The VLT Interferometer - ESO

2.1. SENSITIVITY ESTIMATES 13
2.1.3 Detector or Background Noise Limited Case (1.4-20
/-Lm )
2.1.3.1 Sensitivity of Phased Apertures and Cophased Telescopes in
the Source-Referenced Mode
This case is expected to be achieved by the auxiliary telescopes (assumed to
have D = 2m) at any wavelength equal to or longer than H (1.65 /--lm ), and
by the 8-m telescopes at wavelengths equal to or longer than K ( 2.2 /--lm). In
Tables Ia and Ib we give the magnitudes and the fluxes of objects for which
an acceptable value of A is obtained in the snapshot mode (bright objects) for
a visibility V=1 (unresolved source) and a system efficiency of 'f)(3 = 0.3. The
value of A = 50 is selected as a conservative estimate of what is required to
make an adaptive optics control system with an adequate band-pass (Table Ia).
A more optimistic value of A = 10 is adopted in Table Ib. 1 Other parameters
entering the estimates in Tables Ia and Ib are:
• For the detector noise-limited case (H and K), sensitivities are computed
in Table Ia with a detector read-out noise of 300 electrons, a value consistent
with existing detectors. In Table Ib, a more optimistic value of
30 electrons is adopted, representing most likely the state of the art in 5
years' time.
• For the thermal background noise limited case ( L, M, N, Q), the photon
count is assumed to be A times the background noise computed for a
temperature of 273 K, an emissivity of 1.0 and a diffraction-limited cold
stop appropriate to the coherence area.
• The optical bandwidths are set by atmospheric windows, i.e. 0.18, 0.4,
0.3, 0.4, 4.0, 2.0 pm at H, K, L, M, N, Q. The value of 6.T = To is
set equal to To/V, where the wind velocity is assumed as v = 10 m/s.
As long as To(A) < D, the limiting magnitudes are independent of the
telescope diameter, since the coherence volume V(A)is controlled by the
atmospheric coherence area.
1 When comparing these values with current limits of two-dimensional infrared speckle interferometry,
one should note that this latter technique is still in its infancy and limited by
extraneous noise and calibration limitations, not by the ultimate detector noise. On the other
hand, one-dimensional speckle has reached its theoretical limits.