Stray light analysis for Alternative Mechanism Concept in LISA optical bench
Nardello Marco,Lintz Michel
One of the tasks to be accomplished to guarantee LISA’s successful heterodyne measurements is the correct superimposition of Rx and Tx beams. To this purpose, the first proposed design foresees on the optical bench a movable clip (Active Aperture Mechanism - AAM) coupled with a rotating mirror (Point Ahead Angle Mechanism - PAAM), allowing tilt and positioning correction for Tx.
The UK Astronomy Technology Centre and the University of Glasgow recently proposed an alternative design (LISA-UKOB-INST-TN-007 Alternative OB Mechanism Concept), composed by 4 transmitting elements, which will absolve AAM’s and PAAM’s functions. It amounts to two Risley prism Scanner in sequence. The first one, composed by two prisms independently rotating around their axes, will allow tilting. The second, composed by two parallel plates rotating around the optical axes, will allow translation. This design would permit the use of the same mechanism to implement all the mechanized function on LISA’s optical bench, which would bring simplification in the research and choice of the supplier.
Several tests are necessary to evaluate the performance of the new design, among which is a stray light study. Using FRED software, I developed an optical design to simulate the behavior of the four optical elements in terms of ghost reflection, backscattering and forward scattering. Main stray light paths from AMC elements are: forward direction with subsequent backscattering towards the Rx stop by telescope, and backscatter to the fibre injector with back reflection towards the interferometer pupil.
According to this analysis, the straylight power fraction reaching the interferometer does not amount to critical levels. The main ghost contribution is due to double internal reflection in the optics, but it is low in intensity. Evaluation of scatter is made for different polishing of the optics and for different contamination levels and even in the worst case the amount of stray power in the interferometer will be manageable. To calculate the amount of backscattering from the telescope, a second design detailing telescope’s optics was needed, in order to calculate the amount of stray light collected in the spacecraft interferometer field of view and in the (wider) CAS field of view.
This work demonstrates the feasibility of the system from the stray light management point of view, providing dependable results on stray light power levels in LISA interferometer and CAS.
