Moderate Resolution VUV Rocket Spectrograph

A. W. Mantz, B. L. Sowers, and J. J. Lange

A moderate resolution rocket spectrograph is described for use in the vacuum ultraviolet (VUV) spectralregion from 1050 A to 2000 A. The spectrograph incorporates a Carruthers electronographic camera, aplane grating, and focusing optics to provide a well defined field of view for an extended atmosphericsource. A detailed description of the spectrograph and sample spectra of the fourth positive band of COfrom 1200 A to 1800 A are given.

Introduction

There has been recent interest in studying upperatmospheric molecular emission processes of extend-ed sources in the vacuum ultraviolet (VUV) bymeans of sounding rocket experiments. A major dif-ficulty in interpreting the results of photometric ex-periments is establishing the spectral distribution ofthe emission and identifying the species responsiblefor the radiation, because efficient narrow-band fil-ters do not exist for this spectral region. The aim inthis work was to design and construct a moderateresolution spectrograph for the 1050-2000-A region.Well resolved emission spectra of the fourth positivesystem of carbon monoxide were obtained with thisspectrograph.

The Carruthers electronographic Schmidt camera'was incorporated into the spectrograph design be-cause of its sensitivity, resolution, and proven reli-ability in previous rocket experiments. The numberof reflections was minimized because of the antici-pated low source radiance and the poor reflectivity ofoptical elements in the VUV.

Within allowable weight restrictions, the spectro-graph was designed to maintain optical alignmentthroughout launch and normal recovery of an Aero-bee 170 rocket.

Spectrograph Chamber

The spectrograph was mounted inside the 60-cmlong x 38-cm in diameter Aerobee 170 extension,shown in Fig. 1, and was evacuated to pressures inthe range from 5 Torr to 10 X 10-6 Torr in thelaunch tower. The payload extension was equippedwith a recessed, low profile, 5-cm diam gate valve, a15-cm 20-cm blow-off door, and a 8-cm 10-cm

The authors are with the Air Force Avionics Laboratory,Wright-Patterson AFB, Ohio 45433.

Received 14 June 1973.

film access door. The extension was continuouslypumped in the launch tower by a 5-cm diam diffu-sion pump equipped with a LN2 cold-trap until ap-proximately 90 min before lift-off. At T-90 minutesthe gate valve was closed, and the pumping stationwas disconnected. At an altitude of approximately130 km, a squib was fired, and the blow-off door wasejected. This allowed the spectrograph to achieveoperating pressures in the 10- 6-Torr range quickly.Outgassing in the extension was minimized becauseof continuous pumping in the launch tower.

The film access door permitted the removal of filmon recovery, if necessary, without disassembly of thepayload in the recovery area. The payload extensionwas fitted at either end with cast magnesium bulk-heads. One bulkhead has a MgF2 entrance windowmounted on it in addition to electrical feed-throughs.The spectrograph was mounted to the second bulk-head which was also fitted for electrical feed-throughs.

The entire spectrograph assembly was vibrationtested and passed the Aerobee 170 launch require-ments. The spectrograph maintained proper opticalalignment under accelerations of 15 g rms and 45 gpeak to peak.

Spectrograph

The spectrograph is shown in Figs. 2 and 3. Con-siderable effort went into minimizing the spectro-graph weight (18.2 kg) by removing large amounts ofaluminum from the spectrograph frame and mirrorcells forming a ribbed structure with 6.3-mm thickribs connected by 3.2-mm thick sections. This de-sign proved adequate for Aerobee 170 launch sincethe instrument was recovered with no structuraldamage.

As seen in Fig. 3, radiation enters the payloadthrough the MgF2 window and is focused on the165-,gm 3-mm spectrograph entrance slit by the el-lipsoidal collecting mirror. An off-axis paraboloidcollimates the beam. The collector and collimator

January 1974 / Vol. 13, No. 1 / APPLIED OPTICS 193

i, I I

Fig. 1. Two views of Aerobee extension. Gate valve, blow-offport, film access, and umbilical penetrations are indicated.

Fig. 2. Two views of the spectrograph and the mechanical sup-port, Electrical components are attached to the mechanical

structure.

optical geometry are shown in Fig. 4. The collectingand collimating mirrors are mounted in lightweightfixtures designed to allow necessary alignment ad--justments and to provide mechanical stability in theupper portion of the spectrograph mechanical struc-ture.

The grating-telescope portion of the spectrographis assembled as a unit and is similar in design to theelectronographic Schmidt stellar spectrographs de-scribed by Carruthers.2 After leaving the grating,the beam enters the electronographic Schmidt cam-era that focuses the dispersed radiation onto the allreflective, front-surface, 35-mm diam photocathode.The dispersing element in the spectrograph is a10.2-cm X 12.8-cm plane grating ruled at d-1 = 1200lines/mm, blaze angle 0 = 5.17°, and blaze wave-

length b = 1500 A. The angle of incidence a waschosen to be 37.7°, and the angle of diffraction tobe 52.30 at Xb. This grating arrangement gives atheoretical reciprocal linear dispersion of 67 A/mm.The electronographic camera has an effective aper-ture of 75 mm, f/i optics, and a 200 field of view.The Schmidt corrector plate is LiF; the correctorand the photocathode response allow spectral cover-age between 1050 A and 2000 A. The primary is afront-surface aluminized mirror overcoated with a250-A thickness of MgF2; other mirrors in the systemwere similarly coated.

