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Electronographic Image Tube Development at the Royal Greenwich Observatory
Electronographic Image Tube Development at the Royal Greenwich Observatory
INTRODUCT~ON The Image Tube Department at the Royal Greenwich Observatory was set up two years ago for the development of electronographic image tubes, both for stellar field photometry and for the recording of spectra. The following features were vonsidered to be essential for a successful tube design. I . I1:ase of operation by astronomers after a few minutes instruction. High degree of reliability, especially in the adverse environment of a telescope dome. 2. Large image area for stellar photometry. At least 40 mm in diameter and 90 or 100 mm if' possible. 3 . Also, of course, the absolutely essential requirements of high photocathode sensitivity, good resolution and low background. Although the last requirements are met by existing tube designs, none combines the first two : ease of operation ttnd reliability coupled with large image size. Of the existing tubes. those due to Lalleinandl and t o Krona t have reasonably large image areas but they are riot easy t o operate because the electxonographic plate is introduced into the same vacuum environment as the photocathode. A n observatory using these tubes should have a high-vacuum laboratory for the preparation of the tjubes (including photocathode processing). The S p e c t r a ~ o n4,$~ on the other hand, is simple to operate because of its mica window which enables the electronographic film t o be exposed externally t o the tube. The image size is limited t o about 30 x 15 mm2 since the mica window, which is only 4-pm thick, has to withstand atmospheric prerrsure. The solution we are adopting is to incorporate a mica window but
t
See alro p. 737.
1 SCR also p.
13. .{i
38
D. McMULLAN, J. R. POWELL AND N. A. CURTIS
to protect it from atmospheric pressure by a vacuum lock. This overcomes the limitation on window size set by strength considerations and minimizes the possibilit,y of mica breakage.
MICA WINDOW Figure 1 shows the arrangement a t the output end of a tube. The mica 1, 20 mm in diameter, is stretched tight on its mount 2 which is of titanium and is sealed to the vacuum envelope of the image tube
Fro. 1. Manually operated platelfilm holder with eleotrostatic pull-down of mica window. Diameter of mice, 20 mm.
(not shown). The air pressure on the output side of the mica window is kept a t 1 Tom or less by a mechanical or sorption pump and the electronographic plate or film is introduced through a vacuum lock formed by a gate valve 3. The plate or film is mounted on the holder 4 which is inserted into the bayonet fitting 5 of the plate chamber. When this has been pumped to below 1 Torr through the vacuum line 6, the gate valve is opened and the emulsion is brought up to the mica manually by the control rod 7. The vacuum is monitored by thermocouple gauges 8 and 9. I$ is essential that the emulsion should be in intimate contact with the mica :the first system that has been tried uses electrostatic force t o pull the mica into contact with a glass-based emulsion. An electrode 10,
39
IMAQE TUBE DEVELOPMENT AT THE RGO
having a potential of a few hundred volts, is mounted behind the plate, and the mica, which is aluminized, is pulled into contact with the emulsion. Although the contact is very good, unfortunately i t has not yet proved possible to use this method hecause of the difficulty of detaching the mica in a reasonable time at the end of an exposure when the potential of the back electrode is reduced to zero. Owing to charge absorption in the glass the time taken (mi be many minutes ; attempts to separate the mica mechanically while the glass is still charged can lead to fogging of the emulsion. More recent experiments have indicated that the method operates better with Melinex-based film, which has a very high resistivity.
'\
FIG.2. Film holder for 40-mm mica wintlow. The emulaion is brought into contact with the mica by an air pressure of 6 Tow. Holder is contrr~llctlpneumatically.
An alternative method makes use of low pressure air to press a 50-pm Melinex-based film against tJhemica. This is similar to a system developed for a Spectracon with a circular mica window4* but works a t much lower pressures. The film holder is shown in Fig. 2 ; the film 1 is held in place by a cap 2 (see also the Idiottograph in Fig. 3) and the holder is inserted in the bayonet fitting of the vacuum-lock assembly which has already been described. The emulsion is brought up t o the mica and the space behind the film is pressurized with air t o about 5 Torr, thus pressing the eniulsion into contact with the taut mica window. The film holder shown in Figs. 2 and 3 is for a window 40 mm
40
D . MuXrJLLAN, J . R . POWELL AND N. A. CURTIS
in diameter and is controlled pneumatically by the air cylinder 3. All the operations, viz. pumping down the film holder, opening tthe gate valve, bringing forward the film and pressurizing to 5 Torr, are carried out by an automatic electro-pneumatic control system. Film can be changed in 1 t o 2 min or faster if A sorption pump is used.
FIG.3. Head of the film holdor shown in Fig. 2.
