Reduction of the baseline ripple on spectra recorded with the Parkes radio telescope.

Two sources of baseline ripple on the 64 m Parkes radio telescope at 5 GHz are described in terms of methods for their reduction. The first, off-source, arises even when the telescope is pointing at a cold sky, and can be fairly easily attenuated by subtracting an off-source reference from the signal spectrum. Attention is given to the second source, on-source ripple, which depends on the characteristics of the radio spectrum observed. Methods for correcting on-source ripple are outlined, including a vertex scattering cone, which provides almost total elimination of second and higher order harmonic ripple; a focal plane absorber, which reduces the third harmonic by 50%; shaped vertex planes, reducing fundamental ripple to 0.25% of the continuum temperature; and the two-hybrid-mode feed horn, which increases theoretical energy by about 83%, thereby reducing by 40% the energy left over for scattering.