Radio astronomers, in common with many others who work with radio antennas, have always felt rather handicapped by the basic physical relationship between the size of an antenna (measured in wavelengths) and its resolving power (or beamwidth): ΔΘ ~ λ/D, where ΔΘ is the beamwidth in radians, λ the wavelength and D the diameter of the aperture. The largest fully-steerable antennas used as radio telescopes have dimensions of at most a few thousand times their minimum useable wavelength; as a result the beamwidth of such instruments, and hence the angular resolving power, is at best a few minutes of arc

Moffet, Alan T.

Caltech Authors - Main

Combined URSI/GAP Session, Paper 3.  9:00 A.M., Wedresday,  Schoenberg Hall 
THE WHOLE-EARTH ARRAY - 
DEVJZOPMENTS IN VERY-LONG-BASELINE INTERFEROMETRY 
Owens  Valley  Radio  Observatory* 
Alan T. Moffet 
California  Institute  of  Technology 
Radio  astronomers,  in  common  with  many  others  who  work  with  radio 
antennas,  have  always  felt  rather  handicapped  by  the  basic  physi- 
wavelengths)  and  its  resolving  power  (or  beamwidth): A0 - h/D, cal  relationship  between  the  size  of  an  antenna  (measured  in 
where A0 is  the  beamwidth  in  radians, X the  wavelength  and D the 
diameter of the  aperture.  The  largest  fully-steerable  antennas 
used  as  radio  telescopes  have  dimensions  of  at  most  a  few  thou- 
sand  times  their  minimum  useable  wavelength;  as  a  result  the  beam- 
width  of  such,instruments,  and  hence  the  angular  resolving  power, 
is  at  best  a  few  minutes  of  arc. 
Radio  astronomers  have  for  some  time  evaded  this  limit  by  use  of 
interferometers (1). Small  antennas  at  distances  of  many  thou- 
sands  of  wavelengths  have  been  connected  by  coaxial  cable  or 
microwave  link  to  obtain  information  about  the  angular  distribu- 
tion  of  radio  sources on smaller  angular  scales.  When  many  such 
observations  are  made  with  different  spacings  and  orientations 
of  the small antennas,  two-dimensional  radio  maps  may  be  obtained 
with  resolution  of  the  order  of  the  wavelength  divided  by  the 
largest  spacing  between  the  antennas.  This  process  is known as 
aperture  synthesis (2). By  means  of  this  technique  many  radio 
sources  have  been  mapped  with  resolutions  of - 10 seconds  of  arc. 
Microwave-linked  interferometers  have  been  extended  to  baselines 
of  several  hundred  kilometers,  giving  resolving  powers  of - 0.1 
second  of  arc (3) .  
About  eight  years  ago  it  became  clear  that  wide-band  tape  record- 
ings  might  permit us to dispense  with  the  real-time  link  between 
might  be  arbitrarily  far  apart,  subject  only  to  the  restriction 
the  elements  of  an  interferometer.  If so, then  the  elements 
that  the  radio  source  to  be  studied  must  be  simultaneously  visi- 
ble  at  both  observing  sites.  The  tape  recordings  are  later 
played  back  in  synchronism,  and  the two recorded  signals  are 
cross-correlated  as  are  the  signals  which  come  directly  from  the 
antennas  in  a  real-time  interferometer.  This  technique  has  come 
to  be know0 as  very-long-baseline  interferometry (VLBI). 
For  the V L B I  technique  to  succeed,  it  is  necessary  that  each 
with  sufficient  stability  that  the two receiving  local  oscil- 
recording  station  be  equipped  with  an  atomic  frequency  standard 
*Supported  by  the U.S. Office  of  Naval  Research  and  the  National 
Science  Foundation 
2 
l a t o r s  do n o t  s l i p  i n  phase by more than about a rad ian  in  the  
durat ion of a recording. Rubidium  and  hydrogen  standards  have 
been  used for  this  purpose.   Furthermore i t  i s  necessary that  
the  rec iproca l  of the  recording  bandwidth. Time is kept  with 
the recording medium preserve time with an error  of  less than 
the necessary accuracy by a clock driven from the atomic fre-  
have  been  successfully  used,  with  bandwidths of - 1 NHz. The 
quency standard. Both d i g i t a l  and analog  recording schemes 
by independent groups i n  Canada  and i n  t h e  USA i n  1967 ( 4 ,  5 ) .  
technique was f i r s t  a p p l i e d  t o  s t u d y  of d i s tan t  rad io  sources  
wavelengths as short  as 3 o r  4 cm, giving angular resolutions 
Baselines have been extended to nearly the earth 's  diameter at  
f iner  than  seconds  of  arc.  
have  been  reviewed by Burke ( 6 )  and Cohen (7). It has  been 
Resul ts  from t h e  f i r s t  s e v e r a l  y e a r s '  work with this  technique 
show var ia t ions  of  the  in tens i ty  of their  radio emission, have 
found t h a t  many d is tan t  rad io  sources ,  espec ia l ly  those  which 
components with angular  s izes  uf s 0.001  seconds of arc .  Since 
these sources  are thought to be lo8 t o  l o 9  l i g h t  y e a r s  d i s t a n t  
