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Astronomical Uses of Cascade Intensifies
Astronomical Uses of Cascade Intensifiers W. KENT FORD, Jr. Carnegie Institution of Waahington, Department of Terre&rial Magnetism Waahington, D.C., U.S.A.
INTRODUCTION The Carnegie Image Committee has investigated, over a period of years, various types of intensifying devices for astronomical application~.~ This - ~ paper reviews measurements made at this laboratory on the operating characteristics of a two-stage cascade image intensifier developed by the Electron Tube Division of RCA for the Carnegie Committee.? Some of the equipment that has been built for use a t the telescope is also described.
OPERATINGCHARACTERISTICS The cascade tube described here is the RCA Type C33011. This is a high quality version of the C70056 developmental image tube. The tube includes an S.20 photocathode, a single electron-multiplier of the phosphor-photocathode type, and a final output screen with a fine-grain P.11 phosphor. The cathodes of these tubes have sensitivities of better than 130 pA/lm (2870°K tungsten lamp, unfiltered), with 150 pA/lm and more being not uncommon. The blue sensitivity is typically 9.5 to 10 pA/lm as measured through a Corning 5113 filter of half the stock thickness. It is estimated that the quantum efficiency of a typical tube is 15% or better a t A4200 8. Radiant energy gain measurements have been made by the method described by St~udenheimer.~ This technique consists of measuring the light from a blue source directly through a fixed aperture by means of a photomultiplier. The light radiated from the phosphor screen of the tube is measured with the same photomultiplier. Corrections are made for the difference in geometrical collection of the photometer aperture in the two cases and possible mismatches between the spectral distribution of the blue light and the phosphor output. The most difficult part of the measurement is to eliminate changes in the photometer gain due to the magnetic field used to focus the cascade tube. To some extent this difficulty has been overcome by keeping the photometer and focus-
t Funds have been made available by the National Science Foundation for the production of 20 tubes to be distributed to, and used at, various astronomical observetories. 687
698
W. KENT FORD, JR.
ing magnet rigidly fixed and making calibrations by inserting a small carbon-14 activated P a 1 1 phosphor source. This standard source is used both to calibrate the photometer and to serve as an input source to the intensifier system, and, therefore, also provides the same spectral distribution for the calibration as for the signal. For a P-11source such as this the radiant energy gain is typically 2000-2500. Reynolds shows in a paper? in this volume that the number of developable photographic grains per primary photoelectron can easily exceed unity. The number of photons N emitted from the phosphor screen per photoelectron is the radiant energy gain divided by quantum efficiency of the photocathode or 2000/0.15 in this case. For most of the astronomical observations the transfer lens used had a measured geometrical collection efficiency f (including transmission losses for P-11light) of only 3%. For I I a - 0 emulsions it is estimated that approximately 8 = 150 photons are required on the average per developable grain, Thus, the number k of developable grains per electron incident on the phosphor given by k = NflS is approximately 28 grains per photoelectron. It must be emphasized that although single photoelectrons cannot be detected with certainty, they will contribute statistically to the information in the recorded image. A check on this assumption is given by the fact that ion scintillations are recorded as clumps of blackened grains that are easily recognizable above the background of the emulsion. Assuming 10-30 electrons in each ion scintillation bunch, the above figures would predict 50 or 100 grains per recorded scintillation image. While actual grain counting experiments have 'not been made, visual inspection with high magnification indicates that such numbers certainly are of the right order of magnitude. The resolution displayed a t the phosphor screen of a two-stage tube is 40 lp/mm or better, and is fairly uniform across the 38 mm diameter field. The best that can be recorded on I I a - 0 emulsion is 20 or 25 lp/mm (see Fig. 1). This limitation is set by both the photographic emulsion and the lens. Of course the limiting resolution of each of these elements is much higher, but the modulation transfer of the I I a - 0 emulsion alone is probably not better than 50% a t 30 lp/mm, the relay lens having a similar value. This combination of lens and emulsion, plus, of course, the limited modulation transfer capabilities of the tube itself, brings the limiting resolution down to the observed 20 or 25 lp/mm. In the author's opinion, the best approach to re-imaging from the phosphor screen will probably involve some degree of magnification. Some experiments have been made with a recording lens giving a magnification of two, and for problems where storage capacity is of importance, this scheme works well indeed, but a t the expense of exposure
7 See p. 381.
ASTRONOMICAL USES O F CASCADE INTENSIFIERS
699
time. To compensate in part for the loss of resolution over photographic systems, optical systems giving twice the normal image scale input t o the intensifier system have been used. I n assessing the value of these tubes a useful concept is gain in exposure time for equal resolution. The system described here consistently gives a gain for equal resolution of 10 or 12 times in the blue region of the spectrum. Such gains, while modest compared with the radiant energy gain of several thousand, is consistent with the gain in
FIO.1. Photograph of the Baum test pattern (4 mm indiameter) obtained with the RCA cascade image intensifier. A Burke and Jamesf/2 transfer lens was used at unit magnification, and the image recorded on IIa-0 emulsion.
