  The $^{13}C$ substitutions of molecule $HC_7N$ were observed in TMC-1 using
the J = 12 - 11, J = 13 - 12 rotational transitions in the frequency range 12.4
to 13.6 GHz. We present the first detection the $^{13}C$ isotopic species of
$HC_7N$ in the interstellar medium, based on the average of a number of weak
rotational transitions.
  This paper describes the calibration and data averaging process that is also
used in a search for large cyanopolyyne molecules in TMC-1 using the 100m
Robert C. Byrd Green Bank Telescope (GBT). The capabilities of the GBT 11 to 15
GHz observing system are described along with a discussion of numerical methods
for averaging observations of a number of weak spectral lines to detect new
interstellar molecules.
