II. A. The STar Separator
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I. Introduction A. The goals of PRIMA B. Interferometry Tutorial C. Atmospheric and physical Constraints   II. Description A. The STS B. The PRIMET C. The FSU D. The DDL E. PRIMA Software III. Applications   IV. The Team   V. The Partners   VI. References, links

Table of contents of the STS part: 1) Objectives of the STS 2) Technical requirements on the STS 3) Principle 4) Next milestones 5) Publications about the STS     In the PRIMA facility, the star separator system must pick up two stars in a field of view and send them down to the VLTI for detection of their fringe package and stabilisation. First and foremost, we will see precisely the real objectives of this system before precising what are the PRIMA requirements for its realisation, then we will describe the principle of the proposed STS and finally we will conclude by precising the next milestones. The STS are placed directly at the Coude focus of each telescope, and are thus linked with the framework of PRIMA (cf fig.1):

Fig. 1: Link between the STS and PRIMA

Objectives of the STS

The STar Separator (or STS) is an opto-mechanical system. Its main goal is the separation of two stars at the Coude focus (cf fig.2), a bright guide star and a faint object observed in a narrow angle (typically in a field of view of 2 arcmin) and their collimation into 2 parallel beams, each corresponding to a star. The system has to be very accurate and has to follow the stars (because of the earth rotation) with a precision of 10 mas.

Fig. 2: General scheme of the STS

But this main goal has specific requirements, let's see what they are.

Technical requirements on the STS

a) Calibration:

The STS must first enable the calibration of the fringe tracking system on a bright guide star. It has to be able to send the same star (the primary star) into both feeds without affecting the metrology. Indeed, this particularity will be used to calibrate the whole system on the primary star before observing the primary (PS) and the secondary star (SES) at a same time (cf metrology and differential delay lines for more precisions).

b) Optical path difference:

Because of optical symetric reasons, the optical path difference (OPD) must absolutely be the same in each STS.

c) Tracking precision:

The precision of the pointing of the stars needs to be better than 10 mas as said in the introduction. Because of the earth rotation and alt-azimutal telescope mounts, the stars are moving in the terrestrial referential, and the STS must follow the field rotation at the Coude Focus with this fixed limit of accuracy.

d) "chopping":

One required mode in PRIMA is the "chopping" mode.

The origin of this method comes from the fact that when the telescopes are observing a star, there is some background due to the atmosphere and mirror emissivity added to the brightness. It is particularly important at mid-IR wavelenghts (l ~ 10 µm).

This background comes from different sources:

* the mirrors

* the atmosphere turbulences

* the dectector that, even cooled at liquid helium temperature (T=4.2K) possesses a quantum efficiency ( ratio (Detected photons)/(Incident photons) ) that varies with time.

To solve this problem of background, PRIMA will use the "chopping" mode. Thus, using this method, an object 10e3 to 10e4 fainter than the background can be detected.

The main limit to this technique is the following: when PRIMA is looking on a another point than the star, the beam has different footprint on the upstream mirror (M1 -> M2) leading to a slightly different thermal background that induces a pertubation of the measurement because the cleaning of the background is less effective.

e) The mechanical stability:

Of course, the system needs a very good stability on a mechanical point of view. Precisely, the requirement is to have for the instruments of PRIMA a first eigen-frequency > 100 Hz. That corresponds to a relatively small weight and a high stiffness.

f) The transmittivity:

The system must have a very good transmittivity ( > 80%) to lose the smallest possible quantity of photons.

g) The polarisation:

The system musn't change significatively the polarisation of the incident photons, because if the polarisations of both beams are too different, they will not interfere correctly and there will be a loss of fringe visibility..

h) Other conditions:

The design must have a good thermal stability and a low weight.

i) Summary:

Finally we can resume the main performances required by PRIMA in the following table:

Principle of the STS

The star separator has not reached the Final Design Review (FDR) step yet (the last stage before the manufacturing of the system). The concept as of mid-june 2003 is based on the following corner stones.

a) The sharp roof mirror:

The primary star split during the calibration and the separation of the PS and the SES is done with a sharp roof mirror (cf fig.3).

Fig.3: The sharp roof Mirror

In the first case (case 1 on the figure 3), when the STS is looking only to the primary star, the beam (i.e. the Airy's disk) is focused on the mirror edge and separated into two identical and symetrical beams.

In the second case (case 2 on the figure 3), the primary and secondary beams are directed to the 2 sides of the roof.

b) The spherical mirrors:

This roof focal mirror is made of 2 spherical surfaces (cf S1 and S2 on the figure 4) whose aim is to reimage the pupil on the next mirrors.

Fig. 4: Spherical mirrors and Field Selector Mirror

These field selected mirrors can point to all the points of the spherical mirrors in order to pick up the primary and the secondary stars respectively. Moreover they can tip and tilt very quickly to allow for chopping.

c) Reimaging:

Then we reimage the focal spots on a Variable Curvature Mirror (VCM, cf fig.5) to adjust the exit pupil positions longitudinal position by blowing the mirror, lateral position by tip-tilt of the VCM. Finally, by seeing the figure 5, we can observe all the STS layout. This is motivated by the needs of a good superposition of the pupils in the instrument (cf metrology) and a symmetrical situation for entrance and exit pupil of main delay lines.

Fig. 5: STS Layout

click here or on the picture to see the animation

d) Collimation:

After the VCM the beams are divergent and we want them to propagate toward the VLTI optical train as parallel beams. To collimate the divergents beams, we are using some parabolic mirrors.

Next milestones

I. Introduction A. The goals of PRIMA B. Interferometry Tutorial C. Atmospheric and physical Constraints   II. Description A. The STS B. The PRIMET C. The FSU D. The DDL E. PRIMA Software III. Applications   IV. The Team   V. The Partners   VI. References, links

The next milestones for the STS are resumed in the following table:   Stage Supposed Date Preliminary Design Review (PDR) December 2003 Final Design Review (FDR) June 2004 Preliminary Acceptance Europe (PAE) June 2005   More informations about the STS "PRIMA Technical Description and Implementation", F.Derie, F.Delplancke,A.Glindeman,S.Leveque, S.Menardi,F.Paresce,R.Wilhelm,K.Wirenstrand,Workshop "Hunting for Planets ", Lorentz center, Leiden University, 3-6 June 2002. Slides of the presentation. For the other publications, see the part: References and links.

Send comments to <fdelplan@eso.org> Last update: Jun.31 2003