Baker, D. J.

Bertschi, B. Y

Stair, A. T., Jr.

Steed, A. J.

English

DigitalCommons@USU

Utah State University DigitalCommons@USU Space Dynamics Lab Publications Space Dynamics Lab 1-1-1981 Recent Auroral Measurements Using a Field-Widened Interferometer Spectrometer A. J. Steed D. J. Baker B. Y. Bertschi A. T. Stair Jr. Follow this and additional works at: https://digitalcommons.usu.edu/sdl_pubs Recommended Citation Steed, A. J.; Baker, D. J.; Bertschi, B. Y.; and Stair, A. T. Jr., "Recent Auroral Measurements Using a Field-Widened Interferometer Spectrometer" (1981). Space Dynamics Lab Publications. Paper 123. https://digitalcommons.usu.edu/sdl_pubs/123 This Article is brought to you for free and open access by the Space Dynamics Lab at DigitalCommons@USU. It has been accepted for inclusion in Space Dynamics Lab Publications by an authorized administrator of DigitalCommons@USU. For more information, please contact digitalcommons@usu.edu. Recent auroral measurements using a field -widened interferometer spectrometerA. J. Steed, D. J. Baker, B. Y. BartschiElectro- Dynamics Laboratories, Electrical Engineering Department, Utah State University,Logan, Utah 84322A. T. Stair, Jr.Radiation Effects Branch, Optical Physics Division, AFGL, Hanscom AFB, Massachusetts 01731Utah State University and Air Force Geophysics Laboratory have been developingfield- widened interferometer systems since the late 1960's. These instruments have beendeveloped primarily to remotely sense spectral emissions from the night sky in the nearinfrared. However, the systems also have application for making measurements of any dimextended source.The three generations of field- widened instruments developed have demonstratedincreased throughput gains, and have been used to make excellent atmospheric emissionmeasurements. The first FWI instrument was for visible and near it application. Itemployed a large granite V- groove slide and a worm -gear drive. Results with thisinstrument were reported by Despain, et.a1.1,2 The need for a faster drive system becameobvious for auroral work. Consequently, a second generation instrument using a gas -lubricated bearing was developed in 1973. This instrument has provided excellent resultsand is still making state -of- the -art measurements. Instrument description and measurementresults using this instrument are summarized by Steed3, Haycock'', and Bakers. The currentfield operating configuration of this second generation system is described in this paperand recent auroral results are presented. A third generation rocketborne version of thisinstrument has been developed by Haycock, et.a1.6 Atmospheric measurements have been madeusing this instrument (Huppi7).There are a number of possible configurations for field compensating an interferometer(Bakers). The methods used in all of the USU versions was first suggested by Connes9 andis illustrated in Fig. 1. The plane mirrors of a standard Michelson are replaced byoptical wedges which are mirrored on the back side and retardation is achieved by drivingone of the wedges along its image plane. Second order spherical aberrations areessentially eliminated if compensated properly; the compensated set of theoretical limitingaberrations becomes those illustrated in Fig. 2. This figure shows a comparison of thefield- widened field -of -view (FOV) limits imposed by chromatic aberrations, 4th orderspherical aberrations, and astigmatism as a function of resolving power. A plot of cM =2ir /R, the limit for a conventional Michelson interferometer, is included in this figure forMIRROR SURFACEFIXED PRISMLIGHTINBEAM SPLITTERSUPPORTED INGLASS CUBE1111111,11, IIII II,111111TODETECTORMOVINGPRISMDIRECTION OFMOVEMENTMIRRORSURFACEFig. 