4
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA Solar Spectroscopy with a Cashman Cell ROBERT R. MCMATH AND ORREN C. MOHLER McMath-Hulbert Observatory, University of Michigan,Pontiac,Michigan (Received July 29, 1948) A description is given of a long-focus all-reflecting spectrometer of the Pfund type which has been de- signed and constructed at the McMath-Hulbert Observatory for initial application to the infra-red solar spectrum. The detectors employed thus far are two Cashman cells that permit the recording of the solar spectrum between 1.0,uand 3. 6 u with high resolution. The amplifier was designed and constructed by W. Wilson. Radiation from overlapping orders is effectively eliminated by a pre-dispersing unit. Two plane reflection gratings, ruled 600 lines/mm and 200 lines/mm, are used for the regions L.OA- 2 . 5 u and 2.5A-3. 6 ,u respectively. Observed line separations indicate that approximately 80 percent of the theoretical resolving power has been attained. I. INTRODUCTION APPLICATION of a Cashman lead-sulfide photo- conductive cell to the problem of solar spec- troscopy was started on an exploratory basis at the McMath-Hulbert Observatory early in May, 1947. The preliminary experiments were carried out with the McGregor tower telescope' and Littrow-type spectro- graph. 2 A record of the spectrum of the sun, extending from X13,500 to X24,000, on a scale of 1.6 mm/A had been completed with this equipment by mid-June, 1947.3 This preliminary exploration of a relatively new region of the solar spectrum showed that it was possible to secure very high dispersion "spectra" of good quality. The next step was the design and construction of a long-focus all-reflecting spectrometer of the Pfund type. Figure 1 is a schematic drawing of the optical and recording components of the new instrument. II. TOWER TELESCOPE The path followed by a beam of solar radiation is in- dicated in Fig. 1. Reflection from the coelostat and second mirror serves to direct radiation vertically down the 70-foot tower to a paraboloidal mirror, 12 inches in diameter, of 50-foot focal length, and figured 5 degrees off axis. Two subsequent reflections from plane mirrors direct an image of the sun, about 5- inches in diameter, to the entrance slit SI of the pre-dispersing unit. A McMath-Hulbert telescope drive system 4 controls the rotation of the coelostat and second mirror so that a small feature of the surface of the sun, such as a sunspot, or faculous area, can be followed with pre- cision from sunrise to sunset. An auxiliary drive pro- vides slow motion of the solar image at constant speed 'Robert R. McMath, "The McGregor building and tower telescope of the McMath-Hulbert Observatory," Nature 146, 287 (1940). 2 Orren C. Mohler, "The McGregor spectrograph of the McMath-Hulbert Observatory," J. Opt. Soc. Am. 34, 121 (1944). 3 Robert R. McMath and Orren C. Mohler, "High dispersion solar spectrum in the 10,OOOA to 20,OOOA region," Astronom. Soc. Pac. 59, 267 (1947). 4 Robert R. McMath and Walter A. Greig, "A new method of driving equatorial telescopes," Publ. Mich. Obs. 5, 123 (1934). Robert R. McMath, "The tower telescope of the McMath- Hulbert Observatory," ibid. 7, 42 (1937). across slit Si, to facilitate measurement of the dis- tribution of radiation along a diameter of the sun's disk. III. SPECTROMETER AND PRE-DISPERSING UNIT The first half of the "double monochromator," located between slits SI and S 2 in Fig. 1, is a low-dis- persion spectroscope which serves as a pre-dispersing unit. The dispersing element is a thirty-degree syn- thetic calcium fluoride prism, the remainder of the unit consisting of two plane and two paraboloidal mirrors. A spectrum on a scale of 0.0005 mm/A at X7000 is focused on the slit S2, which is the entrance slit of the main spectrometer. Any desired wave-length region can be selected to pass through S2 by positioning a cali- brated wave-length drum that rotates the second pa- raboloidal mirror. The spectrometer, situated between slits S 2 and S3, follows a design proposed by Pfund, 5 that resembles an 21 COELOSTAT AMPLI RECORDER \FLAT MIRROR SLI CP AXS PARABOLA //C CONCAVE /FLTl MIRRORS / IRRORSI \GRATING 52 1080 CYCLES PER SEC OFF-AXIS PARABOLID FIG. 1. Schematic drawing. McGregor solar tower telescope and all-mirror spectrometer of the McMath-Hulbert Observatory. 5 August H. Pfund, "An infra-red spectrometer of large aper- ture," J. Opt. Soc. Am. 14, 337 (1927). 903 NOVEMBER, 949 VOLUME: 39, NUMBER 11

