A Study of the Solar Spectrum from 7 to 400 A C. B. Farmer and P. J. Key A series of measurements of the solar spectrum covering the wavelength range 7-400 At, made from an observing site in the Bolivian Andes at 5200-m altitude, is described. The experimental technique was designed to give both high resolution (0.3 cm-') spectra for line identification purposes and absolute transmittance data for measured values of the atmospheric water content in the solar paths. Introduction During the summer of 1964 a series of measurements of the solar spectrum was made from a site close to the Laboratorio de Fisica Cosmica on Mt. Chacaltaya in the Bolivian Andes.' The altitude from which the measurements were made was 5200 m, and the wave- length range covered was from 7 ,z to 4 00/u. The site was chosen because of the small solar zenith angles and low atmospheric water contents associated with it, and also the fact that under normal conditions the prob- ability of long periods of clear weather during the dry season (June to September) is high. The aim of the work was twofold, namely, to extend the range of existing solar atlases, particularly that of Migeotte et al., 2 as far into the rotational water vapor band as possible and also to provide estimates of the absolute transmittance of solar energy throughout the wavelength range, for atmospheric water contents de- termined from local psychrometric measurements. One of the major problems in attempting radiometric spectroscopy of the sun, i.e., trying to assign absolute values of intensity to the spectra in addition to ob- serving the detailed line structure, is the determination of the unattenuated solar envelope upon which the transmission spectrum is superimposed. This is equivalent to the reference path spectrum of conven- tional double-beam spectroscopy, and for the purposes of obtaining atmospheric transmittance the reference en- velope can be effectively simulated by the use of a suit- able auxiliary blackbody source. This method requires the assumption of a constant solar brightness tempera- ture with wavelength, an assumption which does not seriously degrade the accuracy of transmittance results alone. The current observations were of two types: first, the solar spectrum was recorded with as high a resolu- The authors are with E.M.I. Electronics Limited, Feltham, Middlesex, England-. Received 16 March 1965 .. : tion as possible to give additional data to the existing atlases. Second, to obtain transmittance data, medium-resolution (1-2 cm-') spectra of each regioh were recorded alternately with blackbody calibration runs, the wavelength ranges being chosen so that they could be recorded in sufficiently short periods of time to ensure minimum change of the absorber conditions. Description of Apparatus A. Spectrometer The spectrometer was of the Czerny-Turner type commonly used in far infrared work, 3 ' 4 with an aperture ratio of f/6 and a focal length of 105 cm. The optical arrangement is shown in Fig. 1. Six diffraction gratings, G, were used, having ruling frequencies from 98.4 lines per mm down to 2.18 lines per mm. Ruled areas were 229 mm X 178 mm, except for the 98.4 lines per mm grating, which was 165 mm square. The gratings were mounted on a table rotated by a worm gear which was driven by a synchronous motor through a multispeed reduction gearbox. Gear- box speed selection was achieved by means of electro- magnetic clutches. The monochromator slits, S2 and S3, were 1.3 cm long and could be opened to a maximum width of 1.0 cm. The Pfund condensing system, Mo and M1i, gave a 5:1 reduction onto the 3-mm diam sensitive area of a Unicam Golay detector, D. Detectors with diamond, quartz, and KRS-5 windows were used. The radiation was chopped (at C) at 12.5 cps with a metal chopper driven by a synchronous motor. The beam between a lamp and photodiode was also chopped to provide synchronizing square waves for the detector amplifier. The elimination of higher-order radiation was achieved by conventional far infrared techniques. Scatter plate filters, made by grinding flat aluminium plates with various grades of carborundum powder, were used at F and F' for-wavelengths beyond 22 1A, black polythene was placed at T, and other transmission filters- at T 2 . Indium antimonide filters, bloomed for September 1965 / Vol.. 4, No. 9 / APPLIED OPTICS 1051
Transcript
A Study of the Solar Spectrum from 7 to 400 A
C. B. Farmer and P. J. Key
A series of measurements of the solar spectrum covering the wavelength range 7-400 At, made from anobserving site in the Bolivian Andes at 5200-m altitude, is described. The experimental technique wasdesigned to give both high resolution (0.3 cm-') spectra for line identification purposes and absolutetransmittance data for measured values of the atmospheric water content in the solar paths.
