be easily detectable.
CONCLUSION Our experience with the image orthicon has convinced us that this tube, and allied photoelectric image devices, can be an important tool in astronomy, and a valuable supplement and ally t o traditional methods of photographic astronomy.
DISCUSSION F. B. MARSHALL: For the movies of satellites, did you say that time-lapse photography was used? For your anticipated search for possible volcanic action, will this include viewing the illuminated side of the moon as well as the dark side? Then will seeing be a problem? Is short exposure time especially important? J. A. HYNEK: Time-lapse photography was used for the asteroid Betulia. Although time-lapse photography could certainly be used for satellites, our telescope drives were not adapted for satellite tracking. We shall concentrate
APPLJCATION OF THE IMAGE ORTHICON TO ASTRONOMY
72 1
on viewing the illuminated side of the moon. The Iikclihood of discovering anything of value on the unilluminated side of the moon is too slight to pursue a t this time with this equipment. R. J. DAVIS: How many television lines correspond to 1’? Do you plan any direct analysis of your video signals, or will you rely exclusively on an intermediate photographic step? I feel that one of the most important advantages of television over direct recording image tubes is its generation of signals that can be processed directly by electronic computer techniques. J . A. HYNEK: One of the virtues of the now television system is that the number of television lines oan be varied from 400 to 1600 scan lines. The Cassegrain focus uses a field of approximately 7‘ which, with the addition of a projection lens for close lunar work, gives us 2’ across the television monitor. We do not plan any direct analysis a t this time, but this approach is certainly a part of our long-range planning. J. D. McaEE: Can you get enough accuracy in your pictures of galaxies to enable you to detect supernovae by superposition of images in opposite phases? J. A. HYNEK: I believe tho answer to this is yes, although this is not the method we intend to use initially. Provided that pictures are not spaced too far apart, this method should be feasible. Much depends on the long-term stability of the image orthicon system being used.
INTRODUCTION Northwestern University operates two modest observatories in New Mexico, separated by some 40 miles, housing 12-in. and 24-in. reflecting telescopes, respectively. Both have been used, largely on an experimental basis, in the application of the image orthicon tube to astroncmica1 problems, as part of a larger planned program of the use of various photoelectric image devices in astronomy. The work was begun under the sponsorship of the George C. Marshall Space Flight Center of the National Aeronautics and Space Administration (NASA), and continued through the support of the Goddard Space Flight Center of NASA, by the National Science Foundation, and by the Washington office of NASA.
IMAGE ORTHICONPERFORMANCE The usefulness of the image orthicon and related devices in astronomy lies particularly in those applications in which speed is essential, especially “real-time” speed, but fine detail resolution is not necessary. It is more a special-applications device rather than a substitute for photography, and at times it can do what photography cannot do, or would be hard pressed to do well. An excellent example to illustrate this is the photography of the field of faint stars immediately surrounding the totally eclipsed Moon (Fig. 1). The Moon and stars have a relative apparent motion of about O.B”/sec. Thus, to obtain both the Moon and the stars in sharp focus requires a very short exposure, far too short to record faint stars. The image orthicon, however, even with a modest 12-in. telescope, shows 14th magnitude stars immediately adjacent to the sharp limb of the totally eclipsed Moon in a 0.25-sec exposure. While this particular example is of limited value in astronomy, it and other examples given later, illustrates a capability which may find application in fields quite foreign to astronomy. A related example, and one of broader application, is the recording of images of faint moving objects; as for instance, asteroids and comets. Here again the problem is the differential motion of objects. To “stop” 713
23-2
714
J. A. HYNEK,
a. BAKOS, J. DUNLAP
AND
w. POWERS
the motion of an asteroid or a comet photographically, invariably involves “guiding” so that the image of the object moving relative to the stars remains sharply defined, thus causing the stars to appear as short tracks. With the image orthicon attached t o the 12-in. telescope, it
FIG.1. Faint stars, 14th magnitude, immediately adjacent to the sharp limb of totally eclipsed Moon (0.25 see exposure).
