MIDI Overview

MIDI (MID-infrared Interferometric instrument) is the VLTI instrument for N-band (8 to 13 microns) interferometry. It is a two-beam recombiner giving values of moduli of fringe visibility (samples in the (u,v) plane) depending on the wavelength (spectral resolution: R=30 or R=230). The "first fringes" were obtained in December 2002. MIDI has been offered for both service and visitor modes since September 2003.

The main features of MIDI for P91 are:

  • Use of PRIMA FSU-A as an external fringe tracker (in VM).
  • Correlated flux mode for faint target, with and without group-delay tracking (in VM).
  • Unit Telescopes (UTs): UT adaptive optics guarantees diffraction-limited image quality on MIDI, for targets brighter than V=17.
  • Auxiliary telescopes (ATs): Tip-tilt correction for targets brighter than V=13.5.
  • Off axis guiding is possible if the target does not meet the above limits. The presence of an appropriate guide star must then be checked and indicated in the proposal in the target notes.
  • Baselines: please look at the VLTI News webpage to get the list of the UT and AT baselines available.
  • Interference fringes recorded in "dispersed-Fourier" mode (long slow scan). Self-fringe tracking at 1-Hz rate (coherencing).
  • Spectrograph optics: either NaCl prism (R=30), or KRS5 grism (R=230).
  • Beam combiner: either "HIGH_SENS" (no simultaneous photometric measurement of beams before combination), or "SCI_PHOT" (simultaneous photometric measurement).

    • TelescopesBeam combinerSpectrographLimit (N mag)Limit (Jy @ 12 μm)
    UTsHIGH_SENSPRISM41
    UTsHIGH_SENSGRISM2.83
    ATsHIGH_SENSPRISM0.7420
    ATsHIGH_SENSGRISM0.3130
    UTsSCI_PHOTPRISM3.22
    UTsSCI_PHOTGRISM26
    ATsSCI_PHOTPRISM0.040
    ATsSCI_PHOTGRISM-0.4460

  • Limiting correlated magnitudes (flux*visibility).
Because of the scarce availability of UTs for MIDI, we recommend that any scientific program proposed for an object can be completed in one night. Only objects with "fixed" features over a long-period (6 months) can be observed with MIDI and the UTs. Depending on the scientific interest considered by ESO, the same object may be observed with different baselines at several weeks of interval to get different (u,v) samples. On the contrary, ATs are suitable for monitoring of the evolution of the visibility of a target over several weeks.

In any case, time will be allocated as 50-minute slots (1 slot per calibrated visibility point, i.e. 2 OBs CAL-SCI).