LISA, The Near Infrared Camera for VINCI

LISA is the near infrared camera for VINCI, the test instrument for the VLT interferometer. VINCI was built in a collaboration of ESO, DESPA, and MPE, and operated at Paranal from 2001 to 2003.

The goals of VINCI are to commission the VLTI subsystems, to measure fringe visibilities (primarily in K-band), and to provide an artificial star for alignment. DESPA provides the main components of VINCI, using technologies already tested in other interferometers. The essential parts of VINCI are: the beam combiner (based on single-mode fluoride glass fibers), the reference sources (for internal tests, future science instruments, and alignment), the image and pupil sensor, and the fringe sensor (the near infrared camera LISA).

The LISA optics and cryo-mechanics are being built at MPE. The detector, a 1024×1024 HgCdTe HAWAII array from Rockwell is supplied by ESO together with the readout electronics. The cryostat houses a filter wheel with 6 positions (open, closed, K-band filter, TBD narrow band filters), the camera lens doublet, and the detector array. The collimator is located outside the cryostat, the beam is deflected into LISA by a plane mirror which can be replaced by a grating for the observation of dispersed fringes. LISA detects the light from 4 single-mode-fibers (6.4µm core diameter) located at the corners of a square of 125µm length in the focal plane of the warm collimator. On the detector array the fiber images are separated by 9 pixels, the image of each fiber is smaller than one pixel. Therefore, a minimum of 4 pixels have to be read. Because of adjustment problems and aberrations, the fiber images may be larger than one detector pixel (17µm square), or they are not centered on the pixel, in this case rectangular areas adapted to the beam size can be read.

Two of the four fibers are outputs of the beam combiner, they carry the complementary interferograms (phase shifted by 180 degrees). The other two fibers carry a small percentage of the flux from the interferometer channels just before beam combination, i.e. their brightness is proportional to the flux coupled into the interferometer arm by the corresponding telescope. The coupling efficiency varies with the image quality (e.g. seeing, adaptive optics correction), thus modulating the amplitude of the interferograms. By dividing the difference of the interferograms by the product of the other two signals, this effect can be removed.

To detect fringes even when the optical path difference changes quickly (which will happen when the interferometer is used for the first time), the pixel containing the fiber images have to be read at a high speed. By locating the fiber images close to the corner of one quadrant, a frame rate of several kHz is feasible.