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IF frequency table         CHdssvv
 
 
A. Overview
 
   This extension table for a uv data set contains relevant information
about the IFs in the raw uv data file.  For these purposes an IF consists
of the output from an IF at the telescope which can be arbitrarily
spaced in frequency form other such IFs.  Examples are the A-C and B-D
IFs of the VLA and the output of independent video converters in VLBI
recorders.  Individual, regularly spaced frequency or delay channels derived
from the correlation of such IFs are not themselves considered IFs.

 
Names: The file name is CHdsssvv where d is disk number, sss=catalog
number and vv = version number.
 
B.  File structure.

     The table entries give the frequency offset to the center of
the IF and the sideband of each IF.
    The file header record contains no KEYWORDs.
 
    Table entries:
 
    Title               Units  code       Description
   ----------          ------- ----  -----------------------------------
     IF NO.                    1I    The IF pixel number of the entry.
     FREQUENCY OFFSET    Hz    1D    Frequency offset from ref. freq.
                                     True = Ref + Offset
     SIDEBAND                  1I    Sideband of each IF
                                     -1 => 0 video freq. is high freq. end
                                      1 => 0 video freq. is low freq. end
 
C. User notes.

  The "code" column is element_count + basic type code.
basic type codes: D=Double precision, E=single precision,
   A=character, J=long integer, L=logical, I=short integer, X=bit.

     The true frequency of the observations are the signed sums of the
reference frequency in the subarray antenna (AN) file, the peculiar
source doppler offset from the source table (SU file) and the IF
frequency offset from this file.
 
D.  Routines to write CH files:
     AIPS utility routine CHNDAT will read of create/write these tables;
CHNCOP will copy all or a subset of the table.
Also see chapter 14 of "Going AIPS" for a detailed description of routines
to access tables files.
 
E.  Routines to access CH files:
     Same as D.
 
