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 P93 are:
- Use of PRIMA FSU-A as an external fringe tracker (in VM). With ATs, the limiting magnitude for FSU-A, at a seeing of 0.6" or better, is K=7.5mag.
- Correlated flux mode for faint target, with and without group-delay tracking (in VM).
- VM should, as a rule, be requested for very faint targets, no guide star existing, target that is embedded in a complex structure in the IR, etc...
- 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 (group-delay tracking).
- 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). CORR-FLUX is an observing mode with the HIGH_SENS beam combiner, in which no photometry is required, at the expense of absolute visibility calibration
- Limiting correlated magnitudes (flux*visibility).
|Telescopes||Beam combiner/Mode||Spectrograph||Limit (N mag)||Limit (Jy @ 12 μm)|
In any case, time will be allocated as 50-minute slots (1 slot per calibrated visibility point, i.e. 2 OBs CAL-SCI).