information to be expected from the ratio of quantum efficiency of the photographic emulsion to that of the photoemissive surface. Furthermore, while photographic exposures of 10-12 h are feasible in principle, it is nevertheless much more profitable to use an image tube so that 10 one-hour exposures on different objects can be obtained. Exposure times are limited in general with this tube by the stability of the local magnetic field. The thermal background is sufficiently low at observing temperatures for exposures of many hours but there is occasional difficulty with troublesome ion scintillations. It has been found that the best cure for the ion scintillations is simply to leave the tube operating with potential applied. Occasionally, the tube was maintained a t a potential of 20 kV for 10 days or so with a subsequent substantial reduction in the ion count.
700
W. KENT FORD,
JR.
IMAGE INTENSIFIER SYSTEM The intensifier system which has been built up for the RCA cascade tube has been adapted to several observing situations. Permanent magnet focusing similar to the system described by Baum in a paper? in this volume is used. The image tube and its magnet is positioned within a 64 in. outside diameter cylinder with the relay lens and focusing device attached by means of spacers to the same system (Fig. 2 ) .
FIG.2. The DTM image tube spectrograph. The cascade tube with its focusing magnet is contained in the protruding cylinder (64 in. in diameter). An ocular for viewing the image of the phosphor screen has been inserted in place of the plate holder. A potential divider is contained in the clear plastic box.
Cooling was not found necessary for the S.20 photocathodes although for special experimental tubes with S.1 photocathodes a simple circulating system has been employed. The weight of the basic intensifier system is approximately 501b. An external potential divider provides the correct accelerating potentials t o the electrodes of the intensifier struoture. t See p. 617.
ASTRONOMICAL USES O F CASCADE INTENSIFIERS
701
ASTRONOMICAL USES The principal application of image intensifiers in astronomy is in photographing the fainter stars in star fields, nebulae and galaxies. For such problems, the intensifier system described here competes unfavorably with photographic plates for most applications because of the limited size of the photocathode and the comparatively large field available photographically with a properly designed telescope. Nevertheless, for certain problems where isolated regions are of interest and faint limiting magnitudes are required, there are indeed interesting applications for a cascade tube. Our own work has been confined to experiments with the Morgan 24-in. reflector at Lowell Observatory. This is an fils instrument and, with a typical cascade tube, 20th magnitude stars can be reached in a 15-min exposure with no optical filters. The speed of the intensifier system can be used to allow exposures through relatively narrow-band filters to isolate spectral regions of interest such as the Ha line. I n spite of the initial interest in use of intensifier systems for recording the images of fainter stars, their most exciting application is in the obtaining of astronomical spectra. Spectrographic applications put greater demands on the resolution and contrast capability of an intensifier system than star-field photography. Figure 3(a) shows a sample of astronomical spectra obtained? with the coudk spectrograph at the 100-in. telescope a t Mt. Wilson. This is the region of the calcium H and K lines taken at relatively high dispersion, 4 6 B/mm. Here the S-distortion of the magnetic focusing system is evident. The spectrum has a deep absorption in the middle of which is a strong emission core. The width of these cores is related to the luminosity of the star5, and the resolution of the intensifier system must be such that the width of these cores can be measured with some degree of certainty. Another frequent type of demand on the capability of the intensifier system is in the detection of weak absorption lines, as in Fig. 3(b). This is the 6700-A region of a main sequence star, and the marked line is the lithium A6707.8-f! line. These spectra were obtained a t Mt. Wilson with the 73-in. Schmidt camera of the coud6 spectrograph of the 100-in. telescope.$ Medium dispersion spectra have been obtained at the 69-in. Perkins telescope of Ohio State and Ohio Wesleyan Universities at Lowell Observatory using the spectrograph shown in Fig. 2. The same system has also been used with an infra-red sensitive tube for exploration of the I-pm spectral region, and a sample spectrogram is reproduced in Fig. 3(c). In Fig. 3(d), a very low dispersion spectrogram of a Seyfert galaxy is
t In collaboration with Drs. W'.
