Producing Large Synthetic Turbulence Plates using MRF Polishing
John Pazder (1), Jean-Pierre Véran (1), Chris Hall (2), Glen Herriot (1), Scott Roberts (1)
(1) Herzberg Institute of Astrophysics, National Research Council, Victoria, BC, Canada (2) QED Technologies, Rochester, NY, USA
In order to exercise real-time AO correction without being on the sky or even on the telescope, most AO systems include a calibration unit that simulates a number of sky sources as well as atmospheric turbulence. The synthetic atmospheric turbulence can be produced either by physically mixing air masses at different temperatures; or by using one or several moving phase plates (in reflection or in transmission), on which the turbulence has been encoded.
The transmitting phase plate design is usually preferred, since it minimizes the space envelope while ensuring that the turbulence has known pre-determined characteristics. Several techniques exist to manufacture such phase plates, including micro-machining using semi-conductor technology (Silios); and applying acrylic paint to a transparent substrate (UCSC). However, none of these techniques appear to be able to produce plates larger than 6 inches or 150mm. This is significantly too small for ELT size AO systems, such as NFIRAOS, which requires a phase plate of dimensions exceeding 600x300mm.
We have explored the feasibility of using the MRF polishing technique to produce large phase screens such as the one required for NFIRAOS. We present the measurements that we have obtained on a 200x200mm pathfinder prototype, manufactured by QED Technologies on a BK7-n substrate. We find that the synthetic turbulence has exactly the prescribed structure down to a spatial scale of 5mm. For scales smaller than 5mm and down to 0.5mm, we measured less structure than prescribed, but only by about 20%, making the generated turbulence useful, even at those scales. Based on these results, we conclude that MRF polishing appears to be a very promising technique for producing large turbulence phase screens for ELT-class AO systems.