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An XUV Image Sensor for Rowland-circle Spectrographs
An XUV Image Sensor for Rowland-circle Spectrographs J. L. LOWRANCE and C. L. JOSEPH Princeton UniverrsifyObservatory, Princeton. New Jersey, USA
INTRODUCTION In space astronomy there is considerable interest in the spectral region shortward of 1200 A. From the standpoint of efficiency, measurements in this spectral region require that the detectors be windowless and that the number of optical reflections be minimized because of the relatively low transmissivity and reflectivity of optical components. The spectrograph of choice for many of these applications is the Rowland-circle type in which the slit, grating and focal plane all lie on a circle. The grating, ruled on a concave aspheric surface, provides the focusing as well as the dispersion with a single reflection. Optical aberrations normally confine the focal plane to a narrow band along the circle. This paper reports on the preliminary design of a windowless oblique magnetic-focus image sensor similar to existing sensors (Picat et a/., 1972; Johnson and Hallam, 1976; Lowrance et al., 1979; Lowrance, 1985; see also Carruthers et al., 7) but configured to match the long narrow image format of a Rowland-type spectrograph. The principal advantage of this image sensor is its use of an opaque photocathode on a smooth surface to obtain the highest quantum efficiency available. A schematic of the detector and spectrograph is shown in Fig. 1. The focusing magnetic field is generated by a long “C”shaped permanent magnet assembly. Since the magnetic field in the gap of such configurations is intrinsically less uniform than fields inside cylindrically shaped magnets normally employed to focus image tubes (Coleman, I979), the magnetic focusing characteristics have been simulated for a suitable distribution of photoelectron energies in order to demonstrate the feasibility of this design. Although simulations of the electromagnetic focusing for a variety of sensor parameters have been made, none of these parameters has yet been optimized to achieve the best possible image quality. The model uses software developed several years ago to design a 2-dimensional windowless intensified charge-coupled device (WICCD) for the Interstellar Medium Absorption
t This volume, p. 18 I . 46 5 ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS VOL. 14
Copyright 0 1988 Academic Press Limited All rights of reproduction in any form reserved ISBN 0-12-014674-6
466
J. L. LOWRANCE A N D C. L. JOSEPH
___.-FIG.1. Rowland-circle spectrograph configuration showing windowless image intensifier with a long narrow format matching the spectrograph image.
Profile Spectrograph (IMAPS) sounding rocket programme. The results of those computer simulations turned out to be a very accurate representation of the operating characteristics of the now flight-proven IMAPS WICCD. In order to have a set of well-defined goals, this paper focuses on the preliminary design of a WICCD for a particular envisaged Rowland-circle spectrograph. IMAGE FORMAT
Studies in support of the next generation of ultraviolet astronomy satellite (the proposed Lyman Mission) have pointed out the high optical efficiency in the XUV of a Wolter-Schwarzschild Type I1 grazing-incidence telescope in combination with a Rowland-circle type spectrograph. It has been proposed that the 910-1250 A spectral range be divided into three 120 A bands to allow the grating efficiency to be optimized for each band. At a spectral resolution of AjAA = 3 x lo4,each band would contain 3600 spectral images of the slit and thus, would require at least 7200 pixels in the spectral dispersion direction to sample properly each 120 A wide spectrum without aliasing.
AN
xuv
I
IMAGE SENSOR FOR ROWLAND-CIRCLE SPECTROGRAPHS
u)
c
C
s
3
467
20
190.1 0
Position (mm)
Position (mm)
FIG.2. (a) Optical ray tracing of 1.2 m diameter Rowland-circle spectrograph with an aspheric grating, A/AA = 30 000.Shown are two wavelengths separated by one part in 15 000; (b)histogram showing the spectrum of Fig. 2(a) convolved with an ideal detector having 20 p n wide pixels.
An 80 cm f/lO telescope has an image scale of 39 pm per arc-sec. The telescope resolution is nominally one arc-sec, dictating a spectrograph slit 2 m for width of approximately 40 pm and a Rowland circle diameter of AjAA = 3 x lo4. The image sensor should have a line-spread profile that is small compared to the 40 pm slit and fit an arc that is 14.4 cm long on a circle 2 m in diameter. The magnetic lens simulation presented below, having a full width at half-maximum (FWHM) along the dispersion of 19 pm compares favourably with this requirement. Ray-tracing of the envisaged spectrograph for the Lyman Mission shows that the spatial resolution along the slit is limited by optical aberrations, as shown in Fig. 2(a) (Moos, 1986; Cash, 1984), while Fig. 2(b) shows this image convolved with an ideal detector, having 20 pm wide pixels and 100% modulation transfer function (MTF).
