Four-channel, moving source scanning spectrometer for use in ballistic range radiometr

Whiting, E. E.

English

NASA Technical Reports Server

By E l l i s  E. Whitin@ 
Vehicle Environment Division 
Ames Research Center, NASA 
Noffett  Field, California 
- q7)7 
lq (q PO \ :* '3 -0 /: 
source f o r  t h i s  type instrument. Proof of t h i s  
observation i s  shown i n  Fig. 1, which i s  a s e l f -  
luminous picture  taken from t h e  s ide of a b lunt -  
-Lqqku hSTFfACT 
< !  $ 
A new moving-source scanning spectrometer f o r  
use i n  bal l is t ic-range radiometry i s  described. 
This instrument uses the radiat ing gas cap of 
bal l is t ic-range models as the source and eritrance 
slit and develop: a scan across the spectrum f r o m  
2,000 t o  10,000 A by vir tue of the m d e l  motion 
along i t s  f l i g h t  path. The spec t ra l  resolution fcr 
f u l l  spec t ra l  coverage v i e s  between 10 and 100 A, 
depending on gas-cap thickness and tes t -sect ion 
window size.  The instrument has been cal ibrated 
spec t ra l ly  f o r  both wavelength and intensi ty .  
Sample experimental r e s u l t s  are shown. 
INTROIWCTION 
The  b a l l i s t i c  range has been a valuable t o o l  
f o r  experimental study of atmospheric entry aero- 
dynamics and heating f o r  several  yeaxs. 
the u t i l i t y  of the b a l l i s t i c  range has been 
extended t o  t h e  stuay of gas kinet ics ,  molecular 
physics and gas radiation. In  these latter appl i -  
cat ions it is  necessary t o  observe spec t ra l ly  the 
radiat ion emitted f r o m  the  gas cap formed ahead of 
the  model. Several spectrometric techniques have 
been used f o r  this task;  however, most of these 
have serious l imitat ions.  For example, a spectro- 
graph gives excellent spectral  resolution, bu t  t h e  
time available f o r  exposure, normally a microsecond 
o r  less ,  i s  not generally suff ic ient  f o r  exposure 
of photographic f i l m .  Narrow band-pass radiometers 
using phototubes have good response charac te r i s t ics  
but  give poor spec t ra l  coverage unless very large 
numbers of radiometers are used o r  large nuuibers 
of tests are  performed with radiometers where 
wavelengths are  v a r i d l e .  There is ,  however, one 
instrument for spectral  data  acquisit ion idea l ly  
sui ted t o  ba l l i s t ic - range  t e s t i n g  that has good 
response, good spec t ra l  coverage, and reasonable 
spec t ra l  resolut ion.  This instrument i s  cal led a 
moving-source scanning spectrometer. The pupose 
of this paper i s  t o  discuss the e s s e n t i a l  features 
of t h i s  type instrument and describe a four-channel 
i n s t w n t  i n  operation at Ames Research Center. 
Recently, 
PRINCIPLE OF OPERATION 
A moving-source scanning spectrometer i s  
simply one that uses the  source itself as t h e  
entrance s l i t  and develops a scan across the spec- 
t r u m  by v i r t u e  of the motion of the source. I n  
December of 1959, Messrs. Woodcock and Jones of 
the  Perkin-Elmer Corporation suggested t o  
St .  P i e r r e  of CARDE' that the luminous .gas cap of 
blunt-nosed mdels at trypersonic speeds in 
ba l l i s t ic - range  t e s t s  would provide a good mving 
*Resee;rch Sc ien t i s t  
nosed model f ly ing  at 7.9 hn/sec in to  air at about 
0.1 atmospheric pressure. 
by an image-converter camera with an exposure time 
of 0.1 gsec. Clearly, the  gas cap, as viewed from 
the  side,  appears much l i k e  an illuminated curved 
sl i t ,  indicating the mving-souxe scanning pr inc i -  
p le  i s  applicable. Spectral  resolution i n  any 
spectrometer depends on the width of the entrance 
and e x i t  s l i ts .  The width of the entrance s l i t  i n  
t h i s  case i s  the width of the gas cap; hence, spec- 
tral resolution will depend on model s ize .  
other regions of the flow f i e l d ,  such as the  wake, 
were luminous, the  moving source would be extensive 
and the mving-source scanning concept would be of 
l i t t l e ,  if any, value f o r  spec t ra l  d e t a i l .  
This picture  was taken 
If 
The principle  of operation of a moving-source 
scanning spectremeter i s  i l l u s t r a t e d  i n  Fig.  2.  
