Optical Bench

PIONIER is a four telescope interferometric combiner, operating in the H band. It has limited spectral capabilities and can observe the H band in ether one or six spectral channels (λ/∆λ ≈ 5 and 30, respectively) across the H band. An integrated optics beam combiner (IOBC) is used for the beam combination (see details below). It is fed by single mode, polarization maintaining fibers. The light paths from the four VLTI beams are modulated and injected into the fibers by the four identical arms of the input and optical path delay unit (IOPDU). Each arm of the IOPDU consists of (in that order, following the direction of the incoming light):

1. a dichroic which collects the H band, while the K band is transmitted to the Infrared Image Sensor (IRIS) which is used for the lab guiding of the telescope beams,
   
2. an optical path delay (OPD) scanning unit which consists of a mirror mounted on a piezo translation stage,
   
3. a tip-tilt mirror which controls the injection into the fiber,
4. a shutter,
   
5. a lithium-niobate plate of 2 mm thickness used to compensate for the polarization phase shift, and
   
6. an off-axis parabola focusing the light into the fiber.
   

The interferograms are temporally encoded by modulating the OPDs in the scanning units.

Integrated Optics Beam Combiner

For a detailed description of the IOBC, see Benisty et al. (2009, A&A 498, 601). The four VLTI beams are each separated in three portions for pairwise combination. For each of the six resulting baselines four phase states are created (static ABCD mode, phase shifts of 0 for channel A, π/2 for channel B, π for channel C, and 3π/2 for channel D). This results in six baselines × four phase stages = 24 outputs of the IOBC. After the IOBC, a dispersive element (prism) can be inserted into the optical paths of the 24 outputs. The spectrally dispersed or undispersed light from the outputs of the beam combiner is then focussed onto and imaged by a detector. The scanning speed of the piezos is adjusted to the frequency frame of the detector in order to sample the fringe contrast as a function of OPD by 512 points. The frequency frame of the detector is set by the operator defined detector integration time (DIT) which accepts values between 0.5 ms and 3.0 ms, according to the target brightness and atmospheric conditions. A DIT of 0.5 ms yields a full scan time of ∼300 ms. Different combinations of the four outputs for each baseline allow to extract the fringes (A-C+B-D) and photometry (A+B+C+D). A dark level is measured by integrating with all shutters closed, while a kappa matrix (flux splitting ratios of the IOBC) is measured by integrating four times on source with only one of the four shutters open at a time.

Detector

The 24 (dispersed or undispersed) outputs of the IOBC are imaged on a fast infrared detector called RAPID. This innovative camera uses Avalanche Photo Diode amplification up to 20 times and fast intra-pixel electronic. The amplification can be adjusted to provide a gain of 0.1 e/adu (HIGH sensitivity) or 0.4 e/adu (MEDIUM sensitivity). The noise RMS is about 25 adu per pixel (including background photon noise and detector noise) at the typical frame rate of ∼1 kHz. The camera is linear up to flux of 4000 adu. The limited dynamic range of the detector (1:100) poses severe difficulties when observing very resolved targets, i.e. targets with small visibilities.