-
MAGNETIC FOCUS REQUIREMENTS The magnetic focal length between the photocathode and target for a uniform field is expressed by the following equation (Johnson and Hallam, 1976; Beurle and Wreathall, 1962):
where V is the total voltage drop experienced by the photoelectrons and B is the strength of the magnetic field. While a magnetic field does not have to be uniform to provide focusing, it is
468
J. L. LOWRANCE AND C. L. JOSEPH
important that all the photoelectrons experience essentially the same magnetic field as they travel from the photocathode to the target. The field between two magnetic pole faces is not uniform. In this case, however, there is uniformity in the long, dispersion direction, except near the ends. Also, the telescope’s 60 arc-sec usable field of view, limiting the image at the photocathode to only 2.4 mm perpendicular to the plane of dispersion, somewhat eases the off-axis magnetic focus requirements. Increasing the magnetic field generally reduces the point-spread function of the imaged photoelectrons. On the other hand, for a fixed electric field strength, equation (1) indicates that increasing the strength of the magnetic field reduces the focal length, and correspondingly the voltage drop experienced by the photoelectrons. Each photoelectron must be accelerated sufficiently to produce a measurable gain of signal electrons. Hence the problem becomes one of balancing an improved point-spread function of the photoelectrons caused by larger B fields against the corresponding decreased signal strength. The electric field is limited by high-voltage breakdown considerations. For the Rowland-circle spectrograph described above, the magnetic field must fill a volume approximately 3 x 45 x 150 mm3in extent. The electric field was taken to be 405 kV m-’ a t an angle 30” to the magnetic field (see Fig. 3). Table I shows the on-axis variation of the magnetic field along Z , the distance from the centre of the magnet gap to the photocathode or target, for the dimensions shown in Fig. 3.
/d>EB Incident light
FIG. 3. Sketch of cross-section of the “C”-shaped permanent magnetic assembly showing location of photocathode and focal plane of oblique-focus image intensifier electron optics. The distance from the nearest pole face to the centre of the photocathode is 33.5 mm. The distance from the nearest pole face to the centre of the target is 24 mm.
AN
xuv
IMAGE SENSOR FOR
ROWLAND-CIRCLE SPECTROGRAPHS
469
TABLE I Magnetic field across the air gap of the magnetic assembly shown in Fig. 3
-35.0 - 30.0 - 25.0 -20.0 - 15.0 - 13.5
- 10.0 -5.0 0.0 5.0 10.0 15.0 19.6
20.0 25.0 30.0 35.0
574 496 44 1 403 377 371 (at photocathode) 360 35 I 348 351 360 377 40 1 (at focus) 403 441 496 574
THECCD The CCD employed in the IMAPS intensified CCD detector is the RCA 501, which is back-illuminated and has 256 x 320 pixels of 30 x 30 pm2. This Rowland-circle version of an electron-bombarded WICCD detector would require chips from other vendors, since these CCDs are no longer manufactured. The actual CCD configuration could be made up of a long narrow row of CCD chips closely spaced such that there would be small gaps in the format that could be filled when necessary by a second exposure. Alternatively, these small gaps might be eliminated by custom-designing the CCD to be butted at the ends or built as one long narrow CCD comparable in length to the diagonal of the Tektronix 2048 x 2048 pixel CCD (80 mm). In any event, each 40 pm slit image (resolution element) should be sampled at least twice in the image read-out process to reduce abiasing artefacts in the data.
MAGNETIC Focus POINT-SPREAD PROFILES The reader is referred to the proceedings of the Eighth Symposium on
470
J. L. LOWRANCE A N D C . L. JOSEPH
FIG.4. Photoelectron energy distribution of opaque Csl photocathode excited by 11.8 eV and 1487 eV photons.