A. Baum and 0. C. Wilson. $ In collaboration with Dr. W. A. Baum.
702
W. KENT FORD, JR.
FIa. 3. Spectra obtained with the RCA cascade image intensifier. (a) H D 154143. 73-in. camera of coud6 spectrograph of Mt. Wilson 100-in. telescope. The spectral region is centered at approximately A3960 i% and includes the broad H and K absorption lines with emission cores. Original dispersion 4.5 A/mm. (b) ADS 923 br. 114-in. camera of the coud6 spectrograph of Mt. Wilson 100-in. telescope. The line LiI X6707.8 A is marked. Original dispersion 4.6 Ajmm. (c) W Ori. 12-in. camera of the DTM spectrograph on the Perkins 69-in. telescope. The 1-pm region is shown. The comparison line at the extreme left is A9665 A, and the pair a t the right is A10798 A and A10844 A. Original dispersion is 45A/mm. (d) NGC 7469 (a Seyfert galaxy with broad emission features). 3-in. camera of the DTM spectrograph on the Perkins 69-in. telescope. Original dispersion is 410 i%/mm.
shown. This was obtained? a t the Perkins telescope with the spectrograph shown and an f/0.87 camera (Super Farron, 76-mm focal length) giving a reciprocal dispersion of 410 A/mm. Laboratory measurements have indicated that the gain of the image tube system is constant over some 5 orders of magnitude for current densities less than A/cm2. This implies that the phosphor screens have uniform efficiency in the current density range from to
t I n collaboration with Dr. Vera Rubin of the Department of Terrestrial Magnetism, Carnegie Institution.
ASTRONOMIUAL USES OF UASCADE INTENSIFIERS
703
A/cm2, which is not in contradiction to the observations of at higher densities. Of course, the image tube system St~udenheimer~ as a whole is non-linear because of the photographic recording.
CONCLUSION The cascade intensifiers developed for the Carnegie Committee have demonstrated their usefulness in a variety of astronomical applications. At the present time, the limitations of the cascade system are primarily those associated with the transfer optics. Because of their sensitivity, reliability, and ease of operation, these tubes are particularly suited for a number of routine astronomical observations. ACKNOWLEDGMENT The assistance of the National Science Foundation in the development of these intonsifiers at RCA for the Carnegie Committee is gratefully acknowledged.
REFERENCES 1. Carnegie Institution of Washington Year Book 61, 295 (1962).
2. Carnegie Institution of Washingt
DISCUSSION M . F. WALKER: Have you made radial velocity measuwments on plates obtained with these tubes? To what extent do the residual aberrations of the tube, such as S-distortion, limit the accuracy of such measurements? How easy will it be to correct for these distortions if “two dimensional” velocity measurements are attempted, or if a long slit is used to derive the rotation curve of a galaxy? w. KENT FORD: The only radial velocity measurements we have made thus far have been with the low dispersion camera where the region measured is limited to the central centimeter or so. There has been no particular difficulty in doing this, but we have not required high accuracy. I do not think that two dimensional measurements on spectrograms made with a long slit will present any great difficulty. It must be remembered that approximately twice the dispersion must be used with this type of tube to obtain resolving power comparable with “unaided” photography. We have found it convenient to measure the plates thus obtained with a microscope having approximately one-half the magnification normally used. J. A. HYNEE: What was the exposure time on the excellent high dispersion spectra you first showed (Fig. 3(a)) and can you give the equivalent exposure for a 6th magnitude star at that dispersion? I would like to compliment you on the excellence of these spectrograms. w. EENT FORD: Thank you. The exposure time for a 6th magnitude star at 4.6 A/mm is 10 or 16 min, depending on the seeing.
704 U. B . WELLUATE:
W. KENT FORD, JR.
What percentage of photoelectrons is registered?
w. KENT FORD: We have not actually measured the number of photoelectrons
registered, so we do not know how many we are losing due to the distribution in the various statistical processes involved. D. J. BRADLEY: You mentioned the good contrast performance of this type of tube. Have you measured the instrumental profile of the tube? w. KENT FORD: No. We have not measured the instrumental profile of the tube alone. We have measured the profile of our spectrograms but this includes, of course, the spectrograph camera, the tube, the transfer lens, and the recording emulsion.
