At frequencies much below 30 Mc./s. radio astronomy is substantially affected by the earth's ionosphere. The principal effects of the ionosphere are to absorb signals from extra-terrestrial sources, and to propagate earthbound interfering signals over long distances. Successful observation at these relatively low frequencies requires (1) a clear observing channel, or (2) operation during the interval immediately preceding sunrise when the maximum usable frequency for oblique-incidence ionospheric propagation has fallen below the operating frequency. Since early attempts to locate a clear channel between 10 and 20 Mc./s. were fruitless, we accepted the condition in (2) realizing that ionospheric absorption would also be minimized, since the observing frequency, under these conditions, is more than three times greater than the ionospheric critical frequency at vertical incidence.
22. Preliminary observations of point sources at 12·5 and 15·5 Mc./S.