Photo-Electronic Image Devices for a detailed description of the computer modelling and electromagnetic ray-tracing programme used in this design study to evaluate the focusing properties of a field between two pole faces of a permanent magnet (Lowrance, 1985). Figure 4 shows the photoelectron energy distribution of an opaque CsI photocathode, excited by 1 1.8 eV photons (Jenkins, private communication). This energy distribution was simulated by choosing five discrete values of photoelectron energy and assigning each a weight in calculating the focused point-spread profile (Lowrance, 1985). For this study, the photocathode-to-target distance L was taken to be -33 mm and the accelerating voltage 12 kV at an angle 4 of 30" to the magnetic field. The magnetic field used in calculating electron trajectories is summarized in Table I. The photocathode is located 13.5 mm from the centre of the gap and the focus lies 19.6 mm from the centre on the other side. These are not necessarily optimum positions, merely the best of two or three trials. Figures 5 and 6 show the resultant line-spread profiles in the dispersion direction and along the slit, respectively. The FWHM is approximately 19 pm wide (and 64 pm along the slit). The point-spread function is astigmatic, but considerably less than the astigmatism of the spectrograph's optical image, shown in Fig. 2. This is understandable, since the magnetic field is uniform in the direction normal to the slit and non-uniform along the slit. It should be noted that the magnetic field uniformity can probably be improved somewhat by shaping the magnet pole faces, by increasing the distance between the pole faces, or by increasing the height of the pole faces. This would undoubtedly reduce the astigmatism along the slit. The position of
AN XUV IMAGE SENSOR FOR ROWLAND-CIRCLE SPECTROGRAPHS
47 1
x
.B
-5900
-5850
-5800
Y ( microns) FIG.5. Line-spread profile in dispersion direction.
x
.B
FIG.6. Line-spread profile along the spectrograph slit.
the photocathode and focal plane in the gap is not necessarily optimum either. However, given the spectrograph optical aberrations, such optimization appears to be unnecessary for the application. Figure 7 shows the point-spread profile and the point-to-point mapping of the image in 0.5 mm steps, off-axis. All of the point-spread profiles appear to be essentially the same as the on-axis one over a range of k 2.5 mm, which is
472
J. L. LOWRANCE AND C. L. JOSEPH
0
2000 -
*
E
0
FWHM spr' size compareo. t o a 40 micron slit
c
0
+
5
a
1000
-
0
-
0 0 -
-1000
0
0 1000 2000 Dispersion Direction (microns)
FIG.7. Variations in the point-spread function and the mapping of points in 0.5 mm steps offaxis. The full width at half-maximum for a typical spot is compared to the projected 40 pm slit width for the Lyman Mission.
more than adequate to accommodate the limited off-axis optical performance of the Rowland-circle spectrograph and telescope described above. Figure 7 also compares the point-spread function to the slit width as seen at the detector. The depth of focus of the electromagnet (Fig. 8) is deep, providing a margin for variation in the magnetic and electric fields and, for example,
60
1 14
RldS orthognal to dispersion
.
18
18
20
Position (rnm 1
FIG.8. Electromagnetic depth of focus for the photoelectrons.
AN XUV IMAGE SENSOR FOR ROWLAND-CIRCLE SPECTROGRAPHS
473
making the detector insensitive to the orientation of the Earth’s magnetic field. The preliminary results presented in this paper demonstrate the feasibility of this design as an image sensor for the proposed Lyman Mission. Configurations yielding higher resolution and less-astigmatic images than those presented here, are likely to be found, since a number of parameters have latitude for a change. In an earlier study the spatial frequency response of an oblique magnetic-focus lens was analysed and found to be more than 90% at 25cyclesmm-‘ for an XUV photocathode and a uniform focus field (Lowrance, 1985). This high resolving power was substantiated by laboratory tests and IMAPS flight data (Jenkins, private communication). ACKNOWLEDGEMENTS The authors are indebted to David Brown and David Rutzel for substantial assistance in the development of the electron-optical ray-tracing programme. This work was funded in part by NASA grants NAG5-616 and NAS5-30110.