was possible to make a time-lapse motion picture of the rapidly moving faint asteroid, Betulia, against the star background, without the stars and asteroid appearing trailed. This would not have been possible by photography with the same telescope, since frames were taken every 4 sec. Figure 2 shows two representative frames from the Betulia film. A search program for faint short-period variable stars in galactic star clusters was conducted for some two years, during which about 25,000 individual photographs of the monitor screen were taken. Several new variables were discovered, but perhaps the aspect of the program of most value was the development of computer techniques for the detection of star variability which take into account the variation
APPLICATION OF THE IMAGE ORTHICON TO ASTRONOMY
715
FIG.2. Two frames of time-lapse film of asteroid Betulia.
of the star field as a whole on successive frames and especially on frames taken on different nights. The light curve of an eclipsing variable in NGC 1893 is shown in Fig 3. Photometric accuracy of a few hundredths of a stellar magnitude (a few per cent) can be attained with the image orthicon, although the
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(h) Fra. 3. Light curve of eclipsing variable in NGC 1893. Time
tube has little to recommend it as a basic photometric device at present. For ultra-violet photometry from an unmanned spacecraft in which the use of a signal generating tube is mandatory, such photometric accuracy is acceptable for very many purposes. But otherwise it should be used as such only where rapid variations in light make ordinary photographic methods impracticable. The overall sensitivity of the image orthicon, as used with the 12-in. telescope, is illustrated by the photograph (Fig. 4)of the monitor screen
716
J . A. IIYNEX, G . BAKOS, J . DUNLAP AND W. POWERS
a8 it displayed the open cluster NGC 6709. The sensitivity uf the system is such that 16ml can be reached a t a read-out rate of 1/30 sec and 17m4
in 8-sec read-outs. With respect to astrometry, the situation is not as good. While positional accuracies of 2" or 3" can be generally attained (angular coverage of monitor display: 18' x 22'), severe sporadic excursions of
FIG. 4. Image orthicon picture of open cluster NGC 6709 using the 12-in.telescope.
accuracy of the order of a minute of arc frequently occur. For a fast moving, faint object which might escape direct photography, however, one must accept what accuracy one can get. The principal program for the 24-in. telescope, just begun, is that of constant lunar surveillance and monitoring of the lunar surface for possible color changes which might be indicative of volcanic activity. The lunar surface is systematically scanned through rotating color filters and a watch is kept for flickering spots which would signal such changes.
ELECTRONIC SYSTEM The electronic system for the lunar program was designed by Powers and constructed a t t,he Dearborn Observatory. It has several features of interest. The scanning system is different from traditional systems: the horizontal sweep waveform generator (Fig. 5 ) free-runs, and during
APPLICATION O F THE IMAGE ORTHICON TO ASTRONOMY
717
each retrace interval the vertical position is stepped a controllable amount; thus it is possible to control horizontal sweep speed and the number of scanning lines per frame independently. The scanning waveforms are generated by analog integrators: when the sweep potential reaches a fixed upper level, a flip-flop inverts and
Clamp input
Retrace pulse output
FIG.6. Horizontal sweep generator.
switches on the retrace current; when the output of the integrator reaches a fixed negative limit, the flip-flop and the input current to the integrator are restored to their original condition. The result is that a sweep waveform is generated with a linearity better than 0.25%, and an amplitude independent of speed. Special switching circuits make it possible to alternate read-out frames with exposures of the photocathode; while the photocathode potential is turned on, the sweep waveforms are clamped to zero, read-out taking place only after an exposure is complete. This procedure prevents the scanning fields from moving the image as it is being stored on the image orthicon target. The exposures may range between 1 msec and 100 sec. Horizontal speed is adjustable between 60 and 1000 psec per line, and the number of lines per frame may be set from 100 t o 9000. One sweep waveform generator drives a visual monitor, a photographic (precision) monitor, and the image orthicon. The deflexion currents required are generated by a wideband feedback circuit near each deflexion coil which makes the coil current, sampled by a small resistor, match the input sweep waveform potential a t all points during the scanning cycle. The deflexioii coil current is under control a t all times, even during fly-back, and there is no ringing or overshoot in either the horizontal or vertical drive circuits.
