The beam pattern of the IRAM 30–m telescope

Abstract

Total power scans across the Moon around New Moon (mostly day time) and Full Moon (night time) at 3.4 mm (88 GHz), 2.0 mm (150 GHz), 1.3 mm (230 GHz), and 0.86 mm (350 GHz) wavelength are used to derive the beam pattern of the IRAM 30-m telescope to a level of approximately -30 dB (0.1%) and, dependent on wavelength, to a full width of 1000 - 1400″. From the reflector surface construction and application of the antenna tolerance theory we find that the measurable beam consists of the diffracted beam, two underlying error beams which can be explained from the panel dimensions, and a beam deformation mostly due to large-scale transient residual thermal deformations of the telescope structure. In view of the multiple beam structure of the 30-m telescope, and of other telescopes with a similar reflector construction of (mini-)panels and panel frames, we summarize the antenna tolerance theory for the influence of several independent surface/wavefront deformations. This theory makes use of different correlation lengths, which in essence determine the independent error distributions, and of the wavelength-scaling of the diffracted beam and of the error beams. From the Moon scans we derive the parameters for calculation of the 30-m telescope beam in the wavelength range 3 mm to 0.8 mm as required for the reduction of astronomical observations, in particular of extended sources. The parameters of the beam are primarily for the time after July 1997 when the reflector was re-adjusted and improved to the illumination weighted surface precision of σT = 0.065 - 0.075 mm. In the Appendix we explain the choice for this analysis of scans taken around New Moon and Full Moon.