Panel Gap Models for the SPT

The South Pole Telescope (SPT) will have a primary mirror made of cast aluminum panels. These panels are mounted on a Carbon Fiber Reinforced Polymer frame, which has a smaller coefficient of expansion than aluminum. When the SPT is in operation at low temperatures (-80 C), there will be a significant gap, perhaps as much as 2mm across, between adjacent panels. This interruption of the mirror surface will result in radiation lost to the beam through scattering and absorption in the gaps and the structure behind.

This website explores this problem using the program HFSS, by the Ansoft Corporation. My copy of HFSS was generously donated to the SPT project for this purpose by the Ansoft Corporation. HFSS is a program which solves Maxwell's Equations numerically in 3 Dimensions. The electromagnetic properties of the space and its boundary conditions are set up using a graphical interface.

The first example shows an incoming plane wave striking a reflector at normal incidence, no gap.

The second example shows a plane wave coming in at an angle.

The third example shows a normal-incidence plane wave with transverse polarization striking a gap.

In the fourth example, the gap is half as wide and the lost power is about half as much.

The fifth example shows is the same as the third, except that wave is polarized parallel to the gap.