VLTI @ Paranal

60th STC meeting

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What does the VLTI infrastructure do?

• Put the light in the one place at the one time.– Inject the image plane in the lab– OPD variations should only be atmospheric– Make the pupils coincide

• All of this has to more-or-less function to make fringe tracking work.

• Not all of this needs to be at peak capability to get science on bright sources since the photons from 8-m telescopes approximate an infinite number in those cases.

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How does VLTI do it?

• Each UT has a MACAO system that concentrates the bulk of the photons within the Airy ring.

• The beam is propagated via the relay optics to the delay lines

• The delay lines correct in ‘open loop’ geometric OPD (telescope and star locations)

• The VCMs on the delay lines move the pupil in the ‘axial’ direction.

• IRIS corrects for drifts in the conjugation between the MACAO reference and the lab reference

• FINITO corrects for atmospheric OPD variations through the delay lines

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Where are we now?

• 6 delay lines installed (4 operational), VINCI, FINITO, IRIS, ARAL, MIDI and AMBER installed and able to execute scientific programs at the bright end.

• 2 ATs operational with tip-tilt correction.• 30 docking stations installed and aligned, 6

docking stations used with siderostats or ATs.• 4 UTs operational with AO correction.• 35 refereed publications including 2 Nature

papers.

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Process.

• Following the review in December 04, the organization chose to pass the VLTI infrastructure ‘as is’ to Paranal.

• This acceptance by Paranal of the system was along the lines of every other delivery to the site from the UTs to the instruments (the review issues become punch list items)

• By taking the VLTI infrastructure over the lines of responsibility were clarified.

• What we did was to establish a team that would be given a simple task. Answer the Big Finito Question (BFQ).

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Difficult areas

• Beam injection. We have frequent flux dropouts. Weather in the tunnel? Spasms in the delay lines? Spasms of polarization? ….

• OPD variations have significant non atmospheric components. Where do they come from, where do they go…

• Delay line alