MOPTOP

Introduction

MOPTOP, the Multicolour OPTimised Optical Polarimeter, is specially designed for time domain astrophysics. It will take the already-novel aspects from the RINGO series of polarimeters 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 allows 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.

Status

ONLINE

MOPTOP was mounted on the telescope for commissioning and calibration in early 2020, just before coronavirus lockdown travel restrictions between the UK and the Canary Islands came into force. The instrument began observing robotically in October 2020.

Instrument Description


Optical schematic of MOPTOP
Based on Shrestha et al. (2002)

MOPTOP is a dual-beam polarimeter. Incoming collimated light first passes through a continuously rotating half-wave plate which modulates the beam's polarisation angle. The polarised light then passes through a wire-grid polarising beamsplitter. This splits the light into the p and s polarised states and sends the now-separate beams through filter wheels to a pair of low-noise fast-readout imaging cameras.

Image acquisition is electronically synchronised to the rotation angle of the half-wave plate. This combination of half-wave plate and beamsplitter provides about twice as much throughput as conventional polarimeters that use polaroid filters as the analyser.

Filters

MOPTOP provides a set of five filter options:

  • Broad Band L ("Luminosity") filter:
    400–700nm
  • B "Blue" filter: 380–520nm
  • G "Green" filter: 490–570nm
  • R "Red" filter: 580–695nm
  • I "Infrared" filter:
    695nm to cutoff defined by Detector QE

Mini MOPTOP
MOPTOP Detector (Zyla 4.2+) QE curve
Mini MOPTOP
MOPTOP LBGR transmission curves
Mini MOPTOP
MOPTOP I band filter transmission curve

Operational Principle


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

The half-wave plate can rotate at two different speeds; in "fast mode" it completes one revolution in 8 seconds, while in "slow mode" one revolution lasts 80 seconds. Observers can choose between fast and slow mode depending on the brightness of the target. Tentatively, if your target is dimmer than mv = 12, you can use slow mode. Using slow mode whenever possible is recommended as the output is a smaller data set which is easier to handle. We also recommend that all the observations are done in zero degree of mount angle.

In fast mode, each of the 16 rotation positions is imaged every 0.5 seconds. Exposure time is 0.45s with 0.05s for readout time. In slow mode, positions are imaged every 5 seconds with exposures lasting 4.95 seconds. The figure at right shows fast mode exposure times and angles for each rotation position. The blue shaded areas in the top figure indicate the waveplate angle for each exposure. The white areas denote the readout time. The plot below shows waveplate angle as a function of time, along with the corresponding EVPA rotation angle of the incoming beam.

Specification

Optical
Performance
  • 16 wave plate angle positions per revolution
  • field of view 7x7 arcmin
  • predicted R-band sensitivity at right →
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
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 (fast mode) or 80 seconds (slow mode)
Beamsplitter
  • ThorLabs Wire-Grid Polarising Beamsplitter 400-700nm

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).