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A Telecentric Camera Lens for BubbleChamber PhotographyK.H. Carnell a , N.C. Gortmans a , W.T. Welford a & C. Pataky ba Physics Department, Imperial College, London, S.W.7b Optical Surfaces Ltd., Godstone Road, Kenley, SurreyPublished online: 11 Nov 2010.

To cite this article: K.H. Carnell , N.C. Gortmans , W.T. Welford & C. Pataky (1968) A TelecentricCamera Lens for Bubble Chamber Photography, Optica Acta: International Journal of Optics, 15:2,187-193

To link to this article: http://dx.doi.org/10.1080/713818066

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OPTICA ACTA, 1968, VOL. 15, NO. 2, 187--193

A telecentric camera lens for bubble chamber photography

K. H. C A R N E L L , N. C. G O R T M A N S and W. T . W E L F O R D

Physics Depar tmen t , Imper ia l College, London, S.W.7

C. P A T A K Y

Optical Surfaces Ltd. , Godstone Road, Kenley, Sur rey

(Received 7 .~une 1967)

Abstract. A 127 mm, F/25, 54 ° telecentric objective has been designed and made for use with the 80 cm liquid helium bubble chamber of the Rutherford High Energy Laboratory.

1. Introduction Bubble chamber pho tography involves digitized coordinate measurements

on the negatives to + 2/xm at present and there is a likelihood of even smaller least counts in the future. T h e photographs are taken on film rather than plates for reasons of operat ing speed and convenience of handling. T h e repeti t ion rate is high, about one picture every 2 sec at present and probab ly one per 100 msec in the near future. T h e lenses have to cover a wide field of view, f rom +30 ° to + 7 0 ° in some chambers now being designed. I t can be seen that

serious distortion errors could occur if the film were not held flat enough in a conventional camera a r rangement , since an error in the film plane produces a lateral displacement in the point at which the principal ray meets it ; to illustrate this, figure 1 shows the geomet ry of the principal ray relative to the film.

' ~ - - about lOOj~rn film base ~ - - ~

vacuum platen Figure 1. Geometry of the principal ray in relation to the film ; an error h in flatness of

the film would produce an image displacement h tan ft.

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188 K . H . Carnell et al.

For these reasons one of the present authors (W.T.W.) suggested [1, 2] that the camera lens should have a telecentric aperture stop, i.e. the entrance pupil should be at the object side principal focus ; this would ensure that the principal rays in image space were all parallel to the axis, in Gaussian approximation.

In the present paper we describe the design, construction and testing of a lens of this kind which is to be used with a 32 in. helium bubble chamber recently commissioned at the Rutherford High Energy Laboratory.

2. Optical design The lens was required to cover a semi-field angle of 27 ° to photograph an

area 80 cm by 40 cm on 35 mm unperforated film, the lens axis being at one corner of this area, and to be in good focus over distances from about 140 cm to 180 cm. Thus 127 mm focal length, F/32 was required. Reasonable distortion correction and chromatic correction over a short wavelength range centred on 4800 3, were required. The general reasoning behind the design can then be explained as follows, by reference to figure 2.

aperture stop ~ > image

principal ray j [/

(a)

ghost surface

meniscus concentric with stop

(b)

r l ~ m 2~ - -

(cl Figure 2. Stages in evolving a telecentric lens design.

If we start with a thin lens with a stop at the object-side principal focus (figure 3 (a)) we obtain telecentricity. There is heavy barrel distortion which cannot be corrected by bending the lens ; however, a strongly over-correcting aspheric figuring provides distortion correction. The next stages are to put in a ghost refracting surface to correct transverse chromatic aberration and to add a meniscus which is concentric with the stop in order to flatten the field, as in

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A telecentric camera lens for bubble chamber photography 189

figure 2 (b) ; the meniscus pushes the image plane fu r the r back wi thou t chang ing the equiva len t focal length, since in Gauss ian approx ima t ion it is equivalent to a th in negative lens at the s top. Final ly a th in negative double t at the s top is used to cor rec t as t igmat i sm and some slight residuals of coma, spherical aberra t ion and longi tudinal ch romat i c aberra t ion, as in f igure 2 (c) ; this changes the field curva tu re cor rec t ion slightly, so tha t a modif ica t ion to the meniscus is needed.

T h e final design, wh ich includes two ghost surfaces in the main e lement , is shown in f igure 3 and the curva tures , etc., are given in the table.

extreme principal roy image I~ane

~" The aspheric has the equation z = --0.5319 x 10-2y2--1.1 × 10-By 4, where y and z are radial and axial coordinates.

N.B. - -The design was carried out for an object plane at a distance of 1770 mm from the stop ; there was included in this a total plane window thickness of 133 mm of BK7 and a thickness of 405 mm of liquid helium (n = 1.025).

I t can be seen tha t an al ternative app roach to the design wou ld be to replace the aspher ic ma in c o m p o n e n t by, p robab ly , three or four s teeply curved spherical componen t s . Th i s was rejected after some trials as it was felt tha t the h igher o rde r t e rms i n t r o d u c e d at the extra surfaces wou ld be difficult to correc t smooth ly . I t is also desirable tha t a lens for bubb le c h a m b e r work shou ld have ve ry stable d is tor t ion characteris t ics , i.e. the d is tor t ion func t ion shou ld no t change wi th small changes in separa t ions due to the rma l effects or mechanica l stress. T h e

d I 1-65 LaK9 r 2 - - 2 5 . 0

d a 1-65 SK16 r 3 278

d z 26.25 air r 4 -- 28.1

d 4 12.0 LaK9 r 5 --40.1

d 5 62 air r 6 plane

d 6 18 LaK9 r 7 -- 100

d 7 12-7 SF5 r s 266

d 8 35 LaK9 r 9 Aspherict

stop plane 4 mm. dia. stop

204

Figure 3. The final design ; the image plane indicated is for magnification --0-0742.

Specification of the telecentric lens (dimensions in millimetres, glass types according to Schott catalogue)

r 1 Plane (Stop plane)

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190 K . H . Carnell et al.

design evolved is ideal in this respect since almost all the main refractive power is concentrated at one surface, the aspheric. The re remains, of course, the problem of manufacturing this aspheric!

Some aberration curves are shown in the figures which follow. Th e transverse colour is difficult to correct in a simple system of this kind on account of the extreme asymmetry about the stop, but in fact it is desirable to reduce the colour range in order to make the antireflection coatings on the chamber windows more efficient, so the colour correction as indicated in figure 4 is adequate. A square top filter transmitt ing 130 ~ width at half maximum (Type B-2 of Baird Associates) is used. Figures 5, 6 and 7 show respectively distortion, departure f rom telecentricity and astigmatism ; the correction for astigmatism is within the Rayleigh limit and coma and spherical aberration are negligible at the small aperture to be used.

Figure 4.

7 0 -

field 27 ° angle 60 -

E 4O -g

g 2O ~

10

I I -20 0 20

Transverse chromatic aberration m/~m for 130 ~ bandwidth centred on 4861 ~.

7O

field 27o--,- - angle 6 0 -

5 0 -

4 0 -

••db•s rrel tortion

E

3 0 - .m ¢0 ¢-

2 0 - ~' O')

E 1 0 -

I i ~ I, . 0 200 400

Figure 5. Distortion in #m ; the abscissa is displacement of the image from the ideal position.

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A telecentric camera lens for bubble chamber photography 191

Figure 6.

7O

60 ̧

E 50- E

--~ 40 ._~ ¢-

. :30 O

E - 2 0

,o/ I I

-1 0 1 Departure from the telecentric condition, degrees.

J; 0

tangenti~ V..50 E s a g i t t a ~

°

o

I I I I -2-1 0 1

Figure 7. Astigmatism in mm.

3 . M a n u f a c t u r e a n d t e s t i n g

The aspheric surface presents some manufactur ing problems : the paraxia radius of curvature is 94 mm and a spherical surface touching at centre and edge has a radius 104.4 mm, the greatest gap between the aspheric and this latter sphere being 0.6 ram. This is a heavy figuring for such a small diameter and a complicating circumstance is that the aspheric is remote f rom the aperture stop, its main functions being correction of distortion and astigmatism ; more usually an aspheric would be at or near to the stop, as in, for example, the Schmidt camera, and the figuring could be checked by a null test for absence of spherical aberration, but in the present case no null test is available f rom the actual mode of working of the aspheric.

I t is always desirable to figure to a null test and a procedure was devised by W. E. James and M. D. Waterwor th to enable this to be done [3]. By numerical trials an auxiliary component was computed which could be put in contact with

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192 K . H . Carnell et al.

0

o~

~ '~ .

~ 8

o~

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A telecentric camera lens for bubble chamber photography 193

the aspheric componen t to produce a system corrected for spherical aberrat ion at two real conjugates in mercu ry green light, as in figure 8 ; the concave outer surface of the auxiliary componen t is a normal incidence surface.

T h e residual angular spherical aberrat ion of the sys tem in figure 8 does not exceed 0.1 mrad at any part of the aperture for the correct shape of the aspheric. I t was thus permissible to figure the aspheric to reduce the spherical aberrat ion exactly to zero, the result being only a slight change in the computed distortion curve,

~ - - ~ i"--- concave mirror

COmpensato r

am splitter

pinhole i l l um ina ted w i th monochromatic light

component 9 (PSK3 test piece)

~ n ~ ( ~ h ! ! aluminized

component 8

Figure 9. Modified Twyman and Green interferometer.

T h e system of figure 8 was eventually used for figuring by incorporat ing it in a modified T w y m a n - G r e e n interferometer , as in figure 9 ; the normal incidence surface was aluminized and a spherical mir ror of suitable curvature was used in the compar ison beam.

T h e final system, of which the components were made by Optical Surfaces Ltd . , shows image quality confirming the ray trace results.

ACKNOWLEDGMENT This work is suppor ted by a grant f rom the Science Research Council .

Un objectif t616centrique de 127 mm de distance focale, ouvert ~ F/25, et ayant une ouverture angulaire totale de 54 ° a 6t6 calcuM et construit. I1 est destin6 ~ ~tre utilis6 avec la chambre h bulles h h61ium liquide de 80 cm du Rutherford High Energy Laboratory.

In dem Rutherford High Energy Laboratory befindet sich eine Blasenkammer mit flfissigem Helium und yon 80 cm Gr6sse. Daftir wurde ein bildseitig telezentrisches Objektiv berechnet und ausgeftLhrt, dessen Brennweite 127 mm bei einem Offnungsver- h~ltnis 1 : 25 betrtigt und das ein Feld von 54 ° auszeichnet.

REFERENCES [1 ] WELFORD, W. T., 1962, Proceedings of 1961 Conference on Optical Instruments (London :

Chapman & Hall). ~2] WELFORD, W. T., 1963, Appl. Optics, 2, 981. [3] JAMES, W. E., and WATERWORTH, M. D., 1965, Optica Acta, 12, 223.

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