high resolution rocket euv solar spectrograph

5
High Resolution Rocket EUV Solar Spectrograph W. E. Behring, R. J. Ugiansky, and U. Feldman The design andperformance of an Aerobee 150 rocket-borne solar spectrograph covering a wavelength range of 10-385 A are discussed. The spectrograph uses a gold-coated replica concave grating of 3-m ra- dius with 1200 grooves/mm at an angle of incidence of 880. The spectra are recorded on glass photo- graphic plates making possible wavelength determination to 0.003 A if known standard wavelengths occur frequently enough. Special attention to scattered light made possible the photographing of the solar spectrum from 60 A to 385 A without using filters to absorb the strong visible and uv sunlight, al- though the solar spectrum was also recorded through metal foil filters. In the laboratory the spectro- graph has been used to record spectra of highly ionized metals with a resolution of 0.03 A or better. Introduction The solar spectrum in the extreme uv (EUV) range has been successfully photographed by a num- ber of scientists. A major difficulty in interpreting the spectrum has been the relatively low resolving power of most instruments and the consequent lack of accuracy in the determination of wavelengths. Our aim in this work was to build a high resolution spectrograph for wavelengths below 400 A. We ob- tained solar spectra with resolution at least a factor of 5 better than previous results. 1 Because of the low intensity of the sun and the poor reflectivity of surfaces in the EUV we used only one optical element (a gold-coated replica grating at grazing incidence). To obtain precise and highly re- solved solar spectra, a design was chosen that utiliz- ed glass photographic plates held very accurately on the focal curve. Within the allowable weight, the spectrograph was designed to maintain its alignment during the vibration and acceleration of launch, and the glass plates were mounted so as to avoid break- age at all the stages of the launch and recovery. An Aerobee 150 rocket (Vam-20 booster) and a type SPC 300 D/B two-axis solar pointing control (Ball Bros. Res. Corp.) were used. Internal scattering of the intense visible and uv solar light was reduced sufficiently to obtain spectra without the use of thin metallic filters. The first two authors named are with the NASA Goddard Space Flight Center, Greenbelt, Maryland 20771; U. Feldman is with Department of Physics & Astronomy, Tel Aviv University, Tel Aviv, Israel. Received 17 July 1972. Spectrograph The cone-cylinder type nose cone permitted the spectrograph to be long enough to use a 3-m radius grating ruled at d-1 = 1200 lines/mm. The angle of incidence a was chosen at 880 permitting spectra down to 10 A. The spectrum was recorded by three photographic plates (230 mm, 230 mn, and 330 mm long) selected so that the gaps between plates oc- curred in regions of few solar spectral lines. The use of glass plates permits measuring the positions of sharp spectral lines within 2-,gm or 3-gtm rms. The spectrograph is shown in Fig. 1 and in Fig. 2 where portions are cut away for greater clarity. The base plate was rigid enough to maintain, the alignment without the aid of the cover. Except for the photographic plates, all materials used in the spectrograph gave off little gas under vacuum. The cover has ten light tight air outlets to permit rapid evacuation of the interior and is highly polished to lessen heating especially if the nose cone failed to cover the instrument prior to recovery. The spectro- graph was kept light in weight (14.1 kg) by machin- ing holes in the plateholder and machining integral radial ribs in the base plate. The plateholder bends the glass plates (1 mm thick, 51 mm wide) to a very accurate circular seg- ment (1.5-m radius). The plates are secured along the top and bottom portions so that the maximum emulsion area is available for spectra. The plate was held accurately between a 0.137-mm thick Tef- lon strip on the rail and a rubber rod set into the backing plate. Figure 3 also shows the holes for the screws that lock each backing plate to the platehold- er, thus making the whole instrument more rigid. Testing showed that under these conditions the plates withstood shock in excess of 50 g and also passed the Aerobee 150 vibration tests successfully. 528 APPLIED OPTICS / Vol. 12, No. 3 / March 1973

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Page 1: High Resolution Rocket EUV Solar Spectrograph

High Resolution Rocket EUV Solar Spectrograph

W. E. Behring, R. J. Ugiansky, and U. Feldman

The design andperformance of an Aerobee 150 rocket-borne solar spectrograph covering a wavelengthrange of 10-385 A are discussed. The spectrograph uses a gold-coated replica concave grating of 3-m ra-dius with 1200 grooves/mm at an angle of incidence of 880. The spectra are recorded on glass photo-graphic plates making possible wavelength determination to 0.003 A if known standard wavelengthsoccur frequently enough. Special attention to scattered light made possible the photographing of thesolar spectrum from 60 A to 385 A without using filters to absorb the strong visible and uv sunlight, al-though the solar spectrum was also recorded through metal foil filters. In the laboratory the spectro-graph has been used to record spectra of highly ionized metals with a resolution of 0.03 A or better.

