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KISS ELL, Kenneth Eugene, 1928- APPLICATION OF AN INFRARED IMAGE TUBE TO ASTRONOMICAL SPECTROSCOPY. The Ohio State University, Ph«D., 1969 Astronomy
University Microfilms, Inc., Ann Arbor, Michigan APPLICATION OF AN INFRARED IMAGE TUBE
TO ASTRONOMICAL SPECTROSCOPY .
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
Kenneth Eugene Kissell, B. Sc., M. Sc.
$ i f i f S'f $ sjc
The Ohio State University 1968
Approved By
L lluxi A d v ise r ACKNOWLEDGMENTS
The acknowledgments which must be made by this investigator are so numerous that many individuals will doubtless go unthanked.
The principle acknowledgment must go to Dr. W. Kent Ford, Jr. of the Department of Terrestrial Magnetism, of the Carnegie Institution of Washington, who from the very outset of this work has been totally selfless in providing apparatus, counsel and encouragement, and to the Committee itself. Second only to Dr. Ford is Mrs. Edna Kluesner, who for the last weeks has tirelessly served as an extension of myself in translating my thought and scribbles into a written document. Mrs.
Rhonda Duvall of the Aerospace Research Laboratories has been of invaluable assistance in reduction of the several hundred spectra to equivalent widths and many other things.
Thanks must go to the staff at Lowell Observatory for their excellent support and hospitality during the observing runs at the
Perkins reflector, particularly to Dr. John Hall and Mr. Henry
G ic la s.
In the conduct of the eclipse observations the author is indebted to the staff at the Long Beach Flight Test Division of McDonald-
Douglas Aircraft Corp., and to Dr. W arren Arnquist and to the late
Dr. Wolfgang Klemperer for enabling the observations to be conducted in 1963 and to Dr. Michel Bader, and Mr. Louis Haughney, Airborne
Sciences Office of the NASA Ames Research Center, for their hospita lity in allowing the investigator to conduct observations from the ii Galileo research aircraft in the South Pacific eclipse of 1965.
Apparatus used in this investigation has been obtained from several institutions. Much of it has been unique in character and provided to the author for extended periods of time. These include the U. S. Army Engineer Research and Development Laboratories, who loaned the thermoelectric cooler; the U. S. Naval Aeronautical
Photographic Engineering Laboratory, who loaned the infrared objective for the spectrographic camera; C. M. Sgt. E. T. Tyson of the USAF Avionics Laboratory, Research & Technology Division, at
Cloudcroft, New Mexico provided the loan of the power supply and regulator. The USAF Avionics Laboratory, especially Mr. Ronald
Ringo of the Navigation and Guidance Applications Branch, also provided valuable counseling on the gyrostablized platform and arranged the transfer of the high sensitivity gyroscopes used on both the ARL and the Douglas Aircraft Co. heliostats.
The author wishes to acknowledge the great assistance and expeditious cooperation of Mr. Darrell Frank who assisted in the mechanical details, the testing, and operation of the eclipse apparatus in 1963. M essrs. Clifford Grube, and Sylvester Ross fabricated the spectrographs. M essrs. Grewell and Mauer of the ASD Technical
Photographic Division devised the framing camera modifications and programmer. Thanks also go to Mr. Lee Wasserman and Dynamic
Devices, Inc., who designed and fabricated the spectrograph tie-down system for the Convair 990 aircraft and cooperated in every way to accommodate design changes during fabrication.
The quite encouragement of Profs. Slettebak, Keenan, and Czyzak of the Perkins Observatory to translate the results into the document is gratefully appreciated. Without the positive and permissive encouragement of my boss, Col. Paul G. Atkinson, Jr. of the Aerospace Research Laboratories the final stages of this work would not have been completed.
My final acknowledgment is to my wife, Theodora, who has been exceedingly patient in all of the m atters in ways which cannot possibly be set forth. VITA
June 28, 1928 Born - Columbiana, Ohio
1949 B. Sc. , cum laude, The Ohio State University, Columbus, Ohio
1949-1951 Research Associate, Rocket Laboratory, The Ohio State University, Columbus, Ohio
1951-1960 Research Physicist, Aerospace Research Laboratories, W right-Patterson Air Force Base, Ohio
1958 M. Sc. , The Ohio State University, Columbus, Ohio
1961-1968 Supervisory Physicist, Aerospace Research Laboratories, W right-Patters on Air Force Base, Ohio
PUBLICATIONS
"M easurement of Detonation Wave Velocities, " co-author L.. E. Bollinger, ISA Journal (May 1957). pp. 2-5,
"Thermodynamic Performance Study of Possible Working Fluids for Non-Chemical Rockets, " Planetary and Space Science, Vol. 4, p. 111-132, January 1961.
"An Image-Converter Objective-Grating Spectrograph for the Airborne Observation of the Infrared Flash Spectrum, " ISA Transactions, Vol. 3, 220-228, July 1964.
"Requirements for a 4-Axis Tracking Mount for Space Vehicle Photo metry, " Space Vehicle Photometry, " Space Research V, North- Holland Publishing Co. 913 (1965).
"Diagnosis of Spacecraft Surface Properties and Dynamical Motions by Optical Photometry, " Space Research IX, North-Holland Publishing Co. (In Press).
FIELDo OF STUDY
Major Field: Astronomy
Studies in Stellar Interiors: Profs. Arne Slettebak and Phillip K eenan
v VITA (CONTINUED)
Studies in Stellar Atmospheres: Prof. Walter Mitchell
Studies in Radio Astronomy: Professors John D. Kraus and Hsein C. Ko
Studies in Spectroscopy: Professors Wave Shaffer and K. Narahori Rao TABLE OF CONTENTS
P ag e ACKNOWLEDGMENTS. ii
VITA v
LIST OF TABLES . . . ix
LIST OF ILLUSTRATIONS xi
C hapter
I. INTRODUCTION .
Historical Background
II. ANALYSIS ...... 13
Relative Efficiency of the Single-Stage Image Tube X 7500-8000 jS R egion Incident Energy on the Deteictor Direct Photographic Recording Observations with I-N Plates Image Tube Recording Extension to Longer Wavelengths and to Multi stage Tubes Examination of the Linearity Assumptions Photocathode Electron Optics The Phosphor Screen Phosphor Response Time
in. LABORATORY EXPERIMENT 54
The Apparatus Calibration Methods Temperature Effects on th^ Photocathode Phosphor Measurements Contrast Loss Simulation of Absorption Ljnes Directivity of Light Emitted by the Phosphor Absorption and Scattering from P -ll Phosphor in Refractive Systems Relaxation Effects in the P -11 Phosphor Interpretation Summary of Laboratory Results
v ii TABLE OF CONTENTS (CONTINUED)
C h ap ter
IV. COMPARISON OF IMAGE TUBE MEASUREMENTS WITH DIRECT PLATES ......
Supergiant Measurements Discussion Luminosity Class la Luminosity Class lb Luminosity Classes Below lb
V. INTENSITY VARIATIONS OF THE OI ABSORPTION FEATURE AT X 7774 IN CEPHEID VARIABLES .
Reduction of Visual Estimates Application to the Calibration of Cepheids A Suggestion for Further Work
VI. APPLICATIONS TO SOLAR ECLIPSE SPECTROSCOPY
Spectrograph Design Aircraft Installation and Tests Eclipse Operation R e su lts Solar Eclipse of 30 May 1965 Description of Apparatus Optical Calibration System Chromospheric Spectra
VII. OBSERVATIONS IN THE 1-MICRON REGION
Atomic Absorptions
BIBLIOGRAPHY
v iii LIST OF TABLES
T able P ag e
1. Predicted Exposure Times to Record the Standard Stars of Code's List, with Various Types of Detectors. Exposure Criteria Used is to Achieve Photographic Density of 0. 6 Above Background Fog ...... 19
2. Summary of Measurements of Cathodoluminescent Efficiency of P-11 Phosphors (Eberhardt, 1961) . . . 28
3. Fraction of Total Flux Emitted by a Lambertian Source which will be Coupled to an Image by Unity Magnification Reimaging Optics of Various Relative A pertures ...... 31
4. Theoretical Smearing of Images Resulting from Earth Ambient Field on Unshielded Image Tubes ...... 45
5. Luminous Flux Transferred to an Image Formed by Perfect Relay Optics as Zones are Added to the Aperture to Increase the Angle Substanded of the Optics by 5-Degree Increments as. seen from the Phosphor Screen (see sketch on p.29 ) ...... 87
6. Comparison of Different Reimaging Optics Tested . . . 89
7. X 7774 Feature Equivalent Width la Supergiants .... 103
8. X 7774 Equivalent Widths - Class lb Super giants. . . . 105
9. X 7774 Equivalent Width - Luminosity Classes II-V. . 108
10. Spectral Classifications Assigned to Stars Unclassified on the MK S y stem ...... 118
11. Bright Cepheid V ariables ...... 121
12. Summary of Spectra Taken of Cepheid Variables. . . . 126
13. Adopted Epochs and Periods for Visual Maxima. . . . 128
14. Variation of Spectral Type and Predicted X 7774 Equivalent Width for 6 Cephei ...... 130
ix LIST OF TABLES (CONTINUED)
T able P ag e
15. Summary of Chromospheric Spectra Obtained by Image Tube Spectroscopy in the Total Eclipses of 20 Ju ly 1963 and 30 May 1965 ...... 164
16. Molecular Features Observed in R Leonis Near Minimum in 1964 and 1965 ...... 170
x LIST OF ILLUSTRATIONS
Figure Page
1. Monochromatic flux distribution from £ Orionis, 15 p Geminorum, and a O rionis ......
2. Absolute sensitivity of Kodak spectroscopic emulsion, Type I-N. (Ref. Kodak Data Book P-9, 1962). . . . 18
3. Exposure times calculated for Code's standard stars for ideal and real I-N emulsion (with reciprocity failure) compared with actual exposure times for program stars, in order to reach D = 0. 6 above background fog ...... 20
4. Reciprocity failure of fresh I-N and hypersensitized I-Z emulsion. Data from Kodak Research Laboratories ...... 22
5. Schematic of Image Tube S ystem ...... 25
6. Sensitivity of I-N, I-M and I-Z emulsions and relative response of the S-l photocathode. Data taken from Reference (3 ) ...... 34
7. Exposure times in order to reach D = 0. 6 above back ground fog calculated from Code's standard stars for an ideal and real II a-O emulsion, used with the ITT FW-167 image tube, compared with actual ex p o su re tim es fo r p ro g ra m s ta r s ...... 36
8. Exposure times to reach D = 0. 6 above background fog calculated from Code's standard stars for hyper sensitized I-Z emulsion and for a single-stage converter camera at X 10, 049A and at 10, 830.&. . 38
9. Variation of relative photon gain with temperature for constant light inputs (approximately 10® photons cm" 2 sec ) of differing wavelength. The change in gain is apparently independent of illuminating wavelength over a wide range ...... 48
10. Relative spectral distribution of the intensifier output phosphor for a constant light input at several temperatures ...... 48
11. Relative gain of P -ll phosphor vs temperature normalized to + 10° C to emphasize independence of color of the heterochromatic flux to the emul sio n ...... 49 xi LIST OF ILLUSTRATIONS (CONTINUED)
Figure Page
12. Failure of luminous efficiency of P -ll phosphor screens at low excitation currents. Upper scale corresponds to the excitation of a resolu tion element at \7774, for A2Ia supergiaiits, in the camera system used in this study...... 51
13. Persistence characteristic of P -ll Phosphor. (Courtesy of English Electric Valve Company L t d .) ...... 53
14. Exploded image-tube camera assembly. Right to left: ITT image tube with insulators and voltage divider, dry ice cold box, Aero-Ektar lens. .... 56
15. Exploded image-tube camera assembly showing cylindrical magnet and relay optics...... 57
16. Image-tube camera assembly and power supply as attached to the Perkins telescope, Flagstaff, A riz o n a ...... 58
17. Enhancement of S-l photocathode sensitivity as a function of tem perature and wavelength for the ITT and RCA image tubes ...... 67
18. Comparison of cooling-enhanced sensitivity with nominal room-temperature sensitivity...... 68
19. Variation of S-l photocathode sensitivity with temperature in ITT FW-167 image converter . . . 69
20. Optical system for evaluating scattered light and profiles of simulated emission lin e s ...... 72
21. Comparison of simulated 100-micron emission lines. scanned as a (1) direct aerial image (2) phosphor image, and (3) by the two relay optics systems used in this study...... *..... 75
22. Effect of increased extent of continuum on edge profile of an emission a re a ...... 76
x ii LIST OF ILLUSTRATIONS (CONTINUED)
Figure Page
23. Comparison of simulated absorption lines with the density of bulk filter material ...... 79
24. Geometry for refractive effects on the location of apparent phosphor source and the trapping of light in an image-tube faceplate for two extreme cases of optical contact ...... 82
25. Distribution of luminous flux measured for the ITT FW-167 image tube compared with that computed for Lambertian scattering without and with internal reflection lo ss e s ...... 85
26. Phosphor output for photocathode exposure times of 1/ 125, 1/ 60, 1/ 30, 1/ 15 and 1/ 8 second. Sweep sp eed 20 m s e c / c m ...... 93
27. Phosphor output when exposed to excitation of a stroboscopic lamp compared to the lamp discharge itself and the response of the monitoring photo multiplier exposed directly to the discharge la m p ...... '...... 93
28. Variation of equivalent width of \7774 with spectral type, luminosity class la...... 100
29. Variation of equivalent width of X7774 with spectral type, luminosity class lb...... 101
30. Variation of equivalent width \7774 with spectral type, luminosity classes II-V...... 103
31. Comparison of image tube and I-N plates on common program sta rs ...... 109
32. Comparison of the measurements of the present pro gram with those of Keenan and Hynek (1950) .... I l l
33. Probably error vs equivalent width by image tube plates and by direct I-N plates ...... 112
34. Mean variation of X7774 feature with spectral type in luminosity class la...... 114
x iii LIST OF ILLUSTRATIONS (CONTINUED)
Figure Page
35. Mean variation of X7774 feature with spectral type in luminosity class lb ...... 116
36. Variation of the X7774 OI feature with light phase in December 1962 ...... 120
37. Visual estimates of 6 Cephei as a function of the phase for the epoch and period adopted in Table 13. Upper curve is a smoothed average of the data using a Gaussian filter technique, and is displace upward 0. 2 magnitudes for c l a r i t y...... 124
38. The intrinsic-color versus temperature relation for 6 Cephei assuming interstellar reddening in B-V of 0. 11, as deduced by O k e...... 125
39. Variation of OI feature X7774 with light phase in 6 Cephei. Dashed curves are predictions based on absorption in non-variable super g ia n ts ...... 131
40. Sensitivity of the T index of Kraft as a function of spectral type as compared with OI X7774 feature for luminosity class lb, using the results of Keenan and Hynek (1950) and the present investigation ...... 134
41. Profiles of filters considered for OI scanning ...... 137
42. Modulation of narrow-band signal from supergiant w ith = 1. 5 R with filters of various profiles and passbands ...... 138
43. Modulation by Thin Films, Inc. filters of two possible profiles by scanning of OI feature in supergiants of 7774 = 0.1 to 2. 5 ...... 140
44. Dependence of chrornospheric arc length on height of emitting atom in the chrom osphere ...... 145
45. Experimental focal curve of Kodak 178-mm f/ 2. 5 Aero-Ektar and compromise focal plate attained by lens tilt...... 148
xiv /'
LIST OF ILLUSTRATIONS (CONTINUED)
Figure Page
46. Schematic plan of image-tube spectrograph showing optical offset angles ...... 149
47. Eclipse spectrograph assem bly ...... 149
48. Neon-Argon calibration spectrum. Selected wave length s are given in Angstrom s ...... 151
49. Image-tube spectrograph and high-voltage power supply as installed in APEQS aircraft at recessed window (courtesy of the National Geographical Magazine - National Geographic S o c ie ty...... ) 151
50. Flash spectrum at second contact, 1/ 25 sec exposure; (b) 1-sec exposure; (c) flash spectrum just after third contact, 1/ 5 sec exposure. Wavelengths are given in angstroms. Originating atoms noted for strongest lines. Asterisks denote lines of the Paschen series of hydrogen...... 155
51. 1965 Solar eclipse spectrograph as installed on the NASA Convair 990 research aircraft. (Courtesy - National Aeronautics and Space Administration) . . 158
52. Chromosphere flash spectrum at second contact, solar eclipse of 30 May 1965...... 163
53. Spectra of selected bright stars in the region A 9000 to All, 000 A ...... 168
54. Spectra of early-type stars in the region A9600 to A 10, 800 A taken with improved image tube c a m e r a ...... 172
55. Spectra of some late-type stars in the region A9600 to A10, 800. Band identifications in R Leonis are discussed in the te x t ...... 173
xv CHAPTER I
INTRODUCTION
During an association with two different groups of research and development engineers at laboratories at Wright-Patterson Air Force Base, the author became interested in the use of electronic imaging devices for the recording of faint images such as are encountered in astronomical spectroscopy. While the work at W-PAFB dealt with sequential scanning electro-optical systems (image orthicons, vidicons, intensifier orthicons, image isocons, etc.), some of the essential features of such signal-generating electronic imaging systems are embodied in the much simplier image tube or image converter tube. Most such systems employ an area photocathode, an electron-optical imaging section, and a target plate which is conjugate to the photocathode through the electron-optical imaging section. In the scope of this paper, these three features define the class of photoelectric imaging detectors. In the image orthicon this target is a secondary-emitting storage target which builds up an image by a net positive charge which is sensed by the partial absorption of a sequential-scanning
electron beam. In the image isocon the positive charge on this
target is sensed by the production and detection of additional
secondary electrons by the sequential-scanning electron beam. In the intensifier orthicon a stage of electron or optical amplification
is placed before the storage target plate to increase the number of
1 incident electrons before the secondary emission storage step of the process. This pre-intensification of the photoelectric image before storage could be accomplished by any of several techniques which will be discussed below in their application to the multi-stage image tube.
In the case of the image tube the target plate is a phosphor which absorbs the accelerated photoelectrons at one of the conjugate planes of the electron-optical system. The same type of area photocathode and an electron-optical imaging section is employed. In the case of another photoelectric imaging system, the Lallemand camera, the target plate is a nuclear-emulsion photographic plate installed directly in the high vacuum with suitable precautions to prevent poisoning of the photocathode by outgassing of the emulsion. Another possibility to prevent the poisoning of the photocathode is the interposition of a very thin but non-permeable layer of some electron translucent material (Leonard window), such as foil, between the electron imaging section and the electronographic emulsion.
It should be noted that the class of photoelectric imaging systems does not include the Vidicon, Ebicon, Uvicon, etc. , type of photoelectronic image detector. Basically, these devices utilize the photoconductive effect for the production of a charge image on the storage target. The photoelectric effect is not utilized. The application of these devices to quantitative measurement of faint sources is also a topic of active pursuit, particularly for spaceborn applications where simplicity and ruggedness are highly desirable. Such photoconductive imaging devices are basically more simple than the image orthicon, more rugged, much less sensitive to shock and vibration, and are also amenable to the use of photoconductive targets with sensitivities outside the wavelength range of photo cathodes. They also provide a tim e- sequential signal in electrical form for convenient transmission. The sensitivity of the simplest variety of vidicons, however, is lower by a factor of 10 to 1000 from that of the image orthicon. More recently the use of intensifier
sections ahead of the photoconductive target has given promise of bridging this gap in sensitivity. These intensifier sections do use a photocathode as the initial mechanism for converting the optical
image into an electronic one. The intensifier vidicons then fall into
the class of devices discussed in this paper.
From the brief descriptions above, it is apparent that one of
the simplest devices embodying the photoelectric imaging principle
is the image tube, where the initial photoelectron image is converted
immediately back to photons at the phosphor target, photons which may be in a different spectral region than the original incident
photons. A more elaborate, but still basically simple, system is the multi-stage image tube where the initial phosphor image is
coupled immediately and directly into a second photocathode for
further acceleration of new photoelectrohs into a second phosphor,
and so on. The basic image tube system involves no complex
electronic circuitry, is relatively insensitive to shock and vibration,
has no moving parts, and has no fundamental requirements for vacuum pumps or cryogenic cooling, being self-contained within its own vacuum environment. Because of these factors, a given image tube can be expected to have a long utilization life (barring accident) and to be stable in the properties of the photocathode, e. g. sensitivity, thermionic emission, etc. , in the properties of the phosphor, and in the internal characteristics, e. g. spurious ionic emissions, residual gas, leakage conductivity of the glass walls,' etc. For these and other reasons which will be developed, the image-tube type of photoelectric imaging device is both appealing as a practical tool for routine use in the observatory, and as a subject of study in the evaluation of quantitative usage of photoelectric imaging devices for measurement of wavelength and of line intensity in astronomical spectroscopy. The use of an image tube as an intermediate step in the record ing of astronomical spectroscopy has several attractive features to offset the additional complications of its operation if it can be shown that no serious errors are introduced in its quantitative usage. These attractions include the high detection efficiencies inherent in the photoelectric process, which can yield from one photoelectron for each four incident photons near the blue peak of an S- 20 photo cathode (Engstrom, I960). In an S- 1 photocathode one can obtain one photoelectron for each 1000 incident photons at 1 micron wavelength. These detection efficiencies must be compared with one developable grain per 200-600 photons incident at X 4500 R on
103 a-O emulsion and one with developable grain per incident 400, 000 photons of 1 micron wavelength with I-Z emulsion. These values for film sensitivity follow from the direct measurements of Reynolds 5
(1966) and the values of absolute sensitivity for 103 a-O and I-Z emulsions (E. Kodak Co. , 1962) in terms of exposure required to 2 achieve a uniform density of D = 0. 6 (exposure in ergs/cm ) with corrections for the lower energy of the 1 micron photons.
At the phosphor the photo electrons produce a quasi- monochromatic image (blue in the P -ll phosphor) for all wave lengths in the original spectrum. This simplifies the spectro photometry in the final photographic image by allowing use of a single effective H & D curve. The generally increased blackening rate in the photographic emulsion reduces the exposure time required to reach a specific image density, thus reducing the reciprocity failure; when added to the use of a Ma" -type blue emulsions, this allows an additional reduction in exposure times, especially in the infrared. It is technically much easier to maintain the photocathode of the image tube at a low temperature to obtain reduced thermionic emission so that the shorter exposures reduce the reciprocity failure on the room-temperature plates than to cool the recording emulsion itself at the focus of a spectrograph in order to minimize the reciprocity failure in the manner pioneered by Hoag (1964).
If, then, no serious degradation is introduced by interposing the image tube between the optical image of the spectrograph camera
and the recording emulsion, e. g. systematic errors in intensity ..Sjcale, losses of contrast or resolution, or introduction of spurious images or unaccountable geometrical distortions, one can achieve a decrease in exposure time of a factor of many tens, at least in some
regions of one spectrum. This would greatly increase the power of 6 telescopes of moderate size so that many problems now undertaken only with instruments of the 2-5 meter category would be opened to instruments one-quarter that size and whole new problems attacked with the major instruments (F. B. Wood, 1958).
Historical Background For a detailed accounting of the historical development of image tubes and their initial use in astronomy, the reader should refer to excellent summaries and bibliographies in the Transactions of the I. A.U. for 1955, 1958, 1961 and 1964, Sub-commission 9a,
Sous Commission des Convertisseurs D'Images, and to the annual reports of the Committee on Image Tubes for Telescopes established in 1953 by the Carnegie Institution of Washington to stimulate the development of photoelectric imaging detectors which could enhance the productivity of existing telescopes. An excellent review of elec tronic image intensification, as of November 1967, has been made by
W. K. Ford, Jr., (1968). A digest of some early work in the USSR is contained in a survey document published by the Aerospace Infor mation Division of the Library of Congress (1962).
It will suffice here to state that developments leading to high quality image tube, competitive with photography, and the motiva tion to adapt them to astronomy, lay in two parallel streams; first, the technological development of photocathodes of good sensitivity and good uniformity in the visible and near infrared ( < 1. 2 microns) and second, the theoretical developments leading to a realization of the true limitations of detection and measurement of faint, low- contrast images. Along with the development of photocathodes was 7 the development of the technical arts of encapsulating these photo cathodes into suitable vacuum tubes and the evolution of electron optics capable of yielding good definition over reasonable areas of a phosphor conjugate to the photocathode. The image tube had been introduced into this country in its most primative form in the mid-1930fs (Zworykin and Morton, 1936); (Morton and Ramberg, 1936). Military needs for night viewing stimulated their development into such practical devices as an image converter, using an S-l photocathode and a green phosphor, so as to allow battlefield use of infrared searchlights. At the same time similar developments were in progress in Germany, also motivated by military applications.
