MIDI (MID-infrared Interferometric instrument) is the VLTI instrumentfor 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.

Important: MIDI will not be available for P95 and after, as the VLTI is being prepared for the 2nd generation instruments.

The main features of MIDI for P94 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).
• Limiting correlated magnitudes (flux*visibility):

   

  Telescopes	Beam combiner	Spectrograph	Limit (N mag)	Limit (Jy @ 12 μm)
  UTs	HIGH_SENS	PRISM	4	1
  UTs	HIGH_SENS	GRISM	2.8	3
  ATs	HIGH_SENS	PRISM	0.74	20
  ATs	HIGH_SENS	GRISM	0.31	30
  UTs	SCI_PHOT	PRISM	3.2	2
  UTs	SCI_PHOT	GRISM	2	6
  ATs	SCI_PHOT	PRISM	0.0	40
  ATs	SCI_PHOT	GRISM	-0.44	60

   

• Limiting correlated magnitudes (flux*visibility).

Because of the scarce availability of UTs for MIDI, we recommend thatany scientific program proposed for an object can be completed in onenight. Hence, proposals for binary or variable stars will be rejected.Only objects with "fixed" features over a long-period (6 months) can beobserved with MIDI and the UTs. Depending on the scientific interestconsidered by ESO, the same object may be observed with differentbaselines at several weeks of interval to get different (u,v) samples. On 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).