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JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Solar Spectroscopy with a Cashman Cell

ROBERT R. MCMATH AND ORREN C. MOHLER

McMath-Hulbert Observatory, University of Michigan, Pontiac, Michigan(Received July 29, 1948)

A description is given of a long-focus all-reflecting spectrometer of the Pfund type which has been de-

signed and constructed at the McMath-Hulbert Observatory for initial application to the infra-red solar

spectrum. The detectors employed thus far are two Cashman cells that permit the recording of the solar

spectrum between 1.0,u and 3.6 u with high resolution. The amplifier was designed and constructed by W.

Wilson. Radiation from overlapping orders is effectively eliminated by a pre-dispersing unit. Two plane

reflection gratings, ruled 600 lines/mm and 200 lines/mm, are used for the regions L.OA-2.5 u and 2.5A-3.6 ,u

respectively. Observed line separations indicate that approximately 80 percent of the theoretical resolving

power has been attained.

I. INTRODUCTION

APPLICATION of a Cashman lead-sulfide photo-conductive cell to the problem of solar spec-

troscopy was started on an exploratory basis at theMcMath-Hulbert Observatory early in May, 1947. Thepreliminary experiments were carried out with theMcGregor tower telescope' and Littrow-type spectro-graph. 2 A record of the spectrum of the sun, extending

from X13,500 to X24,000, on a scale of 1.6 mm/A had

been completed with this equipment by mid-June,1947.3 This preliminary exploration of a relatively newregion of the solar spectrum showed that it was possible

to secure very high dispersion "spectra" of good quality.

The next step was the design and construction of along-focus all-reflecting spectrometer of the Pfund type.

Figure 1 is a schematic drawing of the optical andrecording components of the new instrument.

II. TOWER TELESCOPE

The path followed by a beam of solar radiation is in-

dicated in Fig. 1. Reflection from the coelostat andsecond mirror serves to direct radiation vertically downthe 70-foot tower to a paraboloidal mirror, 12 inches in

diameter, of 50-foot focal length, and figured 5 degrees

off axis. Two subsequent reflections from plane mirrorsdirect an image of the sun, about 5- inches in diameter,to the entrance slit SI of the pre-dispersing unit.

A McMath-Hulbert telescope drive system 4 controls

the rotation of the coelostat and second mirror so thata small feature of the surface of the sun, such as a

sunspot, or faculous area, can be followed with pre-cision from sunrise to sunset. An auxiliary drive pro-vides slow motion of the solar image at constant speed

'Robert R. McMath, "The McGregor building and towertelescope of the McMath-Hulbert Observatory," Nature 146,287 (1940).

2 Orren C. Mohler, "The McGregor spectrograph of theMcMath-Hulbert Observatory," J. Opt. Soc. Am. 34, 121 (1944).

3 Robert R. McMath and Orren C. Mohler, "High dispersionsolar spectrum in the 10,OOOA to 20,OOOA region," Astronom.Soc. Pac. 59, 267 (1947).

4 Robert R. McMath and Walter A. Greig, "A new method ofdriving equatorial telescopes," Publ. Mich. Obs. 5, 123 (1934).Robert R. McMath, "The tower telescope of the McMath-Hulbert Observatory," ibid. 7, 42 (1937).

across slit Si, to facilitate measurement of the dis-tribution of radiation along a diameter of the sun's disk.

