A confocal image slicer for use in high-resolution spectrography in astrophysics is presented. The device improves the light transmission of a high-resolution spectrograph by an (unprecedented) order of magnitude. The production of a prototype is described, and the first astronomical results obtained with the Anglo-Australian Telescope are presented. The confocal image slicer is being patented and may find useful applications in existing high-resolution spectrographs and in similar instruments planned for the new generation of very large astronomical telescopes.
A high-resolution astronomical spectrograph has to sample the image produced by a telescope with a narrow slit, with the consequent rejection of most of the light, which reduces the efficiency of the whole system and limits this kind of research to relatively bright stars.
This problem can be alleviated by reshaping the disklike image of the star into a narrow and long segment that behaves like a spectrograph slit but with minimum rejection of light. This optical manipulation is made by devices called image slicers, of which there have been several designs, starting with the stack of small mirrors first presented by Bowen1
and described in more detail by Hunten.2 A modified version of the Bowen slicer was introduced by Wal-raven and Walraven,3 in which the mirrors were replaced by total internal reflections inside a thin parallel plate. This design was further developed by Simmons et al.4 A different approach was taken by Richardson et al.,5 who proposed a series of superimposing multiple reflections from concave mirrors facing each other. A third option is now provided by modern optical-fiber technology, which permits the use of fiber bundles to reshape the disklike seeing disk into a long and narrow slit, but the process is inherently inefficient, and the transmission at less than 3600 Å is not good enough with current fiber materials. Currently the slicers used most are the Bowen-Walraven and the Richardson.
The author is with the Department of Physics and Astronomy, Optical Science Laboratory, University College London, London WC1E 6BT, UK.
The Richardson slicer requires several optical components, and since it uses mirrors in multiple passes, it is inefficient and difficult to align. The gain factor obtained by using this slicer is reported to be 2.7 with respect of the transmission of an equivalent slit.5
In principle, the Bowen-Walraven slicer is more efficient and easier to implement, but it has the disadvantage of producing a slit that is tilted along the optical axis, since the slicing takes place along the hypotenuse of a 45° prism. Consequently only a short length of the slit within focus tolerance can be used and the full potentiality of the device is wasted. Nevertheless for moderately slow beams (say from ƒ/20) this slicer should perform better than the Richardson, although the author has found no published figures.
In this Note we describe the confocal image slicer, which can be regarded as a new approach to the Bowen-Walraven slicer, because its main disadvantage is overcome.
Figure 1 shows a version of the Bowen-Walraven image slicer developed and used by the European Southern Observatory.6 The stellar image produced by a telescope is projected on face F of a small prism contacted to a thin parallel plate. (The image is typically 1 mm in diameter.) The plate is optically contacted to a 45° prism, which has a tilted side face, leaving a wedge-shaped area of the plate open to the air. The light is trapped by the plate and undergoes multiple internal reflections within its parallel faces. Transmission to the prism will take place only where there is optical contact between the plate and the prism. The rest of the light is internally reflected once more, to be eventually transmitted as the air gap narrows. The tilted boundary between glass and air is called the slicing edge. To illustrate the defocus-
Fig. 1. Bowen-Walraven-ESO image slicer. A right-angle prism is in contact with a thin parallel plate AB, which in turn is in contact with a 45° prism with a tilted side. The input face is labeled F. Total internal reflections in the plate take place wherever there is a glass-air interface, a condition that decreases because of the tilted face of the prism. Nonreflected light is transmitted through the prism toward the exit face CB. The slicing side along AB becomes an optical slit that is seen from the direction of the axis Z as projected along the skew line CD.
ing effect, the individual slices are shown at equal optical paths starting at their respective origins in plate AB. CD represents the projection on air of the sliced slit AB as seen from the observing point O, and the defocusing is a consequence of this line not being perpendicular to the optical axis Z.
Figure 2 shows the confocal image slicer. It involves two more prisms. The output prism is an optical path-length compensator, which has the effect of increasing the optical path length of AC with respect to that of BD (Fig. 1). The angle α is calculated in such a way that the total optical distances traveled by rays Rl and R2 are the same, so the ends of the generated slit appear along a dashed line now perpendicular to the optical axis Z.
From geometrical construction it follows that the condition
is satisfied when
which gives α = 41° for the refractive index n of fused silica at 4400 Å. At this prism angle light is refracted to the air at 70° from the normal to the surface.
The input prism is required for the introduction of the same refraction angle as the output prism, so the
Fig. 2. Confocal image slicer. The output prism, combined with an overall rotation of the assembly, produces a projected output slit (dashed line) perpendicular to the Z axis and therefore in focus. The field lens compensates any lack of parallelism in the plate and any residual misalignment in the assembly. Thus, by decentering it, it is possible to send the light along the Z axis.
light is not deviated by the slicer but only shifted laterally by a few centimeters.
To assemble the slicer, the relative alignment of components requires that the input and output faces be parallel to each other, and any residual misalignment can be compensated by decentering a weak field lens placed at the output. This field lens is required for all slices to be superimposed upon the dispersive element of the spectrograph anyway. (The slices could diverge or converge as a result of a residual lack of parallelism between the faces of the plate, tipically ~ 20 secarc.) It is also required that all refractions in prisms and internal reflections in the parallel plate take place on the same plane. To achieve this, the input and output prisms must be assembled with their triangular faces parallel to the projection of the slicing edge as seen from the direction of the optical axis Z at the output.
