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.
Performances
Please refer to the latest AMBER User Manual for information about typical performances of AMBER.
Limiting Magnitudess
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)
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.
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.
| AMBER |
FINITO |
Kcorr |
Hcorr |
VisK |
VisH |
AM |
Vmag |
Dist |
|
| UT |
LR-HK |
no |
<7* |
<7* |
>10% |
>10% |
<2.0 |
1...17 |
<55" |
| LR-HK |
group
tracking |
<7.5* | <7.5* | <1.5 |
1...15 |
<13" |
|||
| LR-HK MR-K |
fringe
tracking |
<7 |
<7 |
||||||
| MR-H |
fringe
tracking |
- |
<5 |
- |
|||||
| HR-K |
fringe
tracking |
<6 |
<6 |
>10% |
|||||
| AT |
LR-HK |
no |
** <5.1 (4.1, 3.1) |
** <5.5 (4.1, 3.1) |
>5% |
>5% |
<2.0 | -1.7...13.5 | <60" |
| LR-HK |
group
tracking |
** <5.5 (4.5, 3.5) | ** <5.5 (4.5, 3.5) | >15% |
<1.5 |
-1.7...11
|
<15" | ||
| MR-H |
fringe
tracking |
- |
<4 (3, 2) |
- |
|||||
| LR-HK MR-K HR-K |
fringe
tracking |
<5 (4, 3) |
<5 (4, 3) |
>5% |
The table above assumes seeing<0.8" with CLR conditions, for the UTs. For the ATs, the conditions are supposed to be seeing<0.6" with CLR conditions, seeing<0.8" with CLR conditions and seeing<1.2" and THN conditions. THK conditions should not be used for AMBER observations. PHO conditions are not applicable because AMBER does not provide a photometric calibration to a high level of accuracy even under 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.
* 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.
MR or HR 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.