AMBER Instrument Description


1. Instrument overview

AMBER is a spectro-interferometric instrument for the VLTI working in the near-infrared. It is offered in the H-band and K-band. The instrument combines and disperses three beams arriving from three telescopes that can either be UTs or ATs. Per observation, AMBER delivers the visibilities and phases of the science target at three different spatial frequencies and one closure phase. The interferometric data are spectrally dispersed and the AMBER spectrograph has three spectral settings, viz. low (R~30), medium (R~1500), and high (R~12000). As dictated by the van Cittert-Zernike theorem, the basis of stellar interferometry, the fringe contrast and phase are related to the Fourier transform of the source brightness distribution on sky at the observed spatial frequency f=B/λ. Here B is the baseline distance of a telescope pair within the AT or UT configuration. 

AMBER operates either in stand-alone or in conjunction with the FINITO fringe-tracker. In stand-alone mode, AMBER performs self-coherencing which results in centering of the fringe packet on the detector; fringes nonetheless move. FINITO provides phase-tracking in which ideally, the fringes are frozen on the AMBER detector. In self-coherencing mode the DIT are necessarily short, whereas longer DITs can be chosen when observing with FINITO.

2. Instrument layout

Detailed overview of AMBER is organized in three basic components of the instrument:

3. 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.



1. Angular resolution

The angular resolution (f=B/λ) is set by the baseline length. The VLT interferometer offers baselines of ~140 meters for the ATs and about 130 meters for the UTs. As a result, the spatial resolution limit in the K-band will be about 2 milliarcsecond (mas) for the ATs. The spatial resolution is clearly higher at the shorter H-band and accessible by AMBER in the LR and MR settings.  The choice of baseline for a particular program depends on 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. Baselines available for the current Period can be found here.

2. Spectral resolution and range

High resolution K-band (HR-K), medium resolution K-band (MR-K) and medium resolution H-band (MR-H) and low resolution K and H band (LR-HK) are offered as available modes, with Spectral Resolution R of 12000, 1500 and 35, respectively. 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.

3. Performances

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

4.Field of view

The interferometric field of view of the VLT-I  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-band.

5. Limiting magnitudes

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

  • The type of telescopes either UTs or ATs.
  • The spectral resolution (LR, MR and HR).
  • Environmental constraints such as seeing, and atmospheric transparency.
  • The use of the active fringe tracking (using FINITO)
  • The 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.