A working prototype of the confocal image slicer has been made and tested with the Ultra-High Resolution Facility (UHRF), commissioned at the Anglo-Australian Telescope by the Optical Science Laboratory of University College London (UCL). (Providing resolutions of one million, UHRF may be the most powerful grating spectrograph ever built, and the confocal image slicer is a key component to its success.7,8 )
The five components were made of fused silica for efficient transmission of UV radiation and to permit the use of optical contacting techniques for assembly. All active surfaces are flat to a quarter of a wave-
length, and the angles between the faces are within the normal tolerances of optical manufacturing. Following the requirements of the UHRF, the device produces at its output an optical slit 9 mm long and 30 μm wide, corresponding to an entrance aperture of 0.7 mm × 1.0 mm (or 1 × 1.5 arcsec at the plate scale of the Anglo-Australian Telescope's ƒ/37.5 Coudé focus). These numbers indicate that 32% of the available energy in a typical 1.5-arcsec full width at half-maximum stellar image is transmitted to the slicer instead of the 2% transmitted by an equivalent conventional slit.9 The external surfaces (input and output faces) were coated with a single layer of MgF2, calculated for optimum transmission at 4400 Å at the required angle of incidence of 70°, reducing the reflection losses from 18% to 14%/surface. Measurements of transmission and polarization are under way together with a detailed study of the limitations of this device with respect to seeing disk sizes and focal ratios.10
The slicer was mounted on a stainless-steel plate with flexible glue, and the assembly was installed in aluminum housing with mechanical adjustments for optical alignment. This unit also carries a weak field lens placed just after the last prism of the sheer.
The prototype was first tried during the commissioning run of the UHRF at the Anglo-Australian Telescope. Despite that (because of the limited avail-ability of telescope time, part of which was clouded!), it was not possible to achieve perfect optical alignment of the slicer with the optical axis of the telescope; its performance was good, preserving the ultrahigh resolution of the UHRF (Fig. 3) and transmitting all 17 slices successfully. The gain factor over a
Fig. 3. Interstellar NaD2 (5890-Å) absorption toward Beta Orionis. The hyperfine splitting at 1 km/s is clearly observed. The top spectrum was obtained with the confocal image slicer and the bottom one with a slit. Note how spectral resolution is preserved by the slicer. Each CCD pixel (19 μm) covers approximately 0.0027 Å. For clarity the plots have been shifted in the vertical plane to bring them closer together. In addition, the spectra were taken in different atmospheric conditions, so that the vertical scale is not useful for comparing transmissions.
Fig. 4. Atmospheric absorption lines (at ~ 5890 Å) toward Beta Pictoris. The top spectrum was obtained with the confocal image sheer, which is almost 10 times stronger than the one produced by the same instrument with a slit providing the same spectral resolution. This time the vertical scale has been preserved for both plots. There is a slight discrepancy in dispersion as a cylindrical lens is associated with the slicer in the UHRF.8
conventional slit was almost 10 (Fig. 4), which is unprecedented, and encouraging for the development of a final version that could go up to 20 or 25 slices.
The confocal image slicer is being patented for future commercial exploitation by the Optical Science Laboratory.
The author thanks G. Avila of the ESO for fruitful discussions on its Bowen-Walraven slicer. Terry Dines from I.C. Optical Systems Ltd. showed his patience and skills in producing all the components and putting them together by optical contacting. D. Jackson of the Royal Greenwich Observatory produced the antireflection coatings. The fine mechanical components were made at the workshop of the Department of Physics and Astronomy (UCL) by J. Dumper, R. Golay, and K. Smith. The confocal image slicer was developed as part of the UHRF and funded by a grant from the UK Science and Engineering Research Council.
References 1. I. S. Bowen, "The image slicer, a device for reducing loss of
light at slit of stellar spectrograph," Astrophys. J. 88, 113 (1938).
2. D. M. Hunten, "Reshaping and stabilization of astronomical images," in Methods of Experimental Physics, L. Marton, ed. (Academic, New York, 1974), Vol. 12, p. 193.
3. Th. Walraven and J. H. Walraven, "Some features of the Leiden radial velocity instrument," in Proceedings of the Conference on Auxiliary Instrumentation for Large Telescopes, S. Lausten and A. Reiz, eds. (ESO/Centre Européen de Récherches Nucléaires, Geneva, 1972), p. 175.
4. J. E. Simmons, R. M. Drake, and L. V. Hepburn, "Modified Bowen-Walraven image slicer," in Instrumentation in Astronomy IV, D. L. Crawford, ed., Proc. Soc. Photo-Opt. In-strum. Eng. 331, 427-432 (1982).
5. E. H. Richardson, J. M. Fletcher, and W. A. Grundman, "Image slicers," in Proceedings of IAU Colloquium 79: Very Large Telescopes, Their Instrumentation and Programs (International Astronomical Union, Garching, Germany, 1984), p. 469.
6. G. Avila, ESO, Garching bei München, Germany (personal communication, 1992).
7. F. Diego, I. Crawford, M. Barlow, A. Fish, and M. Dryburgh, "Ultra-high resolution facility for the UCL échelle spectrograph," in Proceedings of the ESO Workshop on High Resolution Spectroscopy with the Very Large Telescope, M. H. Ulrich, ed. (ESO, Garching, Germany, 1992), p. 267.
8. F. Diego, I. Crawford, M. Barlow, A. Fish, M. Dryburgh, D.
Walker, I. Howarth, and J. Spyromilio, "The ultrahigh resolution facility at the UCL échelle spectrograph," submitted to Mon. Not. R. Astron. Soc.
9. F. Diego, "Stellar image profiles from linear detectors and the throughput of astronomical instruments," Publ. Astron. Soc. Pac. 97, 1209-1214 (1985).
10. F. Diego and G. Avila, "Optical characteristics of the confocal image slicer," to be submitted to Appl. Opt.