REFERENCES Beurle, R. L. and Wreathall, W. M. (1962) In “Adv. E.E.P.” Vol 16, pp. 333-340 Cash, W. C. (1984). Appl. Opt. 23, pp. 4518-4522 Coleman, C. I., Delamere, W. A,, Dionne. N . J., Kamminga, W., Long, D., Lowrance, J. L. and van Zuylen, P. (1979). In “Adv. E.E.P.” Vol 16, pp. 89-99 Johnson, C. B. and Hallam, K. L . (1976). In “Adv. E.E.P.” Vol. 40A, pp. 69-82 Lowrance, J. L. (1985). In “Adv. E.E.P.” Vol. 64B, pp. 591-599 Lowrance, J. L., Zucchino, P., Renda, G. and Long, D. C. (1979). In “Adv. E.E.P.” Vol. 52, pp. 44 1-452 Moos, H. W. (1986). NASA Proposal for Lyman Mission
INTRODUCTION In space astronomy there is considerable interest in the spectral region shortward of 1200 A. From the standpoint of efficiency, measurements in this spectral region require that the detectors be windowless and that the number of optical reflections be minimized because of the relatively low transmissivity and reflectivity of optical components. The spectrograph of choice for many of these applications is the Rowland-circle type in which the slit, grating and focal plane all lie on a circle. The grating, ruled on a concave aspheric surface, provides the focusing as well as the dispersion with a single reflection. Optical aberrations normally confine the focal plane to a narrow band along the circle. This paper reports on the preliminary design of a windowless oblique magnetic-focus image sensor similar to existing sensors (Picat et a/., 1972; Johnson and Hallam, 1976; Lowrance et al., 1979; Lowrance, 1985; see also Carruthers et al., 7) but configured to match the long narrow image format of a Rowland-type spectrograph. The principal advantage of this image sensor is its use of an opaque photocathode on a smooth surface to obtain the highest quantum efficiency available. A schematic of the detector and spectrograph is shown in Fig. 1. The focusing magnetic field is generated by a long “C”shaped permanent magnet assembly. Since the magnetic field in the gap of such configurations is intrinsically less uniform than fields inside cylindrically shaped magnets normally employed to focus image tubes (Coleman, I979), the magnetic focusing characteristics have been simulated for a suitable distribution of photoelectron energies in order to demonstrate the feasibility of this design. Although simulations of the electromagnetic focusing for a variety of sensor parameters have been made, none of these parameters has yet been optimized to achieve the best possible image quality. The model uses software developed several years ago to design a 2-dimensional windowless intensified charge-coupled device (WICCD) for the Interstellar Medium Absorption
t This volume, p. 18 I . 46 5 ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS VOL. 14
Copyright 0 1988 Academic Press Limited All rights of reproduction in any form reserved ISBN 0-12-014674-6
466
J. L. LOWRANCE A N D C. L. JOSEPH
___.-FIG.1. Rowland-circle spectrograph configuration showing windowless image intensifier with a long narrow format matching the spectrograph image.
Profile Spectrograph (IMAPS) sounding rocket programme. The results of those computer simulations turned out to be a very accurate representation of the operating characteristics of the now flight-proven IMAPS WICCD. In order to have a set of well-defined goals, this paper focuses on the preliminary design of a WICCD for a particular envisaged Rowland-circle spectrograph. IMAGE FORMAT
Studies in support of the next generation of ultraviolet astronomy satellite (the proposed Lyman Mission) have pointed out the high optical efficiency in the XUV of a Wolter-Schwarzschild Type I1 grazing-incidence telescope in combination with a Rowland-circle type spectrograph. It has been proposed that the 910-1250 A spectral range be divided into three 120 A bands to allow the grating efficiency to be optimized for each band. At a spectral resolution of AjAA = 3 x lo4,each band would contain 3600 spectral images of the slit and thus, would require at least 7200 pixels in the spectral dispersion direction to sample properly each 120 A wide spectrum without aliasing.
AN
xuv
I
IMAGE SENSOR FOR ROWLAND-CIRCLE SPECTROGRAPHS
u)
c
C
s
3
467
20
190.1 0
Position (mm)
Position (mm)
FIG.2. (a) Optical ray tracing of 1.2 m diameter Rowland-circle spectrograph with an aspheric grating, A/AA = 30 000.Shown are two wavelengths separated by one part in 15 000; (b)histogram showing the spectrum of Fig. 2(a) convolved with an ideal detector having 20 p n wide pixels.
An 80 cm f/lO telescope has an image scale of 39 pm per arc-sec. The telescope resolution is nominally one arc-sec, dictating a spectrograph slit 2 m for width of approximately 40 pm and a Rowland circle diameter of AjAA = 3 x lo4. The image sensor should have a line-spread profile that is small compared to the 40 pm slit and fit an arc that is 14.4 cm long on a circle 2 m in diameter. The magnetic lens simulation presented below, having a full width at half-maximum (FWHM) along the dispersion of 19 pm compares favourably with this requirement. Ray-tracing of the envisaged spectrograph for the Lyman Mission shows that the spatial resolution along the slit is limited by optical aberrations, as shown in Fig. 2(a) (Moos, 1986; Cash, 1984), while Fig. 2(b) shows this image convolved with an ideal detector, having 20 pm wide pixels and 100% modulation transfer function (MTF).
