Under development is MOPTOP, a Multicolour OPTimised Optical Polarimeter specially designed for time domain astrophysics. It will take the already-novel aspects from the RINGO series of polarimeters (see box below) and add a unique optical dual-camera configuration to both minimize systematic errors and provide the highest possible sensitivity.
MOPTOP's design enables the measurement of polarisation and photometric variability on timescales as short as a few seconds. Overall the instrument will allow accurate measurements of the intra-nightly variability of the polarisation of sources such as gamma-ray bursts and blazars, allowing the constraint of magnetic field models to reveal more information about the formation, ejection and collimation of jets.
MOPTOP will be a dual-beam polarimeter (see schematic at right). Incoming collimated light will first pass through a continuously rotating half-wave plate which will modulates the beam's polarisation angle. The polarised light will then pass through a wire-grid polarising beamsplitter. This will split the light into the p and s polarised states and send them through filter wheels to a pair of low-noise fast-readout imaging cameras. Image acquisition will be electronically synchronised to the rotation angle of the half-wave plate. This combination of half-wave plate and beamsplitter will provide about twice as much throughput as a conventional polarimeter using a polaroid filter as the analyser.
MOPTOP is currently under development, with projected deployment in 2020. A proof-of-concept instrument without filter wheels (Mini-MOPTOP, see right) was evaluated on the LT in August 2019.