In the same manner that the photomultiplier became available to the astronomical community in 1945, so did the early image converters.
Meanwhile, Lallemand (1936, 1937) had begun his pioneering work on electronography. During the same period developments were occuring in the application of information theory ot the recording of images, especially in the quantitative evaluation of images. The early and most far- reaching analysis was that of Rose (1946, 1948) who pointed out the ultimate quantum limitations in low light level detection, introduced the concepts of equivalent quantum efficiency, and outlined the method to analyze the relative performance of various detectors near their threshold of sensitivity, especially the relationships in performance of eye, the image orthicon, and the photographic plate under both "white light" and monochromatic conditions. Thus by 1950, the in gredients were available for the development and application of the photoelectronic detector for the study of faint sources.
The early designs of image tubes employed electrostatic focussing which restricted both the resolution and the useful field of the im age. When the present work was begun in 1962, a tube design utilizing magnetic focussing was introduced as the result of the efforts of the
Carnegie Committee. This tube was in three basic forms, (1) a mica- window contact form being used by L. W. Fredrick and T. E. Houck at Lowell Observatory, (2) a conventional single-stage form using glass faceplates and hence requiring re-imaging optics to copy the phosphor image and (3) a cascaded tube which utilized a thin mica internal membrane sandwiched between a phosphor and a second photocathode to increase the internal gain of the converter. This cascaded tube was also made with glass faceplates; its additional gain was thought to offset the lower efficiency of the coupling optics rela tive to the mica-window contact tube, although its resolution was known to suffer from the added intensifier stage. All of these mag netically-focussed tubes were being supplied by the ITT Laboratories, under direct contract to the Carnegie Committee. Of the three designs the single-stage tubes were proving successful. At this same time, RCA, Lancaster was working on a similar two-stage design based on their experience with two-stage electrostatic tubes which had been quite successful in observatory tests by Fredrick, Houck, and W. K.
Ford, Jr. To cite some specific experience with these devices, Houck
(I960) had undertaken direct photography of star fields with the S-l 9 mica-window tube for comparison with direct photography at 1 micron and had found speed gains in excess of 50 over hypersensitive infrared
emulsions. Fredrick (1961) had utilized this type of tube for spectra
of long period variables, carbon stars, planetary nebulae and the
planets. In all of the diverse applications the advantage of the cooled
S-l image tube was very large, in terms of sensitivity, over conven
tional photography. Only in terms of poorer resolution, limited spec
tral range at a given dispersion (fixed by the 40 mm maximum diameter
of the photocathode) the somewhat troublesome experimental technique
for contacting the film, and the occasional occurrence of spurious
interned emission detracted from this image converter as a routine
tool. The mica-window tube did not, however, lend itself to spectro
photometry since it required the spectra to be taken on quite small
pieces of 35-mm emulsion which were inconvenient for sensitometric calibration. Despite these difficulties, Fredrick demonstrated that
the image tube in the hands of a competent experimentalist was a powerful tool, using only a 24-inch reflector, truly in keeping with
F. B. Wood's predictions. Ford had utilized the two-stage electrostatic tube of RCA for
prism spectra on the Perkins 69-inch reflector while it was still at
Delaware, Ohio. These spectra were in the visible and red region,
the tube being equipped with a multi-alkali photocathode (S-20). The
results of these tests may be cited as typical of the two-stage tube in
comparison with baked 11 a-O emulsion. The image tube, operating
at 18 kv yielded properly exposed spectra in 2. 5 minutes of a KO- star of apparent magnitude 6.3; this compared with 70 minutes using 10 103 a-O at the same dispersion 20 A/ mm. The resolution was £ound to be three times poorer, i.e ., the same quality of spectrum could have been obtained with a camera of one third the focal length. Thus the gain in information rate was only (70/ 2. 5 X 3 X 3) or about 3.
To achieve more information gain, it was necessary to increase the resolution, the gain of the stages, or the 'photocathode sensitivity relative to the direct emulsion. This shows immediately the reasons for the higher gains quoted by Houck and Fredrick since their com parisons were against much slower infrared plates, even thoug;h the
S-l photocathode is less efficient than the multi-alkali cathode.
Baum and Hall and later Fredrick applied this two-stage tube to the problem of short exposure double star photography where the resolution of the tube is relatively unimportant and where the image tube could be operated at higher voltages for higher stage gain since the increased internal emission background is not important for short
exposures. Fredrick was able to measure the separation of 51
Aquilae as 0.46 i 0.05 arc sec, using the 24" Lowell refractor.
The image tube was being applied to solar physics problems,
also, as part of the Carnegie Committee program. Firor and Zirin
(1962) had used one of the single stage electrostatic tubes of RCA
design to obtain spectra of the coronal Fe XIII lines at XX 10,747 and
10,798 using the coronograph of the High Altitude Observatory,, At
16 A/ mm dispersion it proved possible to obtain spectra in one
minute which could only be obtained with 30-minute exposures on
hypersensitized I-Z emulsion. The investigators compared absolute
intensity measures of these lines with those of lines in the vis.ble spectrum originating from other ionization states to place a lower limit on the ionization cross-sections for Fe X, XI, XII, XIII, and
XIV. They concluded that "there are at present observational prob lems for which the use of image converters improve the chance of s u c c e s s ." Thus, early in 1962, the image tube in one form or another gave great promise as an improved detector for recording spectra of faint sources, particularly of stars and nebulae. None of the work of Houck, Fredrick, and Ford had been quantitative except in the sense of measuring resolution and blackening rates. That of Firor and Zirin was quantitative for intensities of emission lines. Other properties of image tubes which might prove important had not been examined. These included the stability of the photocathode spectral sensitivity against temperature variations, linearity of the phosphor brightness as function of excitation current, stability of the geometry of the image, internal scattering of light, and the general ability to calibrate the
resultant plate to allow spectrophotometric results in both emission and absorption spectroscopy. It was expected, of course, that the procedures of photographic spectrophotometry could be adopted directly,
and that the image tube was a very nearly simple linear device inter posed between the spectrograph and the photographic plate. When faced with the large number of prototype photoelectric
devices of varying degrees of complexity and reliability, the image
tube appeared to be the most fundamental device on which a series of experiments could be undertaken and, in fact, the most likely device to bo acceptablo to the average astronomor who seeks to obtain spectra 12 in the simplest possible way. Very few quantitative experiments had been made up to 1962. From the literature and from discussions which the author held with staff members of the Carnegie Institution
Committee on Image Tubes for Telescopes, W. K. Ford. Jr., J. S .
Hall. M. A. Tuve. and L. W. Fredrick, the hard-vacuum image tube in one or several of its possible forms appeared to be most likely of success and adoption for routine application in observatories, whether or not it could reach the detection efficiency of the electronographic
sy stem s. Through the generous cooperation of the Carnegie Committee, the author obtained, for an extended period, a high-quality image tube plus several essential components for an image tube camera, namely
a cylindrical focussing magnet, camera lens, and mechanical parts.
This enabled the writer to conduct a set of laboratory and field experi ments embracing the properties of the photocathode and phosphor of the specific tube, the study of spectral variations in cepheids by both the image tube and by direct photography, the collection of infrared spectra of early and late type stars in the 1 micron region, and the study of the chromospheric spectrum of the sun in the 1 micron region.
These experiments and the results are reported here. While this work was underway, the application of image tubes
in astronomical spectroscopy was burgeoned in response to the
potentialities cited above. The experiments conducted by the experi
menter remain unduplicated at the present. A summary of more
recent work by others on late type stars is contained in the discussions.
The laboratory measurements reported are those which support the
observational programs. CHAPTER II
ANALYSIS
Relative Efficiency of the Single-Stage Image Tube in the \ 7500-8000 iP Region
While examining the application of image tubes to stellar ab
sorption spectroscopy, an analysis was made of the exposure which
would be required by imaging detectors to record the continuum of
stars of different spectral types. The exposure required with a given
telescope/ spectrograph to bring the continuum to a suitable photo
graphic density is one of the important parameters in determining
the advantages of the image tube over conventional photography after
one accounts for loss of resolution. This analysis is presented here
to provide a framework for identifying specific parameters of image
tubes to be determined in the laboratory. The analysis is made in
detail for recording the continuum at the wavelength of the OI triplet
at X7774, one of the experimental programs in this study. The com
parison made here actually was not undertaken until after spectra
were taken both by image tube equipment and by I-N plates on the
same telescope and spectrograph, the Perkins 69" and the original
grating spectrograph. The results of these observations showed
that there was no apparent advantage of the single-stage S-l image
tube with II a-O emulsion over fresh I-N plates, in terms of exposure
time, for A0 to GO stars in the range of + 2 to -4 apparent visual
magnitude, but that an advantage of about a factor 2 did exist
13 14 at stellar brightnesses of + 6 apparent visual magnitude where ex posure time8 for the image tube were of about one hour duration.
As the analysis shows, the advantage of a single-stage image con verter over film cannot be large in the region where I-N emulsion has always been useful; the advantage is derived primarily from the lower reciprocity failure of II a-O as the final recording emulsion.
The analysis confirms the empirical finding. The potential advantage in using an image tube in this region (X7600-9000) lies in use of a two- stage tube, with its additional information gain of 15-20. The analysis is then extended to cover the regions at 1. 0 and 1.1 microns where the
S-l photocathode does have a larger advantage in sensitivity compared to I-Z emulsion, and even a simple image converter does yield an increased sensitivity compared to direct photography, with the possibility again of multi-stage gain.
Incident Energy on the Detector
For the analysis, the star a Cygni is selected since it is an A2Ia supergiant and was one of the program stars for the X 7774 experiments. Data on the continuum of a Cyg were derived from
Code (I960) in the manner shown below. The other stars of Code's
Table 8 were similarly reduced.
Using Code's adopted monochromatic flux equivalent of 3. 8 x
10”^ erg/ cm2/ sec/ A at X5560, one arrives at a flux distribution
shown in Fig. 1 where the flux distributions of three bright program
stars is plotted. At X7774 the monochromatic flux of a Cyg is com
puted to be ^ (7774) = 4. 38 x 10"*® ergs/ cm2/ sec/ A RECEIVED M O N O C H R O M A T IC FLUX X 10 (ERGS/CM -S E C - A ) 4 2 22 20 6 1 18 10 12 4 1 8 4 6 2 0 OGmnrm iad Orionis a i and jO^Geminorum, g 1 --Mncrmtcfu dsrbto rm € rionis, O € from distribution flux Monochromatic - - 1. ig. F MI ) S N O R IC (M H T G N E L E V A W
15 16 Assuming a 5-percent efficiency for the Perkins telescope* for the transmission of the mirror-slit-grating-camera system* the effective telescope collecting area is 1/ 20 that of the uncenter-blocked prim ary.
Aq££ = 11. 56 x 102 cm2 for a 14-inch secondary cell.
The incident flux in a 1-Angstrom element in the focal plane of the spectrograph is then
0(7774) = 5. 07 x 10 ^ ergs/ sec/ A
In terms of light quanta at this wavelength
■L - I p 1 erg = = 2. 56 x 10" ergs/ quantum
0 (7774) = 1. 99 x 10^ quanta/ sec/ A
In order to compare the efficiencies of photoelectronic and photographic imaging detectors, we will consider their utilization in two ways.
1. Direct impingement upon a fresh photographic emulsion of Type I-N.
2. Conversion by an image-tube system to blue light from the phosphor and subsequent coupling to an emulsion of high sensitiv ity in this shorter-wavelength region, namely II a-O.
Direct Photographic Recording
The Eastman Kodak Company data book on scientific emulsions
(Data Book P-9* 1962) gives sensitivity curves for I-N plates, the
most efficient at X7774, for densities 0. 3 and 1.0. These curves can
be interpreted in terms of the spectrophotometric absolute sensitivity*
i. e ., energy required per unit area to achieve a useful exposure for
microphotometry. Interpolating for useful density, D s 0. 5 to 0.6, 17 yields the curve shown in Fig, 2. At X7774 the sensitivity can be deduced as
Log S = 0. 9 where S = __— "—:------7------s r ® Exposure (ergs/ cm )
Thus the total exposure required to bring the continuum to a density suitable for microphotometry is
Total Exposure = 10^"^*^ = 0.125 erg/cm 2
For a resolution element of 1 Angstrom at plate scale of 50 A/ mm and widening the spectrum to 1 mm in height yields an element of
20 microns by 0.1 cm or A = 0. 00020 cm2.
The necessary exposure time for a Cygni is then
t = 0.00020 x 0.125 „Q exp — oTT W -1 - 4 9 s e c for ordinary I-N plates. Using ammonia hyper sensitization, speed gains of 2. 5 to 4 are possible. Thus, we predict that direct photo graph of the X7774 region of a Cygni - should be possible in 15- 25 seconds at a plate scale of 50 A/ mm. Similar calculations for p
Geminorum, an FOV star of m - +4.18 yields 570 seconds. Calcu lations for the remainder of the stars in Code's list yield the data of Table 1, where fresh I-N emulsion without hyper sensitization is con sidered.
The result of these predictions is shown in Fig. 3 for BOIa and
M2Ia stars; since the above calculation does not include reciprocity failure at the low light levels, these data yield an optimistic pre diction. Data on reciprocity failure of the IR sensitized emulsions were not given in the Eastman Kodak Company Data Bulletin 9, but
Mr. W. F. Swann, Special Products Manager at Eastman, kindly arranged for reciprocity failure measurements to bo nuulo in their +1
w f CM E D=0.6 ABOVE e> J«L 0 BACKGROUND FOG 3 8 cu ILIX o 1
> > w z 1. LLI CO 8
6 0 0 0 7 0 0 0 8000 9000 10,000
WAVELENGTH (ANGSTROMS)
Fig. 2. -- Absolute sensitivity of Kodak spectroscopic emulsion, Type I-N. (Ref. Kodak Data Book P-9, 1962) TABLE 1
PREDICTED EXPOSURE TIMES TO RECORD THE STANDARD STARS OF CODE'S LIST, WITH VARIOUS TYPES OF DETECTORS. EXPOSURE CRITERIA USED IS TO ACHIEVE PHOTOGRAPHIC DENSITY OF 0. 6 ABOVE BACKGROUND FOG.
Star Sped Type MV I-N w/ o Reciprocity I-N w/ Reciprocity Image Tube Failure (seconds') Failure (seconds) H a-O (seconds) 10 Lac 0 9 V + 4.91 1780 6870 2790 v Ori BO V + 4 .4 1270 4120 1860 € Ori BO la + 1.75 88 84 87 55 Cyg B3 la + 4.89 910 2490 1240 U Ma B3 V + 1.91 107 111 108 P Ori B8 la + 0. 34 21 12 18 6 Cyg B9. 5 III + 2.97 250 380 280 a Lyr AO V + 0 .1 4 18 10 15 a Cyg A2 la + 1.33 49 37 45 p Tri A5 HI + 3.08 250 360 280 P Ari A5 V + 2.72 190 240 200 p Gem FO V + 4.18 570 1240 720 a Boo F2 V + 4.48 650 1490 830 ir3 Ori F6 V + 3.31 220 300 240 X Ser GO V + 4 .42 540 1140 670 16 Cyg A G2. 5 V + 6. 26 2920 14, 500 5100 51 P eg G4 V + 5.59 1590 5800 2430 16 Cyg B G4 V + 6.37 3250 17,100 5800 a Tau K5 HI + 1.06 10.2 4 .6 8.1 61 Cyg A K5 V + 5 .5 4 950 2680 1320 a Ori M2 la + 1.3 8.7 3.8 6 .9 APPARENT VISUAL STELLAR MAGNITUDE 4 5 6 2 3 0 1 ae ih cul xoue i s o poa sas i order in com stars, failure) progam for es reciprocity tim (with exposure ulsion em actual I-N with real pared and ideal for o ec D 06 bv bcgon fog. background above 0.6 = D reach to F ig. 3. - - Exposure tim es calculated for Code's Standard stars Standard Code's for calculated es tim Exposure - - 3. ig. F OS IE SECONDS) S D N O C E (S TIME E R SU PO X E 12 la XEI NA EXPOS GOI - Ib O G : E R SU O P X E ENTAL EXPERIM - X - ALR OF RE P T - ITY C O IPR EC R F O FAILURE O N Ia O B CIROCIY FIUE I 0 - a II F O FAILURE ITY C O IPR EC R
21 laboratories, on both normal 1-N and ammonia-hypersensitized I-Z em ulsions.
These data are plotted in Fig. 4 for the plate density of D = 0.6 above gross fog. It is immediately apparent that I-N emulsion suffers very badly from reciprocity failure, requiring nearly 10 times as much exposure at 10, 000 seconds than an ideal emulsion. Fig. 3 also shows the modified exposure prediction for image tube spectra including this reciprocity failure in I-N. Also plotted in Fig. 3 are
"experimental exposure times" for some of the program stars ob served between 2 October and 8 October 1963 using I-N emulsions.
The "experimental exposure times" are adjusted values of the expo sures actually used to obtain spectra, the adjustments being to a continuum plate density of 0.6, and to the slit height and slit width as used in the preceding calculation. The method of adjustment is described below.
Observations with I-N Plates
Observations were made of stellar spectra around X7774 on six nights in October 1963 using fresh I-N plates and the Perkins grating spectrograph equipped with the camera. The Gp cam era was of. 175 mm focal length and f/1 .7 5 relative aperature. Using the Johns-Hopkins grating in the first order (blaze at approximately 70.00A) and with the spectrum centered at X 7400A, spectra were obtained with 0. 4-mm widening. These correspond to a 3-mm slit height. While the design focal ratio of the spectrograph collimator was not determinable, its focal length must be in the range of 175 mm x 3. 0/ 0.4 = 1300 mm and probably matches the f/ 18 RECIPROCITY FAILURE RELATIVE TO 100 S E C O N D S \ 10 12 8 4 6 2 1 - emulsion I-Z F ig. 4. - -R eciprocity failure of fresh I-N and hyper sensitized hyper and I-N fresh of failure eciprocity - -R 4. ig. F 10 XP URE TM - ) S D N O C E S -( TIME E R SU PO EX ■■ \ 100 \ ~ 1000
lla-O Z - l N - I 10000 to to 23 focal ratio of the Perkins Cassegrain configuration. The collimator
focal length of the Perkins prism spectrograph was 1290 mm which
indicates that both spectrographs used similar refractive optics. Thus
the resolution element in the I-N spectra is of the order of 1/ 7. 5 of
.the slit-width, 160 mm in this case, or 0.021 mm. This agrees well
with the assumptions in the above calculation if the actual exposures
are increased by 2. 5 to adjust to a standard 1-mm widening. Thus the
exposures required for the program stars to achieve a 0.6 density
continuum were obtained by correcting the actual exposures used for
widening, the slit width, and the relative exposure factor required to
increase or decrease the achieved density to a value of 0.6. While this
is a very questionable absolute photometric procedure, since the see
ing can greatly affect the transmission of starlight through the slit, it
provides a rough test of the calculation.
These data are plotted in Fig. 3, and a family of exposure curves
vs. spectral type is visually fitted through the data. The slope of 1.85
magnitudes/ decade is in good agreement with the slope of 1.7 5 magni
tudes/decade predicted on the basis of the Eastman reciprocity data.
It is much nearer to this value than to the slope of 2. 20 mag/ decade
predicted below for the image tube II a-O combination. It is concluded that the Eastman reciprocity data of Fig. 4 are nearly correct for the
plates used, based on exposures up to 5000 seconds at the telescope.
The calculation shows a reasonable agreement with the recipro
city failure in that the slope of the exposure curves are consistent with
the experiment however it indicates that the transmission of the telescope/ spectrograph system may be less than the assumed 5 per cent. An effective transmission on the order of 2-3 percent is required to bring the model in line with the data in the region of 600-second ex posures. Alternately the absolute sensitivity of the emulsion may have been lower by a factor of two than the value assumed.
Image Tube Recording
To interpose the image tube as an intermediate amplifier stage between the spectrograph camera and the recording emulsion, we presume that it acts as a linear device apart from any quantum fluctu ations which would occur at low photon rates. This ideal linear device will be considered a linear model. The model assumes that a fraction q of the incident photons will be absorbed in the photocathode with use ful effects to produce photoelectrons. All of these electrons will be emitted from the resolution element such that they are focused by the electric/magnetic optics into an equivalent conjugate resolution ele ment, not necessarily of the same size. These electrons are absorbed by a semi-conductor phosphor with an efficiency N for converting the momentum of the electrons into luminous flux. This luminous flux will be emitted in some definable distribution, and some fraction of the flux is collected by auxilliary optics to be focused on a new conju gate resolution element on a photographic emulsion. An alternate model would introduce one or more phosphor-mica-photocathode sand wiches between the initial photocathode and the readout phosphor. Such a system is shown schematically in Fig. 5. The image-converter system employed in all of the spectroscopic measurements of this study was a single-stage ITT tube of the FW-167 type, using an S-l POWER SUPPLY
VOLTAGE DIVIDER WWW— —AWWV— —WWW
CAMERA RELAY OPTICS INTENSIFIER DYNODES PHOSPHOR PLATE
( IX V V V/\ A A A A A A A X X ^ CYLINDRICAL FOCUS MAGNET f ...... Fig. 5.— Schematic of image tube system. i U1 photocathode and a P -ll phosphor. The output of the phosphor was normally coupled to II a-O emulsion with a pair of f/ 1. 5 lenses. The final system developed in these experiments utilized f/ 3.6 optics and baked II a-O plates. Some laboratory experiments were made with a two-stage EGA tube of Type C70056E with S-l input cathode, S-20/ P -ll interstage cathode and phosphor screens and P -ll output screens.
At X7774 the quantum efficiency, photoelectrons emitted per incident photon, of the nominal S-l photocathode is 0. 0036 (Engstrom,
I960). The actual photocathode was 1. 57 times more sensitive than the average S-l surface, based on measurements at ITT Laboratories of the total response in the IR tail. This implies a quantum efficiency o of 0*0056. Thus, the electron flux originating in the 1-A resolution j element of a Cygni is
77 = 1.99 x 105 x 0. 56 x 10”2 = 1110 elec/ sec s
These electrons are then accelerated down the tube to the phosphor screen. The accelerating potential required in a magnetically-focused tube is that which yields a transit time from cathode to screen equal to the gyro period of the electrons about the field lines. (Zworykin and Ramberg, 1949). Expressing the field B in gauss and the potential in til volts the separation of the n conjugate image from the photocathode For the ITT tubes the cathode-phosphor distance was 5.7 cm. The
magnetic £ield used was approximately 395 gauss. To achieve a focus
at the first conjugate plane would have required an accelerating poten tial of 45.6 KVi too high for practical use because of internal emission.
These tubes are designed to focus at the second node. The focus volt
age required is then one quarter of that for first node focus or about
11. 4 kilovolts.
Not all of this energy is available for phosphor excitation. To
prevent optical feedback from the phosphor to the photocathode, an opaque aluminum layer is deposited over the phosphor. This layer
absorbs approximately 10-20 percent of the electron energy, equivalent
' to approximately 2 kilovolts in this case. The incident electron then
expends
P = (V-2) e • 7} watts 8
where e is the electronic charge. For a Cygni this will be
P = (11. 4 - 2. 0) x 1.602 x 10'19 x 1110
or
P = 1.67 x 10"*1 2 watts/ resolution element at X7774A.,
assuming the resolution element still represents a 1 Angstrom interval.