1. Field compensation methodused by Boucharieneand Connes.202 / SPIE Vol. 289 1981 Fourier Transform Infrared Spectroscopy (1981)IO0+r1 I [III I 11 111111 I I 111111AST'GMATISM LIMIT FORAN 8° PRISMaCHROMATIC ABERRATIONIM7 QUARTZ OPTICSD IO'v -ä -awJZ 10'áv- MICHEL SONINTERFEROMETERII-3 ymSPHERICALABERRATIONLIMIT- 20OPERATING RANGE OFUSU FIELD WIDENEDINTERFEROMETERSO-5LLJO -IO 3OZWZQ¡LAIllli 11 1 111,11 I I II 1IL1103 10° 05RESOLVING POWER.5Fig. 2. Field compensated theoreticalaberration limits comparedwith conventional Michelsonsystem.t r r l r nt  i g  fi l -widened int rf r t r tr t r. , . . t iDynamics Laboratories, Electrical Engineering epartment, tah tate iversity,, an, t h   i i i i i  , ss c usetts 01731         widened i  l 19       i    er,        wid       ,          f   r       groo    rm-        l 1 * 2        a    s   in 19  exce re  still making state-of- -art mea   a summarized by Steed 3 , Haycock 4 ,  5 .   i i     ve o th  b  k, l.    7 ).conf fo field compensating an interferometer 8 )   9                           s o theoreti limiti    2    -wi field -vi         s,    %  ir/R, the lim t for a conventional Michelson interferometer, s f for SURFI ED  I N  I  P1    i . . i pensat t reti l err o l it par  it venti al ichel  e ./ SPIE Vol. 289 1981 Fourier Transfor  Infrared t  )Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/15/2014 Terms of Use: http://spiedl.org/termsSteed, A. J., D. J. Baker, B. Y. Bartschi, and A. T. Stair Jr. 1981. “Recent Auroral Measurements Using a Field-WidenedInterferometer Spectrometer.” Proceedings of SPIE 0289: 202–5. doi:10.1117/12.932170. reference. The aberration limits for the field- widened case are: ft = S2M[n(n2- 1) /on](chromatic), n = stMn(2R)1 /2 (spherical), and n = 2nm /tan2a (astigmatism), where S2M = maxsolid angle FOV for a standard Michelson interferometer, n = index of refraction of opticalmaterial, a = wedge angle, and R = resolving power.It is noted that astigmatism varies with wedge angle; only the plot of an 8° wedgewith n = 1.5 is shown in the figure. USU /AFGL instruments have utilized 8° wedgesfabricated from quartz and CaF2.Experience has shown that theoretical FOV limits for the field- widened case aredifficult to achieve since vignetting occurs before the limits can be achieved. Thesystems described achieve full angle fields -of -view in the 9 -12° range depending ondetector and wavelength region. If large throughput is to be achieved, large detectors andfast collector optics are needed to collect the radiation and this can be the FOV limitingfactor.The gains that have been achieved by field widening can also be seen from Fig. 2 wherethe actual operating region of the systems are indicated. It can be seen that S2 gains ofmore than an order of magnitude have been realized which translates to measurement timegains of more than a factor of 100. System specifications are contained in previousreports (Steed3).Photographs of the field interferometer system are given in Figs. 3 and 4. Thesystem is housed in two sections, an optical section shown in its cryogenic enclosure andthe electronics and support equipment which are housed in a portable shipping container.The shipping container also serves as the field operating platform.Fig. 3. FWI optical head in fieldoperational environmentalenclosure.Fig. 4. FWI field support electronicsincluding FFT unit and record-ing system.The basic components in the electronic section are the drive control electronics forthe interferometer, the time -code generator, and an FFT processing system which performs a16,384 point transform and displays the results in less than a second at the end of eachinterferogram scan. The spectral results of each scan are continually updated on thedisplay giving immediate feedback to the system operators on the status of the measurement.The FFT unit can also be set to co -add a number of scans if desired. Interesting spectraldisplays from the FFT unit can be permanently recorded on an XY plotter for futureexamination. The shipping rack also contains instrumentation for supporting experiments(radiometer, photometer, and weather information).The interferograms are recorded on a Hewlett Packard 3968A analog FM instrumentationrecorder mounted on the left side of the rack. Since near it emissions from the night skyexhibit a line profile instead of a continuum, the interferogram dynamic range is notlarge. The dynamic range of the FM