The model i s  shown at  a point in i t s  f l i g h t  past  
the window i n  the test section. A t  any ins tan t  
the instrument ac ts  as a normal mnocbromator; that 
is, the  spectrum dispersed by the grat ing is  
focused, and a s l i t  i n  the  foca l  surface le ts  a 
small portion of the spectrum f a l l  on the photo- 
cathode of a mult ipl ier  phototube. The scanning 
feature  of the  instrument i s  intmchced by the  
model motion along the f l i g h t  path. As t h e  mdel 
moves, the spectrum mves and, consequently, i s  
swept past  the  e x i t  s l i t .  The var ia t ion i n  inten- 
s i t y  across the  spectrum i s  sensed by the photo- 
tube and recorded d i rec t ly  on f i lm v ia  an 
oscilloscope. 
PRIOR ART 
The f irst  known instrument of t h i s  type 
developed f o r  bal l is t ic-range radiometry was con- 
s t ructed in  1960 by M r .  St .  Pierre  of CAFXIE.l 
inst-nt covezed the spectrum f r o m  4,000 t o  
6 , m A  with 40 A resolution. 
was made i n  1962 at Ames Research Center of the 
NASA by Messrs. Victor Reis,' Roger Craig, and 
W i l l i a m  Page, by modifying a Ferrand W-VIS grat ing 
lnonochromator. This inFtrument c o F r e d  the  spec- 
trum from 2,w t o  4,300 A with 100 A resolut ion.  
The present author has b u i l t  i?struments with 5 A 
resolution, but with only 200 A spec t ra l  coverage. 
MI-. D a v i ?  Cooper of Ames has b u i l t  an instrument 
with 25 A resolution and l,,ooO A spec t ra l   overage.^ 
The success of these instruments and the  increasing 
demand f o r  gas-cap spectra with reasonable resolu- 
t ion  encouraged the design of the present instru-  
ment. "he design goal f o r  this i n s t m e @  was to  
cover t h e  spectrum from 2,000 to 10,000 A and main- 
t a i n  reasonable spec t ra l  resolution. The design 
considerations t h a t  l ed  t o  choosing a four-channel 
concept f o r  the design are  discussed i n  the next 
H i s  
A second instrument 
section. 
https://ntrs.nasa.gov/search.jsp?R=19660028350 2020-03-24T02:00:48+00:00Z
DESIGN CONSIDERATIONS 
The pr incipal  fac tors  affect ing the  design of 
a par t icu lar  i n s t m n t  are  spec t ra l  resolution 
and spec t ra l  coverage, f r o m  an opt ica l  viewpint ,  
and signal-to-noise r a t i o  and r i s e  time, f r o m  the 
electronic  viewpoint. Spectral  coverage was the 
primary constraint ,  because t h e  main purpose of 
the  present i n s t r m e n t  was  to cover the spectrum 
f r o m  2,000 t o  10,ooO P\.  Note i n  Fig. 2 t h a t  the 
r e l a t i v e  or ientat ion of the grating, the focusing 
mirror, and the  e x i t  s l i t  i s  fixed. Therefore, 
the d i f f rac t ion  angle that d i rec ts  l i g h t  f r o m  the 
grating t o  the  s l i t  i s  fixed. This f a c t  s implif ies  
the use of the  grat ing equation i n  explaining the  
fac tors  affect ing spec t ra l  coverage. Fig.  3 i l l u s -  
trates the  o p t i c a l  fac tors  involved. Equation (1) 
i n  t h i s  figure shows that t h e  spectral  coverage 
M i s  given, approximately, by the product of the 
angular sweep A i  and the distance between rul ings 
on the grat ing.  I n  b a l l i s t i c  ranges, window s ize  
usual ly  limits the sweep angle t o  less t5an 10'. 
Therefore, t o  produce the desired 8,000 A ?can by 
a s ingle  channel would require b = k0,OOO A, corre- 
sponding t o  a grat ing with 250 lines!mm. 
ber  of lines/mm has a strong e f f e c t  on the  spec t ra l  
resolut ion as shown by Eq. (3)  i n  the lower par t  of 
Fig. 3. This equation shows that spec t ra l  resolu- 
t i o n  i s  improved by using a grat ing with as many 
l i n e s / m  as possible.  
600 lines/mm are available and economical. How- 
ever, with a 600 line/mm grating and ai st i l l  
r e s t r i c t e d  t o  100, the  spectTal coverage given by 
Eq. (1) is  onlyoabout 3,000 A. 
required 8,000 A m u l d  then require more than one 
channel. 
