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Problems in the Use of Image Intensifiers in Astronomical Cassegrain Spectrographs
Problems in the Use of Image Intensifiers in Astronomical Cassegrain Spectrographs D. It. PALMER and A. S. MILSOM Royal Greenwich Observatory, Herstmonceux Castle, Hailshawb, Sussex, England
INTRODUCTION The idea of a Cassegrain spectrograph employing image intensifiers for use a t Herstmonceux and elsewhere first evolved some five years ago. The primary objective was t o extend the magnitude limit for the conventional spectrographic observations that are carried out by the Royal Greenwich Observatory in the low to intermediate dispersion ranges. The design was to be compatible with several different telescopes with Cassegrain focal ratios between f/14 and f/20. The first of the instruments was commissioned in October of last year on the Radcliffe 74-in. telescope a t Pretoria. Initial result,s will be described later. I n order that the system could be updated as and when developments in the detector field made this desirable, a modular form was adopted. Since many of the observations to be made with the spectrograph were for the determination of radial velocities and for spectrophotometric work the type of image tube to be preferred was dictated by tlhe requirements of high resolution over a reasonable field, high overall stability, good geometry and low background as well as high efficiency, and these, together with the desirability of retaining the photometric integrity of the optical image could best be met by an electronographic system such as the Spectracon.lt It was, however, recognized that there were some fields of astronomy where the observer required the ability to carry out quick visual assessment and measurement, a feature not found in the linear characteristic of the electronographic records, from which the extraction of information can only be effectively carried out by the use of photoelectric scanning or grain counting techniques. For such purposes and in order to provide back-up for the electronographic system, provision was made in the
t See p.
13. 769
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D. R. PALMER AND A. S. MILSOM
spectrograph design for a suitable cascade image intensifier. The type of intensifier that would best meet the requirements of the Observatory in this respect is discussed later.
SPECTROGRAPH DESIGN The form of the spectrograph is shown in Fig. 1. It is divided into three main units: (a) the top acquisition and guiding section; (b) the collimator box, incorporating the slit and grating assemblies ; and (c) the camera and detector unit. The guider system has a 45" pierced mirror and is mounted on ballbush slides t o allow lateral translation for on-axis field viewing. The
Colaur and neutral density filter wheels ---
FIG.1. Schematic diagram of the speotrograph for the 1/18 Cassograin focus of t h e Radcliffe 74-in. telescope.
771
USE OF IMAGE INTENSIFIERS Ipi CASSECRAIN SPECTROCJRAPHS
eyepiece is mounted on x-y slides and serves either for slit viewing via a relay lens and flat mirror mounted behind the central hole of the main mirror, or for offset guiding u hen working on faint or extended source objects. To retain the degree of inechanicd stability auhicved in the other Cassegrain spectrographs designed a t the Royal Greenwich Observatory, it was necessary to use a Cassegrain form for the collimator in order that compactness could be maintained, bearing in mind the focal ratios to be used (f/14 tof/20). The slit assembly is bolted on to the collimator box, the latter also housing the grating assembly which has been designed to allow for easy interchange of gratings. The principal problems in the design of the spectrograph were found to lie in the cameraldetector area. In order to obtain maximum efficiency of the system in faint stellar observations, the width of the slit should approximate to the stellar seeing disk. I n addition, the projected slit width should match the resolution of the detector. This condition requires the use of fast camera optics, f/2-2 for the Pretoria spectrograph and f/1.4 for the Herstmonceux instrument. The requirement of an accessible focal plane imposed by the form and size of an image tube system means that such a camera must be of the folded type. The optical parameters which have to be balanced against each other to optimize the system are back focus, field size and the degree of obscuration from the secondary, whilst maintaining sufficiently high resolution to match that of the Spectracon. Chromatic aberration should also be minimized because there is a choice of linear dispersions. The back focus is important in that it dictates the format of the solenoid in the region of the photocathode. The fi2.2 andfll.4 camera designs were described by Wynne and Kidger at the last Symposium2 and more recenhly in a communication t o the Royal Astronomical S ~ c i e t y .Suffice ~ it to say that they are both CassegrainMaksutov systems, the latter being semi-solid. The fl2.2 camera (as shown in Fig. 1 ) uses a Cervit primary mirror separated from the meniscus element by a quartz spacing tube and, in order to achieve maxiinurn back focus, the primary was inadc as thin as was mechanically allowable. The short back focus of this camera was accommodated by McGee in the modification of his standard Spectracon solenoid, where the mu-metal screen is brought right across the front of the tube save for a slot aperture 29 x 6 nim2 opposite the photocat1iode.l With this arrangement it is possible to bring the front window of the Spectracon to a distance of 1 nim from the end of the solenoid without deterioration in image quality and geometry. Even so, the space between the camera cell and the solenoid is no more than 1 mm and this has to accommodate a tilt atljustment of &*", leaving only PEID-B
7
772
D. R. PALMER AND A. 9. MnSOM
$ mni for variations in dimensions from tube to tube. Figure 2 shows the arrangement using the f l l . 4 camera. I n this case the standard Spectracon solenoid, which has a circular aperture in the front mumetal screen, is used. This aperture is of the right size t o accept the stub on the back of the camera, allowing the focal plane to be sufficiently far inside the solenoid to avoid the region of non-uniformity in the magnetic field. The clearance between the rear surface of this stub and the front window of the Spectracon is typically only about 0.3 mm.
7 5 ond 21 O n m / m m )
Fra. 2. Schematic diagram showing thefll.4 camera for the spectrograph a t thef/14 Carsegrain focus of the 98-in.Isaac Newton telescope.
Cooling of the solenoid can cause problems when working a t the Cassegrain focus of large telescopes. The pipes carrying the coolant to the solenoid are currently being run through the telescope axes, but this is rather unsatisfactory in many respects (for example, the extremely long lengths of piping involved increases the risk of a leak as well as the attendant difficulty of locating and curing it should one occur). Other possible long-term solutions are therefore being examined, such as a self-contained refrigeration system local to the spectrograph. One method is to use a Peltier device adjacent to the cathodel. and t o remove the heat this generates together with that produced by the solenoid (the dissipation of the latter being reduced by using singleloop focusing) by conduction through the metalwork of the instrument
USE OF IMAGE INTENSIFIERS I N CASSEGRAIN SPECTROGRAPHS
773
and te1escope.t The unknowns in this technique as applied to the spectrograph are the cooling effect of the Peltier device on the camera optics, the effect of the heat conducted away through the spectrograph case on the optical paths within the instrument, and the effect of the possible degradation of image geometry through using single-loop focusing. This technique cannot be used with the f12.2 camera because there is no space in the solenoid for a Peltier cooler. Scan coils are provided in the solenoid to enable the spectrum to be broadened electronically. The spectrograph slit length is restricted by means of one of a number of circular apertures, selected to match the stellar seeing disk. The same circular aperture is used for the light from the comparison source, when a d.c. bias in the scan generator shifts the image first to one side of the stellar spectrum and then the other. Apart from improving image geometry, electronic broadening has been shown to be useful in photometry in that photocathode blemishes are less troublesome and can more readily be taken into account. Additional apertures over the slit can be selected when required in order to provide sky sampling. I n order t o ensure that the direction of scan is exactly a t right angles to the spectrum, a parameter which is affectNedby reversal of the focusing field and to a lesser extent by a change of tube, two coils mutually a t right angles have been incorporated in the solenoid. The resultant field direction can then be set by the simple adjustment of a helical potentiometer. I n addition to a control which provides variable spectrum broadening, the scan generator incorporates a circuit which enables the separation between the star and comparison t o be pre-set according to the type of observation to be carried out. The generator also has an interface which allows an external source to be used for the scan frequency should a different, or more accurate, one be required. DETECTOR CHOICEOF ALTERNATIVE As already indicated, although the spectrograph was designed primarily around the Spectracon, provision was also made for other types of detector. The decision as to the best alternative is not by any means clear-cut. The requirements are basically, good resolution, good stability and good geometry with low background, as before. I n addition, it would be desirable t o use a photographic emulsion having a reasonably high storage capacity and with good photometric characteristics such as Eastman Kodak IIIa-J. I n order to meet these conditions and t o be able to record individual photon events, it was considered that a 3-stage cascade intensifier coupled t o a high quality transfer lens, such as that designed by Wynne,2 would be
t See p. 13.
774
D. R . PALMER A N D
A.
9. MILSOM
necessary. Alternatively, and hopefully as an eventual better solution (because of its greater compactness), a 2-stage cascade tube with a fibre-optic output window might fulfil the requirements. This possibility is being investigated as a future second generation back-up detector. Extensive laboratory tests have already been carried out on a sample 6-pm fibre-optic plate. The results of these tests can be summarized as follows. 1. The low contrast hexagonal grid visible to the naked eye under certain conditions of illumination is not evident on photographic plates. 2. Fibre grouping was regular, in a hexagonal pattern, although discontinuities appeared under the microscope as dark straight lines about 100 pm long. 3. No blank fibres were detected in the areas examined. 4. Resolutions of 80 Ip/mm were maintained on the output face when examined visually, although there was some reduction in contrast. 5 . The numerical aperture was found to be very large and emergent images could be seen a t grazing incidence, although the resolution was maintained to at least 76" to the normal. 6. A laser beam focused down to 2 pm could be made to fill one fibre with very little stray light in adjoining fibres. 7 . The photographic resolution tests were very disappointing. Several emulsions were tried and the best results were obtained with Ilford N50 and G5, both of the thin film variety (i.e. 5 t o 10-pm thick emulsions). These showed limiting resolutions of about 25 to 28 lp/mm. On the assumption that this loss of resolution is due to the high numerical aperture of the plate under test, plans were made to carry out experiments on plates with lower numerical aperture. Initial results are described below. 8. Tracings across the plate with a microdensitometer using a 20 x 50-pm2 slit showed uniform transmission generally t o within &0.005 D (i.e. f1*270 * Subsequent tests have been carried out on a sample fibre-optic plate with 6-pm fibres and numerical aperture of 0.25. This plate showed resolutions well in excess of 56 lp/mm on IIIa-J emulsion and about this figure on IIa-0. Transmission, however, is down to less than 10% and a compromise may have to be found between this, fibre size and numerical aperture. The conclusion to be drawn from these tests so far is that a good cascade intensifier with a fibre-optic output window of the quality found in the plate used in the first tests but with much lower numerical aperture and suitable fibre size could find quite wide astronomical use.
EXPERIMENTAL RESULTSWITH SPECTROGRAPHS Both spectrographs were put through extensive stability tests in the laboratory and on the Yapp 36-in. telescope at Herstmonceux. With the system recording photographically, no detectable displacements
FIG. 3. Spectra of HD 20794 taken with the Radcliffe spectrograph using (a) a Spectracon with Ilforci G5 emulsion, and (b) direct photography with Eastman-Kodak IIa-0 (baked).
776
D. R. PALMER AND A. 5. MILSOM
(the limit of detection being 0-5 pm) were recorded when the instrument was moved through 6 h of hour-angle a t a declination of +20°. Displacements of no greater than 1 pm/h were recorded when the Spectracon was used, a figure which includes image movements due t o tube instabilities as well as mechanical flexure. Figure 3 shows examples of spectra taken on the 74-in. telescope a t Pretoria. The upper spectrum (a) was taken on Ilford G5 with a Spectracon and for comparison purposes, the lower spectrum (b) shows the same star taken direct on I I a - 0 at the same dispersion through the same optics. Apart from significantly better resolution, the Spectracon film shows far less image spread in the brighter lines of the comparison spectrum, a feature which enables one to use a greater selection of lines for measurement purposes and demonstrates the excellent optical resolution of the camera.
PIG.4. The spectrograph mounted a t the Cassegrain focus of the 98-in. Isaac Newton telescope.
FIQ.5. Two spectra obtained with the spectrograph on the 98-in. Isaac Newton telescope using a Spectracon and Ilford G5 emulsion. In each case the slit width was 100 pm. (a) NGC 449 taken at a dispersion of 21 nm/mm with an exposure time of 150 min. (b) NGC 7662 taken at a dispersion of 3.0 nm/mm also in 150 min.
778
D. R. PALMER AND A. S. MILSOM
Some 40 Spectracon spectra and 18 photographic spectra of standard radial velocity stars were taken through the same spectrograph a t Pretoria (all a t a dispersion of 5*5nm/rnm), and these have been measured and reduced with the following results. 1. The mean internal standard error for 3 different Spectracons turned out to be between & 3 and 1 3 . 5 km/sec. This compares favourably with the 1 3 . 4 km/sec from spectra taken in the direct photographic mode, and also with the figure of A2.5 km/sec for plates taken with the 6 nm/mm camera of the Isaac Newton conventional Cassegrain spectrograph, bearing in mind that the Pretoria figure can be improved when the wavelengths used for the measurements have been fully worked out for the new system. It should be noted that 3.5 km/sec a t 5.5 nm/mm corresponds approximately t o 1 pm on the plate. 2. The mean of the differences, Pretoria velocity minus IAU standard velocity, was determined for each of the three Spectracons and for direct photography. For the latter this turned out t o be $1-9 km/sec whilst two of the Spectracons gave $3.2 and $3.7 km/sec. The third Spectracon, however, showed a systematic difference of +11.9 km/sec, corresponding to 3 pm on the film. The reason for the discrepancy is not a t all clear. Line curvature due t o poor image geometry was first suspected, but an analysis of the spectra taken with different arc-to-star separation did not confirm this and more data will be required before the matter can be investigated further. The standard errors of all these four values were about k 1 . 5 km/sec (or h0.5 pm). 3. Spectracon exposures were made on both 50-pm thick Melinexbased film and also on stripping emulsion. The latter gave results as good as the film-based material. The second spectrograph was brought into use on the Isaac Newton telescope during the summer of this year. F gure 4 shows the spectrograph mounted and Fig. 5 shows two spectra recorded on G5 stripping emulsion with a Spectracon.
CONCLUSION In conclusion one can say that as far as accurate radial velocity measures are concerned, the Spectracon is capable of providing results which have the same 1-pm precision as those obtained with conventional photography. ACKNOWLEDGMENTS Our thanks are due t o the Astronomer Royal, Sir Richard v.d. R. Woolley, F.R.S., for permission to publish this paper, and for his active interest, and encouragement in the project. We arealsoindebtedto ProfessorJ.D. McGee, F.R.S.,
USE OF IMAGE INTENSIFIIRS I N CASSEGRAIN SPECTROGRAPHS
779
of Imperial College for his cooperation in the supply of Spectracons and for his advice and help in this field and to Professor C. G. Wynne, F.R.S., of Imperial College for his work on the opt,ical design of the image-tube cameras.
REFERENCES 1. McGee, J. D., McMullan, D., Bacik, H. and Oliver, M., I n “Adv. E.E.P.”, Vol. 28A, p. 61 (1969). 2. Wynne, C. G. and Kidger, M. J., IN “Adv. E.E.P.”, Vol. 28B, p. 759 (1969). 3. Wynne, C. Q., Mon. Not. R . Astron. Soc. 153, 261 (1971). 4. McMullan, D. and Oliver, M., J . Sci. Instrum., Ser. 2, 1, 1255 (1968).
DISCUSSION J . R m ( : : What were the relativo spends of‘ t.he photographic and eloctronographic systems? A. s. MILSOM : Our findings agree w i t h the tmts carried out a t Imperial College in that the overall information gain with G 5 over baked IIa-O is about ten times. J. A. HYNEK: I noticed what scemed to be considerable longitudinal Sdistortion. Was this on the slide or was i t real? I am impressed that you were able to obtain such excellent radial velocity results despitu this distortion. A. s. MILSOM : S-distort,iori with this tube is about 100 pm at 10 mm radius. Radial velocity measi~rement~s are affected more by non-parallelism between t)he spectral lines alorig t,he dispersion than by S-distortion. w. A . BAUM : Let me add to the comment concerning the measurement of radial vclocit,ies in the presence of S-distortion. This problem was invcstigated by Vera Rubin of the Carnegie Institute of Washington. She found that, velocity errors caused by S-distortion were surprisingly small. Her methods of spectrum measurement and data reduction were conventional. w. M. BURTON : What method was used to press the photographic cmiilsiori in contact with the fibre-optic plate? The loss in resolution observed could be duo to inadequate optical contacting. A. s. MILSOM: To check that we were obtaining good contact between the emulsion and tho fibre-optic plate, a drop of liquid was placed bet,ween the two surfaces. No improvement in resolution was observed. K . E. KISSELL: Was the plate bowed inwards due to vacuum effects? A. s. MILSOM: The fibre-optic plates under test were not mounted in image intensifiers and were not therefore subject to this effect. J. RICKARD : We have had some experience in the use of photographic plates in contact with fibre-optic plates. At least wit,h commercially available fibreoptic plat,es, they are not flat onough for repeat,able performance in resolution when pressed into contact with photographic plates. For this reason we are adopting films rather than plates 80 that t,hay will conform better to tho fibreoptic output plates. A. s. MILSOM : One of the emulsions used ( G 5 ) was coated on a film base. This showed n o better resolution than the glass-based N50 emulsion. However, to be sure of consistently uniform contact with the fibre-optic plate we too propose using film-based material. J. RING : How do you control the numerical aperture of a fibre-optic plate? A . s. WLSOM : This is a manufacturing problem, but I believe it is achieved by varying the relative refractive indices of the fibre material and its cladding.
INTRODUCTION The idea of a Cassegrain spectrograph employing image intensifiers for use a t Herstmonceux and elsewhere first evolved some five years ago. The primary objective was t o extend the magnitude limit for the conventional spectrographic observations that are carried out by the Royal Greenwich Observatory in the low to intermediate dispersion ranges. The design was to be compatible with several different telescopes with Cassegrain focal ratios between f/14 and f/20. The first of the instruments was commissioned in October of last year on the Radcliffe 74-in. telescope a t Pretoria. Initial result,s will be described later. I n order that the system could be updated as and when developments in the detector field made this desirable, a modular form was adopted. Since many of the observations to be made with the spectrograph were for the determination of radial velocities and for spectrophotometric work the type of image tube to be preferred was dictated by tlhe requirements of high resolution over a reasonable field, high overall stability, good geometry and low background as well as high efficiency, and these, together with the desirability of retaining the photometric integrity of the optical image could best be met by an electronographic system such as the Spectracon.lt It was, however, recognized that there were some fields of astronomy where the observer required the ability to carry out quick visual assessment and measurement, a feature not found in the linear characteristic of the electronographic records, from which the extraction of information can only be effectively carried out by the use of photoelectric scanning or grain counting techniques. For such purposes and in order to provide back-up for the electronographic system, provision was made in the
t See p.
13. 769
770
D. R. PALMER AND A. S. MILSOM
spectrograph design for a suitable cascade image intensifier. The type of intensifier that would best meet the requirements of the Observatory in this respect is discussed later.
SPECTROGRAPH DESIGN The form of the spectrograph is shown in Fig. 1. It is divided into three main units: (a) the top acquisition and guiding section; (b) the collimator box, incorporating the slit and grating assemblies ; and (c) the camera and detector unit. The guider system has a 45" pierced mirror and is mounted on ballbush slides t o allow lateral translation for on-axis field viewing. The
Colaur and neutral density filter wheels ---
FIG.1. Schematic diagram of the speotrograph for the 1/18 Cassograin focus of t h e Radcliffe 74-in. telescope.
771
USE OF IMAGE INTENSIFIERS Ipi CASSECRAIN SPECTROCJRAPHS
eyepiece is mounted on x-y slides and serves either for slit viewing via a relay lens and flat mirror mounted behind the central hole of the main mirror, or for offset guiding u hen working on faint or extended source objects. To retain the degree of inechanicd stability auhicved in the other Cassegrain spectrographs designed a t the Royal Greenwich Observatory, it was necessary to use a Cassegrain form for the collimator in order that compactness could be maintained, bearing in mind the focal ratios to be used (f/14 tof/20). The slit assembly is bolted on to the collimator box, the latter also housing the grating assembly which has been designed to allow for easy interchange of gratings. The principal problems in the design of the spectrograph were found to lie in the cameraldetector area. In order to obtain maximum efficiency of the system in faint stellar observations, the width of the slit should approximate to the stellar seeing disk. I n addition, the projected slit width should match the resolution of the detector. This condition requires the use of fast camera optics, f/2-2 for the Pretoria spectrograph and f/1.4 for the Herstmonceux instrument. The requirement of an accessible focal plane imposed by the form and size of an image tube system means that such a camera must be of the folded type. The optical parameters which have to be balanced against each other to optimize the system are back focus, field size and the degree of obscuration from the secondary, whilst maintaining sufficiently high resolution to match that of the Spectracon. Chromatic aberration should also be minimized because there is a choice of linear dispersions. The back focus is important in that it dictates the format of the solenoid in the region of the photocathode. The fi2.2 andfll.4 camera designs were described by Wynne and Kidger at the last Symposium2 and more recenhly in a communication t o the Royal Astronomical S ~ c i e t y .Suffice ~ it to say that they are both CassegrainMaksutov systems, the latter being semi-solid. The fl2.2 camera (as shown in Fig. 1 ) uses a Cervit primary mirror separated from the meniscus element by a quartz spacing tube and, in order to achieve maxiinurn back focus, the primary was inadc as thin as was mechanically allowable. The short back focus of this camera was accommodated by McGee in the modification of his standard Spectracon solenoid, where the mu-metal screen is brought right across the front of the tube save for a slot aperture 29 x 6 nim2 opposite the photocat1iode.l With this arrangement it is possible to bring the front window of the Spectracon to a distance of 1 nim from the end of the solenoid without deterioration in image quality and geometry. Even so, the space between the camera cell and the solenoid is no more than 1 mm and this has to accommodate a tilt atljustment of &*", leaving only PEID-B
7
772
D. R. PALMER AND A. 9. MnSOM
$ mni for variations in dimensions from tube to tube. Figure 2 shows the arrangement using the f l l . 4 camera. I n this case the standard Spectracon solenoid, which has a circular aperture in the front mumetal screen, is used. This aperture is of the right size t o accept the stub on the back of the camera, allowing the focal plane to be sufficiently far inside the solenoid to avoid the region of non-uniformity in the magnetic field. The clearance between the rear surface of this stub and the front window of the Spectracon is typically only about 0.3 mm.
7 5 ond 21 O n m / m m )
Fra. 2. Schematic diagram showing thefll.4 camera for the spectrograph a t thef/14 Carsegrain focus of the 98-in.Isaac Newton telescope.
Cooling of the solenoid can cause problems when working a t the Cassegrain focus of large telescopes. The pipes carrying the coolant to the solenoid are currently being run through the telescope axes, but this is rather unsatisfactory in many respects (for example, the extremely long lengths of piping involved increases the risk of a leak as well as the attendant difficulty of locating and curing it should one occur). Other possible long-term solutions are therefore being examined, such as a self-contained refrigeration system local to the spectrograph. One method is to use a Peltier device adjacent to the cathodel. and t o remove the heat this generates together with that produced by the solenoid (the dissipation of the latter being reduced by using singleloop focusing) by conduction through the metalwork of the instrument
USE OF IMAGE INTENSIFIERS I N CASSEGRAIN SPECTROGRAPHS
773
and te1escope.t The unknowns in this technique as applied to the spectrograph are the cooling effect of the Peltier device on the camera optics, the effect of the heat conducted away through the spectrograph case on the optical paths within the instrument, and the effect of the possible degradation of image geometry through using single-loop focusing. This technique cannot be used with the f12.2 camera because there is no space in the solenoid for a Peltier cooler. Scan coils are provided in the solenoid to enable the spectrum to be broadened electronically. The spectrograph slit length is restricted by means of one of a number of circular apertures, selected to match the stellar seeing disk. The same circular aperture is used for the light from the comparison source, when a d.c. bias in the scan generator shifts the image first to one side of the stellar spectrum and then the other. Apart from improving image geometry, electronic broadening has been shown to be useful in photometry in that photocathode blemishes are less troublesome and can more readily be taken into account. Additional apertures over the slit can be selected when required in order to provide sky sampling. I n order t o ensure that the direction of scan is exactly a t right angles to the spectrum, a parameter which is affectNedby reversal of the focusing field and to a lesser extent by a change of tube, two coils mutually a t right angles have been incorporated in the solenoid. The resultant field direction can then be set by the simple adjustment of a helical potentiometer. I n addition to a control which provides variable spectrum broadening, the scan generator incorporates a circuit which enables the separation between the star and comparison t o be pre-set according to the type of observation to be carried out. The generator also has an interface which allows an external source to be used for the scan frequency should a different, or more accurate, one be required. DETECTOR CHOICEOF ALTERNATIVE As already indicated, although the spectrograph was designed primarily around the Spectracon, provision was also made for other types of detector. The decision as to the best alternative is not by any means clear-cut. The requirements are basically, good resolution, good stability and good geometry with low background, as before. I n addition, it would be desirable t o use a photographic emulsion having a reasonably high storage capacity and with good photometric characteristics such as Eastman Kodak IIIa-J. I n order to meet these conditions and t o be able to record individual photon events, it was considered that a 3-stage cascade intensifier coupled t o a high quality transfer lens, such as that designed by Wynne,2 would be
t See p. 13.
774
D. R . PALMER A N D
A.
9. MILSOM
necessary. Alternatively, and hopefully as an eventual better solution (because of its greater compactness), a 2-stage cascade tube with a fibre-optic output window might fulfil the requirements. This possibility is being investigated as a future second generation back-up detector. Extensive laboratory tests have already been carried out on a sample 6-pm fibre-optic plate. The results of these tests can be summarized as follows. 1. The low contrast hexagonal grid visible to the naked eye under certain conditions of illumination is not evident on photographic plates. 2. Fibre grouping was regular, in a hexagonal pattern, although discontinuities appeared under the microscope as dark straight lines about 100 pm long. 3. No blank fibres were detected in the areas examined. 4. Resolutions of 80 Ip/mm were maintained on the output face when examined visually, although there was some reduction in contrast. 5 . The numerical aperture was found to be very large and emergent images could be seen a t grazing incidence, although the resolution was maintained to at least 76" to the normal. 6. A laser beam focused down to 2 pm could be made to fill one fibre with very little stray light in adjoining fibres. 7 . The photographic resolution tests were very disappointing. Several emulsions were tried and the best results were obtained with Ilford N50 and G5, both of the thin film variety (i.e. 5 t o 10-pm thick emulsions). These showed limiting resolutions of about 25 to 28 lp/mm. On the assumption that this loss of resolution is due to the high numerical aperture of the plate under test, plans were made to carry out experiments on plates with lower numerical aperture. Initial results are described below. 8. Tracings across the plate with a microdensitometer using a 20 x 50-pm2 slit showed uniform transmission generally t o within &0.005 D (i.e. f1*270 * Subsequent tests have been carried out on a sample fibre-optic plate with 6-pm fibres and numerical aperture of 0.25. This plate showed resolutions well in excess of 56 lp/mm on IIIa-J emulsion and about this figure on IIa-0. Transmission, however, is down to less than 10% and a compromise may have to be found between this, fibre size and numerical aperture. The conclusion to be drawn from these tests so far is that a good cascade intensifier with a fibre-optic output window of the quality found in the plate used in the first tests but with much lower numerical aperture and suitable fibre size could find quite wide astronomical use.
EXPERIMENTAL RESULTSWITH SPECTROGRAPHS Both spectrographs were put through extensive stability tests in the laboratory and on the Yapp 36-in. telescope at Herstmonceux. With the system recording photographically, no detectable displacements
FIG. 3. Spectra of HD 20794 taken with the Radcliffe spectrograph using (a) a Spectracon with Ilforci G5 emulsion, and (b) direct photography with Eastman-Kodak IIa-0 (baked).
776
D. R. PALMER AND A. 5. MILSOM
(the limit of detection being 0-5 pm) were recorded when the instrument was moved through 6 h of hour-angle a t a declination of +20°. Displacements of no greater than 1 pm/h were recorded when the Spectracon was used, a figure which includes image movements due t o tube instabilities as well as mechanical flexure. Figure 3 shows examples of spectra taken on the 74-in. telescope a t Pretoria. The upper spectrum (a) was taken on Ilford G5 with a Spectracon and for comparison purposes, the lower spectrum (b) shows the same star taken direct on I I a - 0 at the same dispersion through the same optics. Apart from significantly better resolution, the Spectracon film shows far less image spread in the brighter lines of the comparison spectrum, a feature which enables one to use a greater selection of lines for measurement purposes and demonstrates the excellent optical resolution of the camera.
PIG.4. The spectrograph mounted a t the Cassegrain focus of the 98-in. Isaac Newton telescope.
FIQ.5. Two spectra obtained with the spectrograph on the 98-in. Isaac Newton telescope using a Spectracon and Ilford G5 emulsion. In each case the slit width was 100 pm. (a) NGC 449 taken at a dispersion of 21 nm/mm with an exposure time of 150 min. (b) NGC 7662 taken at a dispersion of 3.0 nm/mm also in 150 min.
778
D. R. PALMER AND A. S. MILSOM
Some 40 Spectracon spectra and 18 photographic spectra of standard radial velocity stars were taken through the same spectrograph a t Pretoria (all a t a dispersion of 5*5nm/rnm), and these have been measured and reduced with the following results. 1. The mean internal standard error for 3 different Spectracons turned out to be between & 3 and 1 3 . 5 km/sec. This compares favourably with the 1 3 . 4 km/sec from spectra taken in the direct photographic mode, and also with the figure of A2.5 km/sec for plates taken with the 6 nm/mm camera of the Isaac Newton conventional Cassegrain spectrograph, bearing in mind that the Pretoria figure can be improved when the wavelengths used for the measurements have been fully worked out for the new system. It should be noted that 3.5 km/sec a t 5.5 nm/mm corresponds approximately t o 1 pm on the plate. 2. The mean of the differences, Pretoria velocity minus IAU standard velocity, was determined for each of the three Spectracons and for direct photography. For the latter this turned out t o be $1-9 km/sec whilst two of the Spectracons gave $3.2 and $3.7 km/sec. The third Spectracon, however, showed a systematic difference of +11.9 km/sec, corresponding to 3 pm on the film. The reason for the discrepancy is not a t all clear. Line curvature due t o poor image geometry was first suspected, but an analysis of the spectra taken with different arc-to-star separation did not confirm this and more data will be required before the matter can be investigated further. The standard errors of all these four values were about k 1 . 5 km/sec (or h0.5 pm). 3. Spectracon exposures were made on both 50-pm thick Melinexbased film and also on stripping emulsion. The latter gave results as good as the film-based material. The second spectrograph was brought into use on the Isaac Newton telescope during the summer of this year. F gure 4 shows the spectrograph mounted and Fig. 5 shows two spectra recorded on G5 stripping emulsion with a Spectracon.
CONCLUSION In conclusion one can say that as far as accurate radial velocity measures are concerned, the Spectracon is capable of providing results which have the same 1-pm precision as those obtained with conventional photography. ACKNOWLEDGMENTS Our thanks are due t o the Astronomer Royal, Sir Richard v.d. R. Woolley, F.R.S., for permission to publish this paper, and for his active interest, and encouragement in the project. We arealsoindebtedto ProfessorJ.D. McGee, F.R.S.,
USE OF IMAGE INTENSIFIIRS I N CASSEGRAIN SPECTROGRAPHS
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of Imperial College for his cooperation in the supply of Spectracons and for his advice and help in this field and to Professor C. G. Wynne, F.R.S., of Imperial College for his work on the opt,ical design of the image-tube cameras.
REFERENCES 1. McGee, J. D., McMullan, D., Bacik, H. and Oliver, M., I n “Adv. E.E.P.”, Vol. 28A, p. 61 (1969). 2. Wynne, C. G. and Kidger, M. J., IN “Adv. E.E.P.”, Vol. 28B, p. 759 (1969). 3. Wynne, C. Q., Mon. Not. R . Astron. Soc. 153, 261 (1971). 4. McMullan, D. and Oliver, M., J . Sci. Instrum., Ser. 2, 1, 1255 (1968).
DISCUSSION J . R m ( : : What were the relativo spends of‘ t.he photographic and eloctronographic systems? A. s. MILSOM : Our findings agree w i t h the tmts carried out a t Imperial College in that the overall information gain with G 5 over baked IIa-O is about ten times. J. A. HYNEK: I noticed what scemed to be considerable longitudinal Sdistortion. Was this on the slide or was i t real? I am impressed that you were able to obtain such excellent radial velocity results despitu this distortion. A. s. MILSOM : S-distort,iori with this tube is about 100 pm at 10 mm radius. Radial velocity measi~rement~s are affected more by non-parallelism between t)he spectral lines alorig t,he dispersion than by S-distortion. w. A . BAUM : Let me add to the comment concerning the measurement of radial vclocit,ies in the presence of S-distortion. This problem was invcstigated by Vera Rubin of the Carnegie Institute of Washington. She found that, velocity errors caused by S-distortion were surprisingly small. Her methods of spectrum measurement and data reduction were conventional. w. M. BURTON : What method was used to press the photographic cmiilsiori in contact with the fibre-optic plate? The loss in resolution observed could be duo to inadequate optical contacting. A. s. MILSOM: To check that we were obtaining good contact between the emulsion and tho fibre-optic plate, a drop of liquid was placed bet,ween the two surfaces. No improvement in resolution was observed. K . E. KISSELL: Was the plate bowed inwards due to vacuum effects? A. s. MILSOM: The fibre-optic plates under test were not mounted in image intensifiers and were not therefore subject to this effect. J. RICKARD : We have had some experience in the use of photographic plates in contact with fibre-optic plates. At least wit,h commercially available fibreoptic plat,es, they are not flat onough for repeat,able performance in resolution when pressed into contact with photographic plates. For this reason we are adopting films rather than plates 80 that t,hay will conform better to tho fibreoptic output plates. A. s. MILSOM : One of the emulsions used ( G 5 ) was coated on a film base. This showed n o better resolution than the glass-based N50 emulsion. However, to be sure of consistently uniform contact with the fibre-optic plate we too propose using film-based material. J. RING : How do you control the numerical aperture of a fibre-optic plate? A . s. WLSOM : This is a manufacturing problem, but I believe it is achieved by varying the relative refractive indices of the fibre material and its cladding.