718
J . A. HYNEK, 0.BAKOS, J . DUNLAP AND W. POWERS
The pre-amplifier (Fig. 6) is of some special interest because of the use of capacitance cancellation as a means for improving the rise-time a t the output of the image orthicon. Dynodes 3, 4 and 6 are coupled together capacitively and are isolated from their potential supplies by resistors; positive feedback from the non-inverting first stage, with a gain of about 1.4, is connected to these dynodes through a capacitor. Since most of the output capacitance of an image orthicon is between the output anode and these three dynodes, this approach reduces the effective output capacitance from its normal value of 12 p F t o a smaller
2N 711 B
--Feedback
Dl
Dz
D3
D,
D,
0
V
To line driver
FIG.6. Pre-amplifier 1st stage.
value; 0.5 p P is achievable with careful adjustment, and the rise-time with a 40-kQ load resistor is actually about 0.2 p e c with an amount of feedback that is considerably less than critical. It seems to be the case that this method prevents the marked emphasis of high-frequency noise from the first stage that usually appears with ordinary high-peaking methods. The rise-time of the first pre-amplifier stage must be very fast: the present one shows a rise-time equal t o 22 nsec, which is the responsetime of the Tektronix oscilloscope used to measure it. The equivalent input noise current is about 20 nA. Blanking of retrace lines in the image orthicon is achieved by switching off the scanning beam during each horizontal and vertical retrace rather than repelling it from the target with a large negative target potential, as is customary. This method of blanking introduces the inconvenience of a white, rather than a black reference-level but two considerable advantages are obtained. First, the image section is not thrown out of focus during the blanking periods by a change in photocathode-to-target potential. Second, the clamping of the video signal to a reference potential, which must take place during retrace to
APPLICATION OF THE IMAGE ORTHICON TO ASTRONOMY
719
prevent shifts in d.c. signal level, is done during a time when no beam is present, and hence the clamp level is not disturbed by beam noise, and there is no line-to-line wandering of the d.c. picture-level. Clamping is done only at one stage in the main video amplifier; from there on to the monitor cathode-ray tube grid, coupling is direct. When an observer wishes to photograph the screen, the press of a button causes the recording camera shutter to open, and transfers control of sweep speed, lines per frame, beam current, and target potential to a second set of controls, which may be pre-set for maximum resolution. Maximum resolution requires sweep rates which are too slow for visual observation over long periods, and it was therefore decided to have a visual mode and a photographic mode of operation t o permit optimum resolution in both cases. After the “take” button is depressed, the camera remains open for the subsequent read-out frame, which is scanned at the rate determined by the second set of controls, and then normal read-out is automatically restored and the film is advanced by a motor. Pictures may be taken as fast as 2 frameslsec, or automatic time-lapse photography at rates from two pictures every second to one picture every five seconds may be instituted by throwing one switch. I n the lunar surveillance program, the virtue of the image orthicon is not its speed (although this does allow very short exposures and thus works to alleviate the effects of atmospheric turbulence, the bane of ground-based astronomy, and allows short exposures through narrow band filters) but its convenience: operation in “real time” so that suspected ephemeral changes can be verified a t once, a virtual impossibility with photography; the ability to alter contrast electronically; and most important, the ability to intercompare in rapid succession the lunar surface as viewed in different colors. To do the latter directly at the eye-piece of a telescope without the interposition of a television system monitor would prove fatiguing in the extreme and would surely lead to many “false alarms” which could not be checked by simultaneous photography, as is done in the present instance.