Photoelectrons leaving the CsI photocathode areelectromagnetically focused with unity magnificationonto 35-mm, Kodak NTB-3 emulsion, nuclear trackfilm. There is a linear dependence between inte-grated photon flux on the photocathode and the den-sity of the recorded electron image with this film,unlike in conventional photography where a nonlin-ear dependence exists. An onboard, mechanicaltimer was employed to advance the film in the trans-port to give sequential exposure times of 1 sec, 3 sec,and 9 sec allowing a total of approximately forty-fiveexposures to be taken during the flight.

-ENTRANCEI WINDOW

Fig. 3. A scale diagram of the spectrograph. The path of a1500-A ray is traced through the optical system.

194 APPLIED OPTICS / Vol. 13, No. 1 / January 1974

I :�' 7, ,I

11 I I I-

I� I �� ti �'

lo-,

lo-2

zL.

LI

10-4 1 I ' I I I I I

1000 1200 1400 1600 1800 2000 2200 2400

WAVELENGTH ()

Fig. 4. A cross section of the ellipsoid and paraboloid of revolu-tion illustrating the portion used to figure the collecting and colli-mating mirrors. The ellipse with foci at F and F' has a semima-jor axis of 48 cm and a semiminor axis of 26 cm. The parabola isdescribed by the equation y

2= 4 ax, where a = VF = 7.6 cm.

The extended radiating source was located at F' for the flightand in the laboratory.

Alignment and TestingThe collector and collimator were initially aligned

and focused in autocollimation as a separate opticalcell; it was found that the spectrograph slit could beused as an accurate Foucault test on the cell. TheSchmidt camera was also aligned and focused elec-trically and optically as a separate unit, and it wasdetermined that the camera had a resolution of 2min of arc at the field center. All optical elements,including the photocathode, were calibrated to de-termine their optical or quantum efficiency, as ap-propriate, as a function of wavelength. The over-allefficiency of the system was then calculated as afunction of wavelength. The results appear in Fig.5. The Schmidt camera was calibrated against asecondary intensity standard, and these data wereused to calculate a peak spectrograph NESR of 7 10-l 0 W/cm 2 sr Afor a 1-sec exposure at 1300 A.

After completing the above measurements the en-tire spectrograph was assembled, and spectra wereobtained from a flowing gas (CO2 :He) dc high volt-age discharge source. The reciprocal linear disper-sion of 67 A/mm with a demagnified 165-gm slit pro-vided a working resolution of better than 11 A. Asample spectrum is shown in Fig. 6.

Fig. 5. Over-all system efficiency calculated from measured ef-ficiencies of the three mirrors, the photocathode, the window, the

Schmidt corrector, and the grating as a function of wavelength.

ji

t:

1200 1600 1800

I (v-3)11 5 2 1

,0 S(vi-2)10 3 2 0

4 3

2 1 0

1 0 ...

(v-O)

Fig. 6. A spectrum and microdensitometer traces of the fourthpositive system of carbon monoxide in the 1200-1800-A region ob-

tained with the electronographic spectrograph.

January 1974 / Vol. 13, No. 1 / APPLIED OPTICS 195

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Results

The sensor was launched at White Sands MissileRange on 29 January 1973 as part of project PLU-MAR. All sensors performed nominally; however,the source for the upper atmospheric release mal-functioned, and the spectrograph collected no data.An extensive failure analysis is currently underway.A second launch is being planned contingent uponpositive identification of the source(s) of the pastfailure and appropriate modification to minimize theprobability of failure on ensuing missions.

The authors acknowledge the tremendous enthusi-asm and effort that John Unertl3 gave to this proj-ect. The detailed mechanical design, fabrication ofthe mechanical support and figuring of the opticswere done in collaboration with J. Unertl and his as-sociates.

We also acknowledge the generous hospitality andsupport afforded us at the Naval Research Laborato-ry by Robert Arritt for vehicle support and byGeorge Carruthers, Talbot Chubb, and Chet Opalwith regards to electronographic camera details.The support given by Harry Merchant, Dave King,and Sam Adams, who assisted in various phases ofthe spectrograph payload construction and assembly,is also greatly appreciated.

This work was supported by the Air Force Avion-ics Laboratory, Wright-Patterson Air Force Base,Ohio.

References1. G. R. Carruthers, Small Rocket Instrumentation Techniques

(North-Holland, Amsterdam, 1969), p. 168.2. G. R. Carruthers, Appl. Opt. 8,633 (1969).3. John Unertl Optical Co., Inc., 3551-3555 East Street, Pitts-

burgh, Pennsylvania 15214.

CALL FOR PAPERS

1974 INTERNATIONAL OPTICAL COMPUTING CONFERENCE

ZURICH, SWITZERLAND APRIL 9-11, 1974

Papers are desired for the 1974 International Optical

Computing Conference sponsored by the IEEE Computer Society. The

Conference will be held in Zurich, Switzerland, April 9-11, 1974.

Areas of interest are, Holographic Imaging, Coded Aperture Techniques,

Speckle Techniques and Holographi Interferometry, and Optical

Computing Systems. A 200 word informal summary of all prospective

papers and a biographical sketch of the author(s) must be

submitted to:

Dr. David CasasentTechnical Program ChairmanCarnegie-Mellom UniversityDepartment of Electrical EngineeringPittsburgh, Pennsylvania 15213 USA

A 1000-2000 word technical digest summary of

all accepted papers is due before 1 February 1974 in camera ready

form for inclusion in the conference digest.

196 APPLIED OPTICS / Vol. 13, No. 1 / January 1974