AMBER Instrument Description


AMBER is an interferometric beam combiner for the VLTI working in the near-infrared (J, H, and K bands). It combines three beams coming from three telescopes. AMBER gives access to the visibilities of the object for three different spatial frequencies and one closure phase.

This information is spectrally dispersed (R~30i.e. low resolution, R~1500 i.e. medium resolution, and, R~12000 i.e. high resolution). The contrast and phase (either in differential or closure phase sense) of the fringes observed on a source with given baselineB and wavelengthλ are related to the Fourier transform of the source brightness distribution at the spatial frequency f=B/λ


Angular resolution and target morphology

The angular resolution (f=B/λ) isset by the available baseline, which can reach about 200 meters for the ATs and about 130 meters for the UTs. The limit will be about 2 milliarcsecond (mas)for the ATs (about 3 mas for the UTs) in the K band. These values are roughly halved for the J-band.

The choice of baseline is also influenced by the expected complexity of the object. For a simple spherically symmetric object (for instance, a stellar disk at a first approximation) a single triangle (3 telescopes) allows to determine the diameter. For a binary star, two triangles may be required if, for example, the orientation is not known. For increasingly complicated objects, various triangles (in size an orientation) will be necessary in order to reconstruct an image by mutli-aperture synthesis.

In the case of the VLTI, the large number of stations for the ATs and the availability of four UTs located in a non-redundant way, provides a very rich scenario of possible baselines. Baselines available for the current Period can be found here.

Spectral Resolution and Range

High Resolution K-band (HR-K), Medium Resolution K-band (MR-K) and Low Resolution K and H band (LR-Kand LR-HK) are offered as available modes, with Spectral Resolution R of 12000, 1500 and 35respectively. This table gives the various spectral configurations available. For each mode the central wavelength and the full covered wavelength is given. Note that for all modes except the LR modes only a part of the full wavelength range can be readout due to the limitation on the available DITs, unless active fringe tracking is used.


Please refer to the latest AMBER User  Manual for information about typical performances of AMBER.

Limiting Magnitudes

Concerning the limits in sensitivity, these depend on a large number of factors:

  • The type of telescopes, can be either UTs or ATs.
  • The chosen spectral resolution (LR, MR and HR).
  • Environmental constraints such as seeing, and atmospheric transparency.
  • The use of the active fringe tracking (using FINITO)
  • The novel polarization control as of P94 (see manual Sect. 2.6.3)

The table below presents the limiting magnitudes as function of the instrumental set-up, ambient conditions and target properties. In particular, the AMBER spectral mode, the correlated magnitudes in K and H ( Kcorr and Hcorr), K and H lowest visibility on the two shortest baselines (VisK and VisH), the Airmass of the target, the Vmag of the guide star and the Distance between the science object and the guide star.

    Note: The magnitude limits are for the correlated magnitude. This correlated flux magnitude is defined as:

    Kcorr = Kmag - 2.5log10(V), where V, is the Visibility of the object.


Spectral mode

Fringe Tracking1

Kcorr limit

Hcorr limit

Vis K

Vis H

Air Mass

Guid. Vmag

Guid. Dist




9.0, 8.0*

9.0, 8.0*








8.5, 7.5

8.0, 7.0







6.5, 5.5



7.5, 6.5 7.0, 6.0




6.5, 5.5, 4.5**

6.5, 5.5, 4.5 **









4.5, 3.5, 2.5







6.0, 5.0, 4.0 5.0, 4.0, 3.0

1: "Phase" tracking is performed by FINITO, "Group" tracking is performed by AMBER self-coherencing.

The table above assumes seeing < 0.6" with CLR conditions, seeing < 0.8" with CLR conditions (UTs and ATs) and seeing <1.2" and THN conditions (ATs only). THK conditions should not be used for AMBER observations. PHO conditions are not applicable because AMBER does notprovide a photometric calibration to a high level of accuracy evenunder optimum conditions.

PIs should make sure, when submitting their proposals, that the proper seeing conditions are selected according to the correlated magnitudes of their objects. The limiting magnitudes correspond to seeing values for observing at zenith and degradation at higher airmass should be taken into account by correspondingly better seeing conditions.

*   Computed for DIT=25ms. Visibility calibration of longer DIT isnot garanteed.

** Computed for DIT=100ms. Reduced by 0.7mag and 1.5mag for DIT of 50ms and 25ms respectively.

MR or HR without FINITO, as well as other special modes, should be properly motivated in the proposal and agreed by the Instrument Scientist before the date of observation.

Field of view

AMBER is a single-modeinstrument, therefore theoretically the field of view is limited to the Airy disk of each individual aperture, i.e. 250 mas for the ATs in K and 60 mas for the UTs in K.

Instrument layout

The AMBER instrument is quite complex but can bebroken down into three subsystems:

AMBER GTO Programme

An extensive presentation of the AMBER GTO programme can be found here. Information on protected objects for any given semester can be found in the general announcement of the relevant Call for Proposals.