SUPERNOVA DETECTION An astronomical program of considerable significance is the search for supernovae by a systematic monitoring of distant galaxies; it is a program for which the image orthicon is well suited but one in which the use of ordinary photography would be extremely awkward. A supernova is a major occurrence in a galaxy, and becomes so bright that fine photographic resolution is not necessary for its detection; a supernova stands out like the proverbial sore thumb. At present, photographic searches for supernovae are tedious, and occasionally weeks pass before a supernova that was there when the plate was taken
720
J . A. HYNE K,
a. BAKOS,
J . DUNLAP AND
W. POWERS
is discovered. By then it is too late to get a spectrogram of the supernova near maximum, or to get its light curve. Thus the problem is the detection in “real time” of supernova explosions in galaxies whose distances are already reasonably well known. If supernovae in these galaxies can be detected near and especially before maximum, the true brightnesses of both type I and type I1 supernovae can be calibrated. Then the discovery of supernovae in very distant galaxies whose distances are only approximately known, will enable the extra-galactic distance scale t o be recalibrated which, coupled with observation of the red-shift, will give a better value of the Hubble parameter .which expresses the rate of the expansion of the universe. Statistics of supernovae are not well known, but it now appears probable that in the more open spiral galaxies a supernova occurs about once every 50 years. A supernova is visible as a bright object for 2 weeks or more. If one galaxy were to be observed once every 2 weeks for 50 years it could be expected on the average that one supernova would be discovered. Thus, if a galaxy is observed only once in a 2-week period, there appears to be one chance in 1300 of discovering a supernova. This means that if it is wished that 10 supernovae should be discovered each year, it would be necessary to observe 13,000 galaxies, i.e. 1000 during each dark of the Moon period. Allowing that one half of the nights will be cloudy, 1000 would have to be observed each week, or some 150 per night, which would be by no means impossible with a computer directed telescope. The feasibility of such a program has been established by trial runs with the 12-in. telescope. From photographs of representative galaxies it is apparent that with exposures as short as 4 sec, a supernova, if present, would be easily detectable.
CONCLUSION Our experience with the image orthicon has convinced us that this tube, and allied photoelectric image devices, can be an important tool in astronomy, and a valuable supplement and ally t o traditional methods of photographic astronomy.
DISCUSSION F. B. MARSHALL: For the movies of satellites, did you say that time-lapse photography was used? For your anticipated search for possible volcanic action, will this include viewing the illuminated side of the moon as well as the dark side? Then will seeing be a problem? Is short exposure time especially important? J. A. HYNEK: Time-lapse photography was used for the asteroid Betulia. Although time-lapse photography could certainly be used for satellites, our telescope drives were not adapted for satellite tracking. We shall concentrate
APPLJCATION OF THE IMAGE ORTHICON TO ASTRONOMY
72 1
on viewing the illuminated side of the moon. The Iikclihood of discovering anything of value on the unilluminated side of the moon is too slight to pursue a t this time with this equipment. R. J. DAVIS: How many television lines correspond to 1’? Do you plan any direct analysis of your video signals, or will you rely exclusively on an intermediate photographic step? I feel that one of the most important advantages of television over direct recording image tubes is its generation of signals that can be processed directly by electronic computer techniques. J . A. HYNEK: One of the virtues of the now television system is that the number of television lines oan be varied from 400 to 1600 scan lines. The Cassegrain focus uses a field of approximately 7‘ which, with the addition of a projection lens for close lunar work, gives us 2’ across the television monitor. We do not plan any direct analysis a t this time, but this approach is certainly a part of our long-range planning. J. D. McaEE: Can you get enough accuracy in your pictures of galaxies to enable you to detect supernovae by superposition of images in opposite phases? J. A. HYNEK: I believe tho answer to this is yes, although this is not the method we intend to use initially. Provided that pictures are not spaced too far apart, this method should be feasible. Much depends on the long-term stability of the image orthicon system being used.