recorder is sufficient to handle the signal -to -noiseratio resulting from the sky source and little or no signal degradation has been observedas a result of the recording system. When performing blackbody calibrations, it isrequired to co -add a number of scans since the needed dynamic range is large for the broadspectral features of the internal calibration sources. During field operations the FFTunit receives its input from the playback amplifiers of the tape recorder so that theSPIE Vol 289 1981 Fourier Transform Infrared Spectroscopy /1981 ) / 203   ld-wi    l = %[n( 2-! /A» 1  and n = 2%/t 2 a  %  c), fi = % )inte n index of refraction of optical   wit wedge angl             /AF                   t z a n d C a F 2 -      -wid               lds -v   9-     r   d,  t t th c be the  b fi c al be seen fr . Fig 2 where  in   se that Q  t              ( 3 )   i           a s as the fiel operat pl rm.  i      i t e    I       t th t ime-c o d e g en er a or, n  n T        ,   O' .       1   1       11 1 1  t b    e        t t t s o the m  c also be set to co add  r                 eter,  Informatie    t  r     um,      th  I  nal -noi              -ad  a number of scans ince e  f     s    i        I  ol.  ectroscopy (1981) /Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/15/2014 Terms of Use: http://spiedl.org/termscontinually displayed spectrum demonstrates that the entire system is operating properlyand data are properly recorded.The FFT unit is a Unigon Model 4516 Real Time Spectrum Analyzer and is used as a fieldcheck on the system operation only. Data reduction is performed at the end of the fieldtrip with a home -based special purpose computer which offers greater flexibility.The interferometer enclosure is fabricated so that the instrument can be cooled toliquid nitrogen temperatures when desired. The enclosure contains a vacuum shell aroundthe instrument and a 30 -liter nitrogen holding tank to facilitate this cooling. Whenoperated cryogenically, the cold boil -off gas from the liquid nitrogen is used to supportthe gas- lubricated bearing and then the dry nitrogen gas is exhausted around the windowinterface between the interferometer and the stack to prevent frost from building up on theinterface window.Fig. 5 shows one 40 second spectral scan of an auroral arc taken with this system inAndoya, Norway, on November 11, 1980. The FWI instrument was pointed at the zenith, and aseparate dual channel radiometer (shown also in Fig. 3) was co- aligned with theinterferometer to provide long -term OH and 0214 emission levels as well as help verify theinterferometer calibration. The instrument was operating at a resolution of 2.5 cm -1 inthe 0.85 to 1.7 pm spectral range. During the arc presence, a co- aligned photometerregistered 30 kR of 3914 A auroral emission in the FOV. The arc was fairly stable duringthe measurement period.In addition to the N2+ Meinel, N2 +1P, NI, and OI enhancements, the He 10830 A line wasobserved to be enhanced during the arc and remained at an elevated level after the arcdecreased. Detailed analysis of the data will be forthcoming in future publications. Withthe instrument throughput it is possible to obtain excellent signal -to -noise results in asingle scan. Transient events such as aurora and twilight transitions can for the firsttime be analyzed at this spectral and temporal resolution.Preliminary measurements with a very recently acquired Ge detector from AppliedDetector Corporation indicates that an instrument sensitivity of better than 5 x 10-15w /cmzsrcm 1 can be achieved with this system at 1.4 um in a 20 second scan. When thisdetector is properly interfaced with the instrument, the FWI should be five times moresensitive than when the measurements reported herein were taken. This work has beensponsored by the Air Force Geophysics Laboratory and the Air Force Office of ScientificResearch.References1. Despain, A.M., D.M. Baker, A.J. Steed, and T. Tohmatsu, "Extended field largeaperture interferometer -spectrometer for airglow surveys," Applied Optics, 