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A Review of Astronomical Applications
A Review of Astronomical Applications J. RING Astronomy Group, Blacketr Laboratory, Imperial College, University of London, England
It is no accident that the seven Symposia on “Photo-Electronic Image Devices” which have taken place at Imperial College, London since 1958, have recorded remarkable developments in astronomical image detectors. One of the principal objectives of Professor J. D. McGee in setting up his research group at the College was to develop an ideal detector for use in astronomy and the Symposia which he instituted, although covering the whole field of photoelectronic technology, have always strongly emphasized astronomical applications. The published proceedings of the Symposia form a valuable and concise record of the improvement to astronomical detectors over the past two decades and it is in this perspective that we can best consider the present “state of the art” as exemplified by the contributors to the 7th Symposium. It is, perhaps, obvious why detector technologists have been attracted to astronomical observation; practically all our information about the extra-terrestrial universe comes from an analysis of the photons collected by telescopes and the efficiency of the process depends strongly on the quality of detecting devices. In 1958 the photographic plate was used by almost all astronomers, although some observers were beginning to use single-point photoelectric detectors such as the photomultiplier tube. Having pushed telescope design to its limit (there is still only one telescope larger than the 5m Hale instrument) astronomers were wasting as much as 99.9% of the photons, collected at great expense, in inefficient detectors. Low quantum efficiency, threshold intensity levels, nonlinearity and limited storage were the more obvious defects of the photographic technique; there were other, more subtle, difficulties. The photomultiplier tube overcame most of these problems at a stroke, but could not compete with the one great asset of the photographic plate-its spatial multiplicity. If the observation being conducted required information from more than about one hundred spatial or spectral elements then, despite its superiority in all other respects and a quantum efficiency 275
276
J. RING
advantage of order 100 times, the fact that the photomultiplier had to obtain its information sequentially rather than simultaneously was a severe limitation. It was clear that what was needed was an imaging device which retained the advantages of the photomultiplier tube, and it is interesting to note that at the 1st Symposium the problem was well understood and the types of device which could satisfy the needs of astronomers were clearly outlined. Baum opened the proceedings with a clear statement of the astronomical requirement and with a warning that a simple improvement of speed was of limited use-it should be accompanied by a greater storage capacity. Lallemand described his electronic camera and reported preliminary astronomical trials; Zacharov and Dowden, members of McGee’s group at Imperial College, described a tube with a thin mica window which was clearly the forerunner of the Spectracon to be described by McGee himself at the 3rd Symposium. There were several papers on the use of “cascaded” image tubes (without the benefits of fibre optics) and a description of a true “cascade” tube with transmission secondary electron multiplication. McGee outlined a scheme for a channeled image intensifier, the precursor of the channel plate systems currently being investigated. Finally there were papers on charge integration and storage tubes, but without the benefits of diode arrays. What has happened in the twenty years between the First and Seventh Symposia? Initially most of the time seems to have been spent on technological problems such as preventing cathodes from dying, increasing resolution, reducing background, improving efficiency and so on. Later on much effort went into obtaining a better understanding of the more important characteristics of image detecting systems. For example, although it was realized quite quickly that nuclear emulsions were less than ideal for astronomical electronography, it was not until the 6th Symposium that Worswick presented a clear analysis of the complete system including the microdensitometer, which showed how difficult it was to achieve photometric accuracies better than 1 or 2%. Throughout the period, all kinds of tubes were being tried in astronomy, but despite efforts by several authors results were often quoted in a way which made it difficult to intercompare them. Slowly the field of devices narrowed, although new components such as fibre optic plates, channel plates and diode arrays were incorporated. At the present time the systems finding favour with astronomers seem to be either electronographic cameras or commercial image tubes feeding a photographic plate or a rapid scanning detector such as a television camera or solid state array. Both types of system can yield a DQE of at least half that of the primary photocathode, can be linear over a wide
A REVIEW OF ASTRONOMICAL APPLICATIONS
277