The num- 
Good rep l ica  gratings with 
To cover the 
Spectral  coverage must also be considered 
from the standpoint of instrument sens i t iv i ty ,  
including grat ing efficiency and phototube response. 
Grating eff ic iency i s  enhanced by blazing f o r  a 
par t icu lar  wavelength, but the eff ic iency decreases 
rapidly at wavelengths away f r o m  t h e  blaze wave- 
length. The response of the photocathode of the 
mult ipl ier  phototube further r e s t r i c t s  spectral  
coverage, since each surface has good response over 
a l imited s p e c t r a l  region. For these reasons a 
multiple-channel instrument appeared b e s t  t o  pro- 
vide the wide spec t ra l  coverage with good response 
and t o  achieve reasonable resolution. There is ,  i n  
addition to t h e  o p t i c a l  requirements, the require- 
ment t h a t  the  electronic  r i s e  time be short enough 
t o  follow the  s ignal  increase as the  m d e l  image 
crosses the  e x i t  slit .  This is  d i rec t ly  re la ted  
t o  the  time required f o r  the model t o  mve a dis-  
tance given by the  radiat ing gas-cap thickness. 
the Ames f a c i l i t i e s  the  m i n i m  gas-cap thickness 
of i n t e r e s t  i s  about 1 mm and t h e  maximum veloci ty  
i s  about 10 m/psec .  
be l e s s  than 100 nsec. 
I n  
Therefore, the r i s e  t i m e  mst 
FIML DESIGN 
The considerations discussed i n  the  pr ior  
section led  t o  the select ion of a four-channel con- 
cept f o r  the  instrument design. Each channel has 
i t s  own grat ing,  focusing mirror, phototube, and 
associatzd e lec t ronics  and covers a spec t ra l  range 
of 2000 A f o r  fill spec t ra l  coverage. The gratings 
selected have 600 lines/mm and the spec t ra l  resolu- 
t i o n o f o r  full spec t ra l  coverage verries f r o m  10 t o  
100 A, depending on gas-cap thickness (model s ize)  
and window width i n  t h e  test f a c i l i t y .  Resolution 
can be increased by using a longer f o c a l  length 
collimating mirror and moving the  instrument away 
from the  f l i g h t  path. The enhanced resolution is  
obtained a t  the expense of the  phototube s ignal  
which becomes weaker, and the resolution i s  u l t i -  
mately l imited by the signal-to-noise r a t i o  of the 
system. I n  addition, the  spec t ra l  coverage i s  
l imited when the  instrument is  moved back from the 
f l i g h t  path as a r e s u l t  of vignett ing at the  t e s t -  
section window. However, the e n t i r e  spectrum can 
be swept out on successive tests. 
A plan view of the completed instrument i s  
shown i n  Fig. 4. Photographs of the instrument 
are  shown i n  Figs.  5, 6, and 7. The voltage- 
divider amplifiers were designed by MI-. Donald 
Humphry of the  Ames' Electronics Research Branch. 
The c i r c u i t  design f o r  these components i s  shown 
i n  Fig.  8. The amplifier i s  a current amplifier 
with a current gain of 100. 
2.0-inch-diameter DuMont 10-stage photomultipliers 
with S - l , S - l l , S - l 3 ,  and 5-20 surfaces. The r i s e  
time of the conbined photomultiplier, voltage- 
divider,  amplifier system has been measured, with 
the S-1 surface, t o  be about 50 nsec, which s a t i s -  
f i e s  the requirement of less than 100 nsec. 
The detectors used a re  
WAVELENWH CALIBRATION 
Relative wavelength cal ibrat ion of the  ins t ru-  
ment was  performed by moving a s l i t  illuminated 
a l te rna te ly  with Hg, Cd, and Ne l i g h t  past  the  
instrument i n  the posit ion of the model f l i g h t  
path. This cal ibrat ion i s  described in  Fig. 9. It 
w a s  found t o  be impractical t o  f i x  the wavelength 
scale precisely before a t e s t  i s  made. Therefore 
it i s  necessary t o  ident i fy  one feature  i n  the 
spectrum t o  es tab l i sh  the absolute wavelength 
scale .  The gratings and focusing mirrors are 
fixed i n  posit ion and a c o m n  collimating mirror 
i s  used f o r  all four  channels, which fixes the  
wavelength increment between channels. Theref ore, 
i f  one l i n e  o r  band can be ident i f ied i n  any chan- 
nel, the  wavelength scales i n  all four channels are  
determined. 
