The VLT Interferometer - ESO

18 CHAPTER 2. SCIENTIFIC ASPECTS
Table IV. Source-referenced Mode Sensitivity
Phasing & Cophasing. A = 50
Seeing V I
0.5" 7.4 8.1
1.0" 5.1 5.9
The key progress in sensitivity at these wavelengths shall be obtained when
Laser Referenced Sources become available. Then, Ac = 7r D2 /4 and the sensitivity
gain is (1.6D/ro(.A»2 over the values of Table IV. The sensitivity becomes
little seeing-dependent and aperture dependent. It is given in Table V.
Table V. Laser Referenced Mode Sensitivity
Diameter V I
2m 13.1 12.7
8m 16.1 15.7
As was discussed in the previous section, the phasing of individual apertures
with an independent reference (laser) provides a possibility of cophasing at an
increased sensitivity. Nevertheless, the technological difficulties of the two steps
described in this section (large number of adaptive actuators and laser phasing
of individual apertures) are such that we do not consider it realistic to go
beyond this. However, even if complete laser referenced phasing is not possible
in the near future, one should not underestimate the gain over the values of
Table IV, which would result from partial phasing. As shown in the infrared
section 2.1.1 above (Table I), the source reference at Hand K is possible with
rather faint objects. Moreover, at these wavelengths, the isoplanatic field is
large enough to offer a significant probability to find a reference source (at H
or K) close to the object of interest at V or H. Indeed, the wavefront correction
will only be partial, putting a small percentage of the total energy in a coherent
Airy disk.
2.1.5 Concluding Remarks
The above values are subject to a number of remarks .
• When a source is resolved by the interferometer, a most likely case indeed,
the visibility V is less than unity. This will not change the pha.sing
condition since Hartmann-Shack wavefront sensors can be used with extended
objects, but will raise the cophasing flux limits given in the tables.
Hence it should be emphasized that a bright (photometric) source can be
a faint interferometric source, if it is large as compared with .AIL.
• Systematic errors may lead to values of fJ as large as 50 %. They will
reduce the limiting sensitivities proportionally.