II. E. PRIMA Software
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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 PRIMA Sofwatre part: 1) Objectives of PSS 2) Technical requirements on PSS 3) Principle   For working the PRIMA facility needs different sub-systems we have seen to allow physical detection of physical parameters and their correction, but in order to coordinate all these data, PRIMA needs a software too in charge of taking care about all these different sub-systems in order to allow reaching objectives of PRIMA, that's why this part will describe the PRIMA Supervisor Software (PSS). In a first time, we will as usually describe the objectives of PSS. In a second time we will speak about the requirements of PSS and before to present the next milestones, we will describe more in details the principle of PSS. The software is in a special room seperated from the laboratory, and involves for example the OPD controllers (cf fig.1).

Fig. 1: Link between PSS and PRIMA

Objectives of PSS

PSS will be in charge of coordinating all the different sub-systems control software to allow general working, what is the last step before the human work on the astronomical observations.

To present a Control Software Overview we can precise that PRIMA Supervisor Software (PSS) is part of PRIMA Control Software (PRICS). PRICS includes the following software components:

• PRIMA Supervisor Software (PSS), responsible of coordinating PRIMA sus-systems.
• PRIMA Metrology Control Software (PMCS), responsible of controlling PRIMET.
• PRIMA Fringe Sensor Unit Control Software (PFCS), responsible of controlling PRIMA FSU.
• PRIMA Star Separator Control Software (PSCS), responsible of controlling PRIMA STS.

PRICS is a part of VLTI Control Software (VLTICS), as we can see it on the fig.2.

Fig. 2: PRICS system context diagram

Requirements on the PRIMA Software

The PSS has for a function to allow working of all the modes of the differents sub-sustems by using their capabilities (sensor, corrector...). To understand what kind of data and of operting modes PSS has to solve, we will summarize the four sub-sytems functions and modes of working. Morevover we have to consider the two modes of the PRIMA facility itself, i.e. the imaging mode or the astrometric mode.

a) The Star Separator (STS):

(i) Reminder:

We have explained in the STS part that he STS is responsible for picking up two objects located at any position in the Coude's field of view and output them as two parallel collimated beams in the VLTI tunnel. It is also in charge of tracking the two objects accurately and independently. For calibration purpose, the STS has to be able to inject the same object into both collimated beams.

(ii) Modes of work:

The STS will work in somes different modes:

• Initialization mode: swithc on, check status and integrity.
• Alignment mode: align optics. STS alignment is done typically once per month or every time the AT moves to a different position or after an earthquake.
• Pre-setting mode: move to a new target in the Coude's field.
  • In single field: one object, one beam.
  • In dual field: two objects, two beams. One beam for each object.
  • In calibration: two objects, two (identical) beams. One of the two objects is fed into both beams.
  • In single object dual feed: one object, two beams. The object's light is split chromatically into two parts before or at the entrance of the STS: one beam for each part.
• Tracking mode: follow the apparent motion of the object(s) in the Coude's field of view (field rotation).
• Chopping mode: as in tracking mode but with the capability of chopping the secondary object.
• Park/Stand-by mode: fix the actuators to a specific position and set the brakes for the moving parts.
• Maintenance mode: to simulate the STS behavior.

  b) Metrology:

  (i) Reminder:

For summarizing the Metrology part, PRIMET is responsible for measuring the internal OPD difference between two stellar beams with a laser divided into 4 beams. These beams are sent from the level of the FSU B to the telescopes where they are retro-reflected above the STS in the pupil planes. Then they are detected back in the laboratory and used to determine the internal differential OPD.