IntroductionThe solar spectrum in the extreme uv (EUV)

range has been successfully photographed by a num-ber of scientists. A major difficulty in interpretingthe spectrum has been the relatively low resolvingpower of most instruments and the consequent lackof accuracy in the determination of wavelengths.Our aim in this work was to build a high resolutionspectrograph for wavelengths below 400 A. We ob-tained solar spectra with resolution at least a factorof 5 better than previous results.1

Because of the low intensity of the sun and thepoor reflectivity of surfaces in the EUV we used onlyone optical element (a gold-coated replica grating atgrazing incidence). To obtain precise and highly re-solved solar spectra, a design was chosen that utiliz-ed glass photographic plates held very accurately onthe focal curve. Within the allowable weight, thespectrograph was designed to maintain its alignmentduring the vibration and acceleration of launch, andthe glass plates were mounted so as to avoid break-age at all the stages of the launch and recovery. AnAerobee 150 rocket (Vam-20 booster) and a typeSPC 300 D/B two-axis solar pointing control (BallBros. Res. Corp.) were used. Internal scattering ofthe intense visible and uv solar light was reducedsufficiently to obtain spectra without the use of thinmetallic filters.

The first two authors named are with the NASA Goddard SpaceFlight Center, Greenbelt, Maryland 20771; U. Feldman is withDepartment of Physics & Astronomy, Tel Aviv University, TelAviv, Israel.

Received 17 July 1972.

Spectrograph

The cone-cylinder type nose cone permitted thespectrograph to be long enough to use a 3-m radiusgrating ruled at d-1 = 1200 lines/mm. The angle ofincidence a was chosen at 880 permitting spectradown to 10 A. The spectrum was recorded by threephotographic plates (230 mm, 230 mn, and 330 mmlong) selected so that the gaps between plates oc-curred in regions of few solar spectral lines. The useof glass plates permits measuring the positions ofsharp spectral lines within 2-,gm or 3-gtm rms. Thespectrograph is shown in Fig. 1 and in Fig. 2 whereportions are cut away for greater clarity.

The base plate was rigid enough to maintain, thealignment without the aid of the cover. Except forthe photographic plates, all materials used in thespectrograph gave off little gas under vacuum. Thecover has ten light tight air outlets to permit rapidevacuation of the interior and is highly polished tolessen heating especially if the nose cone failed tocover the instrument prior to recovery. The spectro-graph was kept light in weight (14.1 kg) by machin-ing holes in the plateholder and machining integralradial ribs in the base plate.

The plateholder bends the glass plates (1 mmthick, 51 mm wide) to a very accurate circular seg-ment (1.5-m radius). The plates are secured alongthe top and bottom portions so that the maximumemulsion area is available for spectra. The platewas held accurately between a 0.137-mm thick Tef-lon strip on the rail and a rubber rod set into thebacking plate. Figure 3 also shows the holes for thescrews that lock each backing plate to the platehold-er, thus making the whole instrument more rigid.Testing showed that under these conditions theplates withstood shock in excess of 50 g and alsopassed the Aerobee 150 vibration tests successfully.

528 APPLIED OPTICS / Vol. 12, No. 3 / March 1973

Page 2: High Resolution Rocket EUV Solar Spectrograph

Fig. 1. A view of the spectrograph and its reflection in its cover.Two test photographic plates are in place. The front of the grat-

ing is partially hidden by the baffle.

accuracy deteriorates because the bending force less-ens. Application of the grating equation shows thatradius errors Ap = 10 Am lead to wavelength errorsof AX = p(d sin3)/2p = 0.014 A, where a is theangle of diffraction and p = R/2 is the plateholderradius. The effect of these errors can be much re-*duced if the wavelength standards are accuratelyknown and closely enough spaced. For the last fewmm of the plate, accurate wavelengths cannot be ob-tained because the bending is ill defined, and theemulsion is often damaged or peels off.