-
MAGNETIC FOCUS REQUIREMENTS The magnetic focal length between the photocathode and target for a uniform field is expressed by the following equation (Johnson and Hallam, 1976; Beurle and Wreathall, 1962):
where V is the total voltage drop experienced by the photoelectrons and B is the strength of the magnetic field. While a magnetic field does not have to be uniform to provide focusing, it is
468
J. L. LOWRANCE AND C. L. JOSEPH
important that all the photoelectrons experience essentially the same magnetic field as they travel from the photocathode to the target. The field between two magnetic pole faces is not uniform. In this case, however, there is uniformity in the long, dispersion direction, except near the ends. Also, the telescope’s 60 arc-sec usable field of view, limiting the image at the photocathode to only 2.4 mm perpendicular to the plane of dispersion, somewhat eases the off-axis magnetic focus requirements. Increasing the magnetic field generally reduces the point-spread function of the imaged photoelectrons. On the other hand, for a fixed electric field strength, equation (1) indicates that increasing the strength of the magnetic field reduces the focal length, and correspondingly the voltage drop experienced by the photoelectrons. Each photoelectron must be accelerated sufficiently to produce a measurable gain of signal electrons. Hence the problem becomes one of balancing an improved point-spread function of the photoelectrons caused by larger B fields against the corresponding decreased signal strength. The electric field is limited by high-voltage breakdown considerations. For the Rowland-circle spectrograph described above, the magnetic field must fill a volume approximately 3 x 45 x 150 mm3in extent. The electric field was taken to be 405 kV m-’ a t an angle 30” to the magnetic field (see Fig. 3). Table I shows the on-axis variation of the magnetic field along Z , the distance from the centre of the magnet gap to the photocathode or target, for the dimensions shown in Fig. 3.
/d>EB Incident light
FIG. 3. Sketch of cross-section of the “C”-shaped permanent magnetic assembly showing location of photocathode and focal plane of oblique-focus image intensifier electron optics. The distance from the nearest pole face to the centre of the photocathode is 33.5 mm. The distance from the nearest pole face to the centre of the target is 24 mm.
AN
xuv
IMAGE SENSOR FOR
ROWLAND-CIRCLE SPECTROGRAPHS
469
TABLE I Magnetic field across the air gap of the magnetic assembly shown in Fig. 3
-35.0 - 30.0 - 25.0 -20.0 - 15.0 - 13.5
- 10.0 -5.0 0.0 5.0 10.0 15.0 19.6
20.0 25.0 30.0 35.0
574 496 44 1 403 377 371 (at photocathode) 360 35 I 348 351 360 377 40 1 (at focus) 403 441 496 574
THECCD The CCD employed in the IMAPS intensified CCD detector is the RCA 501, which is back-illuminated and has 256 x 320 pixels of 30 x 30 pm2. This Rowland-circle version of an electron-bombarded WICCD detector would require chips from other vendors, since these CCDs are no longer manufactured. The actual CCD configuration could be made up of a long narrow row of CCD chips closely spaced such that there would be small gaps in the format that could be filled when necessary by a second exposure. Alternatively, these small gaps might be eliminated by custom-designing the CCD to be butted at the ends or built as one long narrow CCD comparable in length to the diagonal of the Tektronix 2048 x 2048 pixel CCD (80 mm). In any event, each 40 pm slit image (resolution element) should be sampled at least twice in the image read-out process to reduce abiasing artefacts in the data.
MAGNETIC Focus POINT-SPREAD PROFILES The reader is referred to the proceedings of the Eighth Symposium on
470
J. L. LOWRANCE A N D C . L. JOSEPH
FIG.4. Photoelectron energy distribution of opaque Csl photocathode excited by 11.8 eV and 1487 eV photons.