The efficiency of conversion of this kinetic energy into luminous energy
depends upon the composition of the phosphor.(Eberhardt (1961) cites ten different measurements of the absolute efficiency of the P-ll phos
phor (ZnS:Ag), measurements which range from 5.1 to 25 watts/watt
with an electrons incident at lOkv. These values are listed in Table 2. TABLE 2 / SUMMARY OF MEASUREMENTS OF CATHODOLUMINESCENT EFFICIENCY OF P - l l PHOSPHORS (EBERHARDT, 1961) '
Phosphor Type Phosphor Composition Absolute Efficiency €p Watts per Watt %
PH 5.1
P l l 7 .8
P 1H 8 .0
P l l 8 .6
p n 10.
p u (118-2-11) 1 0.5
PH ZnS:Ag 12.4
p n (118-2-4) 12.6
p n ZnS:Ag 20.9
p n ? ZnS:Ag:Al 25
to 0 0 29
Adopting 10 percent as a mean efficiency and accepting Stoudenhcimer's
(I960) assertion that it is nearly independent of the incident energy between 10 and 20 kv, the photon flux emitted by the phosphor will be
equivalent to
P' = 1.67 x 10-1 3 watts/ A element
The aluminum backing screen serves to reflect part of this flux back
toward the exit face of the tube. The measured efficiencies of the
phosphors include this gain, however, since the measurements were made on deposited screens. The luminous flux will be emitted from the screen with some definable angular distribution, as indicated in
' the sketch
If this is assumed to be Lambertian in our model, we can readily
compute the flux incident on the relay lens subtending a half angle of
as viewed from the resolution element. The flux accepted by a lens of angular extent ± 9^ on the normal
to the screen is
2ir I cos 9 sin 6 d 6 o 30 F = v I sin 2 0. u/2 1 out of the total flux P 1 2it I cos 0 sin 0 d 0 = ttI > The common procedure in the construction of fast, unity-magnifica tion optical systems is to use two conventional, high-quality camera lenses of low f/ number and relatively long focal length mounted front-to-front. This utilizes the first lens to produce an image of the screen at infinity and the second lens to focus this image onto the recording film. The longest possible focal length is used to obtain a large useful field without vignetting, vignetting being one of the pen alties incurred by this simple and relatively inexpensive approach. Other disadvantages include the large path length of glass in such systems and the numerous air/ glass surfaces (14 elements in some cases) which yield absorption and reflection losses negating much of the gain over using a single lens at twice its focal length. In order to compare the efficiency of simple refractive systems of these two alternate constructions, assuming their absorption losses are negli gible and their resolution adequate to avoid loss of light from the spectral resolution element, Table 3 gives the fraction of the total flux emitted from a Lambertian source on the axis of the lens system which will be transmitted to the conjugate image element. We observe that only 6 percent of the light leaving the screen can be collected with two-lens f/ 2. 0 system. The energy incident on the photographic plate in each 1-/P element is then irl sin 2 0j = 1.67 x 10"1 3 x 0. 06 watts - 1.00 x 10”7 erg/ sec TABLE 3
FRACTION OF TOTAL FLUX EMITTED BY A LAMBERTIAN SOURCE WHICH WILL BE COUPLED TO AN IMAGE BY UNITY MAGNIFICATION REIMAGING OPTICS OF VARIOUS RELATIVE APERTURES
Single Lens at 2 Two Lenses Front-to-Front i! n o. Sin201 ir sin2 9j Sin2 9 l ir sin 2
0. 5 0.2005 0.6299 0.5000 1. 5708 0.6 0.1477 0.4640 0.4097 1. 2871 0.7 0.1130 0. 3550 0.3372 0.8822 0 .8 0.0889 0. 2791 0. 2808 0.8822 0 .9 0.0714 0. 2243 0.2357 0.7405 1.0 0.0585 0.1838 0.2005 0.6299 1.1 0.0491 0.1543 0. 1712 0.5378 1.2 0.0416 0.1307 0. 1479 0.4646 1.3 0. 0356 0.1118 0.1288 0. 4046 1 .4 0. 0309 0.0971 0.1131 0. 3553 1.5 0.0271 0. 0851 0.1000 0.3142 2 .0 -0.0154 0. 0484 0.0585 0. 1838
In the image-tube system two 50-mm f/ 2. 0 camera lenses were
employed at a working distance of 100 mm (Burke &James, Inc.,
Catalogue No. 14011A). The phosphor images showed at best 35 line pr/ mm when photographed with the wide-open relay optics. (The tube itself exhibited 65 line pr/ mm when observed directly with a
microscope but only half of this was realizable in practice as a result of relay optics deficiencies. Microscopic examination is by optics of small relative aperture .whereas the fast relay optics o suffer spherical aberation through the thick face plate.) The 1 A
resolution element then occupies a width of 30 microns on the photo
cathode, on the phosphor, and on the photographic emulsion if unity magnification is used throughout. If the spectra are widened to the same 1 mm as in the direct I-N case, we have the above flux falling 32 into an area
A = 0.0030 x 0.1 = 0.00030 cm2 and the illuminance on the film is Ot . 1. OOxlO"7 . , P ------*— = 3. 3 x 10 4 ergs/sec/cm 3 x 10“
Note that the dispersion must be 1. 5 times that of the direct plate case to achieve the same image resolution as compared to direct plates. This results in a lower image illuminance than in the direct photographic case where we had
P1 = 5 x 10 = 25 x 10 4 ergs/ sec/ cm2 on I-N emulsion. 0.0002
The Kodak data book does not give a sensitivity curve for II a-O but does give 103 a-O whose sensitivity is closely equivalent to baked II a-O. For a density of 0.6 above the fog levels we have the requirement for
Total Exposure = ta a lo g \ 3 ------W = °- 005 c ta ‘ the exposure time with the image tube system is then estimates as
t = 0.0050 _ exp " 15 sec 3. 3 x 10“4
Thus we discover that the exposure time for the nominal image tube system used, i.e ., a single-stage, S-l image tube using 6-percent efficient optical coupling of the phosphor to high-speed film, is the same as for hyper sensitized I-N emulsion for the observation of spectral lines at equivalent resolutions in the X7000 - 9000 region. 33 This is the result of the relatively high sensitivity (quantum efficiency) of the I-N emulsion. Only by use of more efficient coupling systems or by use of a multiple-stage tube can appreciable advantages be
realized in this region of the spectrum. This leads to the following
general conclusions
1.) Even utilizing mica-window coupling, which deposits over
one-half of the phosphor output into the emulsion, gains of less than
15X are realizable with single-stage image tubes in the wavelength.
This limit of gain is imposed by the limited photon gain of the single-
stage tube, which does not produce enough photons in the phosphor to
guarantee eventual detection of each photoelectron. If a multi-stage
S-l image tube were used, an additional gain may be obtained up to
a limit set by the ratio of detection quantum efficiency of the photo-
cathode and 1-N emulsion. This region is perhaps the least attractive for image tube techniques until the new S-25 photocathodes are per- o fected; the S-25 photocathode extends the S-20 sensitivity to 8500 A .
2.) Beyond X9000 the potential gain from the simple S-l image
converter is by competition with 1-M and then with I-Z emulsion at wavelengths X9500 or longer. At X10, 500 the nominal S-l photocathode
retains 0.15 of its peak sensitivity while 1-Z emulsion is only 0. 01 as
sensitive as 1-N emulsion. Fig. 6 is based on the equating of the image tube/lI-aO sensitivity to that of I-N at X8000 X. The relative ordnate heights then show the nominal gain improvement over I-M and I-Z
which could be expected from the camera system used here. 3.) At shorter wavelengths than 8000 X the S-20 photocathode has a higher quantum efficiency by a factor of 10 to 100 over that of ♦I
1.1Z .1 w 0
0*0.6 Abova Gtom Fog
"S&X30 7000 9000 KWXX) 11,000 12,000 19,000 WAVE LENGTH
Fig. 6. --Sensitivity of I-N, I-M and I-Z emulsions and relative response of the S-l photocathode. Data taken from Reference (3). 35 the S-l surface. In this region a blue sensitive tube should be used in the amplification process. By use of more efficient reimaging optics or by introduction of dynodc intensifier stages, each detected photoelectron will result in a developable image element on the plate. Further gains of blackening are then redundant and serve only to saturate the photographic plate more quickly, reducing the dynamic range of the system. Using the apparently equal sensitivity of the image-tube/II a-O system as compared with I-N plates (in the region of A 7774 and exposure times of 10-100 seconds), we can now predict for Code's stars the exposure times with an image tube system to reach a continuum density of 0.6, allowing for the reciprocity failure of II a-O emulsion. This yields the data plotted in Fig. 7. The slope of the exposure curves is some 2. 20 magnitudes/ decade as compared to the 2. 51 mag/ decade for an ideal emulsion. Data on exposure times of some of the GOlb program stars, derived in the same way as in the direct plates, by adjusting the spectral width to 1 mm, the slit width to a standard 160 microns and the density to 0. 6 above fog, are also plotted in Fig. 7. A family of curves fitted visually to the data ex hibits a slope of 2. 5 magnitudes/ decade. A system transmission efficiency of 2. 7 percent is required to bring the model exposure times into agreement with the measurements, consistent with the result on I-N. One must conclude that the original Perkins spectro graph was an inefficient system.
Extension to Longer Wavelengths and to Multi-stage Tubes
The preceding discussion applies specifically to observations at A 7774 where experimentally the Gp camera with I-N plates and the APPARENT VISUAL STELLAR MAGNITUDE o cluae rm Cd' sadr sar fr niel n r I aO a-O II l a re and ideal an for rs sta standard Code's from calculated fog em ulsion, used with the ITT FW-167 im age tube, com pared with actual actual with pared com rs. sta tube, ram g age im pro FW-167 for es ITT tim the with used exposure ulsion, em Fig. 7 . --E x p o su re tim es in order to reach D = 0.6 above background background above 0.6 = D reach to order in es tim re su o p x .--E 7 Fig. XP URE IE SECONDS) S D N O C E (S TIME E R SU PO EX 103 X— XPRI NT EXP URE Gib G E R SU PO X E L TA EN IM PER EX — —X CIROCIY FIUE N - I F O FAILURE ITY C O IPR EC R - A - XPRI NAL URE: ib F : E R SU O P X E L ENTA IM PER EX ALR OF RECI TY IT C O R IP C E R F O FAILURE O N Ia C B 105104
O' w 37 single-stage converter camera were actually employed. We should also analyze the performance to be expected of the S-l camera at longer wavelengths of astrophysical interest, e. g. the region at 9300- o 9700 A where the water vapor content of cool stars has been measured by Spinrad, et a l., (1966) or in the 1.1 micron region where the coronal emissions of Fe XIII and chromospheric and nebular He I are of impor tance. Referring to the data of Figs. 2, 4, and 6 the relative sensiti vity and reciprocity factor of the photocathode and I-Z emulsions may be obtained at any wavelength. The absolute flux data of Code may be interpolated to obtain the received flux. Examples of the predicted exposure times for the single-stage system used in the present study and for ammonia-hyper sensitized I-Z emulsion are given in Fig. 8 o o for wavelength 10, 049 A,the Paschen 6 line and X10, 830 A, the He I triplet in BOIa and M2Ia stars. The higher quantum efficiency of the photocathode more than compensates for the somewhat better recipro city characteristic of the I-Z emulsion. An average gain of 4 magni tudes at 1. 0 microns is predicted for the single-stage system used in this study. Also plotted in Fig. 8 are some experimental exposure times required to record spectra at one micron, the times having been adjusted to the standard 1-mm widening, and I60fx slit width. While the slope defined by these scattered experimental values agrees well enough, the exposure times are longer by a factor of 6-8 than pre dicted. This is in part due to increased absorption losses in the Aero
Ektar Camera lens which at lp, are twice those at 7800 R, but the remaining factor must be attributed to other optics in the spectrograph APPARENT VIS. MAGNITUDE 4 2 5 6 7 0 3 1 n a 10, 830 at and em ulsion and for a single-stage converter cam era at at era cam converter single-stage a for and ulsion em calculated from Code's standard sta rs for hypersensitized I-Z I-Z hypersensitized for rs sta standard Code's from calculated . g i F 8
830 IT 0 3 ,8 0 1 A -Z I 9 4 0 , 0 I X I0,830 I-Z I 0 3 8 , XI 0 . -- Exposure tim es to reach D = 0.6 above background background above 0.6 = D reach to es tim Exposure -- . M2la J9. XP UR TM ( ) S D N O C E (S TIME RE SU PO EX o<&> M6e F5IV / ' O OAOV ' X X 10, 049A 5 0 1 BOAI 39 and/ or to a more rapid than nominal fall-off in sensitivity of the FW-167 tube.
The addition of an intensifier dynode would provide an additional gain in blackening by a factor of 20 with a slight loss of resolution. An additional 2. 5 to 3. 0 magnitudes of gain can be achieved in this way.
Thus the two-stage image converter offers a predicted gain of 250 to
600 over I-Z emulsion in the 1. 0 - 1.1 micron region. Only the fraction of this gain derived by the first stage has been studied in these measure m ents.
In many practical cases, one of the important contributing factors in the gain of a photoelectronic system is the decrease in re ciprocity failure. We observe that this amounts to a factor of four in the case of I-N emulsions for 3-hour exposures. It is Felgett (1956) who points out in typical British fashion that the low-intensity recipro city failure "although appearing to be a great nuisance in astronomy, is in fact necessary for stability of the emulsion from room temperature quanta and direct thermal excitation... fogging would become very
severe in a few months were it not that reciprocity failure protects the emulsion. "
Examination of the Linearity Assumptions
The interposition of the^simple image tube into the spectrographic
camera introduces several steps into the processing of the signal supplied
by the telescope/ spectrograph, steps which are either not present in
direct photographic recording, or which replace those which are used,
e. g ., the use of II a-O emulsion instead of I-N or I-Z emulsion. It is
the efficiency and linearity of these steps which provide the advantages or disadvantages of the image-tube camera over direct photographic
recording and which might limit its application to certain classes of
problems. It is apparent that certain classes of problems will not be
easily undertaken by the present image-tube cameras since the small
diameters of the photocathodes limit the total wavelength interval which
can be recorded at a specified dispersion and the physical bulk of the
present tubes restricts the introduction of the image tube into many
optical systems.
Beginning at the photocathode we will proceed through the
process to identify those steps whose properties could affect quantita
tive spectrophotometry, and which should be examined by experiment.
- In many cases, these steps had been examined by prior or concurrent
workers, sometimes with conflicting or ambiguous results. The
laboratory experiments conducted in this study and discussed in
Chapter III were selected to answer questions raised in the following
discussion.
Photocathode
The photocathode has a broad wavelength range of sensitivity
and a very large dynamic range over which it can be used, compared
with photographic emulsions. The wide dynamic range of the photo
electric effect empirically established shortly after its discovery and
is stated by Zworykin and Ramberg (1949) as their First Fundamental
Law of Photoelectric Action.
It may be paraphrased as the number of electrons released per
unit time at a photocathode is directly proportional to the intensity of
the incident light "for a range of intensities varying from statistical 41 fluctuations to full sunlight. " Deviations of photocathodes from this law result not from failures of the photelectric effect but to the internal resistance of the photocathode. The constant of proportionality between the photocurrent and the incident intensity can, however, be affected by the chemical condition of the photocathode and its temperature. The secular changes occuring in high sensitivity photocathodes tend to be downward in performance (termed "slumping1') as the result of redis tribution of the more volitile active ingredients, such as cesium, due to heating of the tube, or a gradual change in the chemical composition with time. Examples of such effects are cited by Jedlicka and Vilim
(1966), Theodorou (1966), L>inden(1962) and others. The minimization of such slumping remains one of the black arts of the industry.
The effect of photocathode temperature on sensitivity is more direct. While the cooling of photocathodes to lower the emission of thermal electrons is an almost universal practice, it was only recently that investigations (Spicer, 1958; Young, 1963) have been conducted on the effects of such cooling on the quantum efficiency of the photo cathodes, particularly as a function of wavelength. In addition to demonstrating temperature coefficients in the photocathode sensitivity of the lP21(S-4) and EMI 9558 (S-20) tubes and in the secondard emission gain of their dynodes, Young emphasized the naivety of assumptions that dry ice
"cold boxes" establish the temperature of photomultipliers at the dry ice point or that they regulate the temperature to a value more con stant than 5-10°C. For measurement of equivalent widths in image tube spectrophotometry, variations of photocathode sensitivity would be of no consequence other than affecting the exposure time required to record the continuum, since the spectral range is small. To attempt to relate intensity measurements in separate wavelength regions, how ever, e .g ., intensity comparisons of emission lines within a multiplet or the energy distribution in stellar continuum as compared to some standard, such variations in relative sensitivity must be known or at least controlled. A temperature coefficient in S-l photocathodes was a specific topic for investigation.
The question of response time of photocathode emission is one which has received attention, in part because of the introduction of laser techniques in holography, of high speed photography, and shock tube spectroscopy. Duchet (1966) has shown that the S-l, S-10, S -ll, and S-20 photocathodes have response times less than 1 nanosecond unless the photocathode has somehow been prepared with a high resis tance. For current astronomical applications no serious response time effects exist.
It is recognized that photocathodes and phosphors will not be uniform over their areas, although modern fabrication methods appear to allow uniformity to ±15 percent in a photocathode (Gayot, et al, 1966) and somewhat better uniformity than this is achieved in high-resolution phosphors prepared by either settling or electrophoretic techniques
(McGee, 1966).
Electron Optics
Uniformity and stability of the accelerating potential and magnetic field have been assumed so that the resolution elements of the photo
cathode and phosphor remain conjugate throughout the exposure. 43
Experimentally the focus is maintained very well if the accelerating potential is maintained constant to ±100 volts, easily done with modern
regulated power supplies if coronal discharges are avoided. The uniformity of magnetic field is not essential if some scale variations and
small geometric distortions are tolerable, but in astronomical appli
cations the stability of the field is not assured even if the field is
obtained by a large cylindrical permanent magnet, as was used in the present investigations.
The earth's ambient magnetic field has a strength of 0. 2 to 0.4
percent of the axial field of the cylindrical magnet. Use of the tube on
the telescope for exposures of several hours at a Newtonian or Casse-
grain focus will result in a variation of the ambient field component which can be an order of magnitude higher than can be tolerated in a
high-resolution image tube of long electron path, such as the Spectracon
of McGee and Baum, an electronographic tube of 28-cm length. For this tube Baum (1966) devised a soft-iron shield to enclose the entire
tube and its solenoid magnet and in the process developed empirical scaling laws for such shields. This shield, weighing some 250 lbs, was required even though the tube was installed at a coude focus since
the rotation of the Mt. Wilson 100-inch dome changed the ambient
field at the coudk sufficiently to require this shielding.
Baum gives a very complete analysis of the shielding problem.
To be added to his conclusions for effects on the relatively long
Spectracon tube, is the lesser sensitivity to these extraneous fields
of tubes of shorter acceleration path such as those used here. 44
Consider the tube axis to lie at right angles to the earth's field as in the sketch. A 180-degree reversal of the tube will result in a displace ment of the resulting superposed B fields of I !i A d = I -
i If exposure times on the telescope are of durations of 3 , h 1 , h and 20 , ixi
the maximum smearing of the image will be
A d X 0.5 tan (A t/15) j
due to telescope rotation.
For the ITT, two-stage RCA, and the Spectracon tubes this
yields the values of smearing in Table 4 . 45
TA BLE 4
THEORETICAL SMEARING OF IMAGES RESULTING FROM EARTH AMBIENT FIELD ON UNSHIELDED IMAGE TUBES ■1 II 1 ^ Exposure T elescope ITT RCA Spectracon Time Rotation 1=5.6 cm 1 = 5.7 cm 1 = 28 cm (D egrees) (Minutes) Bfocus ■ 400*- Bfocus = 20°8- Bfocus = 25°*
20 '• " 5 10fX 20f| 78pi 60 15 28|i 57*i 232pi 180 45 8 In 162n 633pi 720 180 228fi 416/1 1800 fx
Thus the effects are not significant for the tubes used in these experi ments for telescope exposures of less than one hour. It would be seldom that the image tube will be aligned normal to the earth's local field so the actual effect is somewhat less than the values of the table. No shielding was used in the experiments reported heref where exposures ranged one hour or less. The longer exposure plates are not noticeably lower in resolution than the ones of a few minutes exposure. Baum concludes his discussion by suggesting that a feedback com pensation system for canceling the earth's field is the best approach for tele 8 cope-mounted image tubes; he reports that Dennison at Mt. Wilson was working on such a system.
The Phosphor Screen The technology of phosphors, their preparation, their deposition, and'theirrreproducibility, also involves the black arts of industrial processes. Each manufacturer has evolved his own techniques. Since the phosphor is a uniformly-packed layer of individually-grown, 46 activated semi-conductor crystals, both the preparation of the phosphor and'its deposition involve careful technique. McGee, Airey and Aslam
(1966) discuss some of the techniques used for high-resolution screens, as do other authors in the Imperial College Symposium volumes. It is concluded that the uniformity which can now be achieved assures that, while the phosphor granularity is certainly one of the ultimate limits to the resolution of image tubes, a tube of selected quality will have a screen of uniform resolution and nearly uniform efficiency in output, the variations in brightness being only some 10-15 percent, based on tests discussed by Beesley and Norman (1966). The granularity of even the finest grain screens (phosphor crystals of mean size of
1. -1. 5 microns) leaves a residual noise of 10-15 percent. This granu larity was more of the order of 40 percent in the settled P -ll phosphors in the image tubes used in this investigation. A more serious problem than variations of luminous efficiency across the tube, which can presumably be accounted for by calibration, is the variation of luminous efficiency with the temperature of the phosphor. Klasens, et al (1948) report on the influence of temperature on the phosphor efficiency in fluorescence excitation. Changes in temperature produce changes in both the color and the efficiency of the fluorescence in zinc sulphide phosphors. Charmin and Hewitt (1966) have discussed the results of tests of the overall gain of an EMI magnetically-focused tube (Type 9694) of four-stage design. This tube utilized S-20 photocathodes and P -ll phosphors for each stage. The overall luminous output of this tube increased by a factor of two in 47 cooling the entire tube from 20°C to - 22°C. This increase was independent of the wavelength of the incident light. Tests of this tube on the sensitivity of the first photocathode (the only one accessible for test) indicated an increase in photoemission of only 3-5 percent over this temperature range, which would not account for all of the increase in overall gain of the four stages. The increase in screen brightness was generally equal at all wavelengths in the phosphor output. Figs. 9 and TO are-taken from Charmin and Hewitt and show the relative photon gain of the tube and the relative spectral distribution of the out put phosphor, for constant luminous input and tube voltage, as a function of the temperature of the isothermal chamber surrounding the tube.
Fig. 11 is derived from these data to show the uniformity of the gain as a function of the color emitted by the phosphor. The uniformity of a single phosphor will be even better since it should be the fourth root of the values plotted in the figure.
The independence of the color of the phosphor from temperature effects has important consequences for spectrophotometry. Cooling of the S-l photocathode is an absolute necessity for spectroscopic applications in order to suppress the thermionic background;controlled cooling can easily stabilize any possible shifts of photocathode sensi tivity. One of the clear advantages of the image tube is that the record ing emulsion sees the same quasimonochromatic light (range of 1000 K) at every point in the image. This permits direct, simple sensitometric calibration of the entire spectrum image for relative spectrometry in I absorption with a single HD curve. If changes in color had occurred 48 2-0
1*6
c
i o* 00 I i WMtanfth | , O.S06 (im
0-4
-10 30 40 M«on tub* ttm ptroturt (*C) Fig. 9. -- Variation of relative photon gain with temperature for constant light inputs (approximately 108 photons cm 2 sec”1) of differing wavelength. The change in gain is appar ently independent of illuminating wavelength over a_wide. range.