INTRODUCTION The Carnegie Image Committee has investigated, over a period of years, various types of intensifying devices for astronomical application~.~ This - ~ paper reviews measurements made at this laboratory on the operating characteristics of a two-stage cascade image intensifier developed by the Electron Tube Division of RCA for the Carnegie Committee.? Some of the equipment that has been built for use a t the telescope is also described.
OPERATINGCHARACTERISTICS The cascade tube described here is the RCA Type C33011. This is a high quality version of the C70056 developmental image tube. The tube includes an S.20 photocathode, a single electron-multiplier of the phosphor-photocathode type, and a final output screen with a fine-grain P.11 phosphor. The cathodes of these tubes have sensitivities of better than 130 pA/lm (2870°K tungsten lamp, unfiltered), with 150 pA/lm and more being not uncommon. The blue sensitivity is typically 9.5 to 10 pA/lm as measured through a Corning 5113 filter of half the stock thickness. It is estimated that the quantum efficiency of a typical tube is 15% or better a t A4200 8. Radiant energy gain measurements have been made by the method described by St~udenheimer.~ This technique consists of measuring the light from a blue source directly through a fixed aperture by means of a photomultiplier. The light radiated from the phosphor screen of the tube is measured with the same photomultiplier. Corrections are made for the difference in geometrical collection of the photometer aperture in the two cases and possible mismatches between the spectral distribution of the blue light and the phosphor output. The most difficult part of the measurement is to eliminate changes in the photometer gain due to the magnetic field used to focus the cascade tube. To some extent this difficulty has been overcome by keeping the photometer and focus-
t Funds have been made available by the National Science Foundation for the production of 20 tubes to be distributed to, and used at, various astronomical observetories. 687
698
W. KENT FORD, JR.
ing magnet rigidly fixed and making calibrations by inserting a small carbon-14 activated P a 1 1 phosphor source. This standard source is used both to calibrate the photometer and to serve as an input source to the intensifier system, and, therefore, also provides the same spectral distribution for the calibration as for the signal. For a P-11source such as this the radiant energy gain is typically 2000-2500. Reynolds shows in a paper? in this volume that the number of developable photographic grains per primary photoelectron can easily exceed unity. The number of photons N emitted from the phosphor screen per photoelectron is the radiant energy gain divided by quantum efficiency of the photocathode or 2000/0.15 in this case. For most of the astronomical observations the transfer lens used had a measured geometrical collection efficiency f (including transmission losses for P-11light) of only 3%. For I I a - 0 emulsions it is estimated that approximately 8 = 150 photons are required on the average per developable grain, Thus, the number k of developable grains per electron incident on the phosphor given by k = NflS is approximately 28 grains per photoelectron. It must be emphasized that although single photoelectrons cannot be detected with certainty, they will contribute statistically to the information in the recorded image. A check on this assumption is given by the fact that ion scintillations are recorded as clumps of blackened grains that are easily recognizable above the background of the emulsion. Assuming 10-30 electrons in each ion scintillation bunch, the above figures would predict 50 or 100 grains per recorded scintillation image. While actual grain counting experiments have 'not been made, visual inspection with high magnification indicates that such numbers certainly are of the right order of magnitude. The resolution displayed a t the phosphor screen of a two-stage tube is 40 lp/mm or better, and is fairly uniform across the 38 mm diameter field. The best that can be recorded on I I a - 0 emulsion is 20 or 25 lp/mm (see Fig. 1). This limitation is set by both the photographic emulsion and the lens. Of course the limiting resolution of each of these elements is much higher, but the modulation transfer of the I I a - 0 emulsion alone is probably not better than 50% a t 30 lp/mm, the relay lens having a similar value. This combination of lens and emulsion, plus, of course, the limited modulation transfer capabilities of the tube itself, brings the limiting resolution down to the observed 20 or 25 lp/mm. In the author's opinion, the best approach to re-imaging from the phosphor screen will probably involve some degree of magnification. Some experiments have been made with a recording lens giving a magnification of two, and for problems where storage capacity is of importance, this scheme works well indeed, but a t the expense of exposure
7 See p. 381.
ASTRONOMICAL USES O F CASCADE INTENSIFIERS
699
time. To compensate in part for the loss of resolution over photographic systems, optical systems giving twice the normal image scale input t o the intensifier system have been used. I n assessing the value of these tubes a useful concept is gain in exposure time for equal resolution. The system described here consistently gives a gain for equal resolution of 10 or 12 times in the blue region of the spectrum. Such gains, while modest compared with the radiant energy gain of several thousand, is consistent with the gain in
FIO.1. Photograph of the Baum test pattern (4 mm indiameter) obtained with the RCA cascade image intensifier. A Burke and Jamesf/2 transfer lens was used at unit magnification, and the image recorded on IIa-0 emulsion.