The grating must be mounted so as to permitprecise and stable positioning, withstand the vibra-tion and acceleration loads, and yet avoid excessivedeformation of its surface. The mount shown in

Fig. 2. A diagram of the spectrograph. The path of a 330-A solar light ray is traced through the slit to the grating and onto the photo-graphic plate.

Loading plates is easily accomplished in total dark-ness.

After adjusting the plateholder and dowel pinningin place, a dial indicator showed that the plateholderrails deviated from the radius by no more than 10gim. Although the plates may average out some pla-teholder errors, they are subject to their own errorscaused by varying glass or emulsion thickness, wavysurfaces, and uneven bending of the glass. The de-viations for the three glass plates at their midline arenot greater than 10 gm and generally are significant-ly less. Within 25 mm of each plate end, the radial

TEFLON TA

GLASSPHOTOGRAPH I

PLATE

LOCKINGSCREW -HOLE

BACKING PLATE

BACKING- PLATE

SCREW

RUBBERPRESSURE

ROD

Fig. 3. A section view of the photographic plateholder.

March 1973 / Vol. 12, No. 3 / APPLIED OPTICS 529

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Page 3: High Resolution Rocket EUV Solar Spectrograph

ENTRANCE PLANE

PINHOLE

PHOTOGRAPHIC PLATE

UU

OPEN L

TIN

ALUMINUM

Fig. 4. A diagram showing the various slit sections and the slightly overlapping spectra from each section. The solar spectrum from 164A to 198 A is shown printed more densely than in Fig. 5 to exhibit the relatively weak spectra from the tin and aluminum filter sections.

Figs. 1 and 2 was successful and introduced surfacedistortion of less than 1/2 fringe of green light.

The slit jaws were made of Haynes Stellite 6K(Co, Cr, W alloy) which is very hard and can beground to produce a good edge. Each jaw (32 mmlong) was straight within 1 gim. The mounting plateof KR Monel was chosen to match the thermal coef-ficient of expansion of the jaws. One edge of eachjaw slides against a face on the mount to maintainparallelism while setting the width. Each jaw is se-cured by a clamp. The translational and angularposition of both the slit and grating mounts were setby micrometers that were later removed.

Because of the 2-min of arc aiming precision thatis obtained by the pointing control and the small an-gular size of the solar disk (32'), it was possible toget four separate spectrograms on the plate by mak-ing the slit long enough to be divided into four sec-tions (Fig. 4). This is only possible because at highangles of incidence there is little focusing of rayslying in the sagittal plane (i.e., strong astigmatism).Each slit section was 6.5 mm long with a 2-mmblank space between sections and could be varied bychoosing the type of filter, length of opening, and ex-posure time. At the long wavelength end of thethird plate the four spectra overlapped slightly. Thetop spectrum used a pinhole, the second an open slit,the third a 900-A thick tin filter, and the bottom a900-A thick aluminum filter. The pinhole, actuallya 0.6-mm long section, permitted some angular reso-lution along one dimension so that the difference be-tween spectra from active and quiet solar areas couldbe observed provided that the rocket precession coneangle was small. The tin filter transmits from 50- 100 A, the aluminum 150-250 A. These spectraaid in sorting out the higher order lines. The trans-

mittance of tin and of aluminum are given by Sam-son2 along with references to the literature.

The strong off-axis aberrations may be seen bycomparing the lines in the Al filter section in Fig. 4with the lines in the open filter section in Fig. 5.These aberrations rendered the Al filter section un-suitable for wavelength measurement. The filterswere made by floating an evaporated film onto aBuckbee Mears etched 0.25-mm Ni mesh having 82%transmission.

A solar sensor (eye-block) mounted on the spectro-graph base plate provides the signals that point theinstrument at the center of the sun. The eye-blockwas aligned to an optical sight. The lens and a neu-tral density filter were mounted in the front rib ofthe baseplate. The lens focused the sun onto a reti-cle engraved on the matte-surfaced front face of aright angle prism that reflects the light for viewing.In the laboratory this sight had been aligned paral-lel to the beam entering the spectrograph with theaid of a traversing theodolite.

Mack et al.3 have shown that the resolving power

of a concave grating is greatest at the grating widthW= 2.507 [R 3/(tana sina + tan: sinf)]1/4 whichthey called the optimum width. The entrance slit isassumed to be infinitesimally narrow. They alsoshow that for this width the resolving power is (i =0.92 Wm/d,.where m is the spectral order. They findthat this maximum resolving power is degraded bynot more than 10% provided that the slit width S <aR X/W. These relations are valid only for light raysconfined to the plane containing the Rowland circle.The values of W, AXh, and S for various wavelengthsare given in Table I. The line width AXh(at 0.405 ofmaximum) was calculated from AXh = X/cM-

The actual grating width and slit width used for a

530 APPLIED OPTICS / Vol. 12, No. 3 / March 1973

Page 4: High Resolution Rocket EUV Solar Spectrograph

Fe VIII 167.495

Fe VIlli 168.176

Fe IX 171.075 _

Fe X 174.534

Fe X 177.243

Fe Xi 179.762

Fe XI 180.407/Fe X 184.542

Fe Vill 185.225 /

Fe Xi 182.173

188.219

Fe X 18835Fe X 193.517Adk

Fe X 19517-X

S Vill 198.561 B

Fe XI 203.835 _

207.124 S

Fe XIV 211.328 213.781 t~0

Fe XV 219.135 A F i

Si IX 227.006 m

Fe XV 227.21 I

_~ 160

170H

180

190

_ 200

210

220

230

Fig. 5. The solar spectrum from 163 A to 230 A in approximateregister with a laboratory spark spectrum of iron. The solar spec-

trum is shown on the right.

particular flight were chosen by referring to the tab-ulated values while also considering the purpose ofthe experiment, the grating efficiency, desired resolv-ing power, and solar spectrum intensity vs wave-length. For example, on the second flight we em-phasized the strong solar lines from 170 A to 370 Aby choosing a gold-coated replica grating (blazeangle 48', blaze wavelength = 130 A at a = 880).Consulting Table I we chose a 3-gm slit width andadjusted the aperture slot to illuminate a gratingwidth of 32 mm.

Table 1. Optimum Values for Grating Parameters

Wavelength W \ ha Sa(A) (mm) (mA) (pm)

10 12.4 0.7 0.2450 19.5 2.3 0.77

100 23.5 3.8 1.3200 28.4 6.4 2.1300 31.6 8.6 2.9400 34.1 10.6 3.5

a Calculated from optimum width W.

Scattered Light

The problem of scattered light in the spectrographis a cardinal one. The intensity ofthe photosphericlight is six orders of magnitude greater than that ofthe corona. In order to photograph the weak coronalEUV spectrum, the scattered visible amd uv lightmust be reduced to a very low intensity. This wasachieved by these means: (a) scrupulous cleanlinessduring the whole of the testing and the prelaunchperiod reduced dust and other deposits on the grat-ing face; (b) most internal parts were plated withblack nickel to absorb extraneous light (this materialhas a low outgassing rate); (c) the spectrograph wasdivided into three separate sections so that any scat-tered light that might get into the slit section or thegrating section would be largely absorbed before itpassed to the plate holder section; (d) a suitablelight trap was constructed for the direct image; (e)for each filter section an adjustable trap attached tothe direct image trap was used to reduce the strongsmall angle scattering from the grating.

We pointed the spectrograph to the sun for expo-sure times up to 20 min at sea level and obtainedvery little scattered light. Above the atmospherethe solar uv from 1000 A to 4000 A is much more in-tense. Consequently the scattered light on the spec-trogram from thie open slit section is quite intense insome regions as may be seen in Figs. 4 and 5.

Alignment and Testing

Using the method of Rathenau and Peerlkamp4 wecalculated the tolerances necessary in order for theadjustment errors to have negligible effect. The tol-erance on rotation of the grating in the plane tangentto its surface is AOg = 10 min of arc (from their Fig.9). Rotation of the grating about the axis parallel tothe rulings must be limited to 0.43 min of arc (fromtheir Eqs. 13 and 14). Rotation in the plane of slitwas set within 1.7 min of arc.

The final adjustments were made by taking spec-tra using a spark source described by Feldman et al.5

For testing the spectrograph, a small vacuum cham-ber was built with this source mounted on one end toilluminate the instrument.

ResultsTo reduce absorption from unwanted gases in the

spectrograph, it was launched with the nose coneevacuated. Since only the comparatively slow SWRemulsion was available for the first flight (30 Sep-tember 1968) the instrumental sensitivity was toolow; and only the 304 A line and a few other strongsolar lines near 25 A and 180 A were recorded. Thegrating had a blaze angle of 20 35' and was quite effi-cient below 50 A.

On the second launch (16 May 1969) a gratingwith a blaze angle of 4 8' was used having muchhigher efficiency above 150 A than the first grating.The peak altitudes were 203.3 km and 201.7 km,respectively. The new Kodak Special Plate 101-05emulsion (approximately 8 times faster than theSWR) was used in this flight. Handling and devel-

March 1973 / Vol. 12, No. 3 / APPLIED OPTICS 531

444

El

I

Page 5: High Resolution Rocket EUV Solar Spectrograph

opment procedures for this plate have been treatedby Houston and Ugiansky.6

Because of the low intensity of the solar EUV linesthe entire pointing time of 271 sec was used for theexposure. About 370 solar emission lines were re-corded from 60 A to 158 A and 163-385 A. Thewavelengths and visual intensity estimates for thesehave been reported by Behring et al.1 ; 180 of thelines were identified as belonging to the spectra ofvarious ions of He, 0, Ne, Mg, Si, S, Ar, Ca, Fe, andNi. The paucity of good laboratory wavelengths forthe solar lines limits the accuracy of the solar wave-lengths given to 0.008 A above 100 A and 0.004 Abelow 100 A. The precision of the solar wavelengthsand the reliability of the identifications can be im-proved when better laboratory wavelength data be-come available. A portion of the recorded solarspectrum with a wavelength scale and laboratoryiron spectrum is shown in Fig. 5.

The sharpest solar lines show a full width at half-maximum of about 0.06 A at 250 A. This places anupper limit on the instrumental broadening in flight.Two lines 0.032 A apart near 244 A were resolved ina laboratory iron spectrum taken with the spectro-graph in flight configuration.

The spectra of many highly ionized elements havebeen recorded with this instrument in the laborato-ry. The analyses of the spectra have been reportedby Feldman et al.,7 Hoory et al.,8 and later papers inpress.

We thank G. C. Dietz and K. R. Saffer for theirvaluable suggestions concerning design features ofthe grating spectrograph. J. Houston and L. Cohen

assisted in the alignment and did much of the test-ing.

U. Feldman carried out part of this work while aNAS-NRC Postdoctoral Research Associate atNASA Goddard Space Flight Center.

References1. W. E. Behring, L. Cohen, and U. Feldman, Astrophys J. 175,

493 (1972).

2. J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectros-copy (Wiley, New York, 1967), p. 188.

3. J. E. Mack, J. R. Stehn, and B. Edlen, J. Opt. Soc. Am. 22,245 (1932).

4. G. Rathenau and P. K. Peerlkamp, Physica 2, 125 (1935).

5. U. Feldman, M. Swartz, and L. Cohen, Rev. Sci. Instrum. 38,1372 (1967).

6. J. Houston and R. Ugiansky, A.A.S. Photo-Bull. No. 2, 20(1970).

7. U. Feldman, L. Katz. W. Behring, and L. Cohen, J. Opt. Soc.Am. 61,91 (1971).

8. S. Hoory, S. Goldsmith, U. Feldman, W. Behring, and L.Cohen, J. Opt. Soc. Am. 61, 504 (1971).

The Fifteenth Annual Rocky Mountain Spectroscopy Conferencewill be held August 20-21, 1973, at the Brown Palace Hotelin Denver, Colorado. Immediately following on August 22-24,1973, is the Twenty-Second Annual Denver Conference onApplications of X-Ray Analysis also at the Brown PalaceHotel in Denver. This conference is sponsored by theDenver Research Institute of the University of Denver.

Papers in all fields of theoretical and applied spectroscopyare invited. Abstracts of not more than 200 words should besubmitted by May 20, 1973. Send abstracts and/or requestsfor further information to Robert H. Heidel, U.S. GeologicalSurvey, Building 25, Denver Federal Center, Denver, Colorado,80225.

532 APPLIED OPTICS / Vol. 12, No. 3 / March 1973