The Atacama Large Millimeter/submillimeter Array ( ALMA ) is a powerful tool for high-resolution mapping of comets , but the main interferometer ( comprised of 50 \times 12 -m antennas ) is insensitive to the largest coma scales due to a lack of very short baselines .
In this work , we present a new technique employing ALMA autocorrelation data ( obtained simultaneously with the interferometric observations ) , effectively treating the entire 12-m array as a collection of single-dish telescopes .
Using combined autocorrelation spectra from 28 active antennas , we recovered extended HCN coma emission from comet C/2012 S1 ( ISON ) , resulting in a fourteen-fold increase in detected line brightness compared with the interferometer .
This resulted in the first detection of rotational emission from H ^ { 13 } CN in this comet .
Using a detailed coma radiative transfer model accounting for optical depth and non-LTE excitation effects , we obtained an H ^ { 12 } CN/H ^ { 13 } CN ratio of 88 \pm 18 , which matches the terrestrial value of 89 , consistent with a lack of isotopic fractionation in HCN during comet formation in the protosolar accretion disk .
The possibility of future discoveries in extended sources using autocorrelation spectroscopy from the main ALMA array is thus demonstrated .