from us ,  the l inear  diameters  of  these components a r e  less than 
a l igh t  year .  In one relatively  nearby  radio  galaxy  (only 3 x 
l o 7  l i g h t  y e a r s  away:) a component with a diameter of perhaps 
a l i g h t  month has been detected (8). 
When tuned to the wavelength of a r a d i o  s p e c t r a l  l i n e ,  t h e  
radio interferometer  can give information about  the s ize  and 
shape of  the regions emit t ing the l ine radiat ion.  Of par t icu-  
lar  i n t e r e s t  have been VLBI s tud ie s  o f  i n t e r s t e l l a r  mase r s .  
These are  regions which emit in t ense  r ad ia t ion  in  the  18  cm 
l i n e s  of t h e  OH molecule  or i n  the  1.3 cm l i n e  of H20. Because 
of their  extremely high surface brightness,  maser amplification 
i n  t h e  i n t e r s t e l l a r  inedium seems to be the only plausible expla- 
na t ion  for  these  objec ts .  Angular  d iaae te rs  of  0.005 and 0.05 
seconds of arc have been observed for the OH sources and 
c 0.001  seconds of a r c  f o r  t h e  Hz0 sources. A t  d is tances  of  a 
few thousand l i g h t  y e a r s ,  t h i s  would correspond to dimensions 
of the OH sources, t he  more dis tant  ones seem t o  have the  
of t h e  order  of t he  radius  of t he  ea r th ' s  o r b i t .  I n  the  case  
greater  diameter,   This  leads us t o  s u s p e c t  t h a t  t h e  measured 
diameter i s  r e a l l y  t h e  r e s u l t  o f  s c a t t e r i n g  i n  t h e  i n t e r -  
s tellar medium. A t  the shorter wavelength of  the H 2 0  l i n e ,  t h e  
s c a t t e r i n g  is expected to be much smaller ,  and the observed 
angular  diameters  are  a lso smaller  ( 9 ,  10). These masers are 
usual ly  located near  s t rong sources  of infra-red emission, and 
i t  is thought that  they are pumped by energy a t  infra-red 
wavelengths. 
Diameter changes are seen  in  many of the sources with variable- 
intensi ty   radio  emission  (11,   12) .   In   the  best-establ ished 
case,  the quasi-s te l lar  source 3C 279, a ve ry  c l ea r  change i n  
the  in t e r f e romet r i c  v i s ib i l i t y  func t ion  w a s  seen over an inter-  
v a l  of four  months, corresponding to a change in  d iameter  of 
3 
9 percent i n  t h a t  time. (The  change in angular diameter w a s  0.14 
x seconds  of arc.)  This  object  has  a  redshift of 0.538 
implying a distance of about 6 x lo9 l i g h t  y e a r s .  A t  t h i s  d i s -  
diameter change is about ten times the speed of l i g h t  (12). 
tance the apparent t ransverse ve loc i ty  implied by t h e  observed 
Although it is qu i t e  poss ib l e  fo r  an  ob jec t  which is expanding 
at a ve loc i ty  c lose  to  tha t  o f  l i gh t  t o  appea r  t o  expand at super- 
l i g h t  v e l o c i t i e s  ( 1 3 ) ,  t h e r e  a r e  s e v e r a l  d i f f i c u l t i e s  i n  a p p l y i n g  
th i s  explana t ion  to  3C 279. The observations are s t i l l  t o o  incom- 
plete  to  def ine the exact  shape of the expanding source, and 
known. If  the source were very much closer  than the dis tance 
these d i f f i c u l t i e s  may go away  when the true source shape is 
would disappear; however we would then have to find a cause f o r  
implied by its redsh i f t ,  t he  problem posed by its rapid expansion 
its redshift  other than the general  expansion o f  the universe.  
m e n t s  t o  rough two-dimensional mapping of the  small components 
VLB observations are now proceeding from simple diameter measure- 
purpose observations are made wi th  pa i r s  of antennas at  locat ions 
of radio sources on t he  sca l e  of  10-3  seconds  of arc .  For t h i s  
all around the world. Stations used have included many d i f f e r e n t  
radio observator ies  and antennas of the  NASA Deep Space Network. 
In a very  rea l  sense  we a r e  making of the whole earth a gigantic 
antenna array. 
References: 
1. 
2. 
3. 
4. 
5. 
6 .  
7. 
9. 
8. 
10. 
11. 
13. 
12 .  
G.W. Swenson and N.C. Mathur (1968 Proc IEEE 56, p. 2114). 
M. Ryle and A. Hewish (1960 Honthly Notices Roy Astron SOC 
B.P. Palmer et&. (1967 Nature 213, p. 789). 
N.W. Broten st. (1967 Science 156, p. 1592). 
C. Bare et&. (1967 Science 157, p. 189). 
B.F. Burke (1969 Physics Today 22, No. 7, p. 54). 
M.H. Cohen (1969 A m  Revs Astron Astrophys 7, p. 619). 
M.H. Cohen ~t. (1969 Astrophys J 158, p. L83). 
B.O. R&&ing,  O.E.B. Rydbeck,  and J.M. Moran (1970  Radio 
Science 2, p. 1227). 
B.F. Burke et&. (1970 Astrophys J 160, p. L63). 
A.R. Whituey et&. (1971 Science 173, p.  225). 
J. Gubbay zs. (1969 Nature m, p. 1094). 
M. Rees (1967 Monthly Notices Roy Astron SOC 135, p. 345). 
120, p.  220). 


The whole-earth array ‒ Developments in very-long-baseline interferometry

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The whole-earth array ‒ Developments in very-long-baseline interferometry

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