High Resolution Rocket EUV Solar Spectrograph

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  • 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.

    SpectrographThe cone-cylinder type nose cone permitted the

    spectrograph 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

  • 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










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

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

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    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 powerof 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


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