III. SPECTROMETER AND PRE-DISPERSING UNIT

The first half of the "double monochromator,"located between slits SI and S2 in Fig. 1, is a low-dis-persion spectroscope which serves as a pre-dispersingunit. The dispersing element is a thirty-degree syn-thetic calcium fluoride prism, the remainder of the unitconsisting of two plane and two paraboloidal mirrors.A spectrum on a scale of 0.0005 mm/A at X7000 is

focused on the slit S2, which is the entrance slit of themain spectrometer. Any desired wave-length region canbe selected to pass through S2 by positioning a cali-brated wave-length drum that rotates the second pa-raboloidal mirror.

The spectrometer, situated between slits S2 and S3,

follows a design proposed by Pfund,5 that resembles an

21

COELOSTAT

AMPLI RECORDER \FLATMIRROR

SLI CP AXS

PARABOLA //C

CONCAVE /FLTlMIRRORS / IRRORSI

\GRATING 52 1080 CYCLES PER SEC OFF-AXISPARABOLID

FIG. 1. Schematic drawing. McGregor solar tower telescope andall-mirror spectrometer of the McMath-Hulbert Observatory.

5 August H. Pfund, "An infra-red spectrometer of large aper-ture," J. Opt. Soc. Am. 14, 337 (1927).

903

NOVEMBER, 949VOLUME: 39, NUMBER 11

R. R. McMATH AND 0. C. MOHLER

TABLE I.

Dispersion of tracing mm/A at X20.000Degrees/hr. Mt. Wilson Grating U. of M. Grating

11 3 0.93 6 1.8

3/10 12 3.6

earlier suggestion of Russell W. Porter' for an as-tronomical telescope. Similar optical systems have beenincorporated in recording monochromators describedby Migeotte7 and Nelson.8 The paraboloidal colli-mating and imaging mirrors have focal lengths of 277inches, and thus provide high linear dispersion at theexit slit S 3 . Two of the three slits, S 2 and S3 , were con-structed with great care to take full advantage of thepossibilities inherent in high linear dispersion. A slitof average quality is adequate for S,. All of the slitsare 18 mm in length.

IV. GRATINGS AND GRATING DRIVE

The gratings used in the spectrometer have beenloaned through the kindness of Dr. I. S. Bowen,Director of the Mount Wilson and Palomar Observa-tories, and of Dr. E. F. Barker, Director of the HarrisonM. Randall Laboratory of Physics of the University ofMichigan. The Mount Wilson grating has a ruled areaof 96X111 mm, containing 66,700 rulings spaced 600lines/mm. The Michigan grating has an area of 94X 121mm, containing 24,130 rulings, spaced 200 lines/mm.Each grating is housed in an individual box that canbe quickly installed on the grating turntable.

The wave-length band, selected from the small-scale spectrum by slit S2 , is swept across the exit slitS 3 of the spectrometer by rotation of the grating turn-table. The movement of the grating in a long-focusspectrometer must be practically perfect, for evenminute irregularities in the motion will seriously impairthe spectral resolution of the instrument. Principles ofdesign responsible for the success of the McMath-Hulbert telescope drives have been carefully followedin the construction of the drive for the grating.

Two of the most important parts of the drivingmechanism were constructed by the Ex-Cell-O Cor-poration of Detroit. These are: a specially selectedpre-loaded ball bearing spindle for the rotation axis ofthe grating; and a precision ground driving screw. Steelbelts communicate the motion of the screw and drivingnut to the grating turntable. (See Fig. 2.)

The rates of rotation provided for the grating arelisted in Table I. Changes in the rate of rotation aremade by shifting a rubber belt that connects a pair of

6 Russell W. Porter, "A new mounting for a reflecting telescope,"Pop. Astronom. 29, 249 (1921).

7Marcel V. Migeotte, "Spectrographie infra-rouge enregis-trcur," Memoires dc a Soci6t6 Royalc de Lidge 1, 1 (1945).

8 Richard C. Nelson, "Direct recording of spectra in the region1.2 ,u to 3,u using the lead sulfide photo-conductive cell," J. Opt.Soc. Am. 39, 68 (1949).

step-pulleys, one on the driving motor and one on thegrating-drive mechanism.