•o -
to
• 40
4000 •>000 6000
Fig; 10.- Relative spectral distribution of the intensifier output phosphor for a constant light input at several temperatures. RELATIVE GAIN (T)/ RELATIVE ' G A IN (+ 1 9 ° C ) 1 . 1.00 1.20 0 .4 1 0 .6 1 0 8 . 1 0 0 0 4 omaie t + 0 C o mpaie needne f oo of color of ulsion. independence em the to phasize flux em to atic C 10* heterochrom + the to alized norm i* 1 eaie an fP-ll popo v temperature vs phosphor l l - P of gain Relative - - 11. Fig* 0 0 2 4 C ° 2 2 - 6 ° C ° 6 - A) (A H T G N E L E V A W 0 0 4 4 0 0 8 4 0 0 6 4
0 0 0 5 >o as the result of uncontrolled temperature variations across the ; phosphor, the photometric calibration of image tube spectra would have been much more difficult. I
i Phosphor Response Time The calculations took no account of possible relaxation effects reciprocity failures in the phosphors themselves. These effects could be of two kinds. A reciprocity failure at very low excitations could result from the requirement of multiple electron excitation of the phosphor to produce a uniform efficiency of cathodoluminescence, i. e ., that the incident electron current density must exceed a certain threshold to achieve the luminous efficiencies normally assigned. A suggestion of this effect has been made by Francis and Stoudenheimer
(I960) who reported a decrease in luminous efficiency which occurs between 10”11 am peres/cm 2 and 4 X 10"13 amperes/cm2 in the P -ll phosphor. Fig. 12 is derived from their paper. For resolution elements of 0. 0002 cm 2 as treated here, this corresponds to the arrived of 500 electrons per second. As we have computed earlier, this corresponds to the flux of an A2 la star on half as bright as a Cyg or approximately stellar magnitude + 2 and to exposure times of about 20 seconds, using the Perkins telescope and spectrograph. The upper region of the transition corresponds to stellar magnitude -3. 0 and exposure times of 0. 2 seconds. Exposure times as short as 60 seconds were used occasionally but for spectra of only 0.4 mm widening. Thus in all of the stellar spectra obtained in this study, the input current density at the phosphor was less by a factor of 8-10 than the one suggested by Francis and Stoudenheimer as lying near a transition 51
Magnitude of F2la Star +4...5 +2. 0 -0 . 5 -3. 0
l0r't to*" EXCITATION CUN RENT DENSITY—AMP/Cm*
Fig. 12 --Failure of luminous efficiency of P -ll phosphor screens at low excitation currents. Upper scale corresponds to the excitation of a resolution element at A 7774, for A2la supergiants, in the camera system used in this study. 52 region to higher phosphor efficiency. Only in the case of quite bright o targets should such an effect be present. The use of the image tube for solar eclipse spectra, where exposures of fractions of a second are o desired, might result in operation in this transition region between the faint continuum and the bright emission lines, or between bright and faint regions of the continuum. For stellar spectrscopy, however, where an image tube might reasonably be used in preference to direct plates because of the faintness of the source, this possible reciprocity failure is of no consequence. In an unpublished study by W.K. Ford,
J r., (1964), no additional reciprocity failure was encountered down to
current densities of 5 X 10 16 amperes, corresponding to exposure times of 6-7 hours under the conditions considered here. Ford con
cludes
it is unlikely that there are deviations from linearity greater than 10 percent for the range of current densities of 3 X 10"1 2 to 3 X 10"1 6 amp/ cm2. .. consistently high values of output flux at current densities above 3 X 10"1 2 amp/ cm2 are in rough agree ment with the increased efficiency found by Francis and Stoudenheimer.
A second mechanism which could operate in the phosphor is the
relaxation time to reach full brightness or to decay from full bright ness. Relaxation effects are well known in decay of phosphors since
persistence~6f the image (or freedom from it) are important in cathode
ray tubes. The persistence appears to be a function of the stimulating
current. Data on the persistence of the P -ll phosphor supplied by
English Electric Valve Co., Ltd. indicates that low excitation currents
of brief duration can increase the persistence by a factor of 30 as
compared to intense excitation for longer intervals. 53 Fig. 13 is reproduced from commercial literature supplied by the
English Electric Valve Co., Ltd. The curve, for P-ll, is considered to fit the case of the image tube phosphor, but data on relaxation from other levels of excitation are not available. The 2 microamp/ cm* excitation is higher by a factor of 100, 000 over the regime of the reciprocity failure noted by Francis and Stoudenheimer. This corre sponds to exposures of 1 microsecond or so in the experimental camera. No data of any type were found for relaxation effects in phosphor rise time. Such effects are not important for stellar spectroscopic applications but might be important in short exposure studies of astrometric binary systems or in flash spectroscopy of solar eclipses.
It is visually apparent that an image appears on the screen when the voltage is applied or when the shutter is open. It is not visually apparent whether it reaches full, steady state. These relaxation effects were then to be investigated.
100
2 — 2.1 A/CM2 • 1/40 SEC. PULSE 53 x 3 6
UJ z
UJ > 5 i a I 0.4 0.1 0.4 1.0 4.0 TIME FROM REMOVAL Of EXCITATION IN MILLISECONDS Fig. 13.-- Persistence characteristic of P-ll phosphor. Source, English Electric Valve Co. CHAPTER III
LABORATORY EXPERIMENTS
The Apparatus Following discussions with Dr. L. W. Fredrick of the Carnegie
Institution, Dr. John S. Hall of Lowell Observatory, Dr. M. A. Tuve
of Department of Terrestial Magnetism, Carnegie Institution of Washington, Dr. W. K. Ford, Jr., of DTM, it was agreed that the author could borrow one of several high-quality infrared image tubes
produced in experimental quantity in 1962 by the ITT Laboratories of Ft Wayne, Indiana. This tube, Type FW-167, has an S-l photo-
.cathode and a P -ll phosphor. Dr. Ford also made available several
camera lenses, a large cylindrical permanent magnet for focussing,
and.miscellaneous structural parts to assist in the assembly of a suitable converter camera. Following tests at the Aerospace Research
Laboratories, the equipment was first taken to Flagstaff in October 1962
for telescope tests. Again Dr. Ford was most helpful in providing both
technical advice on adjustment and testing and data on the performance
to be expected from the final system. The author was encouraged to use the original Perkins grating
spectrograph, the one used by Dr. Ford in his electrostatic cascaded tube experiments in I960. This instrument was equipped with an
original Johns Hopkins grating ruled by Strong with 600 lines per mm
and a first-order blaze at 7000 A. It had been widely used for near-
infrared spectra in the late 1940's (Keenan and Hynek, 1945; Slettebak,
54 1951) but was being superseded by the modern Y Spectrograph. The
older spectrograph had a refractive collimator corrected for the
far red (A-band ?). The physical arrangement of its exit beam was
adaptable to the mounting of a variety of cameras and scanners since
it provided a beam path which was readily accessible outside the
spectrograph shell.
The basic equipment used at Flagstaff in 1962/1963 (shown in
Figs. 14, 15, and 16 consisted of the image tube itself, an f/2. 5 Aero-Ektar lens of 178 mm focal length for imaging the collimated
and dispersed light emerging from the spectrograph, the pair of
50-mm Carl Meyer f/2. 0 camera objectives mounted nose-to-nose
.-as the unity magnification relay optics, the photographic plate holder,
and the FW-167 single-storage image tube in its cast Indux V magnet.
The high voltage was derived from a well-regulated and filtered
commercial supply built by Beta Electric Corporation of Sorenson/
Raytheon.
The mounting arrangement is due to Dr. Ford, who also used
this spectrograph for tests at Flagstaff following the removal of the
Perkins reflector to Arizona in 1961.
In general all of the apparatus used in this study were adaptations
of existing, borrowed equipment. The exploratory nature of the
observations and the special nature and cost of the apparatus argued
against development of special optics or electronics not available in
a commercial form. For example, the camera optics were surplus
aerial lenses from Edmunds Scientific Corporation, the relay optics
were available from the catalogue of Burke and James, Inc., the Fig. 14. -- Exploded image-tube camera assembly. Right to left: ITT image tube with insulators and voltage divider, dry ice cold box, Aero-Ektar lens. 57
Fig. 15. -- Exploded image-tube camera assembly showing cylindrical magnet and relay optics. Fig. 16. -- Image-tube camera assembly and power supply as attached to the Perkins telescope. Flagstaff, Arizona. 59 power supplies and voltage regulators were standard items in the
Sorenson catalog. Such power supplies were found to be entirely
adequate. The combination of an AC line regulator of the 500S type
and the Model 5030-4 High Voltage Supply, which incorporates series-
triode feedback regulation, yielded stability of ±100 volts even in
an airborne environment.
To obtain the required cooling of the photocathode to reduce the
thermionic emission of the S-l photocathode, a simple dry ice system
was used on all but the last observing run. This consisted of an
annular cold box, insulated from the surrounding structure, but in
thermal contact with the photocathode, which would allow the tube to
. be cooled to about -50°C for periods of 20-30 minutes and to remain
below -10#C for more than four hours in mid-winter operation at
Flagstaff. The chief disadvantage of this system was that the entire
converter camera had to be removed from the spectrograph each 2-4
hours for recharging of the cold box. This inevitably resulted in small
changes of focus and non-equilibrium temperatures from plate to plate.
In 1965 a prototype thermoelectric cooler was borrowed from the
Night Vision Laboratory, U. S. Army Engineer Research and
Development Laboratory, Ft Belvoir. This device uses a two-stage
Peltier system of bismuth telluride and can achieve six watts of cooling
with some 50 watts of input power (2 amperes at 28 volts). The hot
junctions are water-cooled, requiring a circulating system. A closed
system composed of a garden fountain pump, fan and a small heat
exchanger was attached to the spectrograph. This system allowed
cooling of the photocathode 45° C below the circulating water tempera 60
ture. In February 1965, with a dome ambient of -8°C, this allowed
operation at -35°C on the photocathode as measured by resistance
gages cemented to the faceplate. This electrical cooler allowed
introduction of a temperature regulator into the camera system so
that the photocathode could be controlled to ± 0. 1 °C, thus eliminating
temperature changes in the optics, in mounting structure, and in the
photoelectric response of the photocathode itself.
The ITT image tube was combined with other optics to record objective prism spectra of the solar flash spectrum. It also was used
on the Perkins reflector with a new spectrograph designed by Or. Ford
(the DTM spectrograph) for additional stellar spectra. In summary,
^ 500-odd spectra were taken on 45 nights of the 74 nights assigned to
the author from 1962 to 1965. The unused nights were in general
Weathered out completely. On only two or three nights, generally the first of a run, were equipment problems responsible for total loss
of the night. Six nights were used exclusively for spectra of Class lb
stars on I-N plates to provide comparisons in the Cepheid problem.
Calibration Methods
One of the key problems in the spectrophotometry of the spectra
taken at Flagstaff was the inability to introduce a sensitometric
calibration spectrogram onto the photocathode of the image tube. The
artifice which was used was to calibrate the II a-O plates in the
conventional way with the Bonsack calibration spectrograph and to
use a mean response function for the emulsion. A study was made of
the polychromatic response of the II a-O emulsion applying the theory
of van Kreveld (1934) (Also see Webb, 1936). It was found that the 61 monochromatic H and D curves could be mapped onto each other with excellent conformity over the range from X 3889 to X 5016. However it was also found that a simple arithmetic mean of the monochromatic
H and D curves for XX 4471 and 4713, wavelengths lying on either side of the peak of the convolved P -11 output and II a-O response functions, gave essentially an identical sensitometric calibration. Since the P-ll output color is essentially constant with excitation potential and temperature, as discussed previously, this procedure was adopted for all plate reductions.
We will see in the following section that the photocathode response of the ITT-image .tube employed in all of the observing programs does
depend somewhat on the temperature in the wavelength regions and temperature regime used in this study. ' This would introduce errors of
10-20 percent in attempts to use these indirect calibrations over large wavelength intervals, say 500 Angstroms or so.
Since most of the reductions are for the case of absorption line equivalent widths, however, changes in relative photocathode sensitivity are unimportant.
Temperature Effects on the Photocathode
As pointed out in Chapter II, it is absolutely essential to cool the
S-l photocathode to temperatures of -20 to -50°C in order to suppress the dark current emission of the cesium-treated photocathode. As was pointed out by Young, the effects of cooling on photoelectric devices
are incompletely known. While Young did evaluate two S -1 photo multipliers as part of more general study of temperature effects on multiplier phototubes, he did not isolate the photocathode effects from
those on the electron multiplier in the particular tests. In addition,
problems of icing cast doubt on the results he obtained at nitrogen
temperatures. The tests at dry ice temperatures showed a negligible
change for one tube and an increase for the other of 40% in the X 8000-
10, 000 region. These results were attributed by Young to changes in
the secondary multiplier; he pointed out that one would expect the
response at the extreme tail of the infrared response to be adversely
affected by cooling.
Because the advantage of the converter image tube over
photography is in the region of the response tail, as shown above, it
.was suspected that an optimum photocathode temperature should exist
as a function of exposure time where the expected reduction of
sensitivity due to cooling would not yet be too great but the dark
current would have dropped sufficiently for the desired exposure. It
was also desirable to know if any large variation occured in the
temperature regime from ambient to dry ice since the cold box design
used in the early tests precluded holding of the photocathode at a fixed
temperature, i. e ., the box is stuffed with dry ice at the beginning of
a series of exposures and then gradually outgasses and warms back
toward ambient as the ice sublimes. Since the ice was not used in a
slurry (because the venting of a slurry would be difficult at the
Cassegrain focus due to shifts in the liquid surface with telescope
position), large temperature gradients can exist between the subliming
mass of CO2 and the photocathode faceplate. Hence the photocathode
operated over a wide range of temperature during the course of the 63 2-1/2 to 3-1/2 hour interval between replenishment. If the photo cathode has a negligible temperature coefficient, these variations are irrelevant, affecting only the integrated dark-current fog level on the plate. To investigate these effects in the specific FW-167 tube used in this program, resistance thermometer gages (Micro-Measurements,
Inc. TG-50) were cemented to the photocathode faceplate. At the invitation of Dr. Ford the converter camera was taken to DTM and set up in a comparison configuration with a vacuum photocell of S-l response, RCA 1 P42, at the exit slit of a Beckman Model DU Monochrometer. An unsilvered flat was used to sample the beam for the monitoring phototube. Two General Radio Model 1230 DC
Amplifiers were used to measure the output currents of the two photo cathode s. The image tube was operated as a simple photocell by shorting the phosphor and central anode ring together. Both cells were energized from 300 - volt battery packs. After preliminary measurements a total of five experiments were conducted, three measurements of photocurrent versus wave length with the uncooled image tube and two runs of photocurrent as a function of wavelength during cooling and warming cycles of the image tube. The source lamp was energized by a variable transformer and its current monitored to stabilize against line and lamp variations. The thermometers resistances were monitored at close intervals by a Wheatstone bridge, as well as the lead wire resistances. The manufacturer supplied a calibration of the resistance gages which represented the nominal resistance vs temperature for gages of that 64
production batch. Two independent gages had been cemented to the
faceplate adjacent to the rectangular area where the spectrum was
recorded. The mean value of the gage resistances and the nominal
calibration of the gage batch was used to determine the temperature
of the photocathode faceplate at frequent intervals. The image tube
data were referred to the monitoring cell to remove by normalization
any errors in resetting the monochromator or any lamp variations
which might affect the incident beam onto both the monitor and the
image tube. All photocurrents were reduced to the signal above the
dark current since the dark current was strongly diminished by the
cooling. The measurements suggested that the photocathode increased
. its sensitivity in the region 1.1 to 1.2 microns by as much as 50
percent when the temperature was reduced to -50 °C, while the
apparent change was only 10-15 percent at 0. 8 to 0. 9 microns for the
same reduction of temperature. Despite great care in attempting to
stabilize all components of the experiments, it was not possible to
avoid condensation of moisture inside the tube housing. The validity
of these measurements was in doubt but the possibility of a shift in
relative sensitivity led to incorporation of a thermoelectric cooler
with an active temperature regulation in those cases where spectro-
photometric data were desired.
After the T. E. cooler was obtained it was possible to repeat
the measurements, at least over the range of temperatures provided
by the cooler. The results of such a series of measurements is shown
in Fig. 17, where the measurements have been normalized to those at
room temperature to eliminate scale variations resulting from the use 65 of a set of interference filters being used as the monochrometer.
This was essential because the photocurrent depends on the passband of the filter as well as on the sensitivity; the passbands varied by a factor of 3 to 5.
Data on two image tube S-1 photocathodes are shown, one the FW-167 manufactured in 1962 by ITT and the cathode of an RCA C-
70056E manufactured in 1966. The RCA photocathode exhibits an even larger dependence on temperature than the ITT cathode. While the magnitude of the temperature coefficient differs, the same trends occur. The greatest effect occurs in the region of 1. 1 microns.
These results are in direct contrast to those reached by Malherbe,
Tessier, and Veron (1966) and reported at the Imperial College
o Sym posium . They had experienced a marked decrease in brightness of the phosphor screen as they cooled the tubes in an isothermal chamber down to temperatures as low as -160°C, and inferred from this a decrease in photo sensitivity. The key difference in the experimental procedures used, other than the necessarily different chemical make up of the two photocathodes, is that they relied upon constant luminous gain of their phosphor as the tube was cooled, while the present measures were of the photo current directly. As we had seen in
Chapter Q, the phosphor can have a temperature coefficient, although the results of Charmin and Hewitt gave an increase in luminous efficiency, rather than a decrease, as the temperature was lowered.
It should be noted that the temperature effects are in reality only a small perturbation of the basic infrared tail of the S-l photocathode. 66
Fig. 18 shows the basic S-l spectral response and the response enhanced by cooling to -22° C in the two photocathodes studied. The effect is only important in that the S-l photosurface,despite its low sensitivity, will allow study of the near infrared beyond the limits of hyper sensitized I-Z. The existence of some response even out to
1.7 microns was the main thesis of the paper of Malherbe, et al. The
results here indicate that with the processing of S-l photocathodes used in this country, at least, the residual tail remains and in fact
increases slightly. It would seem possible to detect Paschen (3 at
12, 818 R and perhaps the new coronal line of Si X at 14, 305 A, predicted by Munch in 1966 and verified by Munch and coworkers in the November 1966 eclipse (Munch, et al, 1967).
The enchancement is important under the marginal conditions of detection which occur in astronomical spectroscopy, since the
increase of 40 to 50 percent in sensitivity may be sufficient to reduce exposure time by a factor of nearly 2 where long exposures, with
consequent large reciprocity failures, are required. This is the case for an image tube exposures of 3 hours or so, where the reciprocity correction is a factor of 2. 0 with 11 a-O emulsion. An increase of 50
percent in sensitivity leads one to the lower reciprocity failure (see Fig. 4) of 1.8. The exposure will be reduced by a factor of 1. 7
(1.5 x 2. 0/1. 8). The cooling essential for long exposures with the
S-l cathode reduces the exposures themselves.
Even greater increases in sensitivity are probable. Fig. 19 plots the results on the ITT cathode of the tests at DTM for three
wavelengths in the region of maximum effect. From the data
I i Sx(T)f Sx (ZQOC) 1. 50 1. 1 4( 1. 1. 3( 1. 1 .1( 1. . . 0 2 ( ( TT -0 C -50 T IT f emper ur n vlnt or h I n RA mae ubes. b tu age im RCA and T IT the r fo avelength w and re tu ra e p m te of F ig . 17. - - E n h an cem en t of S -l photocathode se n sitiv ity a s a function function a s a ity sitiv n se photocathode -l S of t en cem an h n E - - 17. . ig F 2° C C70056E RCA -22°C 9 1-0 .9 0 ( cons) n icro (m X W-6 -2 C -22 -167 FW 8 RCA C 18 + 5 RCA C 5 + t Fig. 18. - - Comparison Comparison - - 18. Fig. t
oia om e eaue sensitivity. l - S perature tem room nominal ARBITRARY RESPONSE OF S-l PHOTOCATHODE 0 . 4 2.0 0 . 3 0 0.8 9 . 0 f o MI ) S N O R IC (M cooling-enhanced sensitivity with sensitivity cooling-enhanced 1.0 —RC -22° C ° 2 2 - E 6 5 0 0 7 C CA R — l l i CALE SC X 0 1 1.1 l - S L A N I M O N T FW- 7 6 -1 W F ITT 1.2 3 . 1
IMAGE TUBE PHOTO CURRENT/ROOM TEMPERATURE MONITOR PHOTOCURRENT 4.0 0 . 5 6.0 0 . 7 8.0 0 . 9 0 4 - 0 5 e eaue nIT W17 mg converter image FW-167 ITT in perature tem F ig. 19 . — Variation of S -l photocathode sensitivity with sensitivity photocathode -l S of Variation — . 19 ig. F 0 3 - EPRTR (C) C ( TEMPERATURE -20 10 0 :-~rrT:.rrrn:ii:y^ 10
20 5 9 . 0 18 .1 1 5 0 . 1 0 3 >o O' 70
available it appears that the effect is nearly linear with temperature down to - 20 • C and the enhancement may continue below - 50 • C.
The most important conclusion, however, is that the photo cathode temperature must be stabilized if spectrophotometry of
emission lines or of continuum gradients is to be attempted to higher accuracies than 25 percent. The thermoelectric cooler has been found to be very satisfactory and simple. Dr. Ford has used chilled silicone fluid at Lowell Observatory as have Drs. Firor and Eddy of the High Altitude Observatory on the 1966 airborne solar eclipse
expedition. A freon evaporator system could also be employed with
suitable regulatory valves to achieve a ± 4 0 C stability which would limit the sensitivity variations to less than 5 percent, which is well within the other spectrophotometric errors.
Phosphor Measurements As discussed in Chapter H, the phosphor is one of the most
important elements in the process and one of the least understood.
While many properties of the phosphor were identified as contributing
to the efficiency of the image tube camera, not all of them have been
investigated here, in part because the phosphor of immediate concern
is still within the vacuum bottle of an excellent image tube which
remains intact and useful. All measurements have been made by using the photocathode as the source of stimulating electrons and the electron optics of the camera have been used to direct the electrons to the phosphor. Thus, the properties of the incident
optical image, the thermionic emission of the photocathode and inner
walls of the tube, and the resolution of the electron optics are 71
contributing to the i'cbu Ub . Tests of the phosphor have included:
a. ) Measurement of contrast loss for simulated emission lines including the effects of scattering in the relay optics. An
investigation of scattered light for absorption lines was also made.
b.) Measurement of the emission directivity of the
phosphor/faceplate combination.
c.) Measurement of relaxation effects for short term events, particularly in the time scale of eclipse observations.
Contrast Loss A single emission line of variable intensity and variable width was simulated by use of a microscopic projector of the Baum -
Imperial College Type (Baum, 1962) to image a precision bilateral
slit, Gaertner Model LI64, onto the photocathode of the FW-167. The projector lamp, filtered by a Wratten No. 47 filter, was located behind the slit. The projector was modified to accept the slit at the position normally occupied by the test graticle so that the minification
ratio of 5:1 was maintained. The projected slit could be adjusted from
a few tenths of a micron to 800 microns by the slit micrometer. The
intensity could be varied by introducing different neutral density
filters into the projector. The test arrangement is shown schematically
in Fig. 20. The slit SI is illuminated by the Lamp L placed behind a diffuser
D and neutral density filters F. The microscope Ml produced a real
image of SI at the location S2. If the image tube photocathode is at
S2 there is a conjugate slit image on the phosphor at S3 and a real F D Wr47 mASvww rfls f i ltd miso lnes. lin ission em ulated sim of profiles g 2.--pia Sse o eautn satrd ih and light scattered evaluating for System - -Optical 20. ig. F l M I
O PM image S4 formed by the relay optics R. A second microscope M2 is placed in front of a fixed focal slit S6. This microscope forms a real image S5 which can be made coplanar with S2, S3, or S4. M2, its focal slit, and a photomultiplier PM with suitable Fabry lens FL were mounted on a massive optical comparator so that the slit image
S5 could be scanned across the images of the test slit and the position of S5 measured to ± 1 micron. The fixed slit S6 was 200 microns in width and 1000 to 10, 000 microns high. The scanning image S5 of this slit was reduced by 10:1 or 5:1 by two different microscope objectives M2. The acceptance beam of this scanning microscope was f/3. 5 for the 5:1 objective but only f/4. 5 for the 10:1 objective. For the purpose of evaluating the quality of the image formed by the relay optics, the 5:1 case, with an effective scanning slit of 40 pi x 200 pi, came closest to accepting the entire diverging beam of the relay optics. The geometric quality of the image formed by the relay optics will suffer increasingly from aberations in the additional outer zones of the optics, zones were not sampled by the smaller relative aperture of the scanning objective. The proportion of scattered light received by scanning objective will be less affected by the limited acceptance angle of microscope optics. Stepwise scans of the slit images at various widths indicated that while the central intensity would increase as the projected source slit increased above the size of the projected scanning slit, due to diffraction, the edge profile of the line source remained invariant: negligible scattering was observed in the wings. The slit image was then focussed onto the photocathode and the direct phosphor image 74
scanned with various projected source widths and projected source
intensities. The relay optics were then installed and the aerial image
produced by the relay optics was scanned at various slit widths. This
was done for both the Elgeet and the Burke & James systems. All
multiplier photocurrents were recorded on a strip chart with the aid
of a Keithley Model 614 Picoammeter.
