design of cassegrain light shields

Design of Cassegrain Light Shields

Andrew T. Young

A Fortran program has been written for the optimum design of light shields for Cassegrain optical systemswith finite field. The techniques employed are described, and results are shown.

IntroductionIn a simple C(assegrain telescope, skylight can be seen

around the secondary mirror from a point in the focalplane. This lowers image contrast and creates diffi-culties if the telescope is to be used photometrically orfor photography of low contrast targets. The usual sol-ution is to insert a tubular shield in front of the primarymirror, so that all the skylight is occulted, but all thelight collected by the primary mirror reaches the focalpoint [Fig. 1(a)].

However, wh en an appreciable useful field is required,the problem becomes more complicated. In order notto vignette the field, the shield tube must be larger indiameter than for the on-axis case. Then, in order toprevent direct skylight from reaching the focal plane, anadditional shield must be added around the secondarymirror. This enlarges the shadow cast by the secon-dary and thus decreases the effective light-collectingarea of the system, but it is the price that must be paidto have a fully shielded, unvignetted field. Fortunately,the additional loss is usually only a few percent.

In order to minimize the light loss, we require that(a) the tubular rear shield extend as far forward as pos-sible, so that the line of sight from any part of the fieldremains near the edge of the secondary mirror (therebyreducing the size of the secondary shield and hence theshadowing); and (b) the secondary shield should extendas far backward as possible, since the further it extendstoward the vertex of the cone of vision passing from thefocal plane through the front end of the rear shield, thesmaller will be the diameter of the front shield and hencethe shadowing.

The limit on condition (a) is set by the intersection ofthe cone of rays converging to the Cassegrain focus withthe inside surface of the hollow cone of rays reflectedfrom the primary mirror. Condition (b) can be met bymaking the front shield conical and extending it back

The author is in the Astronomy Department, University ofTexas, Austin, Texas 78712.

Received 19 Janauary 1967.

along the outer surface of the cone of rays reaching thesecondary mirror from the primary, as in Fig. 1 (b). Al-ternatively, one can use an annular hat-brim shieldaround the secondary [Fig. 1 (c) ]. This is much simplermechanically but increases the shadowing somewhat.If only a small field is required, the secondary mirrorcell may provide enough extra shadowing to serve inthis way.

As we extend the conical front shield back toward theprimary mirror, the size of its shadow, and hence the in-ner surface of the hollow cone of rays reflected from theprimary, increases. This allows the rear shield to ex-tend further forward, which changes the size we requirefor the front shield.

There seems to be no analytic solution for this prob-lem, but a graphical technique has been described.'For the reasons given in the previous paragraph, thegraphical method is a trial-and-error procedure. Thegraphical method has the disadvantage of being tediousand is quite susceptible to error, both because the lay-out quickly becomes cluttered with lines, and becausethe ends of the shields are determined by lines that gen-erally intersect at rather small angles. In fact, themethod described in Ref. 1 becomes indeterminate as thefield angle approaches zero.

The problem of proper light-shield design is becomingmore important because of the increasing use of wide-field (Ritchey-Chretidn) Cassegrain systems. For ex-ample, the Ritchey-Chreti6n focus of the NASA/Mc-Donald 107-in. (2.72-m) telescope calls for an 18-in.(46-cm) field. A numerical technique has thereforebeen developed to solve this problem on the CDC 6600computer.