I"SITi CALIBRATION 
The response of the  instrument w a s  determined 
by cal ibrat ion with an NBS cal ibrated tungsten 
filament lamp. This in tens i ty  Salibration w a s  
performed from 2,500 to 11,000 A and i s  shown i n  
Fig.  lc. The dotted curve shown from 1,700 t o  
2,700 A w a s  estimated by combining estimates of the 
spectral  var ia t ion of quartz transmission, grating 
efficiency, and photocathode sens i t iv i ty  and scal-  
ing theseeresul ts  t o  match the  measured response 
at 2,700 A. 
The in tens i t ies  recorded during the  cal ibra-  
t i o n  were measured by chopping the incident beam 
from the standard lamp a t  80 cps and using a 
synchronous amplifier f o r  readout. lxzring the cali-  
brat ion it w a s  necessary to  use extreme care t o  
minimize t rans ien t  e f f e c t s  in  the  photomultiplier 
Circuitry.  When each reading was taken, the high 
voltage was applied t o  the  voltage-divider c i r c u i t  
and a shut ter  opened t o  l e t  the chopped l i g h t  
s t r i k e  the  photocathode. 
the synchronous amplifier required from 3-5 seconds 
t o  a t t a i n  a steady reading. After the reading w a s  
recorded, the high voltage w a s  turned of f  and the 
shut ter  w a s  closed. By t h i s  technique ca l ibra t ion  
values could be repeated within 10 percent. Con- 
di t ions f o r  tes t -da ta  col lect ion are  somewhat 
different  from the ca l ibra t ion  conditions i n  that 
the t e s t  data a re  recorded as a pulse of 10-20 psec 
duration. Therefore, i n  addition t o  the  above 
procedure, it was necessary t o  !waswe t h e  i n i t i a l  
t ransient  e f f e c t s  i n  the  photomultiplier, w l t a g e -  
divider,  amplifier c i r c u i t  and correct the  cal ibra-  
t ion  data  t o  zero time. 
The integrat ion t i m e  i n  
SAMPLE ExpERlMENTAzl RESULTS 
The instrument described i n  t h i s  paper has 
been i n  operation at Ames Research Center f o r  about 
one year. An experimental spectmm reduced from 
the oscil loscope t races  recorded by the f o u r  chsn- 
nels  is  shown i n  Fig.  ll. T e s t  conditions and a 
mass spectrographic analysis of a gas sample taken 
from the t e s t  section apTroximately three minutes 
before the t e s t  are given i n  the f igure.  
tral resolut ion for this t e s t  i s  about 25 A. The 
mst prominent features  of this spectrum are  the 
band sequences of m(Violet)  from 3,500 t o  4,500 A 
and CN(Red) from 6,000 t o  10,OOO A. 
able ap two C,(Swan) band sequences a t  4,700 and 
5,100 A, $he NH baud at 3,360 8, and the OH bands 
a t  3,lGO A. 
7,775 A and t h e  atomic carbon l i n e  a t  2,478 A are 
also evident. 
p e  spec- 
A l s o  i d e n t i f i -  
A group of atomic oxygen l ines  Pt 
Theoretical  predictions,  assuming chemical and 
thermodynamic equilibrium i n  the gas cap, are  shown 
i n  t h i s  f igure f o r  NO(Ganrma), C0(4+), Cz(Swan), 
m(Viole t ) ,  and CN(Red) . 
were computed by Ms. Henry Woodward of Ames, u s i n g  
the  average &-branch approximation described i n  
R e f .  4. The theore t ica l  spec t ra l  features can, f o r  
the mst par t ,  be recognized in the  experimental 
spectrum. The measured and calculated i n t e n s i t i e s  
of the  CN(Red) and Ca(Swan) systems m e  i n  reason- 
ably good agreement. 
measured and calculated in tens i ty  of t h e  CNfViolet) 
system has been shown t o  be due t o  photocathode 
saturat ion and t o  black-body limiting. A cur-ve f o r  
1/10 black-body radiat ion a t  the wan gas-cap t e m -  
perature of 7,6000 K and a projected mea of 
8.6 inn? i s  shown i n  the f igure.  The problem of 
photocathode sa tura t ion  was eliminated i n  l a t e r  
t e s t s  and agreement between theory and experiment 
has been g r e a t l y  improved. 
theory and experiment f r o m  1,900 to  2,500 A i s  
believed t o  b e  due t o  uncertaint ies  i n  the  cal ibra-  
t i o n  i n  t h i s  s p e c t r a l  region, t o  black-body limit- 
ing, and perhaps t o  the high f number used i n  
calculat ing t h e  in tens i ty  of the C0(4+) system. 