The data were reduced to equivalent peak brightnesses to
account for any intensity gains or losses. Fig. 21 shows the profiles
recorded for a source slit width of 100 microns. A slight degradation
occurs as the result of introduction of first the image tube and then
the relay optics. No appreciable difference is observed between the
two relay optics systems except for an increase in scattered light
and a slight comatic condition which appears on one side when the
Burke & James system is used. The measurements were taken near
but not exactly on the optic axis of the image tube camera. Very
little light is lost from the line into the adjacent continuum.
The second intent of these measurements was to examine the
edge contour of the line, especially the wing, for increases in
scattered light as the slit width was increased. This would simulate
the contribution of an increasingly extended continuum on one side of
an absorption line. A true absorption feature would be the sum of
the effects on the sides. Fig. 22 illustrates the measurements
. obtained as the simulated emission region is increased from 60
microns to 800 microns in width. This corresponds to distances on
the plate of approximately two resolution elements to 25 resolution
elements. The measurements cited here were on a two-stage RCA — =*100 PG2 — = 1 0 0 ,P G 4 A =100 ,PG5 Io=100 DIRECT PHOSPHOR •« 94 AIRIAL IMAGE
3 0 -
20- -
10 -
____ MICRONS ...... Fig. 21.-- Comparison of simulated 100-micron emission line scanned as a (1) direct aerial image (2) phosphor image and (3) by the two relay optics systems used in this study. RELATIVE IN T E N S IT Y profile of an em ission area. ission em an of profile Fig. 22. — E ffect of increased extent of continuum on edge on continuum of extent increased of ffect E — 22. Fig. 0 0 6
—. ■* 400 _ _ _
_ STANCE. MI ) S N O R IC (M . E C N A T IS D 200 0 " " s n icro m 0 0 4 0 - 0 — 8 — SSI IDTH W N IO S IS M E - -
200 " "
200 -4 O' 77
C-70056E IR converter tube rather than the FW-167, and were made
through the Elgeet relay optics. This was done because the scattering
was expected to be more severe and easier to detect in the two-stage
tube. An increase in scattered light to 1 or 2 percent of the emission
region is observed in the wings as the total illuminated phosphor area
increases. This increase in background occurred, however, over
the entire tube and is attributable to the general trapping of light in TKc'faceplate by internal reflection rather than to a local scattering of light within the phosphor to regions immediately adjacent to the
continuum area. From these data it appears that the somewhat soft appearance
of the image tube spectra result from general loss of resolution and
a general scattered light background rather than from local scattering
into the adjacent resolution elements. Hence the equivalent widths of emission lines should be well preserved. The wing profiles of the wide slits are not appreciably altered by the increased extent of the
continuum resulting from the wider slit. Absorption lines of moderate strength should then be preserved in equivalent width since
only the most adjacent continuum under the wings can contribute
scattered light to the core of the line.
Simulation of Absorption Lines In an attempt to examine the effect of scattered light on the cores of weak absorption lines and its effect on image contrast, (light
presumably trapped by internal scattering from the faceplate since the localized scattering was found to be small), absorption lines were
simulated by attaching thin slivers of Wratten neutral-density gelatin 78 filter onto the photocathode faceplate of the ITT FW-167. These were not optically contacted, which caused interference shadows near the edges, but it was possible to measure the central intensity of the absorption region with a scanning slit of 38 microns apparent width.
The continuum against which the filters were observed was formed by placing an intense point source at a distance of 3 meters from the faceplate. Since the gelatin filters are not neutral in the IR the spectral range of the point source was restricted to the visable by interposing a heat absorbing filter in the beam. A mask some 4 mm wide and 40 mm long defined the continuum width and length. The phosphor brightness was about that which would be recorded in 5 seconds on baked II a-O emulsion. Figure 23 shows the recorded absorption at the central core vs the transmission of the main filter sheet measured by direct inter- position before the photometer. The simulated line widths are also tabulated. These were very "broad" features being some 10 to 30 resolution elements wide. The agreement is excellent, with no tendency shown for scattering into weak lines of broad extent. This
confirms the conclusion reached above that local scattering is limited
in its lateral range, reaching to only a few tens of microns (a few
resolution elements) even in a two-stage tube. A measurement which was desirable, but which was easier in
concept than in conduct, is the simulation of a narrowly deep absorption
of known central depth. The presence of dark interference bands at
the edges of the narrow filters, the practical difficulty in cutting filters
narrower than 0. 5 millimeters, and a practical limit of some 30 DENSITY AT LINE CENTER est o Bl le Material M ilter F Bulk of Density g 2. oprsn f i ltd bopin ie wt the with Lines Absorption ulated sim of Comparison - - 23. ig. F 0.095 .0 380 0.320 0.208 .2 400 0.493 0.400 0.320 D AS D FLE DENSI Y IT S N E D FILTER ED R SU EA M DTH T ID W . 03 0;4 0.3 0.2 400 890 650 650 ____ 79
80 microns on the size of the projected scanning slit tended to place a limit on the width of the simulated feature at 200 y or so.
Directivity of Light Emitted by the Phosphor
One of the practical problems of the non-contact image tube, i. e ., conventional faceplate rather than a mica window or a fiber- optics faceplate where one transmits the light to the emulsion by pressing the emulsion against the outer surface of the faceplate, is the design of reimaging optics to couple the phospor image to the recording emulsion. To select the type of optics system to be used, it is of interest to know the directivity of the luminous output from resolution elements in the phosphor image. Since the ITT phosphor is applied to the inner face of a borosilicate glass medium with an index of refraction of approximately 1. 48, a certain amount of forward focussing would be expected in the polar emission pattern as the result of refraction and reflection processes. The internal scatter ing properties of the phosphor crystals and the effect of the aluminum backing layer are also of possible importance in determining the angular distribution of the emitted energy.
If strong forward, directivity is present, the use of refractive optics for reimaging would be favored over Schmidt or other reflective systems which would involve center-blocking, since the central region might contain a high percentage of the emitted flux. Measurements made by Kapany, and Capellaro (1961) indicated that settled phosphors, the type used in current image tubes, tend to behave as non-Lambertian sources in terms of the polar emission pattern within the glass substrate.
The emergence of the rays from the faceplate should restore the 81 Lambertian distribution if the grain scattering and backing effects are not important. Two limiting cases exist for the transmission of the light from the phosphor through the faceplate and thence to the reimaging optics. These are illustrated in Fig. 24. If the phosphor particles are in optical contact with the faceplatei the rays experience no refraction entering the glass medium and diverge directly into the faceplate in a 2ir steradian cone. The extreme rays will suffer total internal reflection as they attempt to leave the outer surface of the faceplate and re-appear only as scattered light. The rays which can escape will lie in a beam peaked forward by the refractive divergence of the rays lying near the critical angle determined by the refractive index of the faceplate. Alternately, if the phosphor particles are optically isolated from the faceplate, a small forward peaking should. occur even though the original directions of divergence are reassumed by tho cscAplng rays. Fltf. 24 indicate ft the location of the virtual phosphor source for the two cases. Since Fresnel reflection will become important in the increasingly inclined rays, the reflective losses both entering and leaving the faceplate will increase with the angle ii. Similar Fresnel reflection losses will occur for the total optical contact case but will operate only once as the ray emerges from the faceplate. For the non-contact case the intensity of the ray which emerges from the faceplate in a given direction may be calculated by considering the redistribution of flux in the internal zone at i of angular width di onto the external zone at 6 of angular width dd. The flux entering I the zone is NO OPTICAL CONTACT TOTAL OPTICAL CONTACT n - M 5r
Fig. 24. -- Geom etry for refractive effects on the location of apparent phosphor source and the trapping of light in an im age- oc? tubc faceplate for two extrem e cases of optical contact. N 83 p art2tr dF - \ I • sin i • di • d 0
= 2ir I • sin i • di (1)
Similarly the flux leaving this same zone is
dF 1 = dF = 2ir • I1 . sin 0 ' d 0 (2) with the exception of the Fresnel reflection losses which must be treated as a small correction. Equating (1) and (2), introducing
Snell's law and its derivative
sin 6 = —n sin i (3)
= 4--fsrsd 1 <4>
I' = — • I • ?°s 6 (5) n 2 cos i
If the initial distribution of I is Lambertian,
I' = — -— - I cos i • 9 = — • I • cos 9 n2 ° cos i n2
Thus the luminous intensity distribution remains Lambertian for the total optical contact case, but the intensity is reduced by the action of the refractive dispersion of the emergent light. This reduction should not be detectable from the distribtuion of the emitted flux.
The totally reflected light must emerge at some other region of the phosphor after rescattering from the phosphor or its backing. Total optical contact phosphors should suffer a rather high contrast loss 84 due to this internal reflection.
In an experiment to compare the angular distribution of the ITT
FW-167 tube with the expectations of these two cases, measurements were made by the author in collaboration with Dr. Ford during a visit to the Department of Terrestrial Magnetism in 1964. The tube in its magnet array was supported on a rotary table along with a constant source, target mask, and optics to project a small uniform spot onto the photocathode. The resulting uniform spot on the phosphor was observed through a small mask on the faceplate about 10 mm in diameter to reduce the effects of scattered light without occulting the emission spot. A photomultiplier was used to measure the i'lux inter cepted by another 10 mm aperture some 32-cm distant. The photo current was recorded while the tube and projector were rotated about an axis passing through the plane of the phosphor. By introducing a constant phosphor source immediately before the second mask, it was found that the multiplier anode current was affected in a reproducible way by the rotation of the image tube magnet, since no mu-metal shield was used on the multiplier. After correction for the disturbance of the electrons in the P. M. dynode chain by the field of the rotated focus magnet, the luminous intensity of the phosphor was found to be as shown in Fig. 25. Also plotted is the Lambertian distribution which would result from a phosphor without optical contact, but corrected for reflection losses, and the emergent distribution calculated for total optical contact. The experimental distribution
is more forward peaked than either of the idealized models. This is
probably the result of scattering by the aluminum backing and the 85
!
i \ j LAMBERTIAN * LAMBERTIAN i I W R E FLECTION f \ L O SS 7 MEASURED ITT t I F W -1 6 7
I1 1 J I F / 1 . 0
f / 1 . 2
5
\i f / 2 . 0 i i f / 3 . 2
Fig. 25. - -Distribution of luminous flux measured for t h e ITT FW-167 image tube compared with that computed for Lambertian scattering without and with internal reflection lo sse s. 8 6 phosphor particles themselves. The effect of the slight peaking is small on the choice of reimaging optics. Table 5 gives the incremental increase in flux transferred to the emulsion by perfect optics as the solid angle of the collecting aperture is increased by
5-degree zones in the case of the uncoupled Lambertian phosphor and the actual ITT phosphor. Also listed are the zonal increments for a 50-percent centerblocked system such as would occur with
Schmidt-type or Bowen-type concentric-sphere systems. The forward peaking is too small to be of serious concern in comparison with internal losses of absorption and reflection, especially the latter, since the scattered light may reappear somewhere else in the image.
Absorption and Scattering from P -ll Phosphor in Refractive Systems During these experiments several optical systems were used in an attempt to improve the useful field, the definition, and the speed of the system. All of these were "available" systems in that they were obtained from vendors without requesting special design considerations. The Biotar system (2 lenses front-to-front, f/1. 5, F = 85 mm) was found to have good resolution on axis but quickly suffered vignetting and loss of resolution toward the edges of the field. For most of this work a specially-mounted pair of lenses (f/2. 0 F = 50 mm, front-to-front), marketed by Burke and James, Inc. (Cat. No. 14011A) for television applications, was used since the system gave equally good axial resolution but better definition at the edges and less vignetting. Another similar Burke and James system (f/2. 6, F = 75 mm) was tested and found rather poor in resolution. All of these systems exhibited color in the transmitted light, looking somewhat 87
TA B LE 5
Luminous Flux Transferred to an Image Formed by Perfect Relay
Optics as Zones are Added to the Aperture to Increase the Angle
Substended of the Optics by 5-Degree Increments as seen from the
Phosphor Screen (see sketch on p. 29).
No Centerblocking 50% Centerblocking Lambertian Phosphor on ITT 6 n=1.48 Phosphor (Degrees) f/no. Faceplate Measured Lambertian ITT Phosphor 5 5.8 0. 0076 0. 0076 10 2.8 0. 0302 0.0302 0. 0226 0. 0226 15 1.9 0.0669 0. 0667 20 1.4 0. 1160 0.1153 0. 0858 0. 0851 25 1. 1 0. 1761 0.1739 30 0.87 0. 2441 0. 2390 0. 1772 0. 1723 35 0.71 0. 3192 0. 3096 40 0. 6 0. 3975 0. 3811 0. 2815 0. 2658 45 0. 5 0. 4747 0. 4489 50 0 .4 2 0. 5509 0. 5125 0.3748 0. 3386 8 8 yellow in appearance. Since the P -ll phosphor is used for its high actinic blue output,
the transmissions of these lenses were compared both in white light
from a fluorescent lamp and in blue light by use of a C-5 filter over
the white source. Vignetting measurements were made by estimating
the area of the lens providing flux as it was rotated on a table. The
results of these comparisons and the performance of a new lens that
became available in 1964 are given in Table 6. The last two columns
of the table list relative efficiency factors defined for two different
spectroscopic applications of the camera system. For the case that
a large wavelength interval is to be recorded, an interval greater than
the unvignetted region obtained in a single spectrum the relative
efficiency factor is
_ (Useful Field Diam. ) .. Blue Trans Eff. - ( 46 mm )------x (f/no. )2
This factor allows comparison on the basis that the entire spectral
region across the 40 mm photocathode is to be recorded, if not in a
single spectrum, then by incremental steps. This efficiency factor
does not account for relative resolution differences, requiring more dispersion to detect a given feature, or for the effects of seal in red light on the contrast obtainable. If the image tube is the limiting element
by having a resolution on its image inferior to that of the reimaging
optics, as is the case in the two-stage tubes, these lenses are of equivalent resolution as is assumed here. If it is desired to record
only a small region of wavelengths, such as for interstellar lines, or TA BLE 6 COMPARISON OF DIFFERENT REIMAGING OPTICS TESTED
R el. Eff. Working Working White Light Useful Field Whole Narrow Lens______f/n o . _____Distance _____Trans _____White + C5 filter Diameter * Spectrum Factor
Two f/1 . 5 85 mm Not Not 12 mm 0. 067 0. 22 Biotar M easured M easured f/1 . 5 0. 60 est 0. 50 est 85 mm
Burke & f /2 .0 50 mm 0.62 0. 54 28 mm 0. 095 0. 14 Jam es i f/1 .0 2"
Burke & f/2 . 6 75 mm 0. 48 0. 16 R esol 0 0 Jam es le s s than f/1. 3/75 mm 30 1pm
E lgeet f / 2 . 4 70 mm 0.59 0. 58 10 0. 025 0. 10 O scillo Navitar f/1. 2/85 mm
E lgeet stopped 70 mm 0. 59 0. 58 40 mm 0. 045 0. 045 O scillo to Navitar f /3 .6 f/1. 2/85 mm 0 Nj # Less than 50 percent vignetting and resolution better than 35 line pr/mm 90 for a single feature such as the Ol triplet at X 7774, as was done here, the useful field diameter becomes unimportant and the relative efficiency becomes simply
_ Blue Trans R el* Eff* - ""(f/'nd: )*
From the standpoint of scattered light and visual quality of the recorded spectra, the Elgeet lens operating at f/3. 6 gave by far the best spectra, but at a price of three times in exposure time.
Relaxation Effects in the P -ll Phosphor Because the FW-167 tube was to be used as a detector in observ ing the flash spectrum in the eclipse of 30 May 1965, the question of equilibrium of the image brightness with the target brightness was of real concern. The solar chromosphere has a scale height for most metallic lines of 1000 to 3000 km (Thomas and Athay, 1961) correspond* ing to an angular extent of 1. 3 to 4 arc seconds. The lunar limb in
1965 advanced over the chromosphere at a rate of 0.48 arc seconds/
second. In some eclipses this may be higher by a small factor. Thus
the scale height was to be covered in 2. 5 to 7 seconds in this relatively
fast eclipse. (Moon near to perigee and the earth near aphelion) The
change of intensity of the spectral lines would be expected to be some
10 percent in 250 m sec., the interval between spectra at the camera
framing rate employed. To measure the response time of the phosphor, two types of
experiments were conducted. The first projected a constant source
either onto a Com pur-type shutter, whose open interval was varied I
91
from some 10 m sec to 140 m sec or more, or onto a rotating sector
whose rate and sector opening were variable. The second test
employed a short duration flash to examine the risetime of the
phosphor and less exactly its persistence.
In the first tests the light which passed the shutter was reimaged
onto the photocathode in a general spot some 1-cm in diameter. The
phosphor image of this area was monitored by an S- 20 photomultiplier
of the RCA C 70038D type coupled to an oscilloscope. The resulting
traces for many settings of the camera shutter are superimposed in
Fig. 26. Two effects can be observed. The first is an apparent rise
to 0. 75 of the DC value in 4 milliseconds or less, the second is the
• slow increase from this value over many trace times to reach the
steady state level. After 100 m sec, the trace has reached 0. 90 of
steady state, but only after about one second do we obtain the ultimate
screen brightness. The e-folding time of this slow rise can be
computed from these data as 330 m sec based on the transition from
0. 86 steady state to 0. 91 steady state in 120 m sec. Approximately
3 e-folding times are required to effect the transition from 0. 86 lmax
to °' 99 'max' Fig. 26 also shows for comparison, the trace produced by the
monitoring photomultiplier observing the shuttered source directly.
At the sweep speed used, it is not possible to see the initial transition
time, which is determined by the mechanical shutter inertia. The
initial rise of the phosphor is essentially the same. The shutter system
allowed measurement of only these long term effects. The mechanical
inertia of such a system was too great to allow establishment of the full 92 source in less than one or two millseconds.
To examine the short response transients, a stroboscopic source
was used (General Radio Strobotac). This was flashed at the lowest
rate (approximately two pulses per second) while the image tube photo
cathode current signal and the phosphor signal were measured. The
incident flash shape was monitored by using an RCA 922 vacuum
photocell mounted directly on the terminals of the oscilloscope so as
to eliminate all possible cable capacitance. Fig. 27 shows the results
of these measurements and also the response of the phosphor monitor
exposed directly to the flash lamp From the figure it is clear that the
rise times of the monitor, the image tube photocurrent, and the
- phosphor are consistent with each other and sufficiently prompt that
the peak value is reached in less than 100 microseconds, the actual
response in the displayed signal due more to cable capacitance than
to the phosphor delay. The multiplier rise time from 0. 10 to 0. 90 is
30 microseconds, as is the rise time of the image tube photocurrent.
The combined multiplier/phosphor rise time using the same definition is 40 microseconds.
In the decay process both the monitoring photomultiplier and the
indicated image tube phosphor brightness exhibit measurable lag, the
difference being attributable to the phosphor relaxation. The multiplier
alone exhibits a response from 0. 90 to 0.10 of the peak value in 130 microseconds whereas the phosphor requires 210 microseconds using
the same definition. The P -11 phosphor prompt decay thus occurs in
some 100 microseconds which is in agreement with the data as cited in Chapter U. A longer decay is exhibited in Fig. 26 including the Fig. 26 -- Phosphor output for photocathode exposure times of 1/126, 1/60, 1/30, 1/15 and 1/8 second. Sweep speed. 20 m se c/c m .
Fig. 27 -- Phosphor output when exposed to light of a stroboscopic lamp compard to the lamp discharge it self and the response of the monitoring photomultiplier exposed directly to the lamp. 94 shutter closure, is to 0.01 of the steady-state value in 50 milliseconds or so.
Interpretation
The cathode luminescent excitation of the P -ll phosphor, at least at high light levels, may be a two-step process where the initial response is prompt (within 70 microseconds) up to some 0. 75 of the
DC brightness. An additional luminous output level is reached in about 1 second or so, this additional brightness accounting for only
10 percent or so of the total output. Absolute spectrophotometry of short duration events by image tubes must consider this possible effect in calibration procedures if the measurements are undertaken by shuttering the light falling on the photocathode. If the phosphor is to reproduce as short duration pulse, such a spectra from a shock tube, it is possible that use of DC arc techniques cannot be used for intensity calibration without introduction of a shutter system.
In the actual eclipse experiment it was decided to expose the photocathode to the sun continuously so that the phosphor was excited to the slowly varying flux of the chromosphere. Any rise or decay times of the order measured in the lab were of no practical consequence in this arrangement since the image stability was high and the response times remained small (0. 3 seconds) compared to the time scale of the events (2-5 seconds) in the chromosphere.
Summary of Laboratory Results
In the present work, four principal experiments were conducted by the author to examine physical effects in image tubes which might
I affect their straight -forward application in absorption spectro photometry in the near 1R. These were temperature effects in S-l photocathodes, effects of local scattering in the phosphor and in the reimaging optics, the effects of scattering and refraction on the direction of the luminous output from the phosphor and its effect on the design of reimaging optics, and the establishment of relaxation effects in the phosphor which are important for measurements of less than 1 second duration. The temperature effects were found at variance with conclusions reached by others. The directivity of the phosphor output was determined for the case of reimaging systems, not for fiber optics coupling, as had been done by others. C H A PTE R IV
COMPARISON OF IMAGE TUBE MEASUREMENTS
WITH DIRECT PLATES
Supergiant Measurements
Keenan and Hynek (1945) have discussed the possibilities of using near infrared spectra for luminosity classification and (1950) have
studied the luminosity dependence of the total absorption of the neutral
oxygen triplet at A 7774 in the atmospheres of stars of types BO to G4 using hypersensitisa'ud 1-N emulsion at plate scales of 50 A/ mm. In
his pioneering survey with the first high-speed IR emulsions, Merrill
(1934) had noticed this OI feature to be abnormally enhanced in super-
luminous stars of type A. Keenan and Hynek established quantitatively
the strong dependence of the total X7774 absorption on MKK luminosity
class and on spectral type. More recently S. B. Parsons (1964) has re
examined this problem measuring the central depth of the feature rather
than total absorption, reaching conclusions not materially different from
those of Keenan and Hynek. Parsons was limited in his discussions of
supergiant stars by having little new material, having observed only
two stars of class la and three of lb. The original work of Keenan and Hynek was based on single plates of six stars of class la and 15 plates
of 14 stars in Class lb. An extension of these measurements was
believed worthwhile to additional stars of established MK classification
and to additional stars classified as "c" on the Mt. Wilson system.
96 97 Most recently an atlas of 1R stellar spectra of 85 stars of quite early types (06-A2) has been published by Andrillat and Houziaux (1967).
These spectra, taken on hyper sensitized I-N plates at dispersions of
40 and 20 A>/ mm, are primarily of dwarfs, and have been published with commentary of great value in evaluating the use of the near IR region for spectral classification. Three-fourths of their program stars were peculiar stars, either shell stars or emission-line stars, and show anomalous strength of the OI feature in absorption or show OI in emission, (Sletteback, 1951). Only three stars of Andrillat and Houz iaux program were normal supergiants in the MK classification,so that their results are limited in use in this present study.
This earlier work showed a clear potential for general use of this feature in spectral classification with a clear separation of luminosity classes la and lb and with a lesser capability for separation
of giants and subgiants. Utilization of this feature for spectral classi fication, particularly for luminosity class and for detection of super-
giants in spectral types B3 to BO, has been limited by the inordinately long exposure times required to record early-type blue-white stars in
the infrared. The use of H y equivalent widths determinable at plate
scales of 50 A/ mm, has proven to be efficient for stars of type B and
early A (Petrie and Munsell, 1949), (Petrie, 1952), Williams, 1936).