Optical DesignBecause we wish to design shield systems for existing

telescopes as well as planned systems, the program mustaccept different types of input information. For ex-ample, we cannot usually measure the Cassegrain focalratio directly, but the focal length can be determined bymeasuring the plate scale. Also, all measurementsalong the optical axis are referred to the edge of the pri-

June 1967 / Vol. 6, No. 6 / APPLIED OPTICS 1063

(a) a _ ___ ____-: - - - _

j: -- SECONDARYMIRROR

CASSEGRAIN PRI MEFOCUS FOCUS

a-

(b)

- SECONDARY-F ~~~~~~....MIRROR F

FOCUS1° CASSEGRAIN REAR SHIELD FN S

FIELD | REAR SHIELD NTEX \SHIELD

a:\

0:

shiel fo_1_fel._c)Syte s _it ______fonsied_ or1

decrbe inth.txt

________ PRIME1CASSEGRAIN R~EAR SHIELD FOCUS

FIELD FRONTSHIELD

mary mirror rather than the vertex, because the edgecan be located mechanically; in most cases the mirror isperforated and the vertex is not accessible. In practicethis does not introduce any complication into the cal-culations, which are carried out in rectangular coordi-nates in a plane including the optical (x) axis; see Fig. 2.The origin is at 0. The position of the secondary mir-ror is also specified by its edge.

The optical system is uniquely specified by the di-ameter RU and the focal length PF (or, alternatively,the focal ratio) of the primary, the linear or angular fieldrequired, and any two of the following three types ofdata: (1) information about the Cassegrain focal ratio,e.g., the effective focal length, the magnification of theCassegrain secondary, or the plate scale; (2) the locationof the Cassegrain focal plane ATIC; and (3) the vertex-to-vertex separation PS of primary and secondarymirrors.

From the given information, all the other parametersspecified in the above list are easily computed. In ad-dition, the equations of the primay rimary and hy-perbolic secondary mirrors are readily found.

The limiting ray RHD at the edge of the field is lo-cated because the angle FRD must be a, the field half-angle. Since the slope (tany = RO/OF) of the ray RFis known, the angle , = y - a of the limiting ray RDis known. The intersection H of this ray with the sec-ondary hyperbola determines the edge of the secondarymirror, hence its position and diameter. The reflectionof the limiting ray RH is along HA, whose intersection

with the primary mirror surface determines the mini-mum perforation required to pass the desired field.(Actually, since AH has a small slope, the intersectionG of AH with the y axis is used.)

Shield DesignThe method of shield design given in Ref. 1 is the fol-

lowing (see Fig. 2). We begin with a trial position ofC, which must lie on UI. The rear shield must thenterminate at B, the intersection of the extreme line ofsight AC with the extreme ray KI. The innermost rayreaching the secondary mirror is then DB, which comesfrom the point E on the primary. But the innermostilluminated point on the primary is Q, the projection ofC at the field half-angle ax. (It is not clear in Ref. 1that this projection is not parallel to the optical axis.)If the shields are correct, EBD should be the reflectionof CQ from the primary, and E should coincide with Q.In general it does not, and the distance EQ is a measureof the correction required.

Analytically, we take the trial position of C to begiven by

YC = I - YA X PS/7S,xc = xI + (c - y)/tano.

(la)(lb)

Equation (la) makes an approximate allowance for thedependence of C on field size and position and is a bet-ter first guess than taking C to coincide with I. Equa-tion (lb) imposes the condition that C lie on UI.

We start the iterative process by using the B positioncorresponding to zero field, with an approximate allow-ance for field size:

XB = XF X TS/(XF + TS) (2)YB = YI X (XB - XT)/TS - YA)

The slope of AC is

tan = (ye - A)/(XC - XA)- (3)

We take the approximate starting values

yE = YQ = yc + XC X tana, (4)

since both E and Q are found by an iterative technique,and approximate values are required at the start.

Y RIM STOP

oafield halt-angle

Fig. 2. Diagram illustrating the method used for large fields(see text).

1064 APPLIED OPTICS / Vol. 6, No. 6 / June 1967

Fig. 3. Diagram illustrating the method used for small fields(see text). Cf. Fig. 2.

The first iteration begins by finding Q accurately:

XQ yQ2/4PF + py Q (xc X tana + Ye) - Q X tanas (5)

Equation (5) is run three times to achieve good ac-curacy. This avoids truncation-error problems thatarise when an. explicit solution for Q is used. A similartechnique is used to find E.

