Fast NIR Polarization Differential Imaging

High-speed polarization differential imaging is achieved by way of synchronizing a high frame rate C-RED ONE camera with polarization modulation.

Key elements of the NIR fast-PDI mode are:

  • Half-waveplate (HWP) for slow modulation
  • Field stop
  • Ferro-electric Liquid Crystal (FLC) modulator
  • Wollaston prism
  • High speed NIR camera

Much of the optical train is shared with the CHARIS-PDI mode. The Wollaston prism is a duplicate of the CHARIS-PDI Wollaston.

Two modes are offered:

  • Mode #1: Double-differential imaging. In this mode, we use the HWP for slow modulation, the field stop and the Wollaston prism.
  • Mode #2: Triple-differential imaging. In this mode, we add the FLC modulator, synchronized with the camera acquisition.
  • Capabilities - Overview

    Basic Parameters
    Field of view1.1''x2.4''
    Plate Scale15.3 mas/pixel
    Camera First Light Imaging C-RED ONE See Camera description
    Wavelength coverage Broadband (950-1860 nm) Mode #1. CHARIS PDI can be used simultaneously in high-res K-band.
    y-band Mode #1. CHARIS PDI can be used simultaneously in low-res broadband or high-res with band filter.
    J-band Mode #1. CHARIS PDI can be used simultaneously in high-res H- or K-band.
    H-band Mode #1. CHARIS PDI can be used simultaneously in K-band.
    H-band with FLC Mode #2. CHARIS PDI can be used simultaneously in K-band.
    SCExAO PDI throughput Mode #1: 46-54% ~15% from atmosphere to detector
    Mode #2: 41-48%
    Camera gain x1 to x50
    Frame rate 1 Hz to 3.5 kHz mode #1, Full frame. Limited by the frame rate of the camera.
    1 Hz to ~1 kHz mode #2, Full frame. Limited by the switching time of the FLC.
    Zero-point magnitude 1ADU = 23.2 H-mag mode #1, gain 1, 1.75 kHz, no coronagraph
    Observation efficiency ~90% mode #1. Limited by the rotation of AO188's HWP (few seconds every 1-2 minutes)
    40 to 90% mode #2. Depending on the frame rate, limited by the switching time of the FLC.

    Throughput

    Fast-PDI is optimized for H-band, but can be used in broadband or y- and J-band without the FLC.

    Fast-PDI Imaging Throughput (relative to non-PDI observations)
    Band HWP Wollaston FLC total
    J (1.24um) 96.1% 93.2% 88.7% 79.4%
    H (1.63um) 94.3% 95.7% 88.5% 79.9%

    Preliminary contrast curves

    Contrast curves were calculated on two targets observe during engineering nights with excellent seeing conditions (~0.4''): the unpolarized standard star psi01 Aqr, and the polarization target R Aqr.

    A raw contrast curve was calculated using only one minute of data on the unpolarized standard star psi01 Aqr, showing that we achieve ~1e-4 raw contrast at ~0.15 arcsec separations, before double-differential processing. The contrast for stokes Q and U were also calculated, showing that we achieve a contrast of a few 1e-6 at 0.15 arcsec with double-differential processing (mode #1, H-band). The addition of a third differential processing (mode #2) should improve this further.

    psi01 Aqr Contrast curve

    Contrast curve calculated from 1 minute of non-coronagraphic data on the unpolarized standard star psi01 Aqr. The contrast of stokes Q and U was also estimated after double-differential processing (mode #1).


    We also produced contrast curves using the non-coronagraphic and coronagraphic images of R Aqr (one minute datasets). The raw contrast is similar in this case without coronagraphic mask, and goes down significantly with the coronagraph, especially at small separations. The green curve was obtained by subtracting two datasets ~5 minutes apart. It shows that the contrast stability over a short timescale is on the order of 1e-6.

    R Aqr Contrast curve

    Contrast curve calculated from 1 minute of non-coronagraphic and coronagraphic data on the polarized star R Aqr. The contrast stability was also estimated by subtracting two datasets ~5 minutes apart.



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