AMBER Instrument Description

Introduction

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~30 i.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 baseline B and wavelength λ are related to the Fourier transform of the source brightness distribution at the spatial frequency f=B/λ

Specifications

Angular resolution and target morphology

The angular resolution (f=B/λ) is set 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 reconstrust 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-K and 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 read out due to the limitation on the available DITs, unless active fringe tracking is used.

Limiting Magnitudes and Performances

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)

 AMBER
 FINITO
 Kcorr
 Hcorr
 VisK
 VisH
 AM
 Vmag
 Dist
UT
 LR
 no
 ≤7*
 ≤7*
 ≥10%
 ≥10%
 ≤2.0
 1...17
 ≤55"
LR / MRK
 yes
 ≤7
 ≤7
 ≤1.5
 1...15
 ≤13"
HRK
 yes
 ≤7
 ≤6
 ≤1.5
 1...15
 ≤13"
AT
 LR
 no
 ≤5.1**
 ≤5.5**
 ≥5%
 ≥5%
 ≤2.0 -1.7...13.5 ≤60"
LR / MRK
 yes
 ≤5
 ≤5
 ≥15%
 ≤1.5 -1.7...11 ≤15"
HRK
 yes
 ≤5
 ≤4
 ≥15%
 ≤1.5 1.7...11 ≤15"

Mode is the Spectral Configuration, Kcorr and Hcorr the correlated magnitudes in K and H, VisK and VisH the K and H lowest visibility on the two shortest baselines, AM the Airmass of the target, Vmag the magnitude in the Visible of the guide star and Dist is the distance from the science object and the guide star.

*   Computed for DIT=25ms. Visibility calibration of longer DIT is not garanteed.

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

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

The table above assumes seeing<0.8", clear photometric condition on the UTs, and seeing<0.6" and clear on the ATs.
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.

Visibility

In order to assess the feasibility of a scientific observation, it is of fundamental importance to consider the Visibility Accuracy (σV) and the minimum Absolute Visibility (Vmin) that can be attained.

The Visibility accuracy: We should distinguish between Absolute Visibility Mode and Differential Mode. The former is measured through the comparison with an external calibrator source, the latter is related to changes in the physical appeareance of the source at various wavelengths within the same observation (such as in the presence of either absorption or emission line features). Currently Vmin>0.1/0.05 (UTs/ATs) is required for succesful operation.

Visibility accuracy

Visibility Mode σV
Absolute 0.03 on the UTs and 0.02-0.1 on the ATs (depending on the object magnitude)
Differential 0.01*
*For Vmin>0.5.

FINITO

Using AMBER with an external fringe tracker. It is possible to reach K=7 and H=7 on the UTs in LR-JHK and MR-K, and K=6 with HR-K. On the ATs the limits are K=5 and H=5 on the ATs in all modes ie for LR-KH, MR-K and HR-K. The limits for the UTs are for Seeing ≤0.8", Clear and Coherence time ≥3ms. The limits on the ATs are given for optimal weather conditions ie. Seeing ≤0.6", Clear and Coherence time ≥3ms. For 0.6"<Seeing≤0.8" and Clear the limit is K=4 and H=4, and for > 0.8"<Seeing≤1.2" and Clear the limit is K=3 and H=3. Depending on the required limiting magnitudes the user should request the proper weather constraints.

Field of view

AMBER is a single-mode instrument, 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 be broken down into three subsystems:

Scientific Objectives

Thanks to the combination of instrument performance, choice of baselines, closure phase capability, and the photon-collecting power of the VLTI, a wide range of astronomical sources can be targeted. What follows is a brief presentation of the major objectives, which are in no way a full listing of all scientific possibilities of the instrument. Most of these objectives need the PRIMA (astrometry, fringe stabilizer, and dual feed) facility or FINITO (fringe tracker) to realise the objectives to their full extent, but even without these, AMBER will be able to make great advances in the following areas:
  • Hot extrasolar planets: Determination of planetary mass, orbital parameters and the spectra of the planet and the star.
  • Active Galactic Nuclei: Spatially resolve the Broad Line Region and to constrain its geometry and kinematics. The ionized disks around the putative Massive Black Hole can be studied to constrain its morphology, size, and, velocity and density field. Measuring the wavelength dependence of the central point source, the shape and size of circumnuclear dust structures as well as additional structures (e.g., the inner region of jets, circumnuclear starburst regions, or bars) in order to test AGN models.
  • Circumstellar material in hot/cold and young/old stars: Constraints on the size and morphology of the disk, including velocity and density fields. Similarly, jets and bipolar outflows can be studied, obtaining sizes, morphology, and, velocity and density fields.
  • Binaries: Direct measurement of actual orbital motions, and the masses of the stars.
  • Stellar structure: Measurements of the radius, ellipticity, surface activity, and, limb-darkening effects.

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.