For later types, A5 - GO, the classification can again be effected by the ratios of metallic to hydrogen lines in the blue and violet regions and
by the G band. Thus, spectral classification of stars of the earliest
types will be more efficient by direct photographic spectroscopy in
the blue, where film sensitivity is higher, than in the infrared. At 7800 A both the continuum brightness of early-type stars and film sensitivities are lower, both conditions increasing the exposure times required to obtain spectra of a given dispersion. Spectral classifica tion will continue to be effected in the blue region despite any advan tages of the OI triplet, unless either a lower dispersion can be used in the IR or an image detector of higher efficiency than I-N emulsion can be used. Alternately, if suitable narrow-band photometry could be devised to measure the OI feature, one might eliminate the necessity to record the spectrum with an image detector. More will be said on this point later. Another problem which has received considerable attention in recent years is that of the absolute magnitude and intrinsic color of
cepheid variables in their role as distance and interstellar reddening
indicators. Since the cepheids are known to lie in luminosity classes lb and lab observations of several of the brighter cepheids was under taken to test the OI feature in as a measure of the luminosity and
spectral type of the cepheids. Because this feature lies near the peak sensitivity of the S-l
photocathode, the comparison of image tube measurements of equiva
lent width with the direct photographic measurements of Keenan and
Hynek was chosen as a principal field experiment on the quantitative
usefulness of the image converter camera for absorption spectroscopy. The anticipated higher efficiency over I-N plates offered hope that the infrared region would be opened for spectral classification.
Spectra of supergiant stars and cepheids were taken on 20 nights
with the image converter camera at a reciprocal dispersion of 60 99
Angstroms per millimeter. Spectra were taken by direct photography
on six nights with the Gp camera on non-hypersensitized I-N plates as
a check on exposure time and general quality of the image-tube spectra.
These direct plates were at a plate scale of 65 AV mm. The plates were
reduced by transmission microdensitometry with calibration sensitom-
etry applied to each plate. Monochromatic sensitometry was used on
the 1-N plates, and the mean heterochromatic sensitometry on the II
a-O plates. The equivalent widths were obtained by Simpson's rule in
integration of the profiles. The mean results of both series of plates are
shown in Figs. 28 to 30, where the data of Keenan and Hynek and of Parsons are also plotted. Table 7 gives the numerical mean values
.. of the image tube and direct plates for 15 stars of luminosity classes la,
Table 8 the results for 36 stars of class lb, and Table 9 the results
of classes II, QI, IV, and V. No direct plates were taken of stars less luminous than lb. The number of spectra of each star is noted where
multiple plates were obtained.
D iscussion
Referring to Fig. 28 we observe that the scatter of the points is
quite large. There is a tendency for the equivalent widths from the
image tube spectra to lie below the values determined by direct plates
but this is not always the case. Fig. 31 compares the results of the
two techniques on stars observed in common. A systematic over estimate is present in the image tube data below W-of 1. oA* and an
underestimate is made above unity equivalent width. The mean linear
relation is found to be 1----- x IT ♦ K & H ASP • I - N 2.5 A. * x. ♦ 2.0 + x x
1 . 5 x I £ 1.0
0 . 5
J. BO B4 B8 AO A4 FO F4 F8 GO G4 G8 KO K4 MO M4 SPECTRAL TYPE Fig. 28. -- Variation of Equivalent Width of A 7774 with Spectral Type, Luminosity Class la. 100 23
2.0 + K & H a S P
• +
0 . 5
BO B4 B8 AO A 4 FO F4 F8 G O G 4 G 8 KO K 4 MO SPECTRAL TYPE i
i F i g . 29. -- Variation of Equivalent Width of a 7774 I w i t h Spectral Type, Luminosity Class lb. i— r— I " i— r — i— t— r -— i ~ i — i— i— i— i— i— i— i— i— ° - t
9 IT-CLASS II X IT -C L A S S III
0 IT -C L A S S V
0
0 0 X X
B i i i i i l i i i t i i t i i 1 BO B 4 B8 AO A4 FO F4 F8 G O G 4 G 8 KO SPECTRAL TYPE
Fig.30 . -- Variation of Equivalent Width A7774 with Spectral Type, Luminosity Classes II-V TABLE 7
A 7774 FEATURE EQUIVALENT WIDTH la SUPERGIANTS
Mean No. I-N Mean No. IT Plates Star Sp Type WI-N WIT Plates W KH a Ab
€ Ori BOIa 0 0 (0) o 2 CMa B3Ia 1.03 P Ori B8 Ia 2. 00 1.63 (2. 0) a Cyg B9Iab 1.79 2. 18 1.72 HR 1035 B9Ia 1.19 HR 1040 AOIa 1.22 2. 53 a Cyg A 2Ia 2.32 2. 19 € Aur FOIap 2.62 (2) 2.02 (6) 2.34 0 Cas FOIa 89 Her F2Ia 1.90 1. 14 F F Aql F5Ia- F 8Ia 1.00 (2) 6 CMa F 8Ia 1.42 (2) 1.31 (4) 1.88 p Cas F 8Ia 2. 28 (2) 2. 11 (2) HR 8752 GOIa 2. 04 1.83 (2) (5) 103 HD 25878 CGI 1. 90 TABLE 7 (CONTINUED)
Mean No. I-N Mean No. IT Star Sp Type P lates Plates WI-N WIT w KH a Ab
R W Cep KOIa 0. 26 (2) 0. 47 VV Cep M2epla 0
a Equivalent width from Keenan & Hynek (1950) b Total Absorption from S. B. Parsons (1964) TABLE 8
X 7774 EQUIVALENT WIDTHS - CLASS lb SUPERGIANTS
' Mean No. I-N . Mean No. IT P lates Star Sp Type WI-N WIT P lates w KH a A»
K Ori B05Ib (0)C (2) (0) ? P er B llb - (0)(0. 1) p Leo B llb (0) (0) 67 Oph B5Ib 1. 12 7} Leo AOIb 1.04 1. 84 a Lep FOIb 1. 22 1.17 (3) 0. 96 16 Vul F2Ib(Case) 0. 61 V Aql j F2Ib 1. 2 41 Cyg F4Ib(Keenan) 1. 35 1. 28 (1. 2) a Per F5Ib 1. 11 0.79 (4) 1. 15 1. 1 a Dra F7Ib 1. 19 0 .7 6 7 Cyg F 8Ib 1. 24 1. 12 1. 16 1. 3 HR 7008 F9Ib 0.72 14 P er GOIb 0.54 (2) P Aqr GOIb 0.83 0. 86 0. 54 HR 207 GOIb 0.65 I
1 *\ V TABLE 8 (CONTINUED)
Mean No. I-N Mean No. IT Sp Type Plates Plates Star WI-N WIT WKHa ^
p Cam GOIb 0 .4 2 0. 47 51 P er GOIb 0.49 o Aqr G2lb 0. 29 0. 53 (2) (0. 6) HR 969 cG2 0. 15 (2) P Dra G2lb 0. 39 (0.5) £ Mon G2Ib 1.00 (3) 31 Mon G2lb 0. 64 (2) 25 Gem G2Ib 0 58 Per cG2 (0. 2) 11 Pup cG2 0. 68 (2) 52 P er cG3 + A5 0 (2) 9 Peg G5Ib 0 0. 39 (0.4) if/ And G5Ib - 0. 1 (2) HR 9053 G5Ib 0 C Gem G8Ib 0. 19 0. 23 (2) £ Cep K llb 0 0 HR 8952 KOIab (0) o O' TA B LE 8 (CONTINUED)
i
J ! ' Mean No. I-N Mean No. IT Star Sp Type P lates Plates Ab WI-N I^IT w KH a
€ Peg K2Ib 0 • jo (3)
a Equivalent width from Keenan & Hynek (1950) b Total Absorption from S. B. Parsons (1964) c Denotes eye estimate TA BLE 9
X 7774 EQUIVALENT WIDTH - LUMINOSITY CLASSES II-V
1 Mean No. IT i Star Sp Type P lates WIT w KH a 1 Ab
p UMa A1V 0. 56 (2) 6 Leo A4V 1.35 0.92 P Tri A5III 0.74 p Ari A5V 0.61 0.88 1. 1 6 Cas A5V 0. 58 0. 85 ^ Leo FOIII 0.48 0. 64 P Cas F2III 0.76 (2) 0. 96 0 .9 a CMi F5IV 0.74 0.43 0 UMa F6III 0. 31 (0. 3)c 1 Peg F6III-IV 0. 39 (0.3) P Leo F 8 V 0.76 0.78 a Equivalent width from Keenan & Hynek (1950) b Total Absorption from S. P. Parsons (1964) c Denotes eye estimate 109
5
2.0
IT
.0
5
0 0 .4 0.8 1.2 1.6 2.0 2 .4
Fig. 31. - -Comparison of Image Tube and I-N Plates on Common Program Stars. n o WIT = 0.25 + 0.735 WI-N
An independent comparison is possible with the measurements of
Keenan and Hynek. Fig. 32 gives the comparison of the image tube measures and the I-N measures for the stars common to the programs.
The mean curve is
Wk k = °- °9 + °* 92 WKH
which is felt to be a satisfactory relation considering the limited
amount of data and the scatter in spectrophotometric measurements.
Before attempting to derive a mean relationship of the X7774
equivalent width from the measurements, the probable errors of the
two series of plates were computed for the stars with two or more measurements. These are shown in Fig. 33. While the statistics
are poor, it is seen that the image tube spectra have increasing un- o certainty for the absorption values below 1. 0 A equivalent width,
leading to the increasing overestimate noted above. The large scatter in € Auriga (p. e. = 0. 38) results perhaps from its having been used
as a test object each time the apparatus was disassembled and the poor plates have been included as well as the better ones. The general run of probable errors is indicated by the curves fitted by eye. In an attempt to arrive at a weighted mean curve for the supergiant stars, classes la and lb, the data of all sources was combined for luminosity class la, since only 34 measurements of
16 stars were available even then. The direct combination is considered
proper since (1) the bulk of the additional measures were from Keenan
and Hynek which showed a small difference in moan values from I l l
• IMAGE TUBE
X DIRECT l-N 2.5
2.0
IT M E A N
5
0
0.5
0 0.5 2.0 2.5 ' Fig. 32 . --Comparison of the Measurements of the present program with ; those of Keenan and Hynek (1950) 0 . 4
0.2 l-T
0.1 l - N
0 1.0 2.0 W
Fig. 33. -- Probable error vs equivalent width by image tube plates and by direct I-N plates. 112 113
the present investigation and (2) the two results of Parsons are gener
ally consistent, one being identical with K and H.
Each observation was weighted by the reciprocal of the p.e. of
its own equivalent width W~(n) or, if only one measure had been made,
by the mean p. e. from Fig. 33. The mean relationship of equivalent
width with spectral type was then obtained by applying a Gaussian filter
to the weighted observations. This procedure averaged each observa
tion with the contribution of several of the adjacent observations (10 on
either side) while additionally reducing the weight of the adjacent point
by an exponential function of half width of 2. 0 subtypes, that is the mean
value was less and less affected by observations of earlier or later types.
~ The mean value W*(n) assigned to each observation was computed from n + 10 -(x(n)-x(k))2(0. 02)2 ^ W(n) e
W(n) = n - 10 n + 10 -(x(n)-x(k))2/ (0. 02)2
where x(n) is the coordinate of spectral type, one sub-type = 0. 01.
Luminosity Class la
The smoothed observations and a visually-fitted curve are shown
in Fig. 34, along with the result of Keenan and Hynek. No significant
differences are noticeable. The high luminosity Cepheid, FF Aquilae,
was not included in the reduction. Of the program stars classified only as c in my finding list, only
one was found to be in the class la, HD 25878. This star has been
studied by Bidelman (1948) and the spectrum found to resemble the
0 7774 < 2.5 2.0 0.5 8 AO B8 i. 4 en aito o 77 faue with feature 7774 A of variation Mean - - 34. Fig. pcrl ye nlmioiy ls la. class inosity lum in type spectral A4 FO PCRL TYPE SPECTRAL 8 GO F8 HYNEK ( ) 0 5 9 (1 K E N Y H & N A N E E K RSN RESULT PRESENT KOG 8 G 4
4 MO K4
M4 114 115 maximum-light spectrum of R Cr B, a "hot carbon star". The measured equivalent width of 1. 90 J? is totally consistent with the
mean curve for normal supergiants at Gila.
Luminosity Class lb.
Only the equivalent widths determined in this investigation
were used in determining the mean curve. The material consisted
of 39 measures of stars. Each measure was assigned a weight in accord with Its probable error. The half-width of the exponential
filter was increased to 3 subtypes to improve the smoothing. The
resulting data and the mean curve as visually fitted to the means of
.the equivalent widths at each subtype are shown in Fig. 35. Also
plotted is the mean variation adopted by Keenan and Hynek. The difference in spectral types B1 to F4 is poorly established. It is
based entirely on three measurements, I-N plates of Keenan and
Hynek of 67 Orphiucis (B5Ib:'W = 1. 22 £ ) and rj Leo (FOIb: W" =
1. 68 J&) and one image tube plate of r) Leo which yielded W = 1. 04
The relationship for stars later than F4 is more strongly dependent
on spectral type than adopted in the previous work. This has two consequences. It reduces somewhat the utility of
the OI feature for luminosity lb separation from giants unless the spectral type is well established. If, however, the luminosity class
can be established as lb, the OI feature becomes a sensitive indica tor of spectral type. One such instance is the variation in intrinsic
colors and effective temperature in cepheids where the luminosity class has been established as lb on the basis of statistical parallaxes
of galactic cepheids and by the application of Wesselink's method for < 7774 . BO 2.0 BO pcrl ye n uioiy as lb. lass C Luminosity in Type Spectral . g i F 5 en aito o A77 Faue with Feature A 7774 of variation Mean - - 35. KO 8 G 4 G 4 A PCRL TYPE SPECTRAL HYNEK (1950) 0 5 9 1 ( K E N Y H & N A N E E K RSN RESULT PRESENT
MO 4 K
116 117 determination of the absolute radius of the star. This application is discussed in the next section.
Of the 36 program stars selected from the super giant lists of
Keenan, five had been classified only as "c" luminosity on the Mt.
Wilson system. These stars and their near equivalent widths are listed in Table 10, along with other stars whose OI absorption was anomalous with the calibration. The measures of the present study
confirm'the MK classifications of Bidelman and Kron with the excep tion of 41 Cygni. The dependency of the OI feature on both spectral type and luminosity do not permit statement of more than a range of
MK type but can exclude most of these stars from Class lb.
Luminosity Classes Below lb.
The data on the few program stars of MK luminosities II-V is
so incomplete that no attempt was made to arrive at the variation of
OI absorption with spectral type. It is observed that both 6 Leonis
and {3 Leonis exhibit an anomalous strength for dwarfs.
Recently, Conti, Greenstein, et al (1967) have studied neutral
oxygen abundances in late-type stars using the forbidden 2 3P -2 1D
transition using coude spectra at 2-7 J&/ mm. This nebular transi
tion is relatively insensitive to temperature and luminosity effects so as to allow inference of oxygen/ hydrogen ratios. Eleven of the
program stars were found in common. In each case the star had been
judged normal in oxygen content. TA B LE 10
SPECTRAL CLASSIFICATIONS ASSIGNED TO STARS UNCLASSIFIED ON THE MK SYSTEM
Program Star Orig. Classification Classification "W^ 7774 OI Classification
14 P er cGO G01bb 0. 54 GOIb HR 969 cG2 G5Ua 0.15 G5II-III 52 Per cG3 + A5 G5U + A, Ba 0 G5III G2I + B C 58 Per cG2 C2III-C5U 11 Pup cG2 0.68 Glib 16 Vul F2Ib or F0pC / F 5lla 0.61 F2III- F3II
a Classification of Bidclman, P. A. S. P. 69» 147, 1957 b Classification of Kron, P. A. S. P. 70, 566, 1958 c Classification of Nassau and Morgan, Ap. J. 115, 475, 1952 118 CH A PTER V
INTENSITY VARIATIONS OF THE OI ABSORPTION FEATURE AT X 7774 IN CEPHEID VARIABLES
The classical cepheids lie within the range of spectral types F5 to KO and in the luminosity Class lb. A list of bright cepheids is given in Table 11, derived from Arp (1958). It was expected that the total absorption of the oxygen feature at X 7774 would vary with the phase of the cepheid'sJ^^^^^^^^ghe extent to which the OI absorp tion would agree the object of the inves tigation. Observa^^^^^^^^^^^^^^Bpepheids were conducted simultaneously wit^^^^^^^^^^^^^^Brgiants of the same lumin osity class. Of the only 6 Cephei, j] Aquilae, £
Geminorum and T MonocSS^^^were observed because of seasonal observing factors. Image tube spectra were obtained on approxima tely 14 nights in late 1962 and early 1963 at 60 a/mm plate scale.
Direct I-N spectra of these same program stars were obtained on 6 nights in October 1963 at 65 R/xnm. in order to compare both the measurements and the exposure times required for recording the same events. Fig. 36 reproduces the spectra obtained in 1962 on
6 Cephei. Since the periods of the variables ranged from 5.^37 to 27. ^01 and the observing runs were for only 7-10 days, the spectra are incomplete in covering the complete light cycle. For the three
stars of longer period, the data proved too few in number to allow a
satisfactory determination of the variation of the OI absorption with
119 120
Julian Date 2,430,000 + 8003. 66 8005.70
8006. 65
8007.67
8008. 69
8010. 66
8011.65
8011.66
Fig. 36. -- Variation of the X 7774 OI feature with light phase in December 1962 C H A PTER V
INTENSITY VARIATIONS OF THE OI ABSORPTION FEATURE AT X 7774 IN CEPHEID VARIABLES
The classical cepheids lie within the range of spectral types F5 to KO and in the luminosity Class lb. A list of bright cepheids
is given in Table 11, derived from Arp (1958). It was expected that the total absorption of the oxygen feature at X 7774 would vary with the phase of the cepheid's total light. The extent to which the OI absorp tion would agree with the spectral type was the object of the inves tigation. Observations of four of these cepheids were conducted
simultaneously with those of stable supergiants of the same lumin
osity class. Of the stars in the table, only 6 Cephei, tj Aquilae, £
Geminorum and T Monocerotis were observed because of seasonal observing factors. Image tube spectra were obtained on approxima tely 14 nights in late 1962 and early 1963 at 60 a/mm plate scale.
Direct I-N spectra of these same program stars were obtained on 6 nights in October 1963 at 65 R/mm in order to compare both the measurements and the exposure times required for recording the same events. Fig. 36 reproduces the spectra obtained in 1962 on
6 Cephei. Since the periods of the variables ranged from 5. ^37 to
27. ^01 and the observing runs were for only 7-10 days, the spectra are incomplete in covering the complete light cycle. For the three
stars of longer period, the data proved too few in number to allow a
satisfactory determination of the variation of the OI absorption with
119 120
Julian Date 2 ,430, 000 + 8003.66 8005.70
8006.65
8007.67
8008. 69
8010. 66
8011.65
8011.66
Fig. 36. -- Variation of the X 7774 OI feature with light phase in December 1962
/ J
TABLE 11
BRIGHT CEPHEID VARIABLES
Star Period Mv (max-min) Spectral Type (Code) Spectral Type (Kraft)
6 Cep 5. 366 3. 5-4.4 F5Ib - G2Ib F5Ib - Glib
V Aql 7. 18 3.7 - 4. 5 • F 6lb - G4Ib F7Ib - G2lb S Sge 8.38 5. 1 - 6. 1 F 6lb - G5Ib F7Ib - G3Ib £ Gem 10. 15 3. 7 - 4. 3 F7Ib - G3Ib F7. 51b - Gl. 51b x cyg 16. 39 5. 9 - 7. 1 F7Ib - G8Ib F7Ib - G9Ib T Mon 27. 01 5. 7 - 6. 9 F7Ia-Ib - Klla-Ib F7. 5Ib-Iab - G8 Ib-Iab 121 122 period. Only the result for 6 Cephei will be discussed in detail since they are representative in value and in scatter and are sufficient in number to allow reasonable averaging to a smooth curve. The equivalent widths for all of the program stars were obtained by microdensitometry using the Gaertner machine at the
Aerospace Research Laboratories. The equivalent widths and plate epochs for 6 Cephei are given in Table 12, along with the phase of variable reckoned from maximum visual light. Because of secular period changes in cepheids are not uncommon it was attempted to establish a current epoch and period using visual estimates by the members of AAVSO. A request to the AAVSO revealed that none of the experienced amateur observers normally record the short period variables. However, a new and enthusiastic member, Mavin E.
Baldwin, Capt, USAF, in New Mexico, had observed 6 Cep and 77 Aql in 1961 and 1962 and, at Mrs. Mayall's request, was kind enough to continue visual observations of all the program stars in 1963 and 1964. While these did not have the precision of a photoelectric series, an examination of the data showed them adequate for the present purpose.
Reduction of Visual Estimates A small computer program was used to predict the brightness of the cepheid on the basis of an established epoch, and the nominal light curve of the variable derived from photoelectric or photographic data at the earlier epoch, using the period established at an earlier epoch. The rms differences of Capt. Baldwin's observations from 123 these predicted values were computed and the differences minimized by perturbing the period slightly with the epoch fixed. The amplitude and zero point of the assumed light curve were then adjusted to give a null residual in the differences and the period search repeated. This lead to the period and epochs listed in Table 13, Also listed are the starting values used and their source. The prototype light curves were obtained from the following: 6 Cep, Stebbins (1945); t\
Agl, Stebbins, Kron, and Smith (1952); £ Gem and T Mon, Payne-
Gaposchkin and Gaposchkin (1938). The resulting fit of the magnitude estimates to the adopted visual light curve is to better than 0. 075 magnitudes rms which is less than the observer's 0 . 1 magnitude step size. Using these new epochs results in shifts of 0. 05 periods were found for 6 Cephei and 7] Aquilae while the corrections amounted to no more than 0. 02 periods for £ Gem and 0. 01 period for T Mon. Fig. 37 shows the adopted variation of apparent magnitude of 6 Cephei compared with the raw observations of Baldwin. The smoothed curve is displaced upward 0. 2 magnitudes for clarity. The light phase listed in Table 12 for each of the spectra were then computed from the adopted epoch and period. In order to compare the X 7774 feature with the spectral class of the 6 Cephei, use was made of the studies by Oke (1961a, 1961b) where the intrinsic colors of 6 Cephei and r) Aquilae had been determined in the course of study of the absolute energy distributions of these stars.
Fig. 38 is reproduced from his second paper ; it gives the "true" effective temperature 0 of 6 Cep plotted against both intrinsic color
(B-V)q and against spectral type as derived by Kraft (I960) from T - 2 ;-o-Y ^ S < g 30 B' : ' : :: 2 tsDaaan c o o. 0 0
a-rOf*6a—o-Baoo-f >—n- • ;n—• cq o
o Pm"}: 1— Brio—E313------Q— 0 3
• - B - ; o - e b i n a c : -03— 03BT31-B-- t '
0 |-Cr3^tB0-EZ3C3tE3C3);.:i 07^33--0,50153 0
-I—0 ~j— is-r.-B —Q- 0.10 0:30 | 0:70. ; : c; 0.00 LIGHT PHASE
Fig. 37. -- Visual estimates of 6 Cephei as a function of the phase for the epoch and period adopted in Table 13. Upper curve is a smoothed average of the data using a Gaussian filter technique, and is displace upward 0. 2 magnitudes for clarity. 124 0.7
0.8
0.9
F5 F6 F7 F8 F9 GO G2
0.3 0.4 0.3 0.6 0.7 0.8 0.9 (B-V)0
Fig. 38.-- The intrinsic-color versus temperature relation for 6 Cephei assuming interstellar reddening in B-V of 0. 11, as deduced by Oke. TA BLE 12
SUMMARY OF SPECTRA TAKEN OF CEPHEID VARIABLES
Star: 6 Cephei
Spectrum No. Julian Date Phase WIT ¥ i- n 2438000 + 075 003. 710 0. 337 0. 59 103 006.690 0. 892 0.99 104 006.697 0.894 0.76 123 007.587 0. 060 0. 97 128 007. 646 0.071 1.05 133 007.694 0.079 1. 00 151 008. 662 0. 259 0. 48 152 008.667 0. 261 (0. 5) 178 009.658 0.446 0. 50 179 009 .686 0.451 0. 50 191 011.664 0.819 0.73 214 012.651 0. 003 1. 26 215 012.656 0. 005 1. 25 611 303. 835 0. 265 0. 68 612 303.843 0. 266 0. 54 TABLE 12 (CONTINUED)
Star: 5 Cephei Phase WIT Spectrum No. Julian Date W I-N
2438000 + 613 303.850 0. 267 0. 68 631 304.807 0.446 0.61 647 305. 829 0. 637 0 .7 2 659 306.664 0.791 1. 20 660 306.673 0.793 0. 94 668 306.850 0.827 0. 84 678 307.671 0. 979 0.99 679 307.697 0.985 0.76 680 307.709 0. 987 1.39 681 307.813 0. 006 1.36 692 308. 746 0.181 0. 64 693 308.755 0.183 0. 68 695 308. 828 0.196 1.00 712 309.668 0. 352 0.55 713 309.683 0.354 0. 66 734 310.705 0. 546 0.48 735 310.719 0. 548 0.43 TA BLE 13
ADOPTED EPOCHS AND PERIODS FOR VISUAL MAXIMA
Star Starting Epoch/Period Adopted Epoch/Period RMS Residual
6 Cep J.D. 2430693. 131 J.D. 2438012.644 0. m 074 5. d 366364 5. d 366210
7] Aql J.D. 2433156.476 J. D. 2438309 . 256 0. m 072 7 .d 176724 7. d 176574
£ Gem J. Eh 24 3646 6.768 J.D. 2438304.56 0. m 054 10. d 153527 1 0 .d 153580
T Mon J.D. 2436298.48 J.D. 2438001.71 0. m 064 27. d 018 2 7 .d 035 129 photometry. Using the variation of effective temperature vs phase as tabulated in Table 3 by Oke (1961b), one can select the individual data points for intrinsic color or for the spectral type. These values of spectral type, interpolated from the curve to an unrealistic accuracy but done so to allow smooth continuity, are also listed in
Table 14 for 6 Cephei for each 0. 05 movement of phase.