Photo-Electronic Image Devices for a detailed description of the computer modelling and electromagnetic ray-tracing programme used in this design study to evaluate the focusing properties of a field between two pole faces of a permanent magnet (Lowrance, 1985). Figure 4 shows the photoelectron energy distribution of an opaque CsI photocathode, excited by 1 1.8 eV photons (Jenkins, private communication). This energy distribution was simulated by choosing five discrete values of photoelectron energy and assigning each a weight in calculating the focused point-spread profile (Lowrance, 1985). For this study, the photocathode-to-target distance L was taken to be -33 mm and the accelerating voltage 12 kV at an angle 4 of 30" to the magnetic field. The magnetic field used in calculating electron trajectories is summarized in Table I. The photocathode is located 13.5 mm from the centre of the gap and the focus lies 19.6 mm from the centre on the other side. These are not necessarily optimum positions, merely the best of two or three trials. Figures 5 and 6 show the resultant line-spread profiles in the dispersion direction and along the slit, respectively. The FWHM is approximately 19 pm wide (and 64 pm along the slit). The point-spread function is astigmatic, but considerably less than the astigmatism of the spectrograph's optical image, shown in Fig. 2. This is understandable, since the magnetic field is uniform in the direction normal to the slit and non-uniform along the slit. It should be noted that the magnetic field uniformity can probably be improved somewhat by shaping the magnet pole faces, by increasing the distance between the pole faces, or by increasing the height of the pole faces. This would undoubtedly reduce the astigmatism along the slit. The position of
AN XUV IMAGE SENSOR FOR ROWLAND-CIRCLE SPECTROGRAPHS
47 1
x
.B
-5900
-5850
-5800
Y ( microns) FIG.5. Line-spread profile in dispersion direction.
x
.B
FIG.6. Line-spread profile along the spectrograph slit.
the photocathode and focal plane in the gap is not necessarily optimum either. However, given the spectrograph optical aberrations, such optimization appears to be unnecessary for the application. Figure 7 shows the point-spread profile and the point-to-point mapping of the image in 0.5 mm steps, off-axis. All of the point-spread profiles appear to be essentially the same as the on-axis one over a range of k 2.5 mm, which is
472
J. L. LOWRANCE AND C. L. JOSEPH
0
2000 -
*
E
0
FWHM spr' size compareo. t o a 40 micron slit
c
0
+
5
a
1000
-
0
-
0 0 -
-1000
0
0 1000 2000 Dispersion Direction (microns)
FIG.7. Variations in the point-spread function and the mapping of points in 0.5 mm steps offaxis. The full width at half-maximum for a typical spot is compared to the projected 40 pm slit width for the Lyman Mission.
more than adequate to accommodate the limited off-axis optical performance of the Rowland-circle spectrograph and telescope described above. Figure 7 also compares the point-spread function to the slit width as seen at the detector. The depth of focus of the electromagnet (Fig. 8) is deep, providing a margin for variation in the magnetic and electric fields and, for example,
60
1 14
RldS orthognal to dispersion
.
18
18
20
Position (rnm 1
FIG.8. Electromagnetic depth of focus for the photoelectrons.
AN XUV IMAGE SENSOR FOR ROWLAND-CIRCLE SPECTROGRAPHS
473
making the detector insensitive to the orientation of the Earth’s magnetic field. The preliminary results presented in this paper demonstrate the feasibility of this design as an image sensor for the proposed Lyman Mission. Configurations yielding higher resolution and less-astigmatic images than those presented here, are likely to be found, since a number of parameters have latitude for a change. In an earlier study the spatial frequency response of an oblique magnetic-focus lens was analysed and found to be more than 90% at 25cyclesmm-‘ for an XUV photocathode and a uniform focus field (Lowrance, 1985). This high resolving power was substantiated by laboratory tests and IMAPS flight data (Jenkins, private communication). ACKNOWLEDGEMENTS The authors are indebted to David Brown and David Rutzel for substantial assistance in the development of the electron-optical ray-tracing programme. This work was funded in part by NASA grants NAG5-616 and NAS5-30110.
REFERENCES Beurle, R. L. and Wreathall, W. M. (1962) In “Adv. E.E.P.” Vol 16, pp. 333-340 Cash, W. C. (1984). Appl. Opt. 23, pp. 4518-4522 Coleman, C. I., Delamere, W. A,, Dionne. N . J., Kamminga, W., Long, D., Lowrance, J. L. and van Zuylen, P. (1979). In “Adv. E.E.P.” Vol 16, pp. 89-99 Johnson, C. B. and Hallam, K. L . (1976). In “Adv. E.E.P.” Vol. 40A, pp. 69-82 Lowrance, J. L. (1985). In “Adv. E.E.P.” Vol. 64B, pp. 591-599 Lowrance, J. L., Zucchino, P., Renda, G. and Long, D. C. (1979). In “Adv. E.E.P.” Vol. 52, pp. 44 1-452 Moos, H. W. (1986). NASA Proposal for Lyman Mission