information to be expected from the ratio of quantum efficiency of the photographic emulsion to that of the photoemissive surface. Furthermore, while photographic exposures of 10-12 h are feasible in principle, it is nevertheless much more profitable to use an image tube so that 10 one-hour exposures on different objects can be obtained. Exposure times are limited in general with this tube by the stability of the local magnetic field. The thermal background is sufficiently low at observing temperatures for exposures of many hours but there is occasional difficulty with troublesome ion scintillations. It has been found that the best cure for the ion scintillations is simply to leave the tube operating with potential applied. Occasionally, the tube was maintained a t a potential of 20 kV for 10 days or so with a subsequent substantial reduction in the ion count.
700
W. KENT FORD,
JR.
IMAGE INTENSIFIER SYSTEM The intensifier system which has been built up for the RCA cascade tube has been adapted to several observing situations. Permanent magnet focusing similar to the system described by Baum in a paper? in this volume is used. The image tube and its magnet is positioned within a 64 in. outside diameter cylinder with the relay lens and focusing device attached by means of spacers to the same system (Fig. 2 ) .
FIG.2. The DTM image tube spectrograph. The cascade tube with its focusing magnet is contained in the protruding cylinder (64 in. in diameter). An ocular for viewing the image of the phosphor screen has been inserted in place of the plate holder. A potential divider is contained in the clear plastic box.
Cooling was not found necessary for the S.20 photocathodes although for special experimental tubes with S.1 photocathodes a simple circulating system has been employed. The weight of the basic intensifier system is approximately 501b. An external potential divider provides the correct accelerating potentials t o the electrodes of the intensifier struoture. t See p. 617.
ASTRONOMICAL USES O F CASCADE INTENSIFIERS
701
ASTRONOMICAL USES The principal application of image intensifiers in astronomy is in photographing the fainter stars in star fields, nebulae and galaxies. For such problems, the intensifier system described here competes unfavorably with photographic plates for most applications because of the limited size of the photocathode and the comparatively large field available photographically with a properly designed telescope. Nevertheless, for certain problems where isolated regions are of interest and faint limiting magnitudes are required, there are indeed interesting applications for a cascade tube. Our own work has been confined to experiments with the Morgan 24-in. reflector at Lowell Observatory. This is an fils instrument and, with a typical cascade tube, 20th magnitude stars can be reached in a 15-min exposure with no optical filters. The speed of the intensifier system can be used to allow exposures through relatively narrow-band filters to isolate spectral regions of interest such as the Ha line. I n spite of the initial interest in use of intensifier systems for recording the images of fainter stars, their most exciting application is in the obtaining of astronomical spectra. Spectrographic applications put greater demands on the resolution and contrast capability of an intensifier system than star-field photography. Figure 3(a) shows a sample of astronomical spectra obtained? with the coudk spectrograph at the 100-in. telescope a t Mt. Wilson. This is the region of the calcium H and K lines taken at relatively high dispersion, 4 6 B/mm. Here the S-distortion of the magnetic focusing system is evident. The spectrum has a deep absorption in the middle of which is a strong emission core. The width of these cores is related to the luminosity of the star5, and the resolution of the intensifier system must be such that the width of these cores can be measured with some degree of certainty. Another frequent type of demand on the capability of the intensifier system is in the detection of weak absorption lines, as in Fig. 3(b). This is the 6700-A region of a main sequence star, and the marked line is the lithium A6707.8-f! line. These spectra were obtained a t Mt. Wilson with the 73-in. Schmidt camera of the coud6 spectrograph of the 100-in. telescope.$ Medium dispersion spectra have been obtained at the 69-in. Perkins telescope of Ohio State and Ohio Wesleyan Universities at Lowell Observatory using the spectrograph shown in Fig. 2. The same system has also been used with an infra-red sensitive tube for exploration of the I-pm spectral region, and a sample spectrogram is reproduced in Fig. 3(c). In Fig. 3(d), a very low dispersion spectrogram of a Seyfert galaxy is
t In collaboration with Drs. W'.
A. Baum and 0. C. Wilson. $ In collaboration with Dr. W. A. Baum.
702
W. KENT FORD, JR.
FIa. 3. Spectra obtained with the RCA cascade image intensifier. (a) H D 154143. 73-in. camera of coud6 spectrograph of Mt. Wilson 100-in. telescope. The spectral region is centered at approximately A3960 i% and includes the broad H and K absorption lines with emission cores. Original dispersion 4.5 A/mm. (b) ADS 923 br. 114-in. camera of the coud6 spectrograph of Mt. Wilson 100-in. telescope. The line LiI X6707.8 A is marked. Original dispersion 4.6 Ajmm. (c) W Ori. 12-in. camera of the DTM spectrograph on the Perkins 69-in. telescope. The 1-pm region is shown. The comparison line at the extreme left is A9665 A, and the pair a t the right is A10798 A and A10844 A. Original dispersion is 45A/mm. (d) NGC 7469 (a Seyfert galaxy with broad emission features). 3-in. camera of the DTM spectrograph on the Perkins 69-in. telescope. Original dispersion is 410 i%/mm.
shown. This was obtained? a t the Perkins telescope with the spectrograph shown and an f/0.87 camera (Super Farron, 76-mm focal length) giving a reciprocal dispersion of 410 A/mm. Laboratory measurements have indicated that the gain of the image tube system is constant over some 5 orders of magnitude for current densities less than A/cm2. This implies that the phosphor screens have uniform efficiency in the current density range from to
t I n collaboration with Dr. Vera Rubin of the Department of Terrestrial Magnetism, Carnegie Institution.
ASTRONOMIUAL USES OF UASCADE INTENSIFIERS
703
A/cm2, which is not in contradiction to the observations of at higher densities. Of course, the image tube system St~udenheimer~ as a whole is non-linear because of the photographic recording.
CONCLUSION The cascade intensifiers developed for the Carnegie Committee have demonstrated their usefulness in a variety of astronomical applications. At the present time, the limitations of the cascade system are primarily those associated with the transfer optics. Because of their sensitivity, reliability, and ease of operation, these tubes are particularly suited for a number of routine astronomical observations. ACKNOWLEDGMENT The assistance of the National Science Foundation in the development of these intonsifiers at RCA for the Carnegie Committee is gratefully acknowledged.
REFERENCES 1. Carnegie Institution of Washington Year Book 61, 295 (1962).
2. Carnegie Institution of Washingt
DISCUSSION M . F. WALKER: Have you made radial velocity measuwments on plates obtained with these tubes? To what extent do the residual aberrations of the tube, such as S-distortion, limit the accuracy of such measurements? How easy will it be to correct for these distortions if “two dimensional” velocity measurements are attempted, or if a long slit is used to derive the rotation curve of a galaxy? w. KENT FORD: The only radial velocity measurements we have made thus far have been with the low dispersion camera where the region measured is limited to the central centimeter or so. There has been no particular difficulty in doing this, but we have not required high accuracy. I do not think that two dimensional measurements on spectrograms made with a long slit will present any great difficulty. It must be remembered that approximately twice the dispersion must be used with this type of tube to obtain resolving power comparable with “unaided” photography. We have found it convenient to measure the plates thus obtained with a microscope having approximately one-half the magnification normally used. J. A. HYNEE: What was the exposure time on the excellent high dispersion spectra you first showed (Fig. 3(a)) and can you give the equivalent exposure for a 6th magnitude star at that dispersion? I would like to compliment you on the excellence of these spectrograms. w. EENT FORD: Thank you. The exposure time for a 6th magnitude star at 4.6 A/mm is 10 or 16 min, depending on the seeing.
704 U. B . WELLUATE:
W. KENT FORD, JR.
What percentage of photoelectrons is registered?
w. KENT FORD: We have not actually measured the number of photoelectrons
registered, so we do not know how many we are losing due to the distribution in the various statistical processes involved. D. J. BRADLEY: You mentioned the good contrast performance of this type of tube. Have you measured the instrumental profile of the tube? w. KENT FORD: No. We have not measured the instrumental profile of the tube alone. We have measured the profile of our spectrograms but this includes, of course, the spectrograph camera, the tube, the transfer lens, and the recording emulsion.