The turntable is driven by a single-phase syn-chronous, 1800 r.p.m., reversible electric motor andappropriate gear reduction. The motor was carefullybalanced dynamically to minimize vibration. To insureboth mechanical and thermal insulation it was mountedon the floor of the spectrograph room, some six feetbelow the optical parts, and adjacent to the isolatedpier that supports the grating and the collimating andimaging mirrors. A ply-wood cover prevents air heatedby the motor from circulating in the optical path. Thedrive shaft connecting the first worm gear reductionunit to the grating drive is so constructed that it cantransmit only rotational motion.

V. NINETY DEGREE OFF-AXIS MIRRORAND DETECTOR BOXES

After emergence from the slit S 3 (Fig. 1) radiation iscollected by a paraboloidal mirror, figured ninety de-grees off-axis. The paraboloid turns the beam throughninety degrees and simultaneously focuses it on thesensitive surface of the detector. The image of thegrating formed in this way is less than one-half milli-meter square. The mirror and electrical shielding boxfor the radiation detector are shown in Fig. 3. Adjustingscrews are provided for centering the small image onany part of the detecting surface. These adjustmentsmust be very positive, for the sensitive areas of thelead-sulfide cells are small and the spot of radiationmust remain fixed. The dimensions of the detectorboxes have been standardized to make possible quickand easy interchange of detectors.

VI. OPTICAL TESTS

All optical parts of the instrument have been testedindividually and in the combinations in which theyoccur in the instrument. The flat mirrors of the spec-

FIG. 2. Grating turntable.

904

SOLAR SPECTROSCOPY

trometer are plane to less than 500A, and the pa-raboloids have been figured with corresponding ac-curacy. In the adjustment of the spectrometer, eachof the two plane-paraboloidal combinations was focusedseparately by the Foucault knife-edge method. Withthe grating installed, an over-all focal setting wasmade, again with the knife-edge.

VII. DETECTORS AND AMPLIFIER

The instrument has been used chiefly with lead-sulfide cells especially built for this application by Dr.R. J. Cashman, of Northwestern University. Dr. Cash-man has very kindly provided cells of both uncooledand cooled types, with envelopes of glass and of fusedsilica, respectively. The silica cell is cooled by a mixtureof solid carbon dioxide and acetone. Amplification of thesignal from the cell is provided by an amplifier de-signed and constructed by W. Wilson9 to match thecharacteristics of Cashman cells. The amplifier operateswith variable band pass at a frequency of 1080 c.p.s.The beam of radiation is interrupted at this frequencyby the sectored disk and synchronous motor labelled"light chopper" in Fig. 1.

The output of the amplifier is recorded on a Leedsand Northrup Speedomax Recorder. A resistance-capacitance network matches the output of the Wilsonamplifier to the Speedomax, and simultaneously adjuststhe over-all time constant to about two seconds. Afterexperimentation with the several speeds available, apaper speed of mm/sec. was found to be most satis-factory.

VIII. MEASURES OF WAVE-LENGTH

The known wave-lengths of solar lines in the visiblespectrum provide satisfactory standards of reference for

FIG. 3. Cell holder.

IKuiper, Wilson, and Cashman, "An infra-red stellar spec-trometer," Astrophys J. 106, 246 (1947).

FLAT HINGEDMIRROR

EXIT SLIT

I 3

K RADIATIONDETECTOR

CROSS WIRES

MICROMETER SCREW

EYEPIECE

FIX ED FLAT MIRROR

FIG. 4. Device for wave-length interpolation.

wave-length measures in the infra-red. Comparison ofvisual and infra-red lines is effected in the following way.The grating drive is stopped when the Speedomax in-dicates the point of minimum deflection correspondingto an infra-red line. A flat mirror, hinged as indicatedin Fig. 4, is then rotated so that the spectrum is imagedon the cross wires, rather than on the exit slit S3 of thespectrometer. The wave-length band transmitted by S2is shifted from the infra-red to the visual region of thespectrum by adjustment of the wave-length dial of thepre-dispersing unit. With the micrometer screw indi-cated in Fig. 2, readings are obtained for the positionsof two visual lines. The two visual lines, of known wave-length, are selected so that their positions bracket theposition of the cross wire corresponding to the center ofslit S3. A micrometer reading for the center of S canbe found by measuring the zero-order image as thoughit were a spectrum line, or by direct measurement of avisual line, centered on the exit slit with the aid of theSpeedomax.

Given the micrometer readings for two visual linesof known wave-length, and the reading for the centerof the exit slit, a visual wave-length corresponding tothe unknown infra-red line can be interpolated. Theinfra-red wave-length of the unknown is the product ofthe interpolated visual wave-length and the ordernumber of the spectrum in which the measures weremade.

This procedure, Rowland's method of overlappingorders, has long been used for wave-length measure-ment. Various effects limit the accuracy attainable bythis method to about one part in 200,000. Accuracy ofthis order is more than adequate for the identificationof solar lines, and sufficient for all ordinary require-ments.

905

AND 0. C. MOHLER

IX. PERFORMANCE TESTS

In the study of the solar spectrum, measurements ofintensities are at least as important as the recording ofwave-lengths. Therefore, a careful study was made ofthe relation between intensity and recorder deflection.Intermediate devices such as rotating sectored disks,

and screens of wire gauze were employed in this study,but all measures were eventually based on the inversesquare law. The response curve of Fig. 5 was obtainedwith a rotating sector and a small tungsten lamp. Devia-tions from linear response are less than two percent fordeflections greater than two centimeters. The equationof the straight line in Fig. 5 is: D+1.9=0.2531, whereD is the deflection in cm and I is the true relative in-tensity. This result indicates that intensity measure-ments may be made directly on the tracings for de-flections greater than two centimeters, if a zero cor-

rection of 1.9 centimeters is added to the observeddeflection.

Table II lists the separations in wave-length and wavenumbers for close pairs of lines just clearly resolved.

The numbers in parentheses are the separations corre-sponding to the theoretical resolving power in the firstorders of the two gratings. It is seen from Table IIthat approximately 80 percent of the theoretical re-solving power has been attained.

Part of the excellent resolution should be attributedto the efficiency of the pre-dispersing unit in eliminatingparasitic light. Figure 6 shows two tracings of the xlband of CO2 at X20,084, the first at the highest dis-persion so far employed and the second at the lowest.In the high dispersion tracing the CO2 lines are com-pletely black at their centers, which indicates that verylittle stray light is present in the spectrometer.

FIG. 5. Graph of deflection versus true relative intensity. Thediameters of the circles are proportional to probable errors.

20079 20093

FIG. 6. Upper: C0 2, co,, at 20,084A in the solar spectrum at highdispersion. Lower: C02, ci, at low dispersion.

X. AUXILIARY EQUIPMENT

The control circuit of the grating drive motor in-cludes a cycling clock that automatically records azero deflection at any chosen interval. After the zerohas been recorded, the spectrometer will resume itstracing of the infra-red spectrum. The instrument hasbeen operated in this way for long intervals, with occa-sional attention for adjustment of the wave-lengthband centered on slit S2.

A carbon arc, of the type employed with 35-mmmotion picture projectors, has proved to be an excellentlaboratory source, for use on cloudy days. A series ofglass absorption cells, graduated in length, has beenprovided through the courtesy of Dr. E. F. Barker.

XI. RESULTS

Both as a laboratory instrument and under as-tronomical observing conditions, the pre-dispersing unitand spectrometer have been thoroughly dependable.Their successful operation demonstrates that it ispossible to scan a high dispersion spectrum withoutloss of optical definition by rotation of the dispersingelement. Test runs in the fifth-order green region (dis-persion 2- mm/A; slits 0.002 mm), with an electronmultiplier, reveal no errors in the grating drive.

The results of numerous investigations already car-ried out with the new instrumentation have been pub-lished elsewhere.' 0

The successful operation of the McMath-Hulbertspectrometer has led to the construction of a duplicateinstrument that is now installed in the spectrographroom of the Snow telescope on Mount Wilson. This

solar investigation is being sponsored jointly by theMcMath-Hulbert Observatory and the Mount WilsonObservatory. Funds for the construction of the MountWilson spectrometer were supplied by the Office ofNaval Research.

10 See articles in Astrophys. J., Phys. Rev., and Pub. Astronom.Soc. Pac., beginning in 1947.

906 R. R. McMATH