From the variation of spectral type with light phase, we can now utilize the calibration of the X 7774 feature in non-variable supergiants to predict the variation of X 7774 in these cepheids. Two cases were considered, the earlier calibration of X 7774 by Keenan and Hynek and the one suggested in the present work. In both cases we 5 assume the 3 SQ levels of the neutral oxygen are in equilibrium population with the effective temperature of the cepheid star, just as they have been found to be in non-variable supergiants by Keenan and Hynek (1950). These investigators recorded the total absorption of 5 o- 5 both the X 7774 feature arising from the 3 S 3 P transitions and the 3 3 X 8446 feature arising from the 3 S-3 P transitions. They concluded that, despite the metastability of the lower state, the blend X 7774 showed the proper intensity relative to X 8446 blend, namely, 1. 67 ± 0. 11 (m. e), compared with the ratio to be expected on the basis of the quantum mechanical statistical weights or 5 to 3. The predicted variations of equivalent width of the X 7774 feature in 6 Cephei are tabulated in Table 14 for each of the calibra tion curves. Fig. 39 gives the predicted variation in equivalent width, the results obtained from the program stars.
While the scatter in the spectral data is quite bad, we can TA BLE 14
VARIATION OF SPECTRAL TYPE AND PREDICTED X 7774 EQUIVALENT WIDTH FOR 6 CEPHEI
6 Cephei Phase Spectral Type W KH ^K K
0.00 F4. 81b 1. 11 1.14 0. 05 F5. 6 1. 05 1. 12 0.10 F6. 8 0,96 1. 04 0. 15 F7. 6 0.90 0. 98 0. 20 F 8 . 25 0.85 0. 88 0. 25 F 8 . 9 0. 80 0. 82 0.30 F9. 6 0.74 0.72 0. 35 GO. 1 0.70 0. 66 0.40 GO. 4 0. 675 0.61 0.45 GO. 7 0.65 0. 58 0. 50 GO. 95 0. 63 0.55 0.55 G 1.2 0.61 0.51 0. 60 G1.4 0. 59 0.48 0. 65 G1.3 0.60 0. 50 0.70 G l. 2 0. 61 0. 51 0.75 GO. 5 0.67 0.60 0. 80 F9. 6 0. 74 0.7 2 0. 85 F 8 . 2 0.85 0.91 0. 90 F 6.4 0. 99 1. 08 0.95 F4. 81b 1. 11 1.14
0.2 6 ehi Dse cre are curves Dashed Cephei. . 0.6 0.4 I PHASE T H LIG OBS ONS N IO T A V R SE B O I O F O N A E KK M H & K PREDICTED PREDICTED I TUBE E G A IM X IET N - l DIRECT • . 10 . 04 . 0.8 0.6 0.4 0.2 1.0 0.8 131 132 reach the conclusion that the OI triplet at X 7774 is formed in equilibrium with the spectral type and effective temperature of the cepheid, at least at maximum light. A sparsity of data on the ascending part of the light curve makes uncertain the conclusion that the OI minimum departs as strongly from the light minimum as does the smoothed mean. The variation of the OI absorption exhibits an amplitude which tends to support the present result between a spectral types F 8 to G4, since the present calibration predicts a somewhat larger amplitude than that from Keenan and Hynek. This result appears to hold for both the image tube spectra and the direct I-N spectra. There seems again to be no clear difference between the absorptions obtained by the two techniques. Application to the Calibration of Cepheids Kraft (I960, 1960b, 1960c), Code (1947), Oke (1961b) and others have worked on the problem of the intrinsic colors, the variation of spectral type with light phase, and the energy distribution in cepheids. Code pointed out the importance of establishing the spectral type of cepheids at maximum light, or more specifically the intrinsic color at maximum light, as either independent of period or at least as a function of period. If this can be established as a uniform property of cepheids, it can be used in galactic studies of interstellar reddening. The question as posed by Kraft was: "Are the unreddened (B-V) colors of cepheids at maximum light consistent with the assign ments of spectral types in the manner of Struve and Code?" The classifications of Struve and Code on the MKK system involved study 133 at moderate dispersion of the cyclic variation of the spectra through the light cycle. Assignment of spectral type at minimum light was on the basis of direct comparison with non-variable supergiants. The spectral type assignment at maximum light is complicated by abnor mal strengths in the hydrogen lines, and some metals. The G band and ratios of Fel and Fell were used by these early workers. Kraft abandoned the use of all lines except the G-band. Using a narrow-band filter (half-width 10 J?) and a superimposed 200 ^ filt e r he measured the depression of the continuum by the G-band. Calibra tion of the system against non-variable supergiants led to curve in Fig. 40, taken from his paper (1960a). Also plotted is the dependence of the OI feature at X7774. Kraft's parameter T has the advantage of being relatively insensitive to the luminosity of the star, as can be observed from his figure. The neutral oxygen feature has at least as high a sensitivity to the spectral type, for a star of luminosity class lb, as has the G-band parameter. If the cepheids can be con sidered to remain in the same luminosity class, it should be possible to use the Ol feature to infer the spectral type throughout the light variation equally as well as G-band determinations. Clearly this cannot be done with the data presented here be cause of the large scatter. New measurements should be made either by a higher dispersion spectrograph or, more efficiently, by a narrow band photometer analogous to the G-band photometer of Crawford (1958) which was used by Kraft. The present measurements support the assump tion of thermal equilibrium in the Ol triplet throughout the Cepheid cycle and indicate that further use of this feature may allow indepen- w (mag) o |a 2 . 0 8 0 1 . 4 1.2 2 . 0 4 0 1.0 KK 0.8 2.000 KH 0.6 0 . 4 1 .9 6 0 V 0.2 F2 F4 F6 F8 GO G2 G4 G6 G8 K0 SPECTRAL TYPE Fig.40* --Sensitivity of the T index of Kraft as a function of spectral type as compared with the Ol A 7774 feature, for luminosity class lb, using the results of Keenan and Hynek (1950) and the present investi gation 134 135 dent check of the spectral class to be assigned to cepheids throughout their light periods. A Suggestion for Further Work As was pointed out by Dunham (1956), low resolution measure ments requiring scanning of small wavelength regions can be done much more efficiently by photoelectric scanning than by spectroscopy to achieve the same statistical accuracy. The single feature of X7774 is an example of this. It should not be necessary to use an actual spectrometer for this problem. Present S-20 photomultipliers and recent developments in interference filters, which allow 0. 25 per cent half-width filters to be fabricated in the far red (4 jR at 8000 A), make direct, photometric measurements look feasible. It would be preferred to obtain a rather square transmission profile of 5-Ang- strom half width rather than a width narrower than 3. 5-Angstrom separation of the three components. It is suggested that the measure ments could be effected by using the interference filter as an on-line/ off-line scanner by rocking it about an axis parallel to the filter face. The signal reflected from the face of the filter could be used as a con tinuum reference, since the spectral region about the Ol triplet is very clean except for a line due to Fel at X7780. 6. Other lines due to Fel lie at 7748. 9 and 7751.1 while lines due to Ni I lie at 7789. 9 and 7748. 3. Thus with the exception of the one nearby line, whose intensity was observed to be essentially constant with spectral type and luminosity over the range of cepheid variability, a region of 40 Angstroms of continuum surrounds the triplet. By a balancing process this continuum, as observed through a 25-40 Angstrom filter, 136 could be used to suppress the continuum and perhaps allow readout of the absorption directly. Such an instrument would have a high lumin ous efficiency since it would require only collimating optics ahead of the filter and a Fabry lens, eliminating slits, grating, etc. The in strument should also be useful for luminosity calibration of super- giants whose spectral class is already known and allow this with rela tively modest telescopes. To examine the modulation to be expected in super giants of different luminosities from scanning such an interference filter over the Ol triplet and the neighboring F el line, a convolution of several filter profiles was made with several blended intensities of the triplet. The laboratory wavelengths were assigned. The Ol lines were repre sented by triangular profiles with individual equivalent widths in pro portion to their statistical weights, 7:5:3. Four filter profiles were used, each profile being considered to apply to half width of 5 J? and 10 j£ . The profiles were a true rectangle, a true triangle, and the profiles communicated by Dr. E. Barr of Thin Films, Inc., as rep resenting the state of the art in multi-layer filters. These profiles, designated Type I and Type II, are shown in Fig. 41 for a 5 A* half power width. The peak transmission was assumed to be 0.70 in all ca ses. Fig. 42 gives the modulation to be expected as a function of center wavelength for a supergiant with Ol equivalent width of 1. 5 corresponding to B5Ia or G4la, for each of the filter profiles in the 5 J9 half-power case, and for the practical filters of 10 J? half-power width. The modulation from off-line to on-line is some 15-20 percent. TRANSMISSION 100 40 60 80 20 0 Fg 4. oie o Fitr cniee o O scanning Ol for considered ilters F of rofiles P 41. Fig. I S M O R T S G N A TP I •TYPE YE II TYPE 137 FRACTION OF CONTINUUM SIGNAL 1 _ 7 * 8 9 7 7 * 4 9 7 7 0 9 7 7 6 8 7 7 2 8 7 7 8 7 7 7 4 7 7 7 0 7 7 7 6 6 7 7 2 6 7 7 8 5 7 7 4 5 7 7 1 \ i. 2 ouain fnro-ad inlfo supergiant from signal narrow-band of Modulation - - 42. • Fig. withTRC j 7774 A j bands. v nnn/L = 1. 5 with filters of various profiles and pass- and profiles various of filters with 5 1. = NE H T G N E L E V A W ENTER C A°) ° (A HPBW A 0 1 & HPBW X 5 © 138 To achieve an accuracy of two percent in the measurement of the on-line case will require detection of 2500 photoelectrons. For the Perkins reflector,, based on Code's fluxes for a Cygni, this could be done in sixty seconds for an A2la star of + 12. 5 apparent visual magnitude. Ten minute integration times would allow measure ments of the Ol feature in supergiants in the Magellenic clouds. This assumes a 50 percent transmission for the new optics when function ing in a slitless mode. Many of the high luminosity stars in the Milky Way could be classified by use of a 24-36 inch instrument. Fig. 43 gives the modulation to be expected from different blended absorptions ranging from 0.1 (the Sun) to 2. 5 ( € Aurigae) for the Type I and Type II filters. The effect of the nearby Fel line is quickly lost as the luminosity rises above the dwarf class. If such an instrument were constructed, it should be calibrated not only against the MK stars used here but against the list of super- luminous stars compiled by Schmidt-Kaler (1961-1962), many of which lie in the southern hemisphere. FRACTION OF CONTINUUM FLUX TRANSMITTED 0 : 1 1.0 by scanning of Ol feature in supergiants of = 0.1 to 2. 5. 2. to 0.1 = of supergiants in feature Ol of scanning by g 4 . ouain y hn l , n. itr o to osbe profiles possible two of filters Inc. s, ilm F Thin by Modulation . 43 ig. F 76 78 7762 7758 7756 AEEGH (ANGSTROMS)WAVELENGTH 7774 \ \ \ \ 1 ' 0 0.5 j / ° - 2 •it* Y E II TYPE FILTER Y E I TYPE FILTER- 7794 140 CH A PTER VI APPLICATIONS TO SOLAR ECLIPSE SPECTROSCOPY At the beginning of the total eclipse of 1870, Prof. C. A. Young of Princeton University first observed the rapid transition of the dark absorption lines, seen against the continuous spectrum of the sun, into a multitude of bright emission lines lasting for only a second or two. In Young's words (Mitchell, 1923)" the moment the sun is hidden (by the moon), through the whole length of the spectrum, in the red, the green, the violet, the bright lines flash out by the hundreds and thou sands, almost startlingly; as suddenly as stars from a bursting rocket- head, and as evanescent, for the whole thing is over in two or three sec onds. " Since this visual confirmation by Young, who had predicted its existence several years previously, the flash spectrum has been stud ied extensively to determine both the chemical composition of the solar atmosphere and the physical processes and conditions existing in the chromospheric region between the relatively dense photosphere and the tenuous coronal regions of the sun. The flash spectrum was first photographed in 1893, and during the following half century, it was repeatedly mapped by numerous astronomers, the most notable being S. A. Mitchell of Leander- McCormick Observatory, who in an epic work (1937) summarized the characteristics of 3500 spectral lines observed in five eclipses from 1901 to 1937. These lines extend over the wavelength interval from 3066 Angstroms, near the ozone cutoff of atmospheric transparency, 141 142 to 8863 A ., near the infrared limit of fast infrared emulsion. While the atmosphere represents an ultimate limit in the ultraviolet for surface observations of the flash spectrum, the wavelength region out to 13, 000 A. remains relatively clear with the exception of the dis crete absorption lines due to terrestrial oxygen and water vapor, par ticularly the latter in the intervals 9300 - 9600 and 11, 100 - 11, 600. In the far-photographic infrared the limit has been set by the exposure time necessary to record the spectrum on the emulsion. Since the exposure time is limited by the brief duration of the~ flash spectrum, the sensitivity of the film is the limiting factor. The absolute sensitivities of current Eastman Kodak emulsions were given in Fig. 6. It is observed that the cutoff of Mitchell's results in 1937 corresponds to the region of rapid drop in current I-N emulsion. While it should be possible to obtain a direct photographic flash spectra to the limit of I-M emulsion from 9600 to 10, 000 A ., this has been reported on only one occasion, a British expedition to Siberia in 1936 (Anonymous, 1937) when 12 emission lines were recorded out to X10, 049. On more recent occasions weather and the unfortunate per ishability of IR emulsions have frustrated similar attempts. For these reasons a relatively unexplored region of the chromospheric spectrum lay open, a region not expected to be richly populated with emission lines, but strong emissions were expected near 11, 000 A. at the wavelengths of 10, 830 A. and 10, 938 A ., due to helium and hydrogen respectively, and at the other members of the Paschen series. These emissions have been recorded by long-exposure coronograph spectra of an artificial eclipse, most recently by exposures of several seconds using an infrared image converter tube. - ) ( i 143 In addition, an image converter tube had been used with success in an airborne eclipse experiment over the Crimea in 1961 where coronal lines were recorded in the 11, 000 A. region by V. A. Kurt (1962, 1963). The investigator was not aware of Kurt's results in early 1963 when he was offered the opportunity to place an experi ment on board the National Geographic Society/.Douglas Aircraft Company eclipse aircraft to observe the 20 July 1963 total solar eclipse over the Northwest Territories, Canada. The c< Ircraft, a specially outfitted DC- 8 ,was to provide a laboratory environment at 40, 000 feet to guarantee low water-vapor content in the air path (no clouds either) at the expense of equipment size and stability. A simple flash spectrum experiment was proposed which would use the existing Flagstaff camera system and which, judging from the success of the British expedition, should readily penetrate to fainter lines despite the limited equipment aperture. Operating experience at Flagstaff (altitude 7200 ft.) had shown that careful cleaning of the image tube and its insulated housing, in clusion of magnesium perchlorate dehydrating agent within the optical system housing, and remedial corona doping of the power supply allowed trouble-free operation at the ambient pressure expected in the aircraft cabin. With the Flagstaff experience as a base, it was decided to use this proven basic camera system with an objective grating for record ing the flash spectrum. A 4-inch square Bausch & Lomb plane reflection grating blazed at 10, 000 A. was placed before the camera lens. The method 144 of chromospheric arcs was to be used to record the flash spectrum since this allowed use of the simplest apparatus. In this method the camera directly images the solar limb after passing the light off the dispersing gratipg with the limb of the moon serves as a curved slit at infinity. Images are formed only at the discrete wavelengths e- mitted by the chromosphere, although they may be superimposed on a continuous background of light scattered by the solar corona or the earth's atmosphere. Depending upon the height of the emitting atoms above the surface of the sun, these images will subtend larger or smaller arcs before the radiating layer vanishes behind the moon. (See Fig. 44 ). The method offers the advantage that instrument pointing accuracy is secondary if the instrument is stable during the exposure; it suffers from the disadvantages that one is unable to introduce a comparison spectrum for wavelength measurement of lines, that the apparent spectral position of the chromospheric crescent will depend on the height of the layer above the sun (i. e ., the "slit" of radiating gas is not the same for all lines as would be the case for a mechanical slit), that the exposure time will be more limited by the continuum background, and that the images are more subject to blurring by motion of the instrument during exposure. The Douglas Aircraft Company had supplied data on the DC -8 aircraft and its anticipated stability in the proposed flight regime. These data indicated that the aircraft stability in roll, yaw, and pitch might be as poor as ^ 1 degree with a period of oscillation on the order of 9 seconds. It was concluded that the pointing insensitivity of the slitless spectrograph would be compatible with this and that fine 145 LUNAR LIMB ./SOLAR Fig. 44.-- Dependence of chromospheric arc length on height of emitting atom in the chromosphere. 146 adjustment of position would be futile. The image motion could be handled by use of short exposure times with the tacit hope that the flash would occur at a point of maximum excursion of the aircraft. Based on the fact that emulsion of the I-M type had successfully recorded the flash spectrum in 1936, with an exposure which could not exceed 2 seconds, an {exposure of 1/ 25 second seemed adequate to . record the same emission lines. The higher efficiency of the image tube at the longer wavelengths made it probable that Xll, 000 region would also be recorded. Longer exposures would be subject to blurring but with luck would show additional fainter features. To relate these new features to known ones required inclusion of wave lengths contained in Mitchell's atlas, i.e ., the region below X8800 should be overlapped. The apparatus was constructed for rigidity, was simple in con cept and operation, and took maximum advantage of proven devices. The aircraft was equipped to supply nominally 60-cys, 110-volt power. A 500-watt Sorensen AC regulator was used to smooth this to the high- voltage power supply. The spectrograph was designed for a fixed grating position. Manual transport of the photographic plate was selected as simpler and more reliable than any motorized or mechan ized.system. Manual tripping of the shutter was expected to be more reliable and provide more control than a pulsed solenoid since the observer could select a stable moment for the actual instant of ex posure. Practiced cooperation by two skilled observers would permit rapid operation in alternately exposing the plate and repositioning the plate. Mr. Darrell F. Frank, an electronics specialist of the Met- 147 allurgy and Ceramics Laboratory at ARL, assisted in the installation and flight operations. Spectrograph Design. The borrowed 600 line / mm grating determined the angular dispersion of the spectrum. Although the image tube has a 40-mm diameter photocathode, the maximum usable length of spectrum was 25-nun since the 50-mm relay optics began to vignette badly at about 13-15 mm from the optic axis. It was decided to utilize the 178 mm Aero-Ektar, partially vignette the wavelengths below 9000 A ., and thus operate at approximately 90 A/ mm plate scale. The strong chromatic aberation of the refractive camera was the principal prob lem. Based on the focal settings of the Aero-Ektar camera at the Perkins spectrograph, an approximate focal curve was deduced. In a conventional spectrograph one tilts or bends the plate to reach an optimum focus along the plate. Because tilting of the entire image tube and magnet assembly was impractical, a compromise was reached for the eclipse spectrograph by moving the camera lens off-axis rela tive to the image tube and by tilting it to yield the focal curve shown in Fig. 45. This amounted to a designed "lens tilt" of 2° 04', which was fixed at this value. The grating was then positioned to allow for this optical axis and a folding first-surface flat placed to allow the solar light to enter parallel to the main image-tube camera. The entire optical system is shown in Fig. 46. Fig. 47 shows the main camera with the spectrograph attached. The spectrograph was constructed of two parallel sections of A ero-Ektar and com prom ise focal plane attained by lens by attained plane focal ise prom com and ero-Ektar A tilt. i. 5 xei na oa cre fKdk 7- m 178-m Kodak of curve focal ental Experim - - 45. Fig. FOCAL DISTANCE (mm) ARBITRARY 6 7 5 0 0 0 9 0 0 0 8 X ANX G S T R O M S 10.000 11,000 f/2. 5 Photographic Plate C a r l M e v e r f/g.O Relay Optic? ITT I m o Q e T u b e , Folding Plot mjL A e r o - E k t a r ♦ 2 5 F i l t e r G r a t i n g Ilex Shutter Fig. 46. --. Schematic plan of image-tube spectrograph showing optical offset angles. Fig. 47.-- Eclipse spectrograph assembly. 150 6061-T6 aluminum pipe welded into an integral unit. All optical parts were located by milling the four ends of the resulting double-barrel structure. The aperture of the spectrograph was determined by the 63-mm opening of the No. 5 Ilex shutter, used without lens elements. The spectrograph was cantilevered onto the image-tube camera by three clamps which secured the assembly to a shoulder on the camera. An access door allowed camera focusing of the assembled instrument. Preliminary tests utilizing a collima^ld beam of a neon/ argon glow lamp showed a satisfactory coverage of the desired spectral range. This spectrum is shown in Fig. 48 Astigmatic effects are clearly visible in the lines at the extremes of the field. The No. 25 Wratten filter placed in front of the camera lens (Fig. 46) to block the second-order blue spectra becomes transmissive at about 5750 A. yielding a few second-order lines in the region of 11,600 - 12, 000. The dispersion curve was found to be nearly linear. Aircraft Installation and Tests. The mechanical installation was effected at Long Beach, Calif ornia; the spectrograph and converter camera was reassembled and tested in a Douglas darkroom. The final weight of the equipment was 273 lb6. for the power supply and 120 lbs. for the converter camera and spectrograph. The installed equipment is shown in Fig. 49 « Apparatus was installed by three days prior to the eclipse to allow flight testing of operation. One crisis arose when it was discovered the French airglow experiment across the aisle contained a pulsed 5000 - gauss magnet, switching ten times per minute. Microscopic examination of the phosphor image failed to show any disturbance in 151 2nd 2nd O rd e r Fig. 48.—Neon-Argon calibration spectrum. Selected wavelengths are given in Angstroms. Fig. 49. - -Image tube spectrograph and high-voltage power supply as installed in APEQS aircraft at recessed window (courtesy of the National Geographic Magazine - National Geographic Society). 