(ii) The metrology system used as a sensor:

In practice, the data furnished by the metrology system are given by the 4 lasers:

DL = (B1-B2) - (B3-B4)

where B1, B2, B3 and B4 are the distances covered by the 4 lasers and should represent respectively:

• B1 the measure internal path covered by the primary object's beam in the first telescope
• B2 the measure internal path covered by the secondary object's beam in the first telescope
• B3 the measure internal path covered by the primary object's beam in the second telescope
• B4 the measure internal path covered by the secondary object's beam in the secondtelescope

The metrology delivers the DL to the differential OPD Controller (dOPDC) that will drive the differential delay lines.

c) The Fringe Sensor Unit:

(i) Reminder:

The FSU consists of two identical beam combiner and detection channel - FSU A and FSU B - located in the VLTI laboratory. The role of each FSU is to:

• Look for the presence of fringes
• Measure the fringe patterne position and store it

  (ii) The different modes of the FSU:

The FSU can operate in the following modes:

• Initialization mode:switch on, check status and integrity.
• Alignment mode: align optomechanical parts. This is done once per night using Leonardo (alignment source of the VLTI) as artificial light source. Optomechanical parts are aligned in order to maximize FSU photometry.
• Star Acquisition mode: this mode is activated once the telescopes are tracking to maximize the light coupling. The flux received from the input beam (beam 1, beam 2 or both) is continuously measured. Beam injection can be optimized.
• Fringe Acquisition mode: the fringe amplitude and the total flux are delivered to the OPDC. The OPD modulo lo and the group delay measurements are computed and delivered to the OPDC. Note that the OPDC will use these data to apply a ramp to the delay lines position. No data is stored.
• Tracking mode selected by the instruments:
  • Fringe tracking mode: activated once the fringe tracking loop has been closed. The OPD modulo lo and the group delay measurements are computed and delivered to the OPDC. The measurements are stored with time stamps on a FSU local memory. Note that the OPDC will use these data to track the fringes.
  • Group delay tracking mode: used when the object is too faint to allow fringe tracking. It is identical to Fringe Tracking mode, except that the OPD modulo lo measurements are not delivered to the OPDC.
• Maintenance mode: to simulate the FSU behavior.

  d) The differential delay lines:

  (i) Reminder:

The DDL are in charge of adjusting the differential OPD between the OPD of the two observed objects in order to control the position of the white light fringes. The differential OPD caries with time with the diurnal motion. The DDL follow this variation accurately. Another important function of the DDL is to re-image the pupil at approximately the same location as if the DDL were not present in the VLTI optical path.

(ii) The DDL used as a tool:

The 4 DDL, one for each beam have also in charge to execute the orders given by the FSU.

Principle of the PSS

In fact, we have seen what are the different roles of the four sub-systems of PRIMA. PSS will coordinate all of each subsystems to allow general working in wanted modes. First will be used the Optical Path Difference Controller (OPDC) and differential Optical Path Difference Controller (dOPDC), le'ts see what are their goals in the PSS work.

a) OPDC:

OPDC is used to stabilize the fringes of the primary object and will drive a delay line to compensate for the following turbulences:

• a static factor which represents the internal OPD, or delay contant, encoutered by the object
• the OPD (up to 120 m) induced by the combination of the object's position and the interferometer baseline
• the OPD induced by the atmosphere
• any predictable slowly varying OPD errors
• any remaining not predictable OPD errors, including high frequency errors

Two sensors are involved in the control loop: the FSU B which can measure the above disturbances and the DL metrology system used to servo the position command sent to the DL Controller.

The control loop can be executed using two different control modes:

• Blind tracking mode, in which the DL follows a theoretical trajectory generated by an OPD Model
• Fringe tracking mode: in which the DL motion relies also on the real time signal provided by the FSU B. FSU measurements are collected by the OPDC that has to filter the signals, apply the optimal control algorithm and distribute the real time position offsets to the DL. Depending on the amplitudes of the noise at the FSU B, the fring tracking can be further divided in:
  • Coherence tracking: where the OPD precision is in order of coherence length of the observed light beams
  • Phase tracking: where the OPD precision is in order of a fraction of the wavelength

OPDC architecture doesn't depend on the type of observations carried out with PRIMA, i.e. imaging or astrometry mode. The OPD reference value is zero.