The error (Q - E) is then used to obtain an im-proved value

Y"" = Y + Aye, (6)

where

A = (YQ - Y) (3 dY_ dyQ)-(dye dye

We readily find that

dyQidyc -1 + tana cot,

(7)

The new position of C determines a new Q, and the pro-cess repeats from Eq. (5). Generally about six fulliterations are required to reduce the change in xc to0.001 cm, which is the criterion for convergence.

In designing a hat-brim front shield [Fig. (c)], westart with the conical results and project AC to L,whose coordinates become the new starting values for C.The iterative process is repeated, but cot: is set equalto zero in order to hold xc = x.

Unfortunately the above method fails for small fields,for as a - 0, AC and KI coalesce and B becomes inde-terminate. Therefore, a second scheme is used whenYA<0.1 Y (see Fig. 3).

In this alternate scheme, we again begin by selectinga trial position for C according to Eq. (1). We thenfind Q as before. Because the ray CQ is from the edgeof the field, its reflection must pass through D. First-order lens theory gives

YD =PF tanaXD = XF

(12)

Neglect of field curvature and distortion is permissiblebecause Q is near the center of the primary mirror; first-order lens theory is good for the inner zones.

We now draw the ray QD and find its intersection Bwith KI; B should be the front edge of the rear shield.Remembering that KI is the reflection of AH about theoptical axis, we find that

XB = (XA X S1 + XQ X S2 - YA - YQ)/(Sl + 2),

YB = YQ + S2(XB - XQ),

where s is again the slope of AH and

S2 = (YD - YQ)/(XD - XQ)

is the slope of QD.

(13)

(14)

(8)

neglecting the fact that the mirror surface is not quitevertical. The other derivative is found from

dyE dy1/, dxB d(tanO)dye dzj3 d(tanO) dye

where

dy= -XF[SI X (XF - XB) + YD - YB]/(XF - X)2,

dXB

dXB

d(tano)d(tano)

dye

and

(SL X XT + YA + YG)/(si + tanO)',

- XA) - (Yc - A)CotI3]/(XC - XA)2,

S = (YH - YA)/(XH - XA)-

0.35

.30 F

(9)

I .25

(D2.203

00

co .15

-J4zo .10I-

.05

n.(10)

Of course xc must also be corrected so that C remains onU'.

A new iteration begins by recalculating tanG from Eq.(3) and finding B exactly from AC and KI:

XB = [XT X tan - (YA + YG)I/[SI + tanG], (11)YB = --YG - S X XB.

/\7 f/8 PRIMARY

\ \\

\\. . , \\ ..\\. .

I 1.5 2 2.5 3 4 5 6 7 8 9 1015 20MAGNIFICATIONF

Fig. 4. Shadowing (fraction of objective area occulted bysecondary mirror and front shield) for three different primaryfocal ratios, as a function of magnification (ratio of secondary toprimary focal length). Each curve is for a fixed linear field.Primary diameter = 100 in. (254 cm). --- 5-in. (12.7-cm) field,

rim. - 5-in. (12.7-cm) field, cone. - - zero field.


. -

5 7 10 I5 20

CASSEGRAIN FOCAL RATIO >

Since point Q marks the inner edge of the illuminatedzone of the primary mirror, it specifies the maximum al-lowable size of the central perforation. The rear shieldmay lie anywhere between the cones bounded by KBand QB; in most cases, the vertex of the inner cone liesto the left and a cylindrical shield can be used.

The front conical shield need not have the vertexsemiangle I3, but should have a vertex semiangle greaterthan a, or its shadow will be larger near the edge of thefield than at the center. However, if the field angle issmall and the front shield relatively short, it often turnsout that even a cylindrical front shield produces lessthan 1% vignetting between center and edge of the field,and is therefore an acceptable engineering compromise.One should also remember that if fins are used to sup-port the secondary mirror and/or shields, they will pro-duce increased shadowing off axis and must be includedin the over-all vignetting.*

30 40

Fig. 5. Shadowing for a 1 angular field as a function of Casse-grain focal ratio, for three different primary focal ratios. - rim.

- cone.

We draw AB and extend it past C. If E is now theprojection parallel to the Y axis of C on AB, the distanceCE = YE-Yc is an indication of the error in C, sincewe want C to lie on AB.Since XE = xc,

YE = YA + (XC - XA)(YA - B)/(XA - XB).

ResultsTo illustrate the dependence of shadowing on various

basic design parameters, a grid of models has been com-puted for primary focal ratios from f/2 to f/8, Casse-grain focal ratios from f/7 to f/30, and three or moredifferent field sizes. In every case the focal plane hasbeen assumed to lie behind the primary mirror a dis-tance equal to the radius of the primary, which is typicalfor many telescopes. The results are shown in Figs.4-6. We can draw the following conclusions-

(1) For a given linear field size, the shadowing ofeither the optimum or hat-brim shields decreases with

1.0

.9(15)

Then in order to remove the discrepancy between C andE we must increase Yc by

Ayc =(YE - y) _ BI dyc j

(16)

where (dyE/dyc) is evaluated numerically at each itera-tion.

The iterations continue until I Ayc I is sufficiently small(10-4 cm is the arbitrary cutoff used in the program).Convergence is usually reached in three or four itera-tions. Unfortunately, this second method breaks downfor large linear fields, on the order of 0.2 times the size ofthe primary mirror. In these cases the slope of KI be-comes more positive than the slope of QD, and the pro-gram converges to an intersection (B) that lies to theright of the secondary mirror. Therefore, both methodshave been retained in the final program, with the cross-over set in a region where both methods give satisfac-tory results.

Although Q is located for a parabolic primary, it is un-likely that a calculation employing a true Ritchey-Chr6tien primary, a spherical or a Dall-Kirkham pri-mary mirror would give appreciably different results fornormal focal ratios. The accuracy of the program ap-pears to be entirely satisfactory for practical purposes.

.7

2.6

X .5

-J.4

is

M.3

.2

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0LINEAR FIELD DIAMETER/APERTURE

Fig. 6. Shadowing as a function of linear field diameter for twodifferent Cassegrain systems. -- rim. - cone.

* A referee has pointed out that in some cases an additionalsolution exists. If si > tana, the internal shields can be elimi-nated by extending the telescope tube forward until the upperlimiting ray from R in Fig. 2 intersects the extension of AH. Al-though this reduces the shadowing it is possible only for smalllinear and angular fields, where shadowing is already very small.The mechanical problems of supporting a long extension tubegenerally make this solution unattractive, since even for zerofield the extension is longer than the tube by a factor of about m,the secondary magnification ratio.


0180

.70 I-

.60 -

Z .40 -

-J .304.

a4E "

.10 I

.00

/I/

/ /-

II

f/4 I

A~~~~~///

II

¼ _ _ /I

'2 I

.6w

increasing secondary magnification, and is only weaklydependent on the primary focal ratio (Fig. 4).

(2) For a given angular field size, the amount ofshadowing decreases strongly with decreasing primaryfocal ratio. There is a Cassegrain focal ratio whichminimizes the shadowing of a given primary; for ourfocal position and a 10 field, this is about f/9 for the hat-brim and f/12 for the optimum shields (Fig. 5). Thereason for the minimum is that the shadowing is largefor small magnifications (at the left of Fig. 5), because alarge secondary mirror is required; but it is also largewhen the focal length becomes long and the linear fieldsize becomes large (at the right side of Fig. 5). Theminimum shadowing for a smaller angular field should,therefore, occur at larger Cassegrain focal ratios.

(3) Figure 6 shows the shadowing as a function oflinear field size for (f32, f/7) and (f/4, f/7) systems.Calculations for an (f/4, f/15) system are similar to the(f/2, f/7) system, since the magnifications are similar.From Figs. 5 and 6 we note that hat-brim (rim) systemsare acceptable only for small linear fields and short pri-mary focal lengths. It is remarkable from Fig. 6 thatthe amount of light removed by an optimum (cone)shield increases, almost linearly with the field diameter.

For the NASA/McDonald 107-in. (2.72-m) telescope,which has anf1/4 primary, optimum shielding of the 10f/9 Ritchey-Chr6tien field shadows 25.6% of the pri-

mary mirror. If this focus were to be used only on axis,the area occulted by a minimum secondary mirror couldbe as small 11.1%. For the 10 Ritchey-Chr6tien field,the optimum (conical) front shield is preferable to arim-type shield in spite of the mechanical problems ofrigidly supporting a structure 56 in. (1.4 m) long andnearly as large in diameter, because the hat-brim shieldwould increase the total shadowing to 34%. The sim-pler shield would thus remove about 11% of the lightcollected by the optimum system. Removal of thefront shield and retraction of the rear shield will increasethe image brightness at the coud6 focus by 10% forspectroscopy, with some increase in background light.

A rim-type shield has been designed and built to pro-vide a 3-in. (7.6 cm) photometric field with the (f/3.5,f/13.6) 36-in. (92-cm) telescope at McDonald Obser-vatory. Here the small field size permits the use of asimple front shield with less than 2% light loss com-pared to an optimum system.

Copies of the Fortran source deck may be obtained bysending five dollars to cover handling costs to the As-tronomy Department, University of Texas, Austin,Texas 78712. Checks should be made payable to theUniversity of Texas.

Reference1. F. Sauer, Sterne Weltraum 4, 141 (1966).

Books continued from page 1062

In spite of the regrettable delay in the availability of the work,the articles in both the organic and inorganic materials do consti-tute a good review of the field. The effect of the delay is also mini-mized by the fact that the study of luminescence has not undergoneany discontinuously sharp forward strides in recent years.

As one might expect, there is considerable concentration on thezinc sulfide phosphors with emphasis on the usual manganese,copper and cobalt activators. In the organic phosphor papersconsiderable emphasis has been placed on the effects of solventsand on solid solutions. There does not seem to be any abovenormal amount of overlap between papers presented here andthose found in other journals dealing with the subject. Theauthors have made generous references to all the world literaturein this field, which results in this book being an excellent startingpoint for the reader of luminescence literature.

While recommending the OSA publication to those interestedin luminescence, it should be pointed out that the papers whichare included are at the research level and the book does notconstitute an introduction to the subject.

GoRDoN E. GRoss

Molecular Spevtroscopy: Supplement 2 to Optics and Spectro-scopy. C. W. GARLAND, scientific editor for W INC translation.Optical Society o2 America, Washington, D.C. 1966. 180 pp.$15.00.

A group of sixty-three papers was issued early in 1963, inRussian, as a supplement to Optika i Spektroskopiya, in order toexpedite publication. It is gratifying to have this Englishtranslation available at last.

The papers reflect the wide variety of interests of Russian spec-troscopists, but show rather less emphasis on solid state studiesthan is to be found in the current pages of Optics and Spectroscopy.The majority of articles deal with aspects of ir and Raman

spectroscopy. Analyses of the vibrational spectra are given fornitromethane and nitroethane, polyvinyl chloride and bromide,cyclohexane, toluene, isopropylbenzene, organogermanium ha-lides, and certain porphyrins. Normal coordinate calculationsare also attempted for a number of these molecules. Hydrogenbonding, a topic as popular in Russia as it is in America, isstudied spectroscopically for the following systems: carboxylicacid dimers, methanol and its solutions with other organic sol-vents, water and dissolved hydrogen, ether solutions, benzene-chloroform mixtures, esters and trifluoroacetic acid, crystallineamino acid anhydrides, phthalimides, and isobutyric acid inaromatic solvents. There are also papers on the solid statespectra of hydrated ferric oxides, strontium and barium nitrates,talc and other silicates, and alums. Interspersed with thesearticles are others on diverse topics, such as the spectra of liquidcrystals, second-order Raman spectra, line widths, and correctionsfor scattered light in ir spectroscopy.

L. A. Gribov, E. M4. Popov and their coworkers have con-tributed several articles on the determination of electroopticalparameters of organic molecules. A lot of this type of work hasbeen published in Optics and Spectroscopy, but it has not receivedthe attention which it merits in the Western world (althoughGribov's recently translated book should help to remedy thesituation). The use of the term electrooptical may be partlyto blame for this, since it is normally associated with Kerr andStark effects, whereas here it refers to the derivatives of the dipolemoment of a molecule with respect to the normal coordinates forvibration. These derivatives are parameters that determine theintensities and polarizations of ir bands. Their observed varia-tions within a group of chemically related molecules can give use-ful information about changes in electron density and covalentcharacter in the various bonds, and Gribov's approach providesa means of extracting this information. Experimental spectro-scopists too often concentrate upon analyzing shifts of a fewwavenumbers in band positions while ignoring the equally valu-able conclusions that can be drawn from the corresponding varia-


tions in band intensities. Among the other theoretical papersare several about Raman spectroscopy, including one by L. N.Ovander on the cause of possible asymmetries in the scatteringtensor, a topic which has generated some subsequent argumentin Russian literature.

My only criticism of this Supplement concerns the long delaybefore it has appeared in English translation. The papers in itwere originally submitted in 1961 and 1962 and have been over-taken by the march of events in some places. For example,the B' - X electronic emission spectrum of SiCl described byOveharenko, Kuzyakov, and Tatevskii has, in the meantime, beenanalyzed independently by R. D. Verma [Can. J. Phys. 42,2345 (1964)], under much higher resolution. On the other hand,an analysis that V. L. Broude gives for the 3200-A absorptionsystem of crystalline naphthalene is complementary to that ofCraig, Lyons, and Walsh [Mol. Phys. 4, 97 (1961)], and its transla-tion at an earlier date would have been welcome. This paper isparticularly interesting in view of current alternative theoriesfor the Davydov splittings and polarization ratios which areobserved in the spectrum of naphthalene.

The quality of translation is uniformly high; only an occa-sional lapse occurs when rendering technical terms into English.For example, "character" should be "manner" on page 3 of thetranslation; "forms" should be "modes" on page 14; and "rare-faction" means "separation" in a footnote on p. 27. However,such minor errors do not impede communication of the fullsense of each article. The Supplement contains a wealth ofspectroscopic material, both experimental and theoretical, andis to be recommended to students and research workers in allbranches of the subject. It should certainly also be in allscientific libraries.

GERALD W. KING

Molecular Spectra and Molecular Structure. Vol. III:Electronic Spectra and Electronic Structure of Polyatomic Mole-cules. By GERHARD HERZBERG. Van Nostrand Company,Inc., Princeton, 1966. 750 pp. $20.00.

The publication of the third volume of MOLECULAR SPEC-TRA AND MOLECULAR STRUCTURE is an event which haslong been awaited with great anticipation by the spectroscopicworld. Rarely has a scientific topic centered so much around a sin-gle person who is at one and the same time its chief chronicler,coordinator and leading exponent. Every spectroscopist is enor-mously in his debt for the way he has shaped the development ofthe subject and for the sound guidance which all have derivedfrom the earlier volumes of this work.

Developments in vacuum uv spectroscopy, the introductionof flash photolysis methods for the study of free radicals, and theimprovement of photoionization and mass spectrometric tech-niques have in the last few decades greatly added to the abilityof the experimentalist to obtain precise information about theelectronic structure of molecules. There has been a correspond-ing growth in the theoretical background of the subject which pre-sents to quantum mechanics the challenge of explaining the be-havior of complicated systems. The reward of these studies is adeeper understanding of molecular binding than that which canbe afforded by a knowledge of interatomic distances, force con-stants, etc., which are composite properties resulting from thecontributions from electrons in many different orbitals. Thenecessarypreliminaries to the elucidation of these topics are clearlyan understanding of the electronic spectra of diatomic moleculesand the rotation-vibration spectra of polyatomic molecules, andthus volume III is the natural development of volumes I and II.It sets out the theory of the electronic structure of polyatomicmolecules and illustrates it with a multitude of experimental ob-

servations in the characteristic fashion of the earlier volumes.Basic types of molecules up to those containing twelve atoms aredealt with, and the important papers on each have been sum-marized and brought into a coherent framework.

The book is divided into five chapters with the headings:I Electronic states, II Electronic transitions, III Buildingup principles; electronic configurations and stability of elec-tronic states, IV Dissociation, predissociation and recombina-tion; continuous and diffuse spectra, V Electronic spectra ofindividual molecules and their interpretation. These are fol-lowed by seven appendices which contain tables of symmetrytypes and characters of the more important point groups of spinfunctions, direct products, and resolution of species on going tolower symmetry. Also included are extensive tables of molec-ular constants of most of the small polyatomic molecules forwhich absorption or emission spectra have been analysed.

Truly this will be the vade mecum of all who wish to explore therealm of molecular structure. Spectroscopists owe a great debtof gratitude to its author for the stupendous effort, meticulouscare, and mature judgment which he has brought to his task.

W. C. PRICE

Multilingual Dictionary of Important Terms in MolecularSpectroscopy. By the Commission on Molecular Structure andSpectroscopy, IUPAC. Obtainable by writing Dr. R. NormanJones, National Research Council of Canada, Ottawa.

"And the whole earth was of one language, and of one speech".Whether there were among those builders of Babel who spoiledcommunication among the descendants of Noah, saying one toanother, "Go to, let us build us a city and a tower, whose topmay reach unto heaven"-whether there were among themspectroscopists, Moses saith not; but the appearance of thepresent volume gives support to the hypothesis. Certainly everyspectroscopist who has discovered only after much misreadingthat his French colleague's vibration de torsion is not his own"torsional vibration" will agree that this book will be of real useto any reader of the international spectroscopic literature.

It sets forth the English, French, German, Japanese (inromaji), and Russian equivalents for some 400 basic terms used inspectroscopy. And to the lexicographers, Drs. Herzberg,Lecomte, Lord, Mecke, Mizushima, and the late Dr. Terenin-how splendidly some of their names come out in the Russiancyrillic and the Japanese katakana!-spectroscopists mayproperly feel real appreciation and gratitude.

Being inexperienced in reviewing dictionaries, I am not surewhether the usual rules apply, and I am obliged to find at leastone error. I confess my disappointment to find that, althoughthose descriptive terms for rocking, wagging, twisting, and scissor-ing vibrations are given, with pictures, my own favorite, theumbrella vibration, with its picturesque Japanese equivalentochoko-shindo, is missing; I shall never know whether my hoped-for vibration en parapluie really exists. And, though I wouldclaim competence in at most one language, I have the recollectionthat "Raman spectrum" has appeared as spectre de diffusionand even spectre de combination as well as spectre Raman;

and likewise that the "self-consistent field" comes out as champself-consistent more often than as champ auto-coherent.

But there is more profit and more pleasure in browsing, which iswhat all sensible people do with a dictionary, this no less than theOxford English Dictionary. If we do this, we find among therandom pleasures some trends worth gentle notice. Where wehave a spectroscopic phrase, ancient or modern, which callsupon a common-life analogy, we do not have borrowing buttranslation: for the "boat-form" and "chair-form" we havephrases involving bateau and Sessel and funa. If we look at

continued on page 1075


design of cassegrain light shields

Documents