range and can have high storage capacity. Each suffers from limitations, but these are fortunately different so that the types of device are complementary. Looming over the horizon at the present time is the prospect of large arrays of diodes, with sufficiently low readout noise to allow them to be used as analogue detectors without intensification, but that story is told elsewhere in this volume. Let us consider the present state of the art in electronography and in the use of commercially available image tubes. Several papers at the 7th Symposium showed that electronographic cameras are now in almost routine use at a number of observatories. WlCrickl- describes a variety of applications of the 81 mm version of the Lallemand camera, which yields high resolution (70 lpmm-’) and very low background ( < 10 electrons pixel-’ h-’). The spatial multiplicity of the camera, with 3 X lo7 pixels of a size well matched to the prime focus of the largest reflectors, makes it an almost ideal instrument for the study with moderate photometric accuracy of faint extended objects. The camera has been used mainly at the Cassegrain focus of the 1 . 9 3 m telescope at Haute Provence where it is now available as a “commonuser” instrument; three similar cameras are being constructed for use with the 3.6 m C. F. H. telescope in Hawaii. WlCrick and his colleagues have shown the considerable merits of the electronographic method in studies of radio-galaxies, clusters of galaxies and quasars. At the Royal Greenwich Observatory, McMullanS and his colleagues have continued to develop large field (80 mm) electronographic cameras and have supplied 40 mm cameras to observatories in Israel, South Africa, Australia and Chile. The cameras are of the mica window type and numerous improvements are reported which simplify the routine use of the device. Operation of the camera is automatic, with discs of emulsion, previously mounted on nylon rings, being loaded in less than one minute. Here too, the electronographic method has been applied mainly to the study of the morphology of galaxies and nebulae. Griboval§ reports progress towards the ambitious aim of constructing a 200 mm diameter electronographic camera at the University of Texas. His Mk I1 experimental laboratory model, with a 50 mm field, has been tested oil the 0 - 7 6 m telescope at the McDonald Observatory. The electronographs of galaxies which were obtained were limited by poor seeing and high sky brightness but the camera performed well. If the cameras mentioned so far go a long way towards refuting early fears that electronographic systems were too complicated and fragile for P See p. 295. $‘See p. 315. I See p. 305.
278
J. RING
use at observatories, the work of Carrutherst must surely finally lay that ghost. He describes the development of a 123 mm camera with 10 p m resolution for use in Shuttle/Spacelab missions, at the focus of a 1 m telescope which will yield an angular resolution of 0.2 arcsec over a useful field of half a degree. The camera will be sensitive in the ultraviolet below 2 0 0 n m and will be focused either by a permanent magnet or by a superconducting solenoid. The intense magnetic field generated by the latter (1 tesla) allows the use of curved photocathodes, which match the focal surface of a telescope or spectrograph, without loss of electronic resolution. Laboratory tests have confirmed the expected resolution whilst disclosing problems of radial non-uniformity in magnetic fields. Attempts are being made to extend the wavelength range of the camera to 300nm. If the remaining technical problems can be overcome it is difficult to imagine any other imaging detector rivalling Carruthers’ system for space surveys from vehicles which permit the recovery of data stored on film. Now that electronographic cameras are coming into routine use, more attention is being paid to the reduction of the resulting data. Hardwick et a1.S describe their analysis of electronographs of the elliptical galaxy NGC 3379, obtained with a Spectracon at the Newtonian focus of the 7 4 in. telescope of the Helwan Institute in Egypt. (Despite the spread of the cameras described by WlCrick and McMullan, there are still many astronomers who do not have ready access to them and the Spectracon continues to be put to excellent use.) After scanning the image with a microdensitometer using a 50 p m square aperture (equivalent to 1.1 arcsec), Hardwick et al. derive isophotal contour maps which are used to select an area for detailed study. A computer rejects extraneous objects such as stars and image defects and then derives luminosity profiles around the galaxy. By averaging pixels within successive annuli a measure is obtained of the excess brightness of the galaxy above sky background, which extends to the outermost regions of the object. In this way, the photometric accuracy of the electronographic method can be greatly improved until the errors are comparable to those of photoelectric photometry. Astronomical applications of commercial image tubes with a phosphor output now seem to be concentrated in two rather different areas. The phosphor may be photographed, in which case the principal gain is that of increased speed over the unintensified plate, or the individual photoelectron events may be detected, when several other important advantages accrue. t See p 283. $ See p 329.
A REVIEW OF ASTRONOMICAL APPLICATIONS
279
The contribution by Craine et al.t describes an excellent system of the first kind. It employs an ITT single stage, magnetically focused tube with a 146 mm diameter photocathode sensitive to 900 nm and an output phosphor deposited on a fibre optic faceplate. 300 photons are emitted from the phosphor for each incident photoelectron and the gain in photographic speed is about 40 when IIa-D emulsion is used. The plate limits the resolution to about 25Ipmm-' but the peak DQE of the complete system is of order 7.5%. The fibre optic faceplate produces some discrete shears (<30 p m ) in the image. Although this camera has fewer pixels than the electronographic ones and is limited in its linearity and storage capacity by the plate, it clearly lends itself well to survey type observations and similar systems are being installed on most large telescopes. With the Steward Observatory system a plate limit of 21" is achieved in two minutes at the Cassegrain focus of a 2.3 m reflector. The authors describe how the camera has been used in a near infrared sky survey and in a study of the variation of intensity and polarization across M82. The polarimetric camera has also been used to discover new BL Lacertae objects. When an image intensifier is coupled to a rapid scanning detector such as a T V camera or CCD array the limitations of the photographic plate are removed completely. Each photoelectron scintillation is directly registered and the resultant imaging, photoelectron counting system can have high resolution, is linear and can have almost unlimited digital storage; in addition the image is available on-line. Photoelectron counting systems are now extremely popular amongst astronomers and their use is rapidly being extended to all major telescopes, although their cost and complexity do not yet allow them to be universally available. The most widely adopted photoelectron counting system is that developed by Boksenberg et ul.4 at University College, London. In their contribution to the 7th Symposium they give a lucid account of the way in which the system achieves the properties mentioned above and go on to describe a ruggedized and improved version which is being developed for use with the Space Telescope. There are however still some annoying limitations even in the photoelectron counting systems. The number of pixels may be limited by the TV detector or by the available computer memory, and the need to detect each scintillation limits the maximum flux to much less than one scintillation per pixel per frame. An alternative form of photoelectron counting system has been developed by Stapinski et al.§.The image intensification is achieved by no t See
p. 339.
5 See
p. 389
$ See p. 355.
280
J. RING
less than six electrostatically focused, fibre optically coupled Varo tubes. The output phosphor is scanned by a Reticon, dual, 1024 diode array. Event centring logic is used to improve the resolution and the system has given good results on the 1 - 9 m telescope at Mt. Stromlo in a variety of spectroscopic observations. It is intended to extend the detector to two dimensional operations by using a CCD to scan the phosphor. A similar system was described by Chaffee,? who made an interesting comparison of its performance with that of a Kron electronographic camera. He uses three fibre optically coupled image tubes and a dual Reticon array. The performance of the detector was established by obtaining spectra of [ Oph using an echelle spectrograph on the 1.5 m reflector at Mt. Hopkins. It is very interesting to note that both detectors achieved statistical uncertainties only 1.6 times those expected from Poisson statistics. When exposed to give 2% photometric precision the performances of both detectors were indistinguishable. At lower signal to noise ratios the Kron camera yields too low a density to be measured with conventional microdensitometers; however when a large range of spectrum must be covered the Kron camera is superior on account of its 6 x lo4 pixels as compared to lo3 for the photoelectron counting system. (The author does not make a comparison at higher signal to noise ratios but presumably the electronographic system is limited both by the difficulty of measuring high densities and by variations in emulsion sensitivity which cannot be corrected for.) The expense and complexity of the first photoelectron counting systems is partly due to their use of cascade intensifiers. Attempts are being made to simplify the device by using microchannel plate tubes to provide sufficient gain for photoelectron detection. Two papers report the development in France of systems based on such an intensifier fibre optically coupled to a television camera. Rosier et al.S give a detailed description of the characteristics of their channel plate tube, mentioning the restrictions on photon gain imposed by optical feedback and the efforts being made to remove this limitation. They give pulse height distributions for photoelectron scintillations with various channel plate voltages. BoulesteixD describes the astronomical tests of a similar system using a Thomson -CSF TH9503 microchannel plate tube coupled by fibre optics to a SIT camera. The images comprise 256x256 pixels with event centring logic and are stored in a computer. The dynamic range extends from 2 to 3000 events pixel-' h-'. Images of HI1 regions through t See
p. 415.
$ See p. 369. § See p. 319.
A REVIEW OF ASTRONOMICAL APPLICATIONS
281
interference filters and interferometers and of the spectrum of the spiral galaxy M51 demonstrate the capability of the system which has a gain in speed (for the same signal to noise ratio) of four over a two stage cascade image tube and of 25 over a 103 a-E plate. It is, at first sight, surprising that these devices work so well. There are many statements in the literature that the channel plate tube can only be used efficiently in the saturated mode, because of the negative exponential pulse height distribution at lower gain. Yet Rosier, working at low gain, claims a DQE of 3.4% with a cathode quantum efficiency of 8.6%. Much of this reduction must occur in the loss of photoelectrons at the entrance to the channel plate and in zero yield on the first collision inside it. Further reflection suggests that the earlier literature may well be misleading. The negative exponential shape of the pulse height distribution, whilst increasing the noise of an analogue detector, is of little importance when pulse counting, provided that a sufficiently large fraction of the photoelectrons yield detectable pulses. Simple calculations suggest that the distribution curve must reach a peak well above zero electron gain and there is one observation' which appears to confirm that this peak occurs at a gain of lo4 when the mean gain is of order 10'. A substantial fraction of the incident photoelectrons will lie at gains higher than this peak. If this is indeed the case, or if the optical feedback at high gain can be suppressed, then astronomers can look forward to a very simple photoelectron counting system consisting of a channel plate tube coupled to a CCD array. The dynamic range limitations of the photoelectron counting systems may be removed, whilst retaining the quantum noise limited performance, by using analogue detectors with sufficient gain to overcome readout noise. All that is necessary is to ensure that the pulse height spread of the intensification system does not increase the noise unduly, by unequally weighting individual photoelectrons. The contribution to the 7th Symposium by Hege et ul.? reports tests of five different systems all based on this principle. A variety of image tubes and optical couplers was used to feed a Reticon dual array. Detailed laboratory comparisons were made and several of the systems were tested on the 2.3m telescope of the Steward Observatory. Performances near to the theoretical limit were achieved at fluxes as low as 0-1 pixel-' sec-' and with a dynamic range of 105. The intensified television camera is regularly used at many observatories with direct display of the image on a monitor. Such a system, which is much less expensive and complicated than the photoelectron t See
p. 397.
282
J . RING
counting devices, can nevertheless be extremely useful in finding and guiding on faint objects. Angel et al.t describe the Steward Observatory system which employs a three stage electrostatic image tube coupled to a vidicon with a 10 sec lag. When used on the 2-3 m telescope the camera detects all the stars visible on the E plates of the Palomar Sky Survey over a field of 4 arcmin. For reasons of convenience, papers on purely solid state detectors are published in a separate section and are not reviewed here. It is clear however that they will have important applications in astronomy, particularly when high signal to noise ratios are required; in such circumstances sufficient photons must be collected in each pixel that their shot noise exceeds the readout noise of the detector. What is the astronomer who is not a detector technologist to make of the plethora of devices described at the 7th Symposium? First of all he should be grateful that he has such a wide choice-it has only been provided by thousands of man-years of effort. He should note that he can record objects comparable in brightness to the night sky using cascaded intensifiers with a television or photographic output stage. H e can obtain photometric surveys of the useful field of paraboloid reflectors with electronographic cameras which are linear, extremely sensitive and capable of accuracies of 1 to 2%. For fainter objects he can study a smaller area of sky or of a spectrum with imaging photoelectron counting systems whose photometric accuracy is limited only by the statistical noise of the number of photoelectrons he can afford to collect. Finally, he can look forward to simpler, more convenient ways of obtaining the results described above, but not to any further dramatic improvements in performance. The ideal astronomical detectors envisaged at the first few Symposia are not yet here, but present systems are so close to that performance that the law of diminishing returns must soon begin to apply.
REFERENCE 1. Chalrneton, V., Acta Electron. 14, 99 (1971)
iSee p. 347.
It is no accident that the seven Symposia on “Photo-Electronic Image Devices” which have taken place at Imperial College, London since 1958, have recorded remarkable developments in astronomical image detectors. One of the principal objectives of Professor J. D. McGee in setting up his research group at the College was to develop an ideal detector for use in astronomy and the Symposia which he instituted, although covering the whole field of photoelectronic technology, have always strongly emphasized astronomical applications. The published proceedings of the Symposia form a valuable and concise record of the improvement to astronomical detectors over the past two decades and it is in this perspective that we can best consider the present “state of the art” as exemplified by the contributors to the 7th Symposium. It is, perhaps, obvious why detector technologists have been attracted to astronomical observation; practically all our information about the extra-terrestrial universe comes from an analysis of the photons collected by telescopes and the efficiency of the process depends strongly on the quality of detecting devices. In 1958 the photographic plate was used by almost all astronomers, although some observers were beginning to use single-point photoelectric detectors such as the photomultiplier tube. Having pushed telescope design to its limit (there is still only one telescope larger than the 5m Hale instrument) astronomers were wasting as much as 99.9% of the photons, collected at great expense, in inefficient detectors. Low quantum efficiency, threshold intensity levels, nonlinearity and limited storage were the more obvious defects of the photographic technique; there were other, more subtle, difficulties. The photomultiplier tube overcame most of these problems at a stroke, but could not compete with the one great asset of the photographic plate-its spatial multiplicity. If the observation being conducted required information from more than about one hundred spatial or spectral elements then, despite its superiority in all other respects and a quantum efficiency 275
276
J. RING
advantage of order 100 times, the fact that the photomultiplier had to obtain its information sequentially rather than simultaneously was a severe limitation. It was clear that what was needed was an imaging device which retained the advantages of the photomultiplier tube, and it is interesting to note that at the 1st Symposium the problem was well understood and the types of device which could satisfy the needs of astronomers were clearly outlined. Baum opened the proceedings with a clear statement of the astronomical requirement and with a warning that a simple improvement of speed was of limited use-it should be accompanied by a greater storage capacity. Lallemand described his electronic camera and reported preliminary astronomical trials; Zacharov and Dowden, members of McGee’s group at Imperial College, described a tube with a thin mica window which was clearly the forerunner of the Spectracon to be described by McGee himself at the 3rd Symposium. There were several papers on the use of “cascaded” image tubes (without the benefits of fibre optics) and a description of a true “cascade” tube with transmission secondary electron multiplication. McGee outlined a scheme for a channeled image intensifier, the precursor of the channel plate systems currently being investigated. Finally there were papers on charge integration and storage tubes, but without the benefits of diode arrays. What has happened in the twenty years between the First and Seventh Symposia? Initially most of the time seems to have been spent on technological problems such as preventing cathodes from dying, increasing resolution, reducing background, improving efficiency and so on. Later on much effort went into obtaining a better understanding of the more important characteristics of image detecting systems. For example, although it was realized quite quickly that nuclear emulsions were less than ideal for astronomical electronography, it was not until the 6th Symposium that Worswick presented a clear analysis of the complete system including the microdensitometer, which showed how difficult it was to achieve photometric accuracies better than 1 or 2%. Throughout the period, all kinds of tubes were being tried in astronomy, but despite efforts by several authors results were often quoted in a way which made it difficult to intercompare them. Slowly the field of devices narrowed, although new components such as fibre optic plates, channel plates and diode arrays were incorporated. At the present time the systems finding favour with astronomers seem to be either electronographic cameras or commercial image tubes feeding a photographic plate or a rapid scanning detector such as a television camera or solid state array. Both types of system can yield a DQE of at least half that of the primary photocathode, can be linear over a wide
A REVIEW OF ASTRONOMICAL APPLICATIONS
277
range and can have high storage capacity. Each suffers from limitations, but these are fortunately different so that the types of device are complementary. Looming over the horizon at the present time is the prospect of large arrays of diodes, with sufficiently low readout noise to allow them to be used as analogue detectors without intensification, but that story is told elsewhere in this volume. Let us consider the present state of the art in electronography and in the use of commercially available image tubes. Several papers at the 7th Symposium showed that electronographic cameras are now in almost routine use at a number of observatories. WlCrickl- describes a variety of applications of the 81 mm version of the Lallemand camera, which yields high resolution (70 lpmm-’) and very low background ( < 10 electrons pixel-’ h-’). The spatial multiplicity of the camera, with 3 X lo7 pixels of a size well matched to the prime focus of the largest reflectors, makes it an almost ideal instrument for the study with moderate photometric accuracy of faint extended objects. The camera has been used mainly at the Cassegrain focus of the 1 . 9 3 m telescope at Haute Provence where it is now available as a “commonuser” instrument; three similar cameras are being constructed for use with the 3.6 m C. F. H. telescope in Hawaii. WlCrick and his colleagues have shown the considerable merits of the electronographic method in studies of radio-galaxies, clusters of galaxies and quasars. At the Royal Greenwich Observatory, McMullanS and his colleagues have continued to develop large field (80 mm) electronographic cameras and have supplied 40 mm cameras to observatories in Israel, South Africa, Australia and Chile. The cameras are of the mica window type and numerous improvements are reported which simplify the routine use of the device. Operation of the camera is automatic, with discs of emulsion, previously mounted on nylon rings, being loaded in less than one minute. Here too, the electronographic method has been applied mainly to the study of the morphology of galaxies and nebulae. Griboval§ reports progress towards the ambitious aim of constructing a 200 mm diameter electronographic camera at the University of Texas. His Mk I1 experimental laboratory model, with a 50 mm field, has been tested oil the 0 - 7 6 m telescope at the McDonald Observatory. The electronographs of galaxies which were obtained were limited by poor seeing and high sky brightness but the camera performed well. If the cameras mentioned so far go a long way towards refuting early fears that electronographic systems were too complicated and fragile for P See p. 295. $‘See p. 315. I See p. 305.
278
J. RING
use at observatories, the work of Carrutherst must surely finally lay that ghost. He describes the development of a 123 mm camera with 10 p m resolution for use in Shuttle/Spacelab missions, at the focus of a 1 m telescope which will yield an angular resolution of 0.2 arcsec over a useful field of half a degree. The camera will be sensitive in the ultraviolet below 2 0 0 n m and will be focused either by a permanent magnet or by a superconducting solenoid. The intense magnetic field generated by the latter (1 tesla) allows the use of curved photocathodes, which match the focal surface of a telescope or spectrograph, without loss of electronic resolution. Laboratory tests have confirmed the expected resolution whilst disclosing problems of radial non-uniformity in magnetic fields. Attempts are being made to extend the wavelength range of the camera to 300nm. If the remaining technical problems can be overcome it is difficult to imagine any other imaging detector rivalling Carruthers’ system for space surveys from vehicles which permit the recovery of data stored on film. Now that electronographic cameras are coming into routine use, more attention is being paid to the reduction of the resulting data. Hardwick et a1.S describe their analysis of electronographs of the elliptical galaxy NGC 3379, obtained with a Spectracon at the Newtonian focus of the 7 4 in. telescope of the Helwan Institute in Egypt. (Despite the spread of the cameras described by WlCrick and McMullan, there are still many astronomers who do not have ready access to them and the Spectracon continues to be put to excellent use.) After scanning the image with a microdensitometer using a 50 p m square aperture (equivalent to 1.1 arcsec), Hardwick et al. derive isophotal contour maps which are used to select an area for detailed study. A computer rejects extraneous objects such as stars and image defects and then derives luminosity profiles around the galaxy. By averaging pixels within successive annuli a measure is obtained of the excess brightness of the galaxy above sky background, which extends to the outermost regions of the object. In this way, the photometric accuracy of the electronographic method can be greatly improved until the errors are comparable to those of photoelectric photometry. Astronomical applications of commercial image tubes with a phosphor output now seem to be concentrated in two rather different areas. The phosphor may be photographed, in which case the principal gain is that of increased speed over the unintensified plate, or the individual photoelectron events may be detected, when several other important advantages accrue. t See p 283. $ See p 329.
A REVIEW OF ASTRONOMICAL APPLICATIONS
279
The contribution by Craine et al.t describes an excellent system of the first kind. It employs an ITT single stage, magnetically focused tube with a 146 mm diameter photocathode sensitive to 900 nm and an output phosphor deposited on a fibre optic faceplate. 300 photons are emitted from the phosphor for each incident photoelectron and the gain in photographic speed is about 40 when IIa-D emulsion is used. The plate limits the resolution to about 25Ipmm-' but the peak DQE of the complete system is of order 7.5%. The fibre optic faceplate produces some discrete shears (<30 p m ) in the image. Although this camera has fewer pixels than the electronographic ones and is limited in its linearity and storage capacity by the plate, it clearly lends itself well to survey type observations and similar systems are being installed on most large telescopes. With the Steward Observatory system a plate limit of 21" is achieved in two minutes at the Cassegrain focus of a 2.3 m reflector. The authors describe how the camera has been used in a near infrared sky survey and in a study of the variation of intensity and polarization across M82. The polarimetric camera has also been used to discover new BL Lacertae objects. When an image intensifier is coupled to a rapid scanning detector such as a T V camera or CCD array the limitations of the photographic plate are removed completely. Each photoelectron scintillation is directly registered and the resultant imaging, photoelectron counting system can have high resolution, is linear and can have almost unlimited digital storage; in addition the image is available on-line. Photoelectron counting systems are now extremely popular amongst astronomers and their use is rapidly being extended to all major telescopes, although their cost and complexity do not yet allow them to be universally available. The most widely adopted photoelectron counting system is that developed by Boksenberg et ul.4 at University College, London. In their contribution to the 7th Symposium they give a lucid account of the way in which the system achieves the properties mentioned above and go on to describe a ruggedized and improved version which is being developed for use with the Space Telescope. There are however still some annoying limitations even in the photoelectron counting systems. The number of pixels may be limited by the TV detector or by the available computer memory, and the need to detect each scintillation limits the maximum flux to much less than one scintillation per pixel per frame. An alternative form of photoelectron counting system has been developed by Stapinski et al.§.The image intensification is achieved by no t See
p. 339.
5 See
p. 389
$ See p. 355.
280
J. RING
less than six electrostatically focused, fibre optically coupled Varo tubes. The output phosphor is scanned by a Reticon, dual, 1024 diode array. Event centring logic is used to improve the resolution and the system has given good results on the 1 - 9 m telescope at Mt. Stromlo in a variety of spectroscopic observations. It is intended to extend the detector to two dimensional operations by using a CCD to scan the phosphor. A similar system was described by Chaffee,? who made an interesting comparison of its performance with that of a Kron electronographic camera. He uses three fibre optically coupled image tubes and a dual Reticon array. The performance of the detector was established by obtaining spectra of [ Oph using an echelle spectrograph on the 1.5 m reflector at Mt. Hopkins. It is very interesting to note that both detectors achieved statistical uncertainties only 1.6 times those expected from Poisson statistics. When exposed to give 2% photometric precision the performances of both detectors were indistinguishable. At lower signal to noise ratios the Kron camera yields too low a density to be measured with conventional microdensitometers; however when a large range of spectrum must be covered the Kron camera is superior on account of its 6 x lo4 pixels as compared to lo3 for the photoelectron counting system. (The author does not make a comparison at higher signal to noise ratios but presumably the electronographic system is limited both by the difficulty of measuring high densities and by variations in emulsion sensitivity which cannot be corrected for.) The expense and complexity of the first photoelectron counting systems is partly due to their use of cascade intensifiers. Attempts are being made to simplify the device by using microchannel plate tubes to provide sufficient gain for photoelectron detection. Two papers report the development in France of systems based on such an intensifier fibre optically coupled to a television camera. Rosier et al.S give a detailed description of the characteristics of their channel plate tube, mentioning the restrictions on photon gain imposed by optical feedback and the efforts being made to remove this limitation. They give pulse height distributions for photoelectron scintillations with various channel plate voltages. BoulesteixD describes the astronomical tests of a similar system using a Thomson -CSF TH9503 microchannel plate tube coupled by fibre optics to a SIT camera. The images comprise 256x256 pixels with event centring logic and are stored in a computer. The dynamic range extends from 2 to 3000 events pixel-' h-'. Images of HI1 regions through t See
p. 415.
$ See p. 369. § See p. 319.
A REVIEW OF ASTRONOMICAL APPLICATIONS
281
interference filters and interferometers and of the spectrum of the spiral galaxy M51 demonstrate the capability of the system which has a gain in speed (for the same signal to noise ratio) of four over a two stage cascade image tube and of 25 over a 103 a-E plate. It is, at first sight, surprising that these devices work so well. There are many statements in the literature that the channel plate tube can only be used efficiently in the saturated mode, because of the negative exponential pulse height distribution at lower gain. Yet Rosier, working at low gain, claims a DQE of 3.4% with a cathode quantum efficiency of 8.6%. Much of this reduction must occur in the loss of photoelectrons at the entrance to the channel plate and in zero yield on the first collision inside it. Further reflection suggests that the earlier literature may well be misleading. The negative exponential shape of the pulse height distribution, whilst increasing the noise of an analogue detector, is of little importance when pulse counting, provided that a sufficiently large fraction of the photoelectrons yield detectable pulses. Simple calculations suggest that the distribution curve must reach a peak well above zero electron gain and there is one observation' which appears to confirm that this peak occurs at a gain of lo4 when the mean gain is of order 10'. A substantial fraction of the incident photoelectrons will lie at gains higher than this peak. If this is indeed the case, or if the optical feedback at high gain can be suppressed, then astronomers can look forward to a very simple photoelectron counting system consisting of a channel plate tube coupled to a CCD array. The dynamic range limitations of the photoelectron counting systems may be removed, whilst retaining the quantum noise limited performance, by using analogue detectors with sufficient gain to overcome readout noise. All that is necessary is to ensure that the pulse height spread of the intensification system does not increase the noise unduly, by unequally weighting individual photoelectrons. The contribution to the 7th Symposium by Hege et ul.? reports tests of five different systems all based on this principle. A variety of image tubes and optical couplers was used to feed a Reticon dual array. Detailed laboratory comparisons were made and several of the systems were tested on the 2.3m telescope of the Steward Observatory. Performances near to the theoretical limit were achieved at fluxes as low as 0-1 pixel-' sec-' and with a dynamic range of 105. The intensified television camera is regularly used at many observatories with direct display of the image on a monitor. Such a system, which is much less expensive and complicated than the photoelectron t See
p. 397.
282
J . RING
counting devices, can nevertheless be extremely useful in finding and guiding on faint objects. Angel et al.t describe the Steward Observatory system which employs a three stage electrostatic image tube coupled to a vidicon with a 10 sec lag. When used on the 2-3 m telescope the camera detects all the stars visible on the E plates of the Palomar Sky Survey over a field of 4 arcmin. For reasons of convenience, papers on purely solid state detectors are published in a separate section and are not reviewed here. It is clear however that they will have important applications in astronomy, particularly when high signal to noise ratios are required; in such circumstances sufficient photons must be collected in each pixel that their shot noise exceeds the readout noise of the detector. What is the astronomer who is not a detector technologist to make of the plethora of devices described at the 7th Symposium? First of all he should be grateful that he has such a wide choice-it has only been provided by thousands of man-years of effort. He should note that he can record objects comparable in brightness to the night sky using cascaded intensifiers with a television or photographic output stage. H e can obtain photometric surveys of the useful field of paraboloid reflectors with electronographic cameras which are linear, extremely sensitive and capable of accuracies of 1 to 2%. For fainter objects he can study a smaller area of sky or of a spectrum with imaging photoelectron counting systems whose photometric accuracy is limited only by the statistical noise of the number of photoelectrons he can afford to collect. Finally, he can look forward to simpler, more convenient ways of obtaining the results described above, but not to any further dramatic improvements in performance. The ideal astronomical detectors envisaged at the first few Symposia are not yet here, but present systems are so close to that performance that the law of diminishing returns must soon begin to apply.
REFERENCE 1. Chalrneton, V., Acta Electron. 14, 99 (1971)
iSee p. 347.