MOPTOP

Introduction

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.

RINGO novel aspects

fast rotating element for high time resolution
very low noise cameras optimising signal to noise
dichroic mirrors for multi-wavelength capability

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.

Instrument Description

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.

Filters

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 Detector (Zyla 4.2+) QE curve.

MOPTOP LBGR transmission curves.

MOPTOP I band filter transmission curve

Availability

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.

Operational Principle

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Operational Principle (from Shrestha et al. (2002)).

Specifications

Optical performance	predicted accuracy: See plot below 16 wave plate angle positions per revolution Field of View 7 x 7 arcmin
Cameras	Nikon AF Nikkor 50mm f/1.4D imaging lens Andor Zyla sCMOS detectors - science grade CMOS 4.2 megapixel 6.5 µm pixels 82% peak QE 0.9e^- read noise ~0.1–1 Hz frame rate 7x7 arcminute field of view
Half-wave plate	ThorLabs Achromatic half-wave plate, 400-800nm Rotation rate 7.5 or 0.75 rotations per minute (user selectable) Exposure Time per waveplate position 0.45 seconds or 4.5 seconds (linked to rotation rate) Time to complete one full rotation 8 seconds or 80 seconds (linked to rotation rate)
Beamsplitter	ThorLabs Wire-Gird Polarising Beamsplitter 400-700nm

Predicted R band sensitivity

Exposure Time vs Polarization accuracy for sources of magnitude 12-17

Pipeline

At time of writing, the specification for the MOPTOP pipeline will probably be similar to that of RINGO3. See the RINGO3 pipeline information for an idea of what the MOPTOP pipeline will resemble.

A data reduction receipe for MOPTOP can be found in Shrestha et al. (2020).