WO1994015240A1

WO1994015240A1 - Optical apparatus

Info

Publication number: WO1994015240A1
Application number: PCT/GB1993/002589
Authority: WO
Inventors: Francisco Diego
Original assignee: University College London
Priority date: 1992-12-22
Filing date: 1993-12-17
Publication date: 1994-07-07
Also published as: GB9226661D0

Abstract

A confocal image slicer for a high-resolution astronomical spectrograph has a parallel plate beam splitter (14) with an output prims (13) to equalise the optical paths travelled by component beams. An input prism (11) compensates for angular deflection of the beam.

Description

Optical apparatus A high-resolution astronomical spectrograph needs to sample the image produced by a telescope with a very narrow slit, with 5 the consequent rejection of most of the light, thus reducing the efficiency of the whole system and limiting this kind of work to relatively bright stars.

This problem can be alleviated by re-shaping the disk-like image into a narrow and long segment that behaves like a

10 spectrograph slit but with minimum light rejection. This optical manipulation is made by devices called image slicers, of which there are two designs: the Richardson (based in superimposing multiple reflections from concave mirrors - E.H. Richardson, J.M. Fletcher and W.A. Grundman. Image Slicers,

15 1984. Proceedings of the IAU Colloquium No.79: Very Large Telescopes, their Instrumentation and Programs, Garching.) and the Bowen-Walraven (based in multiple total internal reflections inside a thin parallel plate. (D.M.Hunten, 1974. Reshaping and Stabilization of Astronomical Images. Methods of Experimental

20 Physics, vol.12, p.193. L.Marton, ed., J.E.Simmons, R.M.Drake and L.V.Hepburn, 1982. Modified Bowen-Walraven Image Slicer. SPIE proceedings vol.331, Instrumentation in Astronomy IV p.427.)

The Richardson slicer requires several optical components

25 and as 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.

In principle, the Bowen-Walraven slicer is more efficient

30 and easier to implement but it has the disadvantage of producing a "slit" which is tilted along the optical axis, as the slicing takes place along the hypotenuse of a 45 degree prism. Consequently only a short length of the slit within focus tolerance can be used and the full potentiality of the device is

35 wasted. We have devised a confocal image slicer which overcomes the main disadvantage of the Bowen-Walraven slicer.

According to the present invention there is provided apparatus for altering the cross-sectional profile of a beam of optical radiation comprising input means adapted to receive an input beam of radiation, beam splitter means to separate said beam into a plurality of component beams of different cross-sectional profile from that of said input beam and output compensator means adapted substantially to equalise the optical paths travelled by said component beams.

The invention will now be particularly described, by way of example, with reference to the accompanying drawings, in which:- Figures 1 and 2 are schematic views of a prior art image slicer; Figure 3 is a modification of the device of Figures 1 and

2; Figure 4 is a schematic diagram of apparatus in accordance with a specific embodiment of the invention; and Figure 5 is an explanatory diagram

Referring now to the drawings, Figures 1 and 2 are schematic views of a Bowen-Walraven image slicer. A stellar image produced by a telescope is projected on face A of a thin parallel plate 1; (the image is typically one millimetre diameter). The plate is optically contacted to a 45 degree prism 3 which has a tilted side face 5, 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. The light will be transmitted to the prism only where there is optical contact between the plate and the prism.

The rest of the light is internally reflected once more, and will eventually be transmitted as the air gap narrows. The tilted boundary between glass and air is called "slicing edge"

7. To illustrate the defocusing effect, the individual slices are shown at equal optical paths starting at their respective origins in plate AB. CD represents the projection of the sliced slit AB as seen from the observing point and the defocusing is a consequence of this line not being perpendicular to the optical axis Z. In an alternative arrangement shown in Figure 3, the input face is provided by a small prism contacted to the parallel plate. This arrangement simplifies the manufacture and has been used by the European Southern Observatory.

Figure 4 shows a confocal image slicer in accordance with a specific embodiment of the invention. This includes input prisms 9,11 and an output prism 13 which serves as an optical path-length compensator and has the effect of increasing the optical path length of AC with respect to the one of BD (Figure 1). The angle α is calculated in such a way that the total optical distances travelled by rays Rl and R2 are the same, so the ends of the generated slit appear on a plane (dotted line) perpendicular to the optical axis Z (see Appendix A). The input prism 11 introduces the same refraction angle as the output prism 13, so the light is not deviated by the slicer, but only shifted laterally by a few millimetres. A reflective plate 10 placed just before the input prism facilitates acquisition and guiding during the exposure. A small hole 12 on this plate allows the stellar image from the telescope to go through. In this way the confocal image slicer can easily be implemented in existing spectrographs. The lateral shift can be compensated by moving the telescope until the image falls on the aperture of the reflective plate, although some sort of modification may be required in the acquisition and guiding system in order for it to have access to this off-axis position. The relative alignment of components requires the input and output faces to be parallel to each other and any residual misalignment can be compensated by decentring a weak field lens 15 placed at the output. This field lens is required to superimpose all slices on the dispersive element of the spectrograph anyway. It is also required that all refractions in prisms and internal reflections in the parallel plate 14 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.

In a working prototype of the confocal image slicer the five components were made of fused silica, for efficient transmission of ultraviolet radiation and to allow the use of optical contacting for assembly. All active surfaces were flat to a quarter of a wavelength and the angles between faces were within normal tolerances of optical manufacturing. The slicing edge was made tilted by 6.8 degree, which combined with a parallel plate thickness of 0.75mm produces slices 0.12 x 1.0 mm. By adjusting the angle of the slicer it was possible to reduce the slice width from 0.12 to 0.04mm and produce up to 17 slices. The optical work included also assembling the components by optical contact, in which the pieces are held together by molecular attraction, without the need of any glue. The very low coefficient of thermal expansion of fused silica contributes to the success of this technique.

The external surfaces (input and output faces were coated with a single layer of MgF₂, calculated for optimum transmission at 4000μ at the required angle of incidence of 70 degrees. The coatings reduce reflection losses and polarisation. The slicer was mounted on a stainless steel plate using flexible glue and the assembly was installed in an aluminium housing with mechanical adjustments for optical alignment. This unit also carried a weak field lens for optical alignment placed just after the last prism of the slicer. The complete assembly covered by a Dekker reflective plate.

In preliminary tests the gain factor over a conventional slit was measured between 10 and 15, indicating that a final version could go up to 20 or 25 slices. In order to have the composite slit AB projected on a plane orthogonal to the optical axis, the optical distances AC and BD (Figure 5) must be equivalent.

From geometrical construction, it follows that the condition AC/n = BD is satisfied if tanα= l/(n² - 1), which gives α = 41° for the refractive index n of fused silica. At this angle the light is refracted to the air at 70 deg from the normal to the surface.

Claims

Claims 1. Apparatus for altering the cross-sectional profile of a beam of optical radiation comprising input means adapted to receive an input beam of radiation, beam splitter means to separate said 5 beam into a plurality of component beams of different cross-sectional profile from that of said input beam characterised in that in includes output compensator means (13) adapted substantially to equalise the optical paths travelled by said component beams. 102. Apparatus for altering the cross-sectional profile of a beam of optical radiation as claimed in claim 1 characterised in that it includes angular deflection means (11) to compensate for angular deflection by the output compensator means (13.)

3. Apparatus for altering the cross-sectional profile of a beam 15 of optical radiation as claimed in claim 3 characterised in that said beam splitter means comprises a reflective plate (14) and said angular deflection means and said output compensator means comprise prisms (11,13) assembled with their triangular faces parallel to the projection of the slicing edge of said beam 20 splitter means when viewed from the direction of the optical axis at the output of said apparatus.

4. Apparatus for altering the cross-sectional profile of a beam of optical radiation as claimed in claim 1 characterised in that a reflective plate (10) is positioned before said input means.

25 5. Apparatus for altering the cross-sectional profile of a beam of optical radiation as claimed in claim 4 characterised in that said reflective plate (10) includes an aperture for the passage of a beam of radiation.

6. Apparatus for altering the cross-sectional profile of a beam

30 of optical radiation as claimed in claim 1 characterised in that it includes a lens (15) for optical alignment positioned after said output compensator means (13).