b) dOPDC:

The differential OPD controller is used to stabilize the fringes of the secondary object and will drive either a DL (in phase 1) or a DLL (in phase 2) to compensate the following disturbances (that have lower amplitude when previous OPDC loop is closed):

• the difference between the internal OPD encountered by the primary object and the secondary object (DL)
• the OPD induced by the combination of the object's position and the interferometer baseline can only reach 60mm in the PRIMA operating conditions ( Ds = 1 arcmin, B = 200 m )
• the atmospheric OPD (or piston) anisoplanetism (DA). Its standard deviation is typically ten times lower than the standard deviation of A (atmospheric OPD for an individual star)
• any predictable slowly varying diffrential OPD errors
• any remaining non predictable differential OPD errors
• the residual OPD of FSU B (from the first OPD control lool, i.e. primary channel)

After that contrary to the OPDC we have to consider with the dOPDC in which mode we have to work with PRIMA. In particular in astrometric mode, the sensors involved in the control loop are: FSU A that can measure the above differential disturbance, PRIMET which is used to measure the internal differential OPD between the two channels (DL) and the DDL metrology system. In imaging mode, the sensors involved in the control loop are: PRIMET and the DDL metrology system.

The control loop will be executed using a blind tracking mode. A blind tracking trajectory will be generated based on the prediction of static slowly varying differential OPD errors. The optimum control strategy adopted will essentially depend on the amplitude of the sensors' noise, on the amplitude and frequency of the differential disturbances and on the PRIMA observing mode.

c) System configuration:

PRIMA system shall be deployed in two phases. Before the beginning of the observations, the Observation Preparation Software will provide the following information to the VLTI system:

• the position of the primary scientific object and secondary scientific object called the scientific pair.
• the position of a reference objecte for the adaptative optics (AO) associated with the scientific pair.
• the position or one (or more) calibration pair.
• the position of one AO's reference object for each calibration pair
• the two telescopes used for the observations
• the DL required
• The OPD model for the primary scientific object
• the differential OPD fot the secondary scientific object

 

(i) Phase 1:

In the first one, there shall de 4 delay lines (two fixed and two tracking, respectively DL1, DL2 and DL3, DL4 on the fig.3) and no DDL.

Fig. 3: PRIMA Phase 1 Optical Paths

PRIMA operationnal procedure can be split into 4 stages:

• Initialization
• Presetting
• Calibration
• Measurement

During Initialization, PSS initializes itself and PRIMA sub-systems. After the Initialization, PRIMA starts tracking the pair with the STS what is the Presetting phase. Follows the Calibration phase in which PRIMA operates on the primary object only (the primary object is injected in both channels) to calibrate its components and to reset the metrology system. Finally, at Measurement stage, PRIMA operates with both primary and secondary objects (primary in channel B and secondary in channel A) and measurements are recorded. The above procedure has to be repeated for all the calibration pairs and at the end for the scientific pair (strating from Presetting).

The whole process is repeated every 30 min (or earlier of the seeing condition changes).

(ii) Phase 2:

In the second one two DLs (one fixed and one tracking, respectively DL3 and DL1 on the fig.4) and 4 DDLs (three fixed and one tracking, respectively DDL1,DDL3,DDL4 and DDL2 on the fig.4) shall be used:

Fig. 4: PRIMA Phase 2 Optical Paths

The phase 2 is similar to the 1 with the exception of the number of DLs and DDLs.

d) Data Flow:

Fig. 5: PRIMA Data Flow

We can see on the fig.5 the main data flow trough PRIMA sub-systems. OPDC reads data produced by FSU B and computes the offset for DL1 while dOPDC receives the input from the instruments FSU A, PRIMET and FSU B and computes the offset for either DL2 (PRIMA phase 1) or DDL2 (PRIMA phase 2).

At the end of the observation period, data measured by FSU B and PRIMET are sent to the instrument, trough PSS, in order to be post processed.

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

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