Introduction
During the summer of 1964 a series of measurementsof the solar spectrum was made from a site close to theLaboratorio de Fisica Cosmica on Mt. Chacaltaya inthe Bolivian Andes.' The altitude from which themeasurements were made was 5200 m, and the wave-length range covered was from 7,z to 400/u. The sitewas chosen because of the small solar zenith angles andlow atmospheric water contents associated with it, andalso the fact that under normal conditions the prob-ability of long periods of clear weather during the dryseason (June to September) is high.
The aim of the work was twofold, namely, to extendthe range of existing solar atlases, particularly that ofMigeotte et al.,2 as far into the rotational water vaporband as possible and also to provide estimates of theabsolute transmittance of solar energy throughout thewavelength range, for atmospheric water contents de-termined from local psychrometric measurements.
One of the major problems in attempting radiometricspectroscopy of the sun, i.e., trying to assign absolutevalues of intensity to the spectra in addition to ob-serving the detailed line structure, is the determinationof the unattenuated solar envelope upon which thetransmission spectrum is superimposed. This isequivalent to the reference path spectrum of conven-tional double-beam spectroscopy, and for the purposes ofobtaining atmospheric transmittance the reference en-velope can be effectively simulated by the use of a suit-able auxiliary blackbody source. This method requiresthe assumption of a constant solar brightness tempera-ture with wavelength, an assumption which does notseriously degrade the accuracy of transmittance resultsalone.
The current observations were of two types: first,the solar spectrum was recorded with as high a resolu-
The authors are with E.M.I. Electronics Limited, Feltham,Middlesex, England-.
Received 16 March 1965 .. :
tion as possible to give additional data to the existingatlases. Second, to obtain transmittance data,medium-resolution (1-2 cm-') spectra of each regiohwere recorded alternately with blackbody calibrationruns, the wavelength ranges being chosen so that theycould be recorded in sufficiently short periods of timeto ensure minimum change of the absorber conditions.
Description of Apparatus
A. Spectrometer
The spectrometer was of the Czerny-Turner typecommonly used in far infrared work,3' 4 with an apertureratio of f/6 and a focal length of 105 cm. The opticalarrangement is shown in Fig. 1.
Six diffraction gratings, G, were used, having rulingfrequencies from 98.4 lines per mm down to 2.18 linesper mm. Ruled areas were 229 mm X 178 mm, exceptfor the 98.4 lines per mm grating, which was 165 mmsquare. The gratings were mounted on a table rotatedby a worm gear which was driven by a synchronousmotor through a multispeed reduction gearbox. Gear-box speed selection was achieved by means of electro-magnetic clutches.
The monochromator slits, S2 and S3, were 1.3 cm longand could be opened to a maximum width of 1.0 cm.The Pfund condensing system, Mo and M1i, gave a 5:1reduction onto the 3-mm diam sensitive area of aUnicam Golay detector, D. Detectors with diamond,quartz, and KRS-5 windows were used. The radiationwas chopped (at C) at 12.5 cps with a metal chopperdriven by a synchronous motor. The beam between alamp and photodiode was also chopped to providesynchronizing square waves for the detector amplifier.
The elimination of higher-order radiation wasachieved by conventional far infrared techniques.Scatter plate filters, made by grinding flat aluminiumplates with various grades of carborundum powder, wereused at F and F' for-wavelengths beyond 22 1A, blackpolythene was placed at T, and other transmissionfilters- at T 2 . Indium antimonide filters, bloomed for
Fig. 1. Optical arrangement of spectrometer. B, blackbodysource; Si, auxiliary entrance slit; Ml and M3, plane mirrors;M2 and M2', interchangeable concave mirrors; F and F', planemirrors or scatter plate filters; C, chopper; S2, entrance slit;T, and T, transmission filters; M1
4 , plane mirror; 5 and M6 ,main spherical mirrors; G, grating; S3, exit slit; i\17, MgI9, andMlo, plane mirrors; R and R', reststrahlen filters or plane mir-
rors; M and Ml,, concave mirrors; D, Golay detector.
maximum transmission at various wavelengths, wereused in the 7-22 region, and beyond 180 powdertransmission filters of the type described by Yamadaet al.5 were employed. Reststrahlen filters, used inpairs, were mounted at R and R'; for wavelengthsbelow 14 ,u they were replaced by plane mirrors. Itwas thought that the degree of filtering necessary whenusing a laboratory source might not provide adequaterejection against the much more intense shorter wave-length solar radiation. Only in the 20-40 , region wasthis found to be the case, however, and an increasedthickness of black polythene eliminated the unwantedradiation.
The grating and filter combinations used in eachwavelength range are listed in Table I.
The spectrometer components were mounted on arigid aluminium base casting and enclosed in an airtightbox with detachable panels giving easy access for ad-justments. Controls for slit-width adjustment andfilter changing passed through airtight seals in thepanels. Residual water vapor absorption was mini-mized when carrying out blackbody reference runs bythe use of a recirculating air drying system.
B. Blackbody Source and Sun-Tracking SystemIt was considered that it would be very difficult to
build a 1.5-cm aperture blackbody and maintain auniform brightness temperature at an aperture ratio off/6, as required to match the spectrometer aperture.Hence a 3-cm aperture source, B, uniform in brightnesstemperature over an f/10 cone, was used, imaged downin a 5:3 ratio by a concave mirror, M\12, onto the entranceslit. The operating temperature, usually around1250'K, was controlled to within -410 C. At wave-lengths beyond 20 u, for which a dry optical path insidethe instrument is essential, a polythene window wasused; a similar piece of polythene was inserted in theoptical path when observing solar spectra of the sameregion.
Solar radiation was collected by a 30-cm diam f/6Newtonian telescope, used in conjunction with a planesun-tracking mirror driven by a clockwork motor.
The telescope produced a 1.5-cm diam image of thesun at an auxiliary slit S, maintained at a slightlygreater width than the actual spectrometer entranceslit S2. The purpose of S, which was mounted on amassive metal block, was to prevent as much as pos-sible of the unwanted heat due to the image of the solardisk from reaching the scatter plates and the input slitof the monochromator. It also provided a convenientpoint at which to observe the behavior of the sun-tracking system. S was then imaged at unit magnifica-tion onto S2 via a plane mirror Ml and a concave mirrorM2'. M, 1M2, and M2' were all kinematically mountedand could be interchanged without readjustment forobserving either the blackbody or the sun. A generalview of the equipment at the observation site is shownin Fig. 2.
C. Electronics and Recording EquipmentThe 12.5-cps signal from the Golay detector was fed
to a high-stability transistorized amplifier and syn-chronous rectifier system, and the dc output from thiswas displayed on a Honeywell strip chart recorder.Integration times of up to 85 sec were used. Signallevels could be read and compared from the recorderdeflections and the amplifier attenuation, the settingsof which were calibrated in 1-dB steps to an accuracy of1%.
D. Measurement of Water ContentThe water content of the air close to the observing
site was monitored with a conventional frost-pointmeter. Radiosonde balloon flights which were madesome 30 miles distant from the observing point gavethe humidity and temperature as a function of altitude.From these data the water content profile was deter-mined by a method which is explained in Fig. 3, which
Table I. Grating and Filter Combinations for EachWavelength Range
Fig. 2. General view of equipment at the observation site.
shows the variation of water vapor density with altitudefrom two typical radiosonde records, together withthe associated density measurements made on themountain. The radiosonde profile was modified bylinking the site density to it in the manner shown, andthe resultant profile integrated to give the total zenithpath water content. An investigation of the possibleinaccuracy incurred by this procedure showed that theerror in the values of the total water content deter-mined in this manner should be not more than 8%.Furthermore, good correlation was found to exist be-tween the site and radiosonde humidities over a periodof about six weeks prior to the commencement of theroutine spectral measurements, and it was felt, there-fore, that the method adopted for the determination ofthe zenith path water content was justified.
From the few investigations which were made oftransmittance as a function of air mass in specificwindow regions of the spectrum, it is apparent that thebulk of the total air path water content is concentratedwithin the first few hundred meters immediately abovethe mountain. In fact, the direct use of the local frost-point meter measurements would have incurred onlyslightly greater inaccuracy in the estimates of totalwater content.
Measurement Conditions and Procedure
In addition to the usual difficulties of design andoperation of equipment for use at high altitudes, acomment should be made on the particular problemspresented by the unusual atmospheric conditions whichoccurred during the 1964 season. The site had beenused by one of the authors' in 1962 for horizontal pathmeasurements over distances up to 10 km, and theevidence from this work was that very little atmosphericscattering occurs at this altitude at wavelengths greaterthan 4 A. The visual appearance of the sky from themountain at that time was a deep blue color which con-tinued from the zenith almost to the horizon. Duringthis season, however, there was evidence of a high-altitude dust layer originating probably from a volcanic
eruption which occurred between the two observingperiods, and which caused a much more marked grada-tion of the sky color from the zenith to the horizon.Also, a pale pink or white halo was observed around thesun during most of the period of the measurements.Probably as a result of the presence of this dust layer, athin haze, which often developed into a layer of high-altitude cloud by mid-afternoon, was present duringmany of the days of observation. For this reason, themeasurements of transmittance gave results which aresomewhat lower than those indicated by other workerswho have also investigated the shorter wavelength partof the range. These conditions prevented makingsatisfactory measurements of transmittance in thewindow regions of the spectrum for a wide enough rangeof air mass values to enable estimates to be made of thevariation of solar brightness temperature over the wave-length range.
For the purpose of obtaining transmittance data, ob-servations were made whenever possible on days ofmaximum clarity. For each wavelength range aspectral record of the sun was followed by a record ofthe blackbody calibration spectrum, a run taking ap-proximately 30 min. In this way each solar run hadassociated with it a blackbody spectrum recorded underidentical conditions of slit-width, scanning speed, andtime constant. Thus, the sun and blackbody sourcewere directly compared in terms of their spectral bright-nesses, and in calculating finally the transmittance forthe atmospheric paths a constant solar brightness tem-perature of 5000'K was assumed for the whole of thewavelength range covered. The facts on which this vassumption is based are taken from the results of Saiedyand Goody,6 Farmer and Todd,7 Bastin et al.,5 andBadinov.9 In terms of the accuracy of the final trans-mittance results, an error of 2% (i.e., 100'K) in theassumed solar temperature implies a worst transmit-tance error (at the shortest wavelengths) of 2%. Asmall correction was included to take into account theeffect of the extra mirrors in the optical path of thesolar radiation.
In the case of the high-resolution spectra, the finalresolution was determined not by the instrumentationbut by the total recording time available during theobservation periods. The maximum resolution was
W 7 -
-j~~~~~~~~~~~~0-
4 N. No.57830...4 .64.
1 1 0 1 L I I 01! 1.0 10*0
WATER VAPOUR DENSITY (gm/M3)
Fig. 3. Variation of water vapor density with altitude fromradiosonde data (solid lines) and observation site densities
set, therefore, such that, at a scanning speed sufficientlyslow to ensure negligible time constant distortion of theline shapes, a number of runs could be made over eachof the intervals into which the wavelength range wasdivided to establish confidence in line identification.
Photometric Records
A. High-Resolution Measurements
In Fig. 4, high-resolution spectra covering the range7-40 are presented. These have been selected astypical data from all the records obtained. Between7 and 23 , all the lines observed may be identifiedfrom the atlas of Mligeotte et al. No significant de-partures from the features of this atlas are observed,except for the expected difference due to the somewhatlower values of atmospheric absorber concentrations, aresult of the higher altitude of the observation site.From 23 to 40 4, solar spectra from a terrestrial sitehave not previously been observed as far as the authorsare aware. Atmospheric absorption is entirely domi-nated by the pure rotational spectrum of water vapor,and the strongest lines or groups of lines are readilyidentified from published laboratory spectra. How-ever, a number of weaker transitions which are notnormally seen in laboratory results become evident inatmospheric absorption spectra; for this reason fre-quencies for the majority of the rotational water vaportransitions have been indicated by vertical markings atthe top of each panel [Figs. 4(h) and 4(j) ]. Thefrequencies assigned to those transitions, and the valuesof line strength used in selecting those that are in-dicated, are taken from the most recent data of Benedictand Kaplan. 0
Unfortunately, on those occasions when high-resolu-tion spectra for this region were being taken, the watervapor absorption was too great to enable the highestpossible resolution to be maintained beyond 32 .Therefore, in Fig. 4 (j) (i) a lower resolution spectrumfrom 30 g to 39 g is shown. This was obtained duringthe course of making the absolute transmittance meas-urements and at a time when both the water contentand zenith angle were small, with consequent reductionin the absorption.
Two interesting features occurred in the spectrabeyond 23 u. The first of these is a line which appearsat 31.5 /i (317.5 cm-') and which does not correspondwith any of the predicted transitions. The second is at27.84 4 (359.2 cm-') and, while there may possibly beslight error incurred by difficulty with frequency cali-bration at this point, the observed strength of this lineis not sufficiently great to enable it to be identified withthe 10-,-9-, transition at 358.49 cm-'. Laboratorymeasurements with a long path multitraversal cell arecurrently being made in an endeavor to establish theorigin of these lines.
At wavelengths greater than 40 4 no solar energycould be detected, until the atmospheric window cen-tered at 345 1u (29 cm-') was reached. This window isdiscussed in more detail below. It has been felt useful,for the sake of completeness, to include laboratory data
obtained with the spectrometer covering the region be-tween 30 and 400 .
B. Transmittance Data
The method adopted for the measurement of spectraltransmittance was designed to provide a continuousrecord of transmittance with wavelength for knownamounts of water vapor, rather than to attempt to giveextinction coefficients for specific selected transmissionwindows. Measurements were, in fact, made over aswide a range of air masses (zenith angles) as possible,but the data obtained for the large air mass cases il-lustrated that direct multiplication of zenith water con-tent by the secant zenith angle factor, to obtain slantpath total water, is by no means a valid procedure forobservation sites of the type used for this work. Thesite is essentially a glacier-covered mountain locatedhigh over an otherwise arid plain, so that determinationof water content from local frost-point and radiosondedata, as described previously, is fairly accurate for smallzenith angle cases, but for large zenith angles the verti-cal water content profile and its variation with distanceaway from the site is very much more important. Thedata showed further evidence that the bulk of the watervapor was located close to the mountain, but that whilethe zenith path content remained fairly constant overlong periods of observation, there was rapid and con-siderable variation of water content in the path for largezenith angles, depending, of course, on the detailedterrain over which the path was located. Therefore,attempts to obtain extinction coefficients and to ex-trapolate to zero air mass the intensity values in specificwindow regions are not justified.
The medium-resolution transmittance spectra, repro-duced here as Fig. 5, are restricted to those measure-ments with which can be associated an accurate esti-mate of the water content of the path. The brokenline over each record represents the specified transmit-tance level; the ordinate scale is linear, and the baseline of each panel represents the true zero. Point bypoint reduction of the data to give a constant transmit-tance scale would have been extremely laborious andunnecessary. Table II lists the conditions under whichthe data presented in this paper were obtained.
The over-all accuracy of the final transmittancevalues is estimated to be 5%. Examination of thespectra will show that our measured transmittancevalues at the shorter wavelengths (i.e., from 7 u to 13 I)are slightly lower than would be expected from theresults of, for example, Yates and Taylor," Gebbie etal.," and Gates and Harrop.'I On the other hand, thetransmittance levels for wavelengths beyond 15 A showreasonable agreement with those of Yates and Taylor.This is consistent with the expected effect of the high-altitude dust layer mentioned previously.
C. The Window at 345 ,
Results of measurements made over the range from200 jL to 400 At, and including the small window at 345At, are reproduced in Fig. 6. The measured peak
Table I. Air Mass and Water Content Data forFigs. 4, 5, and 6
ZenithAir mass water
Date (base pressure contentFigure (1964) 535 mb) (pr. mm)
4(a)(b) July 21 0.69 1.8
(c)(d) (i)
4(d) (ii) Sept.8 0.61 1.8
4(d) (iii) 2.54-1.32(e) (i) 1.22-0.80(e) (ii) Sept. 0.80-0.64 3.2(f) (i) 0.66-0.61(f) (ii) 0.61-0.58(f) (iii) 0.58
4(g)(i) 0.66 1.6(g) (ii) Aug. 13 0.61-0.69 1.8(g) (iii) 0.69-0.87 1.9
4(h)(i) 0.72-0.61(h) (ii) Sept.3 0.61-0.58 2.0(h) (iii) 0.58
4(j) (i) Sept.4 0.61 1.7
5(a) (i) 0.80-0.77(a) (ii) 0.77-0.74(a) (iii) Sept. 1 0.74-0.72 2.0(b) (i) 0.72-0.69(b) (ii) 0.69-0.66(b) (iii) 0.66
5(c) July 9 0.72 1.65(d) Sept.4 0.61 1.76 June 26 0.64 1.9
transmittance for this window is 0.05 for total watercontents varying from 1.8 pr. mm to 2.3 pr. mm.
The total extinction produced in the window by allthe rotational transitions between 1000 cm-' and 0.74cm-' was calculated assuming the Van Vleck and Weiss-kopf line shape,' 4 and the resulting predicted spectra forthe two extreme water contents (i.e., 1.8 pr. mm and 2.3pr. mm) are shown in Fig. 6. The effect of collision-induced absorption in nitrogen and oxygen 5 has alsobeen included in this calculation. It is seen that theobserved window is both narrower and of lower trans-mittance than would be expected. Two further traceshave been added to Fig. 6; these are laboratory spectraobtained using a quartz mercury lamp and the cali-bration blackbody source, the amplifier gain levels forthese traces having been set to give comparable re-corder deflections. The different brightness tempera-tures of the two sources (i.e., approximately 4500'Kand 12500K, respectively) are, therefore, reflected in thedifferent noise levels of the two spectra.
The two laboratory spectra were run in order to seewhether the absorption of ozone at 370 Mt could be
detected, as a result of its having been produced in theinstrument by the presence of the mercury lamp.There is some evidence that this is so, and, therefore,the fact that ozone is present in all solar paths maypossibly account for the large difference between thepredicted and measured shape and transmittance of the34 5-,u atmospheric window. Laboratory measurementsin which the Chacaltaya atmospheric paths are simu-lated, but under conditions where the individual ab-sorbers can be isolated, are currently being made in anattempt to clarify this situation.
The authors wish to acknowledge the help of V.Roberts, without whose efforts the expedition could nothave been mounted in time to enable the work to becarried out during the 1964 season. The assistance andcooperation of R. Vidaurre, the Director of the Ob-servatory on Chacaltaya, and his staff is greatly ap-preciated. Thanks are also due to our colleagues atE.M.I. who assisted in the preparation of the equip-ment, in particular to T. Lewis who gave valuable helpduring the measurements program, and F. R. Robin-son and N. Whitlock in analyzing the records.
References1. P. J. Berry, C. B. Farmer, and D. B. Lloyd, Appl. Opt. 4,
1045 (1965).
2. M1. Migeotte, L. Neven, and J. Swensson, Final ReportContract AF61 (514)-432, Part I (1956); Part II (1957).
3. H. Yoshinaga, S. Fujita, S. Minami, A. Mitsuishi, R. A.Oetjen, and Y. Yamada, J. Opt. Soc. Am. 48, 315 (1958).
4. P. J. Hendra, R. D. G. Lane, and B. Smethurst, J. Sci.Instr. 40, 457 (1963).
5. Y. Yamada, A. Mitsuishi, and H. Yoshinaga, J. Opt. Soc.Am. 52, 17 (1962).
6. F. Saiedy and R. M. Goody, Monthly Notices Roy. Astron.Soc. 119, 213 (1959).
7. C. B. Farmer and S. J. Todd, Appl. Opt. 3, 453 (1964).8. J. A. Bastin, A. E. Gear, G. 0. Jones, H. J. T. Smith,
and P. J. Wright, Proc. Roy. Soc. A278, 543 (1964).9. I. I. Badinov, private communication.
10. W. S. Benedict and L. D. Kaplan, computed line pa-rameters for water vapor from 1000 cm-' to 0.74 cm-'.Data kindly made available prior to publication.
11. H. W. Yates and J. H. Taylor, NRL Rept. 5453 (1960).12. H. A. Gebbie, W. R. Harding, C. Hilsum, A. W. Pryce, and
V. Roberts, Proc. Roy. Soc. A206, 87 (1951).
13. D. M. Gates and W. J. Harrop, Appl. Opt. 2, 887 (1963).14. J. H. Van Vleck and V. Weisskopf, Rev. Mod. Phys. 17,
227 (1945).
15. H. A. Gebbie, N. W. B. Stone, and D. Williams, Molec.Phys. 6, 215 (1963).
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