Detection of $^{13}C$ Isotopic Species of $HC_7N$

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.