The f numbers used i n  the  calculations a r e  shown i n  
table I. T h i s  f igure demonstrates the u t i l i t y  of 
t h i s  instrument f o r  identifying radiat ing species 
i n  the gas cap and measuring the radiant pwer 
emitted i f  proper care i s  used i n  cal ibrat ion and 
t e s t i n g .  
The theore t ica l  values 
The difference between the 
The differenceobetween 
1. St.  Pierre ,  Claude: The Visible Emission From 
1/2" Hypervelocity Models Keasured With a 
Moving Target Scan Monochromator. CARDE Tech. 
Memo AB-59, Masch 1960. 
2. Reis, Victor H. :  Osci l la tor  Strengths f o r  the  
N 2  Second Posi t ive and N2+ F i r s t  Negative 
Systems From Observations of Shock Layers 
About Hy-personic Project i les .  J. Quant. 
Spectr. Radiative Transfer, vol. 4, no. 6, 
1964, P-  783- 
3. Cooper, David M.: High-Resolution Experimntal  
Spectra and Theoretical  Studies of Equilibrium 
Radiation From a Blunt Body Traveling at 
37,000 Ft/Sec. AIAA Paper 66-104. 
4. W i l l i a m s ,  M. J.; and Treanor, C .  E.: A Method 
f o r  Calculating Diatomic Spectra Using a 
Dig i ta l  Computer. Rep. QM-l626-A-5, Cornell 
Aero. Lab., 1962. 
TABLE 1.- F NUMBERS USED I N  THEORETICAL SPEcIliA 
C O b P ~ ~ I O N S  SHOWN IN FIGURE ll 
Species F number 
0.02" CNV 
CN 
c2?-) .034 
CO( 4+) 15 
.0072 
* Corresponds t o  a heat of formation = 8.2 eV 
n n 
Y O a L  - - V F -  - - - - F L M  PATH TEST SECTlON 
/WINDOW 
U U 
WTOWLTIPLIERS CJI) 
EECTRONKS-Y 
EXIT SLIT WLDERS 
LIGHT WIELDS 
LIGHT W L E  
Fig. k.-  Four-chamel spectrometer. 
Flg.  1.- I m y e  ccnverter photcg-aph of model i n  
:light (exposure t i m e  = 0.10 psec). 
TEST SECTION 
# GUN A+-----+ 
, ’ QUARTZ WINM)W 
J l  
\FOCUS MIRROR 
COLLIMATING MIRROR 
Y I g .  2.- DiKrac sf mvirg source s c ~ ~ n i c g  
spectroneyer.  
b 
V’ 
sin I : sin e -nr/b 
n= I FOR DIFFRACTION INTO FIRST ORDER 
FOR FIXED DIFFRACTION ANGLE iei 
&,-A2=b [sin i2-sin i l l  
OR FOR SMALL i 
A r o b A i  
Fig. 5.- Searzing spectrome’er. 
I 
Fig. t.- Scanning spect,rometer, entrance port ,  
t r i g g e r  phototube , and ccll impting n i r r o r .  
Fig .  3. - Voltage-divldnr, zxplifier c i r c u i t .  
)i I+ 16 00 
WAVELENGTH, 
i 
)11*800 
C H A W L  )L, 
b I 2000 
0 2 4 m  
0 3 6000 
0 4 Boo0 
f =.788 rn 
101 - I /  
W e 
wTE: 
100 - S M  EViDEWn THAT 
TWIN DDYN IN OATA 
W E  TO SCI\TTERED 
LIGHT l I 1 t ,  
10-2- , 
ESTIYATED 
aRVE 
10-3L I I I I I I I I I t 
1000 so00 5oca 7000 saa llD0 
WMLENGTH. 4. 
Fig. 10.-  In%ensi?;y calibrat-An of  s caming  spec- 
trome?;er adius?;ed t o  hlgh wltzge = 1000 V. 
YAVELEYGTH. i 
Fig. 11.- Experimental spe:tr.m recorded by f0.z- 
channel scanning spectrometer compared with 
t h e o r e t i c a l  spectrirni caic.Aa%ed for e q u i l i b r i ~ m  
cond5tions. 
UODEL POSITION ALONG FLIGHT PATH, crn 
Fig. 9. - Wavelength c a l i b r 3 x i o n  of scanning 
spec:ro?.e:er. 


A new four-channel scanning spectrometer for ballistic-range radiometry

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server