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 provides a set of 5 filter options:
- Broad Band L ("Luminosity") filter. Cut on 400nm. Cut off 700nm.
- B "Blue" filter. Cut on 380nm. Cut off 520nm.
- G "Green" filter. Cut on 490nm. Cut off 570nm.
- R "Red" filter. Cut on 580nm. Cut off 695nm.
- I "Infrared filter". Cut on 695nm. Cut off defined by Detector QE.
MOPTOP is mounted on LT although robotic control software is still under development. We anticipate robotic commissioning in May 2020 and the instrument should be available for general use by the start of Semester 2020B.