HOLRED.

A machine to reproduce and photograph real from holograms taken in the 15 foot bubble chamber at Fermilab.

List of collaborating institutions : —

Birmingham University, Brussels University, CERN, Imperial College London, Max Planck Institute, Munich, Oxford University, Rutherford Appleton Laboratory, CEN Saclay.

Presented by P. R. Nailor, Imperial College London.

Abstract.

The need for high resolution , large volume coverage of the 15 foot chamber has already been described elsewhere in these proceedings. The aim of this paper is to decribe the design criteria and philosophy behind a machine to reproject and photograph the real images of neutrino interactions from holograms taken there in the coming run. Detailed analysis of the vertex region of these events will be done from the phototgraphs. 1.0 THE BASIC RATIONAL BEHIND THE MACHINE

HOLRED is a machine in which holograms , taken in the Fermilab 15 foot bubble chamber with a situated in the centre of the , are replayed by placing the film in the identical position in a fisheye window and illuminating them with a laser beam which is the time-reversed version of the original bubble-chamber reference beam.

The real images which are thus projected into the space on the other side of the fisheye are then viewed with a vidicon camera and monitor and interactions of interest are aligned in the centre of the field of view by the operator. Finally, the vidicon is replaced by a 70 mm. film camera , and the photographic recording of the event is subsequently analysed on conventional bubble chamber scanning machines ,

In the paragraphs below we explain the criteria upon which we based this particular design of a holographic replay machine.

1.1 Real Or Virtual Replay

Previous experience with successful real image replay in a test of two beam bubble chamber (ref 1), with the machine HOLMES (ref 2), and in tests in BEBC of the current recording technique (ref 3). have all pointed to this method as a convenient way of obtaining the maximum resolution in the replayed images. In particular, the necessity of correcting for the aberrations in bubble images induced by the bubble chamber fisheye makes virtual image replay difficult. The fisheye lens introduces aberrations into the images of bubbles which are significant on the scale of resolution that we wish to achieve and we to use a fisheye from BEBC as an integral part of the replay system, where it becomes a phase conjugator , taking out the distortions as the replayed images retrace paths that are time-reversed with respect to the recording situation. The full description of the choice of optics and laser system is discussed later.

1.2 Move The Film And Use A Stationary Vidicon For Scanning, Or Vice Versa ? The laser illuminated hologram projects the space as seen through the fisheye. This space is equivalent to real bubble chamber space demagnified by a factor of around 13 (although the exact demagnification is highly nonlinear and the lower half of the chamber is subject to a much higher local demagnification in the z direction, perpendicular to the film, of the order of 50 times). Consequently , great precision is required to move the film whilst keeping its orientation with respect to the reference beam and post-film optics correct. We decided that once this orientation is achieved to high accuracy, the film should not move , rather the vidicon and a large mirror, described later, be positioned so that the event of interest is viewed on the television screen.

1.3 Why Take Photographs Of Holographic Images , Rather Than Measure On-line ?

Due to the limited film , and residual spherical aberration , the longitudinal resolution of any image is poor and hence the region of image focus is large in depth.

Fig. 1 Spherically aberrated pencil of rays

Hence accurate measurements will be confined to the transverse plane.

More importantly, this is to be a machine used by all the European laboratories involved in the physics experiment at Fermilab. For cost and time effective use , we will take photographs from the real image on 70 mm. film and distribute these to all members. This film can be mounted and scanned on standard Bessymatic or similar tables, possibly alongside the conventional view of the same event. There, standard scanning techniques can be used, such as the edge of method of producing the anamorphic transformations described elsewhere in these proceedings. 2.0 THE OPTICAL DESIGN,

In order to produce as accurate a time-reversed duplicate of the recording situation as possible , four basic problems must be solved : —

1 . There must be a correction for the lack of a fisheye/ fluid interface on replay.

2. The reference beam must be a time-reversed replica of the original to within ~1mrad for the extreme ray.

3- Since a real fisheye is used in the replay optics, the same wavelength CW laser is needed as was used to make the hologram.

4. Great accuracy is required in the positioning of all elements in the optical train.

2.1 Refractive Index Change On Replay.

For complete phase conjugation, the real image should be reprojected into a space with refractive index 1.088, to match the fisheye/fluid interface . No suitable means exists of achieving this, and we must project into air. There are two practical solutions:

2.1.1 A Floating Meniscus Lens. -

Fig. 2 The floating meniscus lens. Raytracing results for on axis point sources are given below. (Delta is the rms value of the disc of least confusion for the given pencil) Record Replay Replay no meniscus with meniscus Depth of source (m.) Depth delta(yum.) Depth delta 1.00 0.71 '40 0.81 "4 2.00 1 .05 "100 1.34 "1 2.50 1 .16 •110 1.58 "1 3.00 1.24 "140 1 .72 "5

Here the meniscus was optimised for the midplane of the chamber at 2.00 m.

Comparing the results with and without the correcting lens, it is clear that a correction is necessary if we wish to resolve images to better than 100 yum in the replayed tracks.

The floating meniscus is however impractical since it is very difficult to implement mechanically , and it destroys the spherical symmetry of the fisheye with its second surface no longer concentric with the rest of the lens. This leads to a degradation of off-axis imagery and would involve constant repositioning during the running of the machine.

2.1.2 A Thicker Outer Fisheye Lens Element. -

fig. 3 Thick lens-cap , concentric with the fisheye

This optical setup provides excellent point to point imagery (delta less than 1 jsm for the entire chamber). It retains the spherical symmetry of the fisheye, since it is concentric, and hence it can be a stationary element in keeping with the philosophy of the machine. The projected space is still demagnified, as shown in fig. 1 below.

'C

i..

•••••. / \

\

\M

\ Fig. 4 Comparison of real bubble chamber space (full lines) and demagnified replay space (dotted lines).

The replayed position of any point P in the bubble chamber space lies on a line PC joining P to the fisheye centre C, at a distace D1 from C, with D' given by :

D ' » DjV 1_ /• — //ill " nzl \

where D = original distance PC R = outer fisheye radius on recording R'= outer fisheye radius on replay yu • relative refractive index (glass/chamber medium on recording) ft - relative refractive index (glass/replay medium) 2.2 The Correct Reference Beam Wavefront

Without an accurately time-reversed reference beam, we introduce significant spherical aberration , coma and astigmatism into the holographic image (refs 4,5.); these distortions increase for points off axis.

In the holographic system described in ref. 3 • the reference beam source point is at the bottom of the bubble chamber, thus the reference beam has passed through the fisheye on recording the hologram , so we need an inverse fisheye to produce the phase conjugate beam.

The fisheye produces mainly 1st and 3rd order spherical aberration, so a four surface condenser will suffice to reproduce this (ref 6):

fig. 5 The reference beam lens assembly.

By raytracing a parallel beam through the condenser, fisheye and chamber fluid, we aim to produce a point focus at the site of the laser entry window into the chamber , where the reference beam is formed. L1 was chosen as a diffraction limited achromat and computer optimisation was employed to design L2 and set positions such that the entire lens array was best form. The end result was a focal spot of "25 yum at 4.13 m. from the film (geometric raytrace). The reference wavefront after the fisheye has negligible wavefront aberration. However strict tolerances are placed on the manufacturer of the lens L2, with the demand that the radii of curvature of the lens be within "0.1Í of the nominal values.

2.3 The Correct Wavelength.

The necessity for the correct wavelength arises from two problems. Firstly , illuminating a hologram of the sort that we are making with the wrong wavelength will introduce both astigmatism and some spherical aberration into the images (ref 4). We can partially correct for this by tilting the film with respect to the reference beam by a different angle for every point in the chamber, but this goes against the design philosophy of having the film static. Secondly, we also introduce distortion due to the fisheye, since it will not act as a phase-conjugator for wavelengths far from 694.3 nm. , the wavelength used to make the holograms.

The simple solution to this is to use a CW dye laser, tuned to 694.3 nm. The linewidth of this laser should be narrow so as to suppress chromatic blurring of the image.

Finally raytracing was again used to determine the tolerances on the positioning of all elements. The combined effects showed that errors of around 10 ymm in positioning any pre-fisheye element introduced "15 /mm into delta for a mid-plane image. We therefore set the positioning tolerance at 25 um. to comply with manufacturing constraints.

3.0 THE SUB-SYSTEMS COMPRISING HOLRED.

A sketch of the machine is given below, and figs. 7,8 show the overall engineering drawings for the assembly which is mounted on a solid concrete base .itself isolated from the floor by Celulon elastomer slabs. Holred is composed of the following sub-assemblies : — rV L•ai e y

L l^L-Pfttk

vid.'<-cor»

1 O r*\ rv\ •for fY>aw»^if>a.r\eg

70 mn\ Cabera «"e-frac ti. ve.

Co«-re cto<

_.U^L.Jkt.^ L

fig 6 of the HOLRED layout.

3.1 The Laser System.

This comprises a Spectra Physics 375B dye laser using DCM dye dissolved in propylene glycol carbonate, and equipped with an ultra fine étalon. It is pumped by a Spectra Physics 165 argon ion laser ( 4 W all lines).The combination gives "300 mW with a linewidth of ~7GHz. This has proved to be sufficient power for reprojecting the test holograms that we have from Fermilab.

3.2 Beam Shaping Optics,

Here we have an 800 mm. achromat used with a custom designed meniscus lens.

91

3.3 Film Transport And Fisheye With Corrector

The film transport has both coarse and fine positioning controls, to enable accurate alignment of the film with respect to the reference beam to within the previously stated tolerances. This positioning is done relative to 10^m fiducial crosses placed on the film when the hologram is recorded. These crosses are projected onto a self- luminous crosshair of 10 yum width. Both are then transferred onto a vidicon ,for monitor display, by a Selfoc lens. The beam shaping optics and fisheye with the thick outer fisheye corrector lens are all mounted on the same Micro-Controle assembly.

3.1* The Y-mirror

This mirror tracks above the fisheye assembly in such a way as always to project out the vector from the film centre to the vertex being studied at the same height, parallel to the base of the machine onto the observation vidicon.

3.5 The X-Z Stage And .

The observation vidicon and 70 mm. camera are mounted onto a stage that can move in X and Z directions (as defined in the diagram) on a system of rails. This stage can rotate so that when the desired vertex image has been optimised using the vidicon, the 70 mm. camera will be swung around and the image recorded. Data box information is displayed numerically by an LED array and projected optically onto the film after the reconstructed event has been photgraphed. The same holds for four image plane reference fiducials also placed onto the film. 4.0 THE OPERATION PROCEDURE .

1. Use the film transport to align the desired frame to the reference beam.

2. Take coordinates of the vertex of interest from previous scan of the conventional film and transform them into HOLRED space.

3. The computer drives the Y-mirror and X-Z stage to the calculated point ( including possibly some predetermined offset from preliminary manual alignment using test holograms).

4. The operator is given control of the stages to optimise the image on the vidicon and a photograph is taken for each region of interest.

The machine will be sited at the Rutherford Appleton Laboratory in England and is planned to be on-line by the end of April 1985.

Referenoea

1. H. Bjelkhagen et al., Nucí. Instr. and Meth. 220 (1984) 300; and these proceedings.

2. K. Geissler, HOLMES — A Holographic Film Measuring Machine; these proceedings.

3. H. Bjelkhagen et al., Holographic Recording of Cosmic Ray Tracks in BEBC; to be published in Nucl. Instr. and Meth. ; and these proceedings.

4. R. W. Meier, Magnification and Third-order Aberrations in Holography; j. opt. soc.amer. 57 51 1967.

5. R. Glanville et al. Factors Affecting the Quality of Holographic Image Construction and Measurements; CEGB, Marchwood Engineering Laboratories, internal report.

6. M. Peters, University of Hawaii internal report; and these proceedings.