Design of the Cone at the Centre of the Subreflector Abstract
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ALMA Memo # 545
Design of the Cone at the Centre of the Subreflector
Richard Hills
2005-11-16
The distribution of the power reflected from the subreflector is calculated for a variety of cases. It is found that good suppression of the reflections can be obtained by placing a cone in the centre, but that this does need to be a good deal larger than is called for in the present specification. The size needed depends on whether or not the subreflector is tilted to optimise the gain when we use off-axis feeds. The question of whether the cone should be a separate component or made as an integral part of the subreflector is also discussed.
The recommendation is for a cone of 60mm diameter and for this to be built into the surface of the subreflector. This does however require that we use a slightly different alignment of the subreflector for each receiver. If this cannot be done then the diameter of the cone needs to increase to about 72mm. This larger size will produce a loss in gain which is of order 0.5%. Perhaps more importantly, however, this analysis has revealed that using off-axis feed positions (e.g. bands 5 and 6) without tilting the subreflector is already producing a significant loss of gain, as well as an increase in the spill-over onto the ground, which together will produce a loss in the sensitivity of ALMA of several percent. This means that having the ability to adjust the tilt of the subreflector is highly desirable anyway.
Finally it is noted that there is a significant reflection back to into the focal plane due to diffraction from the outer edge of the subreflector. This can be suppressed by giving this edge a slightly non-circular form and a variety of ways of doing this are discussed.
View a pdf version of ALMA Memo #545.