KRONELECTRONIC CAMERA Two Kron Electronic Cameras2 have been built at the Observatory from drawings kindly supplied by Dr. Kron of the U.S. Naval Observatory, Flagstaff. I n place of the liquid-nitrogen-cooled plate holder used in the original design the mica window and film holder shown in Figs. 1 and 2 have been mounted on the camera. The seal between the titanium mica-window mount and a stainless steel adaptor att,ached to the Kron coin-valve block is of the gold wire compression type. The mica window is 20 mm in diameter (and the photocathode 40 mm, there being of course a 2 : 1 demagnification). One of the tubes and its control system has been mounted a t the Cassegrain focus of the 36-in. telescope a t Herstmonceux. In Fig. 4 the film holder is being inserted into the film chamber and the 50-mm
IMAGE TCBE DEVELOPMENT AT THE RGO
41
pneumatically operated gate valve? ran be clearly seen. The electropneumatic control system, including the 5-Torr air supply, is mounted on the panel to the left. An existing photographic plate-holder is used for optical acquisition and guiding of the telescope and the image is brought to the photocathocle of the tube by a 45O plane mirror. A holder for UBV filters is mounted at the front of the tube.
KFCJ,~ trihc with mica w i r r t l o ~attnchmcnt mountctl at C'awegrain focur of :%in. tele3cope. Pneumatic contnil panel t o the loft of the photograph.
FIG.4.
Electronographs of star clusters have been taken but unfortunately rather high background in the tube has so far precluded exposures longer than about>$ h on L4 emulsion. The background is high parbly because the mien window innkes it necessary to use a higher accelerating potential, at least 36 kV, compared with the 30 kV or less used by Kron. However. this is not the full explanation because the background in the t u h even at lower voltages is much higher than that reported by Kron. Modifications are Iieing inade aimed at reducing the background kind very recent results indicate that coating the internal glass surfnces with chrornic. oxide reduces the hackground by at least, f' Manufucturd by V.A.T. Akti~,iib.r'-.ell.lchaft,Hltitg, S w it z e r la i~ ~ l.
42
D. McMULLAN, J. R. POWELL AND N. A. CURTIS
a factor of ten. If this is confirmed then the tube as described should operate efficiently as a stellar field photometer since the other parts of the system, including the film holder, have been entirely satisfactory.
PHOTOCATHODE PROCESSING The method of photocathode processing adopted diffem from that described by Kron,2 which is limited to the S.9 type. Photocathodes of the S-11 type have been processed and the method could be used for the S-20.
9
FIG. 6. Photocathode-forming evaporator head (for S.11). 1, Electrical leads; 2, antimony evaporator; 3, shield between antimony and manganese evaporators; 4, shield; 6, caesium generator; 6, collar. (Not to scale.)
The procedure for the S-11 is as follows. The tube is assembled on the pumping system with the photocathode-forming evaporator attached to the back of the coin-valve block. This evaporator comprises the head assembly shown in Fig. 5 which is connected by long wires 1 to three steel tubes sliding on tungsten rods ; these are sealed through the end of the glass tube shown in Fig. 6. This system provides electrical connections to two filaments bearing antimony 2 and manganese (hidden in Fig. 5 by the dividing shield 3), and to a common
IMAGE TUBE DEVELOPMENT AT THE RCO
43
line. Both the filaments are shielded by the cylindrical shield 4 so that only the required photocathode area receives the antimony and manganese. After the whole evacuated ~ystenihas had N thorough hake-out it is cooled and a bulb containing potassium chlorate is heated to produce an oxygen pressure of 0.05 Torr in t h e tube. By bringing a Tesla coil into contact with the cathode-end of the tube a discharge is produced for a few seconds in order to clean the sapphire substrate. The oxygen is pumped away and tthe evaporator probe is moved by magnets (acting on the steel cylindrical contacts mentioned above) into the tube until
FIG. 6. Photooathode.forming evapiirator mountod on ooin-valve block of Kron tube. The tung&en rods extend the full length of tho large diameter Pyrex glasv tube.
the filaments are near the centre of curvature of the sapphire substrate. Manganese is evaporated until the optical transmission is 86% and is then oxidized by an electrical discharge in oxygen produced as before. The oxygen having been pumped away, antimony is evaporated until the transmission is 75%. The tube is now heated to 150 "C for 1 h and caesium is generated by passing current, through the channel 5 which consists of a thin-walled nickel tube, slitted and filled with a caesium chromate and silicon mixture. The electrical circuit is made through the common line and the collar 6 which touches the anode of the tube. It
44
D. McMULLAN, J. R . POWELL AND N. A. CURTIS
should be noted that since this collar obstructs the anode aperture and reduces the pumping speed, additional holes must be made in the base of the anode. When the photosensitivity reaches its maximum and starts t o fall, caesium generation is stopped and the tube is allowed to cool slowly. Small amounts of caesium are added, if necessary, during cooling to maintain optimum sensitivity. A maximum quantum efficiency of 11 yoat 445 nm has been achieved. This is rather lower than expected and is probably due t o insufficient cleanliness in the system. Pneumatlc actuator
Gate valve
Mica window
High voltage connection
I I
I
Appendage pump
FIG.7.
,
Reslstor chaln
Silica face-plate
Cross.soction of magxictically focused 40-rnm electronographic image tube.
MAGNETICALLY FOCUSED TUBE For a larger image tube, focusing by parallel electric and magnetic fields would seem t o be essential, particularly as a flat photocathode is highly desirable from optical considerations. A tube of this type is under development and a cross-section is shown in Fig. 7. As in the case of the Kron tube it is of demountable construction which makes it economical t o construct the tube t o fine tolerances since reprocessing is possible many times during its life. The envelope is of fused silica 130 mm in diameter with a 40-mm photocathode formed directly on the face-plate. The method t o be employed for processing the photocathode without contaminating the interior of the tube is described later in the paper. The tube high vacuum is maintained by an ion appendage pump. The electrode assembly is formed from titanium annuli spaced by soda-lime glass cylinders 10-mm long. These being slightly conducting form closing surfaces of uniform potential gradient between the electrodes. The whole structure is fused together with solder glass.
IMAGE TllBE DEVELOPMENT AT THE RGO
45
Metal-oxide-glaze resistors (30 x I00 Mil), forrriirig the potential divider, are mounted directly on t h e electrode structure. The 40-kV high-voltage photocathode connection is made by bringing the high-voltage cable through a glass tube, tshe insulation being provided by the tube vacuum and the silica envelope. There are no exposed surfaces a t high potential and satisfactory operation should be obtained under the most humid conditions and at observatories a t the highest altitudes. The 40-mm mica window is sealed to a thtiiium mount with solder glass and is protected from atmospheric pressure by a gate valve as has already been described. The film holder is of the type shown in Figs. 2 and 3 and is pneumatically operated. The whole tube is bolted to the back flange of the solenoid which in turn will be bolted rigidly to the telescope.
46
I).
McMULLAN, J. R. POWELL AND N. A. CURTIS
Preliminary Testa A tube, as described, has been assembled. Figure 8 shows the electrode structure and Fig. 9 the completed tube before assembly with the gate valve. An ultraviolet-sensitive palladium photocathode in the form of a test pattern has been mounted in this tube and enables the electron optics, mica window and film holder system to be tested. The best resolution attained was 70 lp/mm but a detailed investigation of the electron optics is needed because this resolution is not maintained
FIQ.9. Completed 40-mm tube before attaohment of vacuum-lock gate valve.
over the full field. Preliminary measures show the distortion to be reasonably small, the maximum deviation of points at the edge of the field from their correct position being about 0.25 mm. Theoretical and computer studies on this type of electron-optical system are being carried out in the Image Tube Department and are reported in a separate paper.? t
See p. 545.
IMAGE TUBE DEVELOPMENT AT THE BUO
47
Photocathode Processing The processing of the photocathode directly on the face-plate of the magnetically focused tube presents some ditriculty since it is essential that the electrodes should be protected from alkali-metal vapours. An alternative would be t o use a transferred photocathode but this suffers from the disadvantage of having two additional vacuum-glass interfaces and therefore more scattered light before the photocathode. A special processing rig has been designed and a cross-section of this is shown in Fig. 10. The t8ubeto be processed 1 is bolted on at the bottom of the rig and a glass tube 2, containing the antimony source and the alkali-metal channels, is lowered by a winch 3 until it is in contact with the face-plate, thus forming a dosed processing compartment. At this stage the mica window is of course not yet mounted in
FIQ.10. Cross-sectionof photocathode processing rig. The ram and bellows assembly is shown to the left of the rig.
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D. McMULLAN, J. R . POWELL AND N . A. CURTIS
position. The tube and the rig are baked and the photocathode processed; the electrical connections to the evaporators are by copper braids 4 which are indicated only schematically in Fig. 10. The glass tube with the evaporators is then winched u p and a 0-in. all-metal gate valve 5, forming part of the rig, is closed. The top section (winch, processing tube and cylindrical enclosure) can now be unbolted. The different sections of the rig are pumped separately by ion pumps 6, 7 (for the gate-valve interspace) and 8. The image tube is also pumped by its own appendage pump 9. The residual gas pressures in the system can be measured by a mass spectrometer 10. After the top section has been removed the whole rig is rotated about a horizontal axis bringing the tube to the top. A pneumatically operated ram 1 1 , sealed by a long bellows 12 is bolted on after the mica-window mount 13 has been placed on the end of the ram. This assembly is pumped and baked. The gate valve can then be opened
FIG.11. Prooesving rig (uiidor construction) in photocathode-processing position.
IMAGETUBEDEVELOPMENTATTHERUO
49
and the window mount sealed into position in the tube base 15 with a cold indium seal, pressure being exerted by t.hree air cylinders 14. Finally the rani is used once more t o push an “0”-ring-sealed plug into the bore of the mica-window mount in order t o protect t h e mica from atmospheric pressure while the tube is being removed from the rig and joined t o the vacuum-lock gate valve and film holder. The protective plug is withdrawn through the vacuum lock. Figure 11 shows the rig, whirh is still under construction, in t h e processing position with the tube at the bottom. In Fig. 12 the rig has been rotated and is in the sealing position with the bellows section attached.
FUTUREPROJECTS The processing rig has been designed t o accommodate tubes with mica windows up t o 120 mm in diameter. The immediate target is the 40-mm tube already described but experiments are in hand on larger windows. 80-mm windows should not present difficulties : large
Pit:. 12. Processing rig (under c r m ~ t r c ~ion) ct
iii
window-waling pr)nitiorl.
50
D. McMULLAN, J. R . POWELL AND N. A. CURTIS
enough pieces of mica have already been split in the Department to the required thickness, 4 pm or less, and a window of this size has been successfully mounted. As soon as the 40-mm design has been proved an 80-mm tube will be attempted. For spectrographic use smaller tubes are required with a rectangular photocathode and mica window. A preliminary design is shown in Fig. 13 for one with a 40 x 5 mm2 photocathode. The method of construction and processing will be as already described for the larger tubes. The overall diameter of the silica envelope will be about 80 mm or slightly larger than the encapsulated Spectracon.
-/,*,’I
I-
FIQ. 13. Cross-section of proposed electronographic tube with reotangular photocathode and mica window for speotrographic use.
ACENOWLEDUMENTS This paper is published with the permission of the Astronomer Royal, Sir Richard v. d. R. Woolley, F.R.S. Thanks are due to Messrs. D. Harman and J. Pharoah for mechanical design and to Messrs. D. Bonnick, W. Mathews, D. Mayhew and E. Wilson for technical assistance.
REBERENUES 1. Lallemand, A,, I n “Adv. E.E.P.”, Vol. 22A, p. l ( l 9 6 6 ) . 2. Kron, G. E., Ables, H. E. and Hewitt, A. V., 1% “Adv. E.E.P.”, Vol. 28A, p. 1 (1969). 3. McGee, J. D., Khogali, A., Ganson, A. and Baum, W. A., I n “Adv. E.E.P.”, Vol. 22A, p. 11 (1966). 4. McGee, J. D., McMullan, D., Bacik, H., and Oliver, M., In “Adv. E.E.P.”, Vol. 28A, p. 61 (1060). 6. McGee, J. D. and MoMullan, D., J . Sci. Inetrum. 2. 36 (1969).
IMAGE TUBE DEVELOPMENT AT THE ROO
51
DISCUSSION Your System appears to w e a single piece of film and therefore provides a singlc exposuro on one evacuation ofthe film charnber. Have you any plam for a film magazine for multiple c?xposures? D. McMULLAN : The design of an ant,omatic plate-changing system is high on our list of priorities. Tho p b e ca.sset,tewould be evacuated and plates could then be changed rapidly wit.hout.having to wait, for t.he vacuumlock to be pumped. Such plate-changing systems are commonplace in electron microscopy. J. RING : I hope the larger t.ube you describn will still be compact and simple enough to be operated at t,ho prime focus. If you have to work at the Cassegrain, the increased plate scale will largely cancel t,he gain in field area over that of a Spectracon a t bhe prime focus. D. MCMTTLLAN : I agree that operation at t,he prime focus is essentJial. As an interim measure it should be possible to mount t.he 4-cm tube at the prime focus of the Isaac Newton telescapo and still leave room for an observer whoss main task would be plate changing. M. F. WALJIER : What do you expect will be, ult.imatoly. the size of the auxiliary equipment, needed for the film- or ,plate-changingprooedurc? Can t,his be reduced to be cornpatiblc with spacct limit.at,ionin imtallat.ion a t the prime fociw of largo reflectors? D. MCMULLAN : An 8-cni tuba with trritomtttic plat,e-chtmgingwill be smaller than an observor and probably about the same weight. For operation without an observer in the prime focus cage, not only must plato changing be autJoniaticbut other ftmctions must be remotely controlled, in particular optical focusing, filtm changing, and offset guiding. These features are being included in the design of the 8-cm tube. J. D. MCGEE: What effect do you expect. t.he Cerenkov radiation in the silica end-window to have on background? D. MCMULLAN : Any light produced in the window due to cerenkov radiation will be insignificant, compared to the sky background when t,he tube is usvd at the prime focus for stellar field photometry, oxcept possibly when very narrow band filters &re being employed. Trouble might be encountered when the tJubo is being used for spectrographic recording ; however it, should be remembered thac large optical components made of silica are often used in spectrograph cameras (e.g. Wynne, C. G. and Kidger, M. J., I n “Adv.E.E.P.” Vol. 28B, p. 759, 1969). J. BAUDRAND : How long is the baking before evaporating the photocathode? And what temperature is reached? D. MCMULLAN: About 3 days at 400 “C. F. DELORI : Could you say something about the focusing solenoid? D. MCMULLAN : At the moment we are using a uniform winding (aluminium foil), screened with two coaxial mu-metal cylinders, and with mild steel end-plates. J. A. HALL:
INTRODUCT~ON The Image Tube Department at the Royal Greenwich Observatory was set up two years ago for the development of electronographic image tubes, both for stellar field photometry and for the recording of spectra. The following features were vonsidered to be essential for a successful tube design. I . I1:ase of operation by astronomers after a few minutes instruction. High degree of reliability, especially in the adverse environment of a telescope dome. 2. Large image area for stellar photometry. At least 40 mm in diameter and 90 or 100 mm if' possible. 3 . Also, of course, the absolutely essential requirements of high photocathode sensitivity, good resolution and low background. Although the last requirements are met by existing tube designs, none combines the first two : ease of operation ttnd reliability coupled with large image size. Of the existing tubes. those due to Lalleinandl and t o Krona t have reasonably large image areas but they are riot easy t o operate because the electxonographic plate is introduced into the same vacuum environment as the photocathode. A n observatory using these tubes should have a high-vacuum laboratory for the preparation of the tjubes (including photocathode processing). The S p e c t r a ~ o n4,$~ on the other hand, is simple to operate because of its mica window which enables the electronographic film t o be exposed externally t o the tube. The image size is limited t o about 30 x 15 mm2 since the mica window, which is only 4-pm thick, has to withstand atmospheric prerrsure. The solution we are adopting is to incorporate a mica window but
t
See alro p. 737.
1 SCR also p.
13. .{i
38
D. McMULLAN, J. R. POWELL AND N. A. CURTIS
to protect it from atmospheric pressure by a vacuum lock. This overcomes the limitation on window size set by strength considerations and minimizes the possibilit,y of mica breakage.
MICA WINDOW Figure 1 shows the arrangement a t the output end of a tube. The mica 1, 20 mm in diameter, is stretched tight on its mount 2 which is of titanium and is sealed to the vacuum envelope of the image tube
Fro. 1. Manually operated platelfilm holder with eleotrostatic pull-down of mica window. Diameter of mice, 20 mm.
(not shown). The air pressure on the output side of the mica window is kept a t 1 Tom or less by a mechanical or sorption pump and the electronographic plate or film is introduced through a vacuum lock formed by a gate valve 3. The plate or film is mounted on the holder 4 which is inserted into the bayonet fitting 5 of the plate chamber. When this has been pumped to below 1 Torr through the vacuum line 6, the gate valve is opened and the emulsion is brought up to the mica manually by the control rod 7. The vacuum is monitored by thermocouple gauges 8 and 9. I$ is essential that the emulsion should be in intimate contact with the mica :the first system that has been tried uses electrostatic force t o pull the mica into contact with a glass-based emulsion. An electrode 10,
39
IMAQE TUBE DEVELOPMENT AT THE RGO
having a potential of a few hundred volts, is mounted behind the plate, and the mica, which is aluminized, is pulled into contact with the emulsion. Although the contact is very good, unfortunately i t has not yet proved possible to use this method hecause of the difficulty of detaching the mica in a reasonable time at the end of an exposure when the potential of the back electrode is reduced to zero. Owing to charge absorption in the glass the time taken (mi be many minutes ; attempts to separate the mica mechanically while the glass is still charged can lead to fogging of the emulsion. More recent experiments have indicated that the method operates better with Melinex-based film, which has a very high resistivity.
'\
FIG.2. Film holder for 40-mm mica wintlow. The emulaion is brought into contact with the mica by an air pressure of 6 Tow. Holder is contrr~llctlpneumatically.
An alternative method makes use of low pressure air to press a 50-pm Melinex-based film against tJhemica. This is similar to a system developed for a Spectracon with a circular mica window4* but works a t much lower pressures. The film holder is shown in Fig. 2 ; the film 1 is held in place by a cap 2 (see also the Idiottograph in Fig. 3) and the holder is inserted in the bayonet fitting of the vacuum-lock assembly which has already been described. The emulsion is brought up t o the mica and the space behind the film is pressurized with air t o about 5 Torr, thus pressing the eniulsion into contact with the taut mica window. The film holder shown in Figs. 2 and 3 is for a window 40 mm
40
D . MuXrJLLAN, J . R . POWELL AND N. A. CURTIS
in diameter and is controlled pneumatically by the air cylinder 3. All the operations, viz. pumping down the film holder, opening tthe gate valve, bringing forward the film and pressurizing to 5 Torr, are carried out by an automatic electro-pneumatic control system. Film can be changed in 1 t o 2 min or faster if A sorption pump is used.
FIG.3. Head of the film holdor shown in Fig. 2.
KRONELECTRONIC CAMERA Two Kron Electronic Cameras2 have been built at the Observatory from drawings kindly supplied by Dr. Kron of the U.S. Naval Observatory, Flagstaff. I n place of the liquid-nitrogen-cooled plate holder used in the original design the mica window and film holder shown in Figs. 1 and 2 have been mounted on the camera. The seal between the titanium mica-window mount and a stainless steel adaptor att,ached to the Kron coin-valve block is of the gold wire compression type. The mica window is 20 mm in diameter (and the photocathode 40 mm, there being of course a 2 : 1 demagnification). One of the tubes and its control system has been mounted a t the Cassegrain focus of the 36-in. telescope a t Herstmonceux. In Fig. 4 the film holder is being inserted into the film chamber and the 50-mm
IMAGE TCBE DEVELOPMENT AT THE RGO
41
pneumatically operated gate valve? ran be clearly seen. The electropneumatic control system, including the 5-Torr air supply, is mounted on the panel to the left. An existing photographic plate-holder is used for optical acquisition and guiding of the telescope and the image is brought to the photocathocle of the tube by a 45O plane mirror. A holder for UBV filters is mounted at the front of the tube.
KFCJ,~ trihc with mica w i r r t l o ~attnchmcnt mountctl at C'awegrain focur of :%in. tele3cope. Pneumatic contnil panel t o the loft of the photograph.
FIG.4.
Electronographs of star clusters have been taken but unfortunately rather high background in the tube has so far precluded exposures longer than about>$ h on L4 emulsion. The background is high parbly because the mien window innkes it necessary to use a higher accelerating potential, at least 36 kV, compared with the 30 kV or less used by Kron. However. this is not the full explanation because the background in the t u h even at lower voltages is much higher than that reported by Kron. Modifications are Iieing inade aimed at reducing the background kind very recent results indicate that coating the internal glass surfnces with chrornic. oxide reduces the hackground by at least, f' Manufucturd by V.A.T. Akti~,iib.r'-.ell.lchaft,Hltitg, S w it z e r la i~ ~ l.
42
D. McMULLAN, J. R. POWELL AND N. A. CURTIS
a factor of ten. If this is confirmed then the tube as described should operate efficiently as a stellar field photometer since the other parts of the system, including the film holder, have been entirely satisfactory.
PHOTOCATHODE PROCESSING The method of photocathode processing adopted diffem from that described by Kron,2 which is limited to the S.9 type. Photocathodes of the S-11 type have been processed and the method could be used for the S-20.
9
FIG. 6. Photocathode-forming evaporator head (for S.11). 1, Electrical leads; 2, antimony evaporator; 3, shield between antimony and manganese evaporators; 4, shield; 6, caesium generator; 6, collar. (Not to scale.)
The procedure for the S-11 is as follows. The tube is assembled on the pumping system with the photocathode-forming evaporator attached to the back of the coin-valve block. This evaporator comprises the head assembly shown in Fig. 5 which is connected by long wires 1 to three steel tubes sliding on tungsten rods ; these are sealed through the end of the glass tube shown in Fig. 6. This system provides electrical connections to two filaments bearing antimony 2 and manganese (hidden in Fig. 5 by the dividing shield 3), and to a common
IMAGE TUBE DEVELOPMENT AT THE RCO
43
line. Both the filaments are shielded by the cylindrical shield 4 so that only the required photocathode area receives the antimony and manganese. After the whole evacuated ~ystenihas had N thorough hake-out it is cooled and a bulb containing potassium chlorate is heated to produce an oxygen pressure of 0.05 Torr in t h e tube. By bringing a Tesla coil into contact with the cathode-end of the tube a discharge is produced for a few seconds in order to clean the sapphire substrate. The oxygen is pumped away and tthe evaporator probe is moved by magnets (acting on the steel cylindrical contacts mentioned above) into the tube until
FIG. 6. Photooathode.forming evapiirator mountod on ooin-valve block of Kron tube. The tung&en rods extend the full length of tho large diameter Pyrex glasv tube.
the filaments are near the centre of curvature of the sapphire substrate. Manganese is evaporated until the optical transmission is 86% and is then oxidized by an electrical discharge in oxygen produced as before. The oxygen having been pumped away, antimony is evaporated until the transmission is 75%. The tube is now heated to 150 "C for 1 h and caesium is generated by passing current, through the channel 5 which consists of a thin-walled nickel tube, slitted and filled with a caesium chromate and silicon mixture. The electrical circuit is made through the common line and the collar 6 which touches the anode of the tube. It
44
D. McMULLAN, J. R . POWELL AND N. A. CURTIS
should be noted that since this collar obstructs the anode aperture and reduces the pumping speed, additional holes must be made in the base of the anode. When the photosensitivity reaches its maximum and starts t o fall, caesium generation is stopped and the tube is allowed to cool slowly. Small amounts of caesium are added, if necessary, during cooling to maintain optimum sensitivity. A maximum quantum efficiency of 11 yoat 445 nm has been achieved. This is rather lower than expected and is probably due t o insufficient cleanliness in the system. Pneumatlc actuator
Gate valve
Mica window
High voltage connection
I I
I
Appendage pump
FIG.7.
,
Reslstor chaln
Silica face-plate
Cross.soction of magxictically focused 40-rnm electronographic image tube.
MAGNETICALLY FOCUSED TUBE For a larger image tube, focusing by parallel electric and magnetic fields would seem t o be essential, particularly as a flat photocathode is highly desirable from optical considerations. A tube of this type is under development and a cross-section is shown in Fig. 7. As in the case of the Kron tube it is of demountable construction which makes it economical t o construct the tube t o fine tolerances since reprocessing is possible many times during its life. The envelope is of fused silica 130 mm in diameter with a 40-mm photocathode formed directly on the face-plate. The method t o be employed for processing the photocathode without contaminating the interior of the tube is described later in the paper. The tube high vacuum is maintained by an ion appendage pump. The electrode assembly is formed from titanium annuli spaced by soda-lime glass cylinders 10-mm long. These being slightly conducting form closing surfaces of uniform potential gradient between the electrodes. The whole structure is fused together with solder glass.
IMAGE TllBE DEVELOPMENT AT THE RGO
45
Metal-oxide-glaze resistors (30 x I00 Mil), forrriirig the potential divider, are mounted directly on t h e electrode structure. The 40-kV high-voltage photocathode connection is made by bringing the high-voltage cable through a glass tube, tshe insulation being provided by the tube vacuum and the silica envelope. There are no exposed surfaces a t high potential and satisfactory operation should be obtained under the most humid conditions and at observatories a t the highest altitudes. The 40-mm mica window is sealed to a thtiiium mount with solder glass and is protected from atmospheric pressure by a gate valve as has already been described. The film holder is of the type shown in Figs. 2 and 3 and is pneumatically operated. The whole tube is bolted to the back flange of the solenoid which in turn will be bolted rigidly to the telescope.
46
I).
McMULLAN, J. R. POWELL AND N. A. CURTIS
Preliminary Testa A tube, as described, has been assembled. Figure 8 shows the electrode structure and Fig. 9 the completed tube before assembly with the gate valve. An ultraviolet-sensitive palladium photocathode in the form of a test pattern has been mounted in this tube and enables the electron optics, mica window and film holder system to be tested. The best resolution attained was 70 lp/mm but a detailed investigation of the electron optics is needed because this resolution is not maintained
FIQ.9. Completed 40-mm tube before attaohment of vacuum-lock gate valve.
over the full field. Preliminary measures show the distortion to be reasonably small, the maximum deviation of points at the edge of the field from their correct position being about 0.25 mm. Theoretical and computer studies on this type of electron-optical system are being carried out in the Image Tube Department and are reported in a separate paper.? t
See p. 545.
IMAGE TUBE DEVELOPMENT AT THE BUO
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Photocathode Processing The processing of the photocathode directly on the face-plate of the magnetically focused tube presents some ditriculty since it is essential that the electrodes should be protected from alkali-metal vapours. An alternative would be t o use a transferred photocathode but this suffers from the disadvantage of having two additional vacuum-glass interfaces and therefore more scattered light before the photocathode. A special processing rig has been designed and a cross-section of this is shown in Fig. 10. The t8ubeto be processed 1 is bolted on at the bottom of the rig and a glass tube 2, containing the antimony source and the alkali-metal channels, is lowered by a winch 3 until it is in contact with the face-plate, thus forming a dosed processing compartment. At this stage the mica window is of course not yet mounted in
FIQ.10. Cross-sectionof photocathode processing rig. The ram and bellows assembly is shown to the left of the rig.
48
D. McMULLAN, J. R . POWELL AND N . A. CURTIS
position. The tube and the rig are baked and the photocathode processed; the electrical connections to the evaporators are by copper braids 4 which are indicated only schematically in Fig. 10. The glass tube with the evaporators is then winched u p and a 0-in. all-metal gate valve 5, forming part of the rig, is closed. The top section (winch, processing tube and cylindrical enclosure) can now be unbolted. The different sections of the rig are pumped separately by ion pumps 6, 7 (for the gate-valve interspace) and 8. The image tube is also pumped by its own appendage pump 9. The residual gas pressures in the system can be measured by a mass spectrometer 10. After the top section has been removed the whole rig is rotated about a horizontal axis bringing the tube to the top. A pneumatically operated ram 1 1 , sealed by a long bellows 12 is bolted on after the mica-window mount 13 has been placed on the end of the ram. This assembly is pumped and baked. The gate valve can then be opened
FIG.11. Prooesving rig (uiidor construction) in photocathode-processing position.
IMAGETUBEDEVELOPMENTATTHERUO
49
and the window mount sealed into position in the tube base 15 with a cold indium seal, pressure being exerted by t.hree air cylinders 14. Finally the rani is used once more t o push an “0”-ring-sealed plug into the bore of the mica-window mount in order t o protect t h e mica from atmospheric pressure while the tube is being removed from the rig and joined t o the vacuum-lock gate valve and film holder. The protective plug is withdrawn through the vacuum lock. Figure 11 shows the rig, whirh is still under construction, in t h e processing position with the tube at the bottom. In Fig. 12 the rig has been rotated and is in the sealing position with the bellows section attached.
FUTUREPROJECTS The processing rig has been designed t o accommodate tubes with mica windows up t o 120 mm in diameter. The immediate target is the 40-mm tube already described but experiments are in hand on larger windows. 80-mm windows should not present difficulties : large
Pit:. 12. Processing rig (under c r m ~ t r c ~ion) ct
iii
window-waling pr)nitiorl.
50
D. McMULLAN, J. R . POWELL AND N. A. CURTIS
enough pieces of mica have already been split in the Department to the required thickness, 4 pm or less, and a window of this size has been successfully mounted. As soon as the 40-mm design has been proved an 80-mm tube will be attempted. For spectrographic use smaller tubes are required with a rectangular photocathode and mica window. A preliminary design is shown in Fig. 13 for one with a 40 x 5 mm2 photocathode. The method of construction and processing will be as already described for the larger tubes. The overall diameter of the silica envelope will be about 80 mm or slightly larger than the encapsulated Spectracon.
-/,*,’I
I-
FIQ. 13. Cross-section of proposed electronographic tube with reotangular photocathode and mica window for speotrographic use.
ACENOWLEDUMENTS This paper is published with the permission of the Astronomer Royal, Sir Richard v. d. R. Woolley, F.R.S. Thanks are due to Messrs. D. Harman and J. Pharoah for mechanical design and to Messrs. D. Bonnick, W. Mathews, D. Mayhew and E. Wilson for technical assistance.
REBERENUES 1. Lallemand, A,, I n “Adv. E.E.P.”, Vol. 22A, p. l ( l 9 6 6 ) . 2. Kron, G. E., Ables, H. E. and Hewitt, A. V., 1% “Adv. E.E.P.”, Vol. 28A, p. 1 (1969). 3. McGee, J. D., Khogali, A., Ganson, A. and Baum, W. A., I n “Adv. E.E.P.”, Vol. 22A, p. 11 (1966). 4. McGee, J. D., McMullan, D., Bacik, H., and Oliver, M., In “Adv. E.E.P.”, Vol. 28A, p. 61 (1060). 6. McGee, J. D. and MoMullan, D., J . Sci. Inetrum. 2. 36 (1969).
IMAGE TUBE DEVELOPMENT AT THE ROO
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DISCUSSION Your System appears to w e a single piece of film and therefore provides a singlc exposuro on one evacuation ofthe film charnber. Have you any plam for a film magazine for multiple c?xposures? D. McMULLAN : The design of an ant,omatic plate-changing system is high on our list of priorities. Tho p b e ca.sset,tewould be evacuated and plates could then be changed rapidly wit.hout.having to wait, for t.he vacuumlock to be pumped. Such plate-changing systems are commonplace in electron microscopy. J. RING : I hope the larger t.ube you describn will still be compact and simple enough to be operated at t,ho prime focus. If you have to work at the Cassegrain, the increased plate scale will largely cancel t,he gain in field area over that of a Spectracon a t bhe prime focus. D. MCMTTLLAN : I agree that operation at t,he prime focus is essentJial. As an interim measure it should be possible to mount t.he 4-cm tube at the prime focus of the Isaac Newton telescapo and still leave room for an observer whoss main task would be plate changing. M. F. WALJIER : What do you expect will be, ult.imatoly. the size of the auxiliary equipment, needed for the film- or ,plate-changingprooedurc? Can t,his be reduced to be cornpatiblc with spacct limit.at,ionin imtallat.ion a t the prime fociw of largo reflectors? D. MCMULLAN : An 8-cni tuba with trritomtttic plat,e-chtmgingwill be smaller than an observor and probably about the same weight. For operation without an observer in the prime focus cage, not only must plato changing be autJoniaticbut other ftmctions must be remotely controlled, in particular optical focusing, filtm changing, and offset guiding. These features are being included in the design of the 8-cm tube. J. D. MCGEE: What effect do you expect. t.he Cerenkov radiation in the silica end-window to have on background? D. MCMULLAN : Any light produced in the window due to cerenkov radiation will be insignificant, compared to the sky background when t,he tube is usvd at the prime focus for stellar field photometry, oxcept possibly when very narrow band filters &re being employed. Trouble might be encountered when the tJubo is being used for spectrographic recording ; however it, should be remembered thac large optical components made of silica are often used in spectrograph cameras (e.g. Wynne, C. G. and Kidger, M. J., I n “Adv.E.E.P.” Vol. 28B, p. 759, 1969). J. BAUDRAND : How long is the baking before evaporating the photocathode? And what temperature is reached? D. MCMULLAN: About 3 days at 400 “C. F. DELORI : Could you say something about the focusing solenoid? D. MCMULLAN : At the moment we are using a uniform winding (aluminium foil), screened with two coaxial mu-metal cylinders, and with mild steel end-plates. J. A. HALL: