JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Monochromatic Camera for Photography in the Far UltravioletW. E. BEHRING, J. M. JACKSON, S. C. MILLER, JR., AND WILLIAM A. RENSE*

University of Colorado, Boulder, Colorado(Received August 24, 1953)

An ultraviolet monochromatic camera has been constructed which avoids the use of quartz or lithiumfluoride optics. The camera makes use of two gratings mounted in the Wadsworth-type arrangement in sucha way that the dispersions neutralize. The instrument is, thus, a double monochromator, a feature helpfulin reducing stray light. Three forms of the camera have been developed, differing only in the manner inwhich the focus of the final image is achieved.

The image is not purely monochromatic but contains a narrow range of wavelengths determined mostlyby grating constants and slit widths. A description of the chromatic composition of the image is given andpossible applications, including the study of solar limb darkening in Lyman-alpha radiation, are discussed.

I. INTRODUCTION

IN connection with the problem of photographing thesun's disk in the far ultraviolet (around 1216A)

from rockets sent above the ozone layer of the atmos-phere, a monochromatic camera has been designed thatavoids the need for filters or for lithium fluoride optics.The camera makes use of gratings mounted in theWadsworth-type arrangement. Three forms of themonochromator have been studied. These are shownin Fig. 1. In A, light from a distant source strikes aconcave spherical grating G1, which disperses the lightand directs it to the Wadsworth focal surface where aslit S, located on the normal to the grating, permitslight in a limited wavelength band to pass on to thesecond concave grating G2, whose constants are identicalwith those of the first grating. The slit acts as a sourcefor the second grating, and the latter disperses the lightagain, but if the symmetry of the arrangement in Fig.1A is maintained, the dispersion is opposite to thatof GI, so that when the emerging light is focused bythe concave mirror M, a true image of the object isformed. For a given angle of incidence this final imagehas a chromatic composition that is determined by theangular diameter of the object, the constants of thegratings, and the width of the slit. The chromaticcomposition will be treated in detail elsewhere.

In Fig. B the optical setup for the second form ofmonochromator is shown. Light strikes a sphericalconcave grating G1, where it is dispersed, then passesthrough a slit to grating G4, whose lines are ruled on aconcave ellipsoidal surface. The latter differs in itsaction from G2 (Fig. 1A) in that it not only neutralizesthe dispersion of G1, but effects its own focusing.

In Fig. IC is shown a third form of the mono-chromator. Here, the light strikes a plane grating G3,is dispersed, and is in part intercepted by a concavespherical grating G2, which not only neutralizes thedispersion of the first grating, but forms an image inaccordance with a Wadsworth-type arrangement. The

* The research reported in this paper has been sponsored by theGeophysics Research Directorate of the Air Force CambridgeResearch Center, Air Research Development Command underContract AF19(609)-631.

final image resembles that of the first form of mono-chromator, but because there is no slit, the wavelengthrange in the image must be determined by the gratingsizes. On the other hand, no third element, such as amirror, is required for focusing purposes. An importantadvantage of the above types of grating monochromatorover other types employing only one grating lies in thereduced scattering that results from the double mono-chromatic feature. This advantage is especially im-portant in applications to solar ultraviolet spectroscopy.

II. ABERRATIONS AND OPTICAL ADJUSTMENT

The theory of the aberrations and optical adjustmentof the monochromatic cameras sketched in Fig. 1 has

FIG. 1. Sketches of three forms of solar monochromatic camera.In each case the gratings are used in the Wadsworth-type arrange-ment, and the second grating neutralized the dispersion of thefirst grating. In A, a concave mirror is necessary to obtain focusof the final image, but in B and C focus is achieved directly fromthe second gratings. All gratings are spherical, concave, exceptG3 in C, which is plane, and G4 in B, which is ruled on an ellipsoidalconcave surface.

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VOLUME 44, NUMBER 3 MARCH, 1954

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BEHRING, JACKSON, MILLER, AND RENSE

WAVE LENGTH * FIG. 2. Coordinatesused in studying chro-matic composition ofimage. The coordinatex is measured in radians.The narrow strip con-sists of light of a range ofwavelengths determinedby grating constants andwidth of slit.

been completely developed by one of the authors.'The method of Beutler2 was employed because of itsadvantages in dealing with aberrations such as astigma-tism and coma. A characteristic function, correspondingto that of Beutler's for a single grating, was derivedfor each system. This function, in the form of a seriesalong with Fermat's principle, led to the calculation ofthe essential optical properties. For a given resolvingpower the instrument constants and allowable adjust-ment errors could be determined from the final equa-tions. For examplet image quality for combination Ain Fig. 1 is tolerable for apertures up to 3 cm.

The quality of the image, when in good focus, is notchanged appreciably for grating translations of aboutone millimeter. The most critical adjustment is therotation of a grating about an axis perpendicular to itssurface so that the lines are parallel to the planethrough the slit and the center of the grating. Here0.70 error is tolerable. The grating constants mustmatch to within 130 lines/cm.

III. CHROMATIC ASPECTS OF MONOCHROMATORIMAGES

The appearance of the image obtained with any ofthe cameras of Fig. 1 is the same as that of an ordinarycamera employing a color filter except in one importantrespect. Images cast by the former are composed ofnearly monochromatic strips parallel to the gratinglines, with the mean strip wavelengths changingcontinuously through a restricted range as one considerssuccessive strips across the object.

Figure 2 illustrates the nature of the image (of acircular object emitting white light) formed by themonochromator in Fig. 1A. For purposes of describingthe chromatic makeup of the image it is convenient topretend that the image is divided into infinitely narrowstrips parallel to the lines of the grating, the coordinateof any one of which is designated by x along an axisperpendicular to the strips through the center of theimage. The numerical value of x is taken to be the angle

I Stanley Miller, Special Report No. 2, Upper Air Laboratory,University of Colorado.

2 H. G. Beutler, J. Opt. Soc. Am. 35, 311 (1945).t The errors discussed here are for 100-cm gratings of 30 000

lines/inch, and for an angular resolution of 10-4 radian on thefirst-order solar image at 1216A.

in radians between the center line to the image and thelight beam which is imaged at the middle of the stripin question. R is the radius of the grating, a the gratingconstant, 7 the order, w the slit width, a the angle ofincidence of the center of the object on the first grating,and a the maximum angular diameter of the object ina direction perpendicular to the grating rulings.

Neglecting aberrations, one can readily compute thewavelength makeup of the image. To describe the image,the following notations are used:

AX=Range of wavelengths passed by slit for eachinfinitesimally narrow object strip ultimately focused asstrip at x in image.

X=Mean wavelength of light forming image of stripat x in image.

AX=Range of mean wavelengths ('s) of the imagestrips across the entire image.

AXT=The total range in wavelengths present in theentire image (a measure of the degree of monochroma-ticity of the image).

Applying the grating equation one obtains, neglectinghigher order terms:

4 AX= 2ow/nR

u = r sin (a+x)/nA = a/n

AXT= AX+AX.

A photograph of the sun taken with the equivalentof the setup in Fig. 1A is shown in Fig. 3. Sunspots arevisible, as well as many Fraunhofer lines. The G band(due to CH) at X4310A runs through the center of theimage. The presence of the Fraunhofer lines reflects thefact that the image is constituted of strips parallel to

FIG. 3. A photograph of the sun in visible light near 4310A(the G band). The total wavelength range on the image is about151A. Fraunhofer lines and sunspots are both apparent, illustratingthe peculiar nature of the camera image.

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MONOCHROMATIC CAMERA

the grating lines with differing mean wavelengths.For this photograph, /vX= 1A, X for the central strip=4310A, AX= 150A, and AXT= 151A.

IV. APPLICATIONS OF MONOCHROMATOR

Because of the rather unusual chromatic character ofthe images formed by the monochromatic camera,applications of the instrument are more restricted thanwould otherwise be the case. The monochromaticcamera was developed with the express purpose ofphotographing the sun in the light of the Lyman-alphaline from rockets sent to high altitudes. At altitudesover 80 km the intensity of Lyman-alpha radiation'-'is of the order of 0.5 erg/cm 2 /sec. With a camerasimilar to B in Fig. 1, and for S.W.R. or 103-0-uvsensitized film, it is estimated that an exposure of afew seconds is required to photograph the sun in thelight of Lyman-alpha. This estimate is made on thebasis of measurements of grating reflection factors for1216A radiation striking at nearly normal incidence.Scattered visible light would present difficulties butcalculations and actual tests show the amount reachingthe final image to be small enough (about 20 percentof Lyman-alpha radiation) to permit acceptable ultra-violet photography.

To study limb darkening for the wavelength corre-sponding to the mean wavelength of the central strip,

3 Tousey, Watanabe, and Purcell, Phys. Rev. 83, 792 (1951).4 Byram, Chubb, Friedman, and Litchman, J. Opt. Soc. Am.

42, 876 (1952).5 Wm. A. Rense, Phys. Rev. 91, 299, (1953).

ti .8

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IW .5.

COSE ABBOT, CONTINUUM

.3 MONOCHROMATOR, -BAN g

.2-

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0 .1 .2 .3 A .5 A .T .8 .9 LOCOSINE G

FIG. 4. Limb-darkening study made from photograph of Fig. 3,illustrating one application of monochromatic camera.

one can make a microphotometric tracing along thecentral strip. An analysis of such a tracing for theoriginal of Fig. 3 yields the curve of Fig. 4. This curveindicates the variation of intensity for the G-bandabsorption region with the cosine of , the angulardistance from sun's limb (as measured from the centerof a spherical sun). Abbot's values for the solar con-tinuum in the same region are shown for comparison.Methods more accurate than the above are of courseavailable for limb-darkening study in the visible, butwhen extreme ultraviolet images of the sun are to beanalyzed, such as those from photographs taken inrockets, the monochromatic camera is especially useful.

The writers are greatly indebted to Y. Ohman forhis many suggestions and ideas. The help of W. B.Pietenpol, 0. Myers, J. M. Hood, L. Shaw, and R. A.Nidey is acknowledged with pleasure.

231March 1954