10(8) :1870,1971.2. Despain, A.M., F.R. Brown, Jr., A.J. Steed, and D.J. Baker, "A large aperturefield- widened interferometer -spectrometer for airglow studies," Aspen Int. Conf. FourierSpectroscopy AFCRL -71 -0019, A.F. Cambridge Research Lab., Bedford, 293 pp., 1971.3. Steed, A.J., "Field- widened interferometry at Utah State University (USU),Society of Photo -Optical Instrumentation Engineers, Vol. 191, pp. 2 -14, 1979.4. Haycock, R.H., and D.J. Baker, "Infrared prism interferometer -spectrometer using agas -lubricated drive bearing," Infrared Phys., 14:259, 1975.5. Baker, D.J., A.J. Steed, and A.T. Stair, Jr., "Development of infraredinterferometry for upper atmospheric emission studies," Applied Optics, Vol. 20, No. 10, 15May 1981.6. Haycock, R.H., "Rocketborne cryogenically cooled field- widened interferometer forthe 2 to 8 um spectral region," Society of Photo -Optical Instrumentantion Engineers, Vol.191, pp. 120 -129, August 27 -28, 1979.7. Huppi, R.J., A.J. Steed, and E.R. Huppi, "Cryogenically cooled Fourier transformspectrometers," Society of Photo -Optical Instrumenation Engineers, 1981 InternationalConference on Fourier Transform Infared Spectroscopy, University of South Carolina,Columbia, South Carolina, June 8 -12, 1981.8. Baker, D.J., "Field- widened interferometers for Fourier spectroscopy," (Chapter2), Spectrometric Techniques Vol. 1, ed. by G. Vanasse, Academic Press, Inc., New York,1977.9. Bouchareine, P., and P. Connes, "interferometer with a compensated field ofspectroscopy by the fourier transform," Le Journal de Physique et le Radium, 24(134),1963.204 / SPIE Vol. 289 1981 Fourier Transform Infrared Spectroscopy (1981)    ri and is used as a field  o  t   -based special purpose computer hich  r      de      -liter nitro      the cold boil    lubrica  t      fr bui up on the .  a   i  , 11 1980. a th z an a           alig    long-term OH and 62 l A sion    i w operating at a resolution of 2.5 cm" 1    y   a aligned  A i t  d  + N ~f~lP, NI, nd OI ements, A lin wa         i fu    excellent signal -noise      t f               o   0"" 1  /cm 2 srcm~' 1 can be   y     i   i                     . ,     ter-spectro    10(8)    A St   r,   widened er-spe   In    FCRL-71-0019, A.F. Cambridge R  2 pp 1971   wide       -Optical 191, p. 2 ,  and D. Bake "Infrared prism interferometer using a -lubricated drive bearing," 1 1975.            etry r  2 10 15  "Rocketborne cryogenically c oled field- inter for     y  o -Optical   120-129, 27-28,  ,      Photo-Optical              , June 8 2,    widened interferometers r S    Vo 1,     ,            S      l   / SPIE Vol. 289 1981 Fourier Transform Infrared SpectrDownloaded From: http://proceedings.spiedigitallibrary.org/ on 10/15/2014 Terms of Use: http://spiedl.org/terms2U900600300o154µm-OH3,1 ;OH(2,0)N2+M (O,IN2IP(1,3).L...:.Lt4,044,1.i.al.A....,u_11M...61.!A4....:_..t_6500900600300OOININI(A)N2lF(1,2)OZ(IgOH(8 5)(B)N2+M( !,N.,IP,1,2;+4u3a....,..'OH(7,4)OH(6,3).!2µm670069b071007 075007700790081008300850087008900WAVENUMBER (CM-1)1.15f.. mN2+M ( 0,0 )HelOH(5,2)N2 IP (O,O)NI(A CONT.)NIO90µmAURORA.,......,_.. w.....u4.A.JaWiraJJ..v,,..+uy.YJ1.M1eL'JwyJL1101411L,1_t1VLi4161Jl1.W.OH (4, I)(B CONT.)NON - AURORA870089009100930095059700990001001030005001070010900100WAVENUMBER (CM -t )Fig. 5.Plot of spectral emission from auroral arc (A) compared with no auroraairglow emission (B).Spectra Awas taken approximately 3 minutes later than spectraBafter the arc formed.Both were taken looking atthe Zenith on November 11, 1980, from Andoya, Norway.Measurement time was 40 seconds for each scan.INTENSITY R-CM INTENSITY (R-CM)CD ^JOr\>.T:3/. 255 7557 Fourier Transform Infrared Spectroscopy (1981) / 205Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/15/2014 Terms of Use: http://spiedl.org/terms

Recent Auroral Measurements Using a Field-Widened Interferometer Spectrometer

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1122&context=sdl_pubs

Recent Auroral Measurements Using a Field-Widened Interferometer Spectrometer

Abstract

Similar works

Full text

Available Versions

DigitalCommons@USU