152 resolution but fluctuations were observed in the current meter of the high-voltage power supply. A momentary shut-down of the magnet pulser at the time of totality was arranged with Prof. Blaumont of Service d'Aeronomie to remove all possibility of its interference with the quality of the image-tube focus. Oscilloscopic check of the air craft 57.14-cps power showed no spurious pulses which would be passed through the regulator and power supply. During the three dry runs, one over San Diego, one over Oregon on the way to Edmonton base of operations, and the last over the eclipse track itself the day prior to event, the sun was acquired and plates taken through heavy filters and small apertures to check for light leaks and focus on the continuous spectrum from the entire solar disk. No operational difficulties were encountered in these tests, and the operating routine and checklist were established. A final focus was made the night before the eclipse by using the mercury flood lamps which illuminated the Edmonton International Airport and sub sequently correcting to the infinity setting. Eclipse Operation. Take off was at 17:10 U. T. with the eclipse interception at 20:37 U. T. Dry ice was stuffed into the cold box at about 18:30 and the cold box topped off after 20:00 to insure a low temperature of the photo- cathode. The plate'-holders were loaded in a changing bag in one of the forward-latrine temporary darkrooms. All personnel were in their place at 20:00 hours to allow peaking of the special Sperry auto pilot to the actual altitude, speed, and load conditions. At T-2 minutes the aircraft was on course and altitude and was slaved to the 153 auto-pilot. The sun was acquired in the guiding camera viewfinder and showed remarkable stability. As the crescent diminished, the author noted the anticipated appearance of the inner corona on the opposite limb of the moon a few seconds before totality, signaled Prof. Blaumont to interrupt his experiment, called for Mr. Frank to open the dark slide, and shortly after the crescent changed into the string of Baily's beads behind the mountainous edge of the moon, be gan a series of four spectra, one at 1/ 25-second and three at 1-second exposures. It was desirable to record the flash spectrum on two separate plates to provide for possible catastrophic events. A second plate holder was inserted during the 142 seconds of the central phase and the exposure time changed to 1/ 5 second. This necessitated a small change of focus for the relay optics to account for the differences in the two plate holders. As a result of some delays in this process, third contact was just missed and a strong bead of the photosphere already visible when two additional exposures were hastily made. This unfortunate and disturbing circumstance led the author to fail to ask for movement of the plate and these last two spectra were nearly superimposed. By good fortune the chromospheric arcs of each are separately visible although in the presence of strong halation from the continuum. Following third contact, two■ sensitometric calibration exposures were made to register a continuous spectrum of the sun using the photospheric crescent as a source. Both spectra proved to be badly underexposed, however, and this phase of the experiment a total loss. 154 Fortunately, the presence of a modest coronal or chromospheric continuum is evident on the emission spectra, which it is hoped will yield satisfactory calibration of the photocathode sensitivity under the conditions of the eclipse. The eclipse plates were then unloaded from the plate holders, stored in dry ice, and upon return to Long Beach were taken to Lowell Observatory for sensitometric calibra tion of the emulsion and processing. R esults. The first two spectra taken at second contact are shown in Fig. 50 with the spectra taken at third contact shown above. The strong lines visible in all the spectra are due to Hel at \10, 830, Ca II at \8498, 8542 and 8662, and the Paschen series of hydrogen at X8598, 8751, 8863, 9015, 9259, 9546, 10, 049, and 10, 938. The high stability of the aircraft at the time of second contact is testified by the sharpness of the second spectrum taken with a one-second ex posure. Many weaker lines of short arc length are visible in the original plate. A total of 17 fainter lines were measured which had not been reported previously in the flash spectrum. A total of 21 lines were recorded redward of 9000A. The line intensities could be reduced to a scale based on the continuum intensity and then, following establishment of the relative photocathode response, reduced to a scale relative to one of the hydrogen lines. Such an indirect pro cedure is necessitated by the failure to obtain the photospheric spectrum after third contact. Wavelength measurement and identifi cation of the atoms originating the weaker lines were made and the Fig. 50. -- Flash spectrum at second contact, 1/25 sec exposure (b) l = sec exposure; (c) flash spectrum just after third contact, 1/5 5 sec exposure. Wavelengths are given in angstroms. Originating atoms noted for strongest lines. Asterisks denote lines of the Paschen series of hydrogen. 156 results are listed in Table 15. Solar Eclipse of 30 May 1965 The results of the 1963 eclipse, preliminary and limited as they were by lack of spectrophotometric calibration, were encouraging enough to make a similar attempt in 1965. The investigator was invited to fly on the NASA International Quiet Sun Year Expedition organized by the Airborne Sciences Office, Aeronautics and Space Administration, Ames Research Center. A new spectrograph of slitless design but allowing a slit to be interposed at a focal plane conjugate to the photocathode, was constructed to circumvent the problems experienced in 1963. This work was done in collaboration with Dr. P. L.Byard of McMillin Observatory. The principal difficulties and deficiencies encountered in 1963 w ere (1) limited ability to take numerous exposures on a photographic plate, ( 2) limited sharpness of long-exposure spectra due to aircraft motion, (3) inability to establish the instants of exposure, since timing apparatus was not incorporated, (4) inability to hold the S-l photocathode at a constant, known temperature for sensitometric purposes, and (5) failure of an attempt to record the solar limb after third contact to provide a relative spectrophotometric calibration against the photosphere. No attempt could be made in 1963 to record the coronal spectrum because of image instability and lack of a defin ing slit to limit the coronal light accepted by the camera to a single, known region. Description of Apparatus. 157 The new instrument, to be used in both a slitless and slit mode, was designed around the original image tube employed in 1963. The spectrograph evolved into the form shown in Fig. 51 . A gyro- stabilized heliostat was employe d to stabilize the image at the focus of the 11 cm f/ 12 telescope onto a polished slit. An identical spherical collimator was used to illuminate a reflection grating blazed at 1.12 microns first order. Dispersed light from the 400 line/ mm grating was focused on the FW-167 image tube by a 305-mm £/ 3. 5 Infrared Aero Ektar loaned to the investigator by the U. S. Navy Aeronautical Photographic Engineering Laboratory for use in this experiment. This unique lens was built by Eastman Kodak to be achromatic at 6560 and 9000 JZ. With proper allowance for color curve it gives excellent mon ochromatic resolution to 11, 500 To provide flexibility in exposure times and large data storage capacity, a pulse-operated framing camera was employed using East man II a-0 film on a 35-mm base. This camera was modified to allow recording of data from the full 40-mm diameter of the image tube on a 1:1 basis by removing part of the shutter blade and housing so as to permit working on the diagonal of the 24 x 36 mm frame. A special set of micro-switches was installed to allow flexible programming of the camera from either a pulse generator for flash events, pre programmed timers for coronal exposures, or a manual override control. The Elgeet 86 -mm focal length Oscillo-Navitar was used for the reimaging optics operating at unity magnification with a £/ 3. 6 true aperture ratio to eliminate vignetting. The S-l photocathode was cooled to -18* 0 degrees Centigrade 158 Fig. 51. -- 1965 Solar eclipse spectrograph as installed on the NASA Convair 990 research aircraft. (Courtesy - National Aeronautics & Space Administration). i i 159 by use of a thermoelectric cooler kindly loaned to the investigator by the U. S. Army Engineering Research and Development Labora tory for use on this experiment, rather than the uncertain and un controlled dry ice system. The cooler required approximately 50 watts of 28 vdc to maintain a differential of 40-42 degrees C. with the cabin air. This cooler used a circulating liquid to remove heat from the hot junctions. A circulator/ radiator system employing a water-ethylene glycol mixture was incorporated to provide secondary exchange with the cabin air. The temperature was controlled and measured through a network of resistance gauges cemented to the photocathode faceplate of the image tube. Periodic checks of the faceplate temperature with a Wheatstone bridge showed identical conditions to a constancy to better than 0. 05° C. throughout both calibration and eclipse flights. The gyro stabilized heliostat was identical with that used on the spectrograph designed and built by the Douglas Aircraft Company and is described in detail elsewhere. (Whittaker, 1966; Whittaker & Burdin, 1966) . It used rate-integrating gyros obtained from the X-20 research aircraft (Dynasoar Program) as the basic sensing elements. Its performance was quite adequate to stabilize the image to the satisfaction of the experimenter monitoring the slit image. Data records from the gyro platform servo errors indicate the rms image stability of the ARL instrument to be 4 arc seconds in roll and 15 arc seconds in combined yaw and pitch. This compared with 0.8 degree roll and 0. 3 deg nee pitch and yaw amplitudes of the air craft itself during 5-10 second intervals. 160 By strengthening the magnetic field to 450 gauss, the focus voltage of the image tube was raised to 13, 900 kv, so as to obtain more luminous gain. This potential was again derived from a standard, stabilized supply fed from the 60 cps inverter system of the Convair 990 research aircraft. The aircraft frame and spectro graph formed the ground for this system with the 13. 9 kv positive potential applied to the phosphor of the image tube. Optical Calibration System To provide wavelength and spectrophotometric calibration, a collimated comparison lamp system was provided beneath the helio- stat so that light from either a Geissler discharge tube or a ribbon filament lamp could be injected into the telescope by repositioning the heliostat to a proper incidence angle. Neon, helium, argon, and mercury discharge tubes were used to cover the wavelength interval. The ribbon filament lamp was energized from a regulated power sup ply employing potential leads from the lamp for the feedback system. A current of approximately 6.8 amperes at one volt yielded a fila ment temperature of some 1730° F. as measured before and after the calibration spectra with a disappearing-filament optical pyro meter. The filament supply was adjusted to yield this nominal temperature on the calibration and eclipse flights so as to allow inter comparison of eclipse data to the photospheric data taken through attenuating filters on the calibration flights. With the regu lator in operation, no change of filament temperature was detectable within the limits of the pyrometer measurements. Filters of 161 approximately density 4.1 were combined with an aperture stop of approximately 1/ 4 square inch to yield equivalent densities from the filament lamp and the uneclipsed solair continuum at the same ex posure time. A step-filter of evaporated rhodium on a quartz base (Jarrell - Ash) was used immediately before the slit to provide the necessary step attenuation for calibration of the combined photocathode/ phosphor/ film combination. This filter was mounted on the slit decker and was slipped into place when needed. To reject the second-order yellow light, including the D3 line of helium, which had been detected in 1963, a glass filter thought to be a Corning 2540 was installed immediately in front of the image tube faceplate and within the photocathode cooler. After the eclipse flight it was found that this filter was instead a Corning 2600 filter which had mistakenly been selected for installation. The 2600 is a notch filter which is already strongly absorbing at 10, 000 A, and which is much less than 1 percent transmissive at 11, 000. This error went undetected in part because of the extreme intensity of the Hel 10, 830 line in the laboratory test source and in part by the expected gradient of the solar continuum to the IR on the test spectra. Be cause of this strong absorption past 10, 500 A. lines detected in a 0. 2 secchd exposure in 1963 were not recorded in 220 seconds in 1965 although the Hel 10,830 line is still strongly present. Chromospheric Spectra Because a fixed spectrograph was used without an image rota tor, the direction of dispersion was fixed in position angle relative to 162 the sky and hence to the solar disk. This angle was some 120 degrees from the north point of the sun or, at the epoch of the eclipse, approximately at a position angle of 135 degrees from the northern extremity of the solar axis of rotation. This results in the chromospheric arcs lying at an angle of 60 degrees from the direc tion of dispersion. To operate in a slitless mode the slit was opened to its full width {4 mm) and the crescent placed in the opening. Some 50 slitless spectra of 300-milliseconds exposure were obtained at second contact and 19 spectra of 47 0-milliseconds exposure obtained at third contact. One of the spectra at second contact is reproduced in Fig. 52 . Table 15 summarizes the wavelength measurements and tentative identifications of all lines detected in the 1963 and 1965 experiments as well as those listed in Mitchell's chromospheric atlas (Mitchell, 1937),, and the results of Kurt (1962, 1963). The wavelengths were obtained by using the identifiable lines to define a mean dispersion curve and to interpolate for unknown lines before attempting identification. A total of 28 chromospheric lines were recorded between 9000 and 10,456 Of these 16 are common to the results of 1963 yielding a total of 35 lines beyond 9000 A ., three of them coronal, detected in the two eclipses. In addition, six lines were measured in the chromosphere lying between 8600 and 9000 A. which are not reported in the compilation of S. A. Mitchell. One of these is unconfirmed in 1965 leaving a total of 40 to 41 lines reported here as having been detected in eclipse for the first, second, or third time. Fig. 52. -- Chromosphere flash spectrum at second contact, solar eclipse of 30 May 1965. 163 TA BLE 15 Summary of Chromospheric Spectra Obtained by Image Tube Spectroscopy in the Total Eclipses of 20 July 1963 and 30 May 1965 NASA IQSY Identification Kurt M itchell (1965) APEQS and Multiplet Laboratory X Coronal v Chromospheric 8445. 5 0 1(4) 8446.4 X 8497.5 Ca 11 (2) 8498.0 X X 1 • * • • • 8542. 3 Ca II (2) 8542. 1 XX 8598.4 8597.6 H Paschen 14 8598.4 8611.0 Fe I (339) 8611.8 8662. 1 8663.0 Ca II (2) 8662. 2 X X 8688.2 8688 .3 Fe I (60) 8688.6 8728. 7 Si I (79) 8728.4 8735.4 X 8750. 5 8748. 7 H P 12 8750.5 X 8790 .6 Si I (79), Fe I (1267) 8791.3/8790. 6 8806.5 8805. 8 Mg I (7) 8806 . 8 X 8823. 2 8821.9 Fe I (60) 8824. 2 8838. 0 Fe I (339) 8838.4 / • • • • • 8849.0 x » - I O' 8862. 8 8863.1 H P l'l 8862.9 TABLE 15 (Continued) NASA IQSY APEQS Identification Laboratory A Kurt M itchell (1965) ______and Multiplet Coronal Chromospheric 8944.8 Fe I (1301) 8998.7 Fe I (339) 8999.6 9014. 6 9015. 5 H P 10 9014.9 9062.0 9062.3 Cl (3) 9061.48 9079 .2 9079. 8 Cl (3) 9078.3 9088 .3 9087 .8 C I (3), Fe I (339) 9088 . 6/ 9088 .3 9095. 0 9095. 0 Cl (3) 9094.9 9112.3 9112. 5 C I (3) 9111.9 9190.6 9213. 0 9213. 8 SI(1) 9212.9 9229. 1 9229. 0 H P 9 9229.0 9264.8 9264. 3 0 1 (8 ) (9261/9263/9265) 9289.3 Fe I (1298) ? 9289.4 9359.6 Fe I(203) ? 9359.4 9369.9 Fe I (202) ? 9405.6 C I (9) 9405.8 9413.9 Fe I (1298, Si I (14) 9414.1/9413.6 9546.4 9546.0 H P 8 9546.0 9596.5 K (?) 165 9600.4 Fe I (1283) ? 9602 . 1 TA B LE 15 (Continued) NASA IQSY APEQS Identification Laboratory A Kurt Mitchell (1965) and Multiplet Coronal Chromospheric 9605.4 C I (2) ?, Mn I (60) ? 9603.1/9608. 6 9620.9 9621.5 Cl (2) 9620.9 9658.1 9659.6 Cl (2) 9658.5 9674.3 9738.9 Fe I (1296) 9738. 6 9911 (coronal) 10, 049.4 10, 050.4 H P 7 (8 ) 10, 049.4 10, 216. 6 Fe I (1247) 10, 216.4 10, 327. 3 10, 329. 3 Sr II (2) 10, 327. 3 10.456.4 S I (3) 10,457 10.686.4 CI(1) 10, 683. 18 10, 685. 55 10.693.4 CI(1) 10, 691. 36 Detected Fe XIII (IF) 10, 748. 6 x Detected Fe XIII (IF) 10, 798. 0 x 10, 830. 0 10.829.5 He 1(1) 10, 830 X 10, 938. 1 H P 6 (8 ) 10, 938. 1 X 11304.* 11355. * 11386.* 166 *Reference 8 suggests these lines are due to an instrument anomaly. CHAPTER VII OBSERVATIONS IN THE 1-MICRON REGION On two observing runs efforts were devoted exclusively at obtain ing spectra at wavelengths of 10, 000 £ or greater. The second of these runs was intended as a test of the improved converter camera with the Army thermoelectric cooler, the special Navy IR Aero Ektar, and the Elgeet relay optics. As will be seen this system performed very well indeed. Employing the 178-mm f/2. 5 Aero Ektar, incorporate ding the tilt mount designed for the 1963 solar eclipse and the Burke and James optics, spectra of both early type stars B1 to K and of M-stars were obtained at 67 A/mm with the Perkins grating spectrograph in March 1964. Using the 305-mm f/3. 5 IR Aero Ektar and the DTM spectro graph, spectra were obtained at 49 A/mm on 5 nights in February/ March 1965. Preliminary experiments at these wavelengths had shown that Mira variables such as o Ceti or R Leonis could be observed out to 1.1 to 1.15 microns with exposure times of 30-60 minutes. Fig. 53 shows spectra taken of early type stars with the original experi ment and spectra of Mira and R. Leonis. o Ceti was observed on 7 nights including the night of predicted maximum, 5 March 1964, and the night of 29 February when Deutsch reported its actual maximum. Comment on the early type stars will be reserved for that of the 167 FOIa M2Ia TiO Fig. 53. -- Spectra of selected bright stars in the region A 9000 to All, 000 R. 168 169 higher resolution plates obtained on the second run, since only atomic lines were detected and most of these were lost in the low resolution. The most conspicuous features in Mira are the emission features of Paschen y and Paschen 6 . Paschen 6 is so completely masked by the or band of atmospheric HgO that it is not possible to infer emission. There is a suggestion of an emission core in Paschen 7), X9015, and broad emission at Paschen d,X8863. R Leonis, observed only eight days after predicted minimum in 1964, exhibits many stellar band structures throughout the entire region X0.88 to 1.1 microns. Earlier spectra of Mira and R Leonis, exposed to reach the region near 1.13 microns, show the 0 band of telluric water in great detail. In R Leonis band heads other than telluric, judged by comparison with Mira, can be recognized (despite the poor resolution) near the wavelengths listed in the first column of Table 16. All but the first three of these bands were reobserved with the improved camera five weeks after the next predicted minima in 1965. The band head loca tions are seen much more distinctly in the spectrogram reproduced in Fig. 54. Additional bands at X9982, 10333, and 10362 can be discerned in the original and are listed in the second column of Table 16 along with improved wavelengths of the previously recognized band heads. Most of these bands have been identified by Lockwood (1968) as TiO, using spectra taken at this same plate scale, 49 A mm, using an S-l mica-window image tube. The identification of Lockwood are given in Column 3 of the table. Exceptions are the band feature near X10460 which is, of course, the beginning of the extensive VO band identified by M cKeller (1956) in R Leonis. TA BLE 16 MOLECULAR FEATURES OBSERVED IN R LEONIS NEAR MINIMUM IN 1964 AND 1965 1964 Minima 1965 Minima Identifi c ation Source 8859 TiO V A(0, 0) A. Wylies 8938 TiO V A(l, 1) Phillips 9208 TiO Spinrad & Newburn, Jr 9635 Ti(32) Blend Babcock & Moore 9684 9685 Telluric Babcock & Moore 9725 9725 TiO 'it - 'A(O-l) Lockwood 9810 9814 (1- 2) it 9900 9902 " (2-3) ii 9986 (3-4) 10, 025 10,025 TiO 'tt-'E (1-0) ii 10,135 10, 128 (2- 1) ii 10, 270 10, 245 " (3-2) ii 10,336 10, 362 TiO 'ir-'E (4-3) Lockwood Prediction 10,460 10,460 VO M cKeller i71 Lockwood identifies each of seven band heads with TiO bands previously measured in the laboratory (Pettersson and Lindgren, 1962) or detected in low resolution scans of the Taurus infrared object by Wing, Spinrad, and Kuhi (1967). The feature at X 10362 agrees with Lockwood's prediction of X 10364 for the 4-3 transition of the band of TiO. As judged from reproductions of Lockwood's spectra, the quality of the spectra taken in the present investigation are some appreciably better. It is not surprising that this fainter bandhead was missed on his plates. The 5-4 and 6-5 transitions of this band, lying at XX 10478 and 10593, are lost in the strong VO band system. Of the shorter wavelength bands, two are identified as TiO bands of the ' ir-'A system. The first was reported by Wyller (1963) as the only TiO feature (the O, O transition) recognized in a coude spectrum of a Herculis taken at 31 A mm. on hypersensitized 1-Z emulsion (4-hour Exposure, unwidened). The feature at 8939 A is identified here with the (1, 1) transition which is the second strongest head according to Phillips (1950). X 9208 was identified as TiO by Spinrad & Newburn (1965) after Kiess. Atomic Absorptions The spectra of Figs. 54 and 55 are the best spectra obtained. The image-converter camera consisted of the IR Aero Ektar, the FW-167 single-stage tube operating at 11. 3 Kilovolts, the Elgeet relay optics operating at f/3. 6 true f/no. The photocathode was normally operated at 35 C. and the entire apparatus allowed to reach thermal equilibrium by beginning the cooling cycle some 3 hours before sunset so that thermal drifts of focus would be minimized. 1T5IV M ars a Aur G8 III a Boo K2IIIp a Sco M llb + B a Ori M2Iab / O Cet M6e R Leo M7e TiO pf6 VO Fig. 55. -- Spectra of some late-type stars in the region XX9600 - 10,800. Band identifications *— ■ - v l in R Leonis are discussed in the text. u> 174 Points of these spectra have been compared directly with points of 16 A mm coude X-Z plates of 20 odd late-type stars taken by Spinrad on the Lick 120 reflector which are in the process of interpretation by R. F. Wing. Dr. Wing has kindly allowed this investigator to examine these prints and his eye estimates of line intensities of the atomic absorption features for the spectra of Antares, Betelgeuse, Mira, and R. Leonis. Over the wavelength range of 9750 to 10,800 every feature classified as "medium" or stronger by Wing is visible on the prints reproduced here. No attempt has been made to identify these features here. Previous investigators (Wyller, 1963; Griffin 1964; Vaughan and Zirin 1968; Lockwood, 1968; Fay, Fredrick and Johnson, 1968) have published line lists and intensities lists which will doubtless be super- ceded by Wing when his reductions are complete. McCarthy, Treanor, and Ford (1966) have published descriptions of Mira-type spectra identifying several atomic transitions. In only two ways are the 1-micron spectra obtained here signifi-— cantly different from those of Spinrad. The spectra of Mira were obtained at or nearer to maximum; while Paschen y is in emission in Spinrad's spectra of Mira variables, Paschen 6 and higher transitions are not. Second, the spectra of R Leonis were obtained at minimum and show more clearly the TiO bands cited above. It is much more clear in the sequence of spectra by Spinrad, however, the manner in which the atomic lines of Sr II, Si I, and Ti I at 10327, 10398, 10493, 10585, and 10661 weaken as the star drops in temperature and the blanketing of the TiO bands and the reduction of Ti I abundance become effective. Two features which remain strong even at the 175 latest type are the feature at 10, 398 and one at 10,423 which may be due to 2-volt Fe 1 transitions. Referring to the spectra of the. stars of earlier type, one can observe that very few features of stellar origin can be seen of the dispersion used here. The Paschen 6 line is seen to be luminosity dependent as one would expect, becoming quite sharp in supergiants. It is quite weak in spectral types K2-M2. The strong feature at X10, 049 in R Leonis is attributable to Ti I (95) with perhaps some Paschen contribution. Griffin commented that he was unable to recognize either P^ or Pg in a Boo. Even in spectral type G (Capella, G811X, Mars, Gl. 5V modified), the Pg line is weak. Its great strength in B, A, and F stars is similar with the behavior of the Balmer lines. Recently Vaughan and Zirin (1968) reported observations of He I 10,830 in late type stars using image tube spectra, Fabry-Perot interferometry and scanning spectrophotometry. Seven of the stars on their program are contained in the spectra here but unfortunately my plates of 1965 were not exposed long enough to yield useful densities at X10,830. Examination of these plates shows the atomic lines of Mg I (37) at 10,811 and Si I (5) at 10,827 as strong features against the weak continuum. The spectrum of (3 Orionis A taken in 1964 at much lower resolution does show moderately strong absorption feature at 10,832 relative to the laboratory neon spectrum. P Ori A was the one B star examined by Vaughan and Zirin which showed 10,830 absorption o of 1. 2 A equivalent width and was cited by them as a key part of the evidence that it may be associated with binary systems (4 of 5 stars observed in spectral types F0 to M5 in which the 10,830 line exceeded 176 strengths 500 milliangstroms were binary. Vaughan and Zirin indicate zero shift of the 10, 830 feature from its rest wavelength in the star, but the combined radial velocity of |3 Ori and the component o of the orbital velocity would yield a Doppler shift of + 1. 5 A since the star was observed low in the west near midnight. Vaughan and Zirin comment that no other stellar feature could be detected near 10,830 in (3 Ori A. Arp, H, C ., The7Me?t^prin.'g'^Ru^'sell^Diagram, Hbd, der Physik, LI, p. 122 Baum, W. A ., Advances in Electronics and Electron Physics, Vol. 22A, pp. 617-628, Academic I^ress Baum, W. A ., Advances in Electronic» XVI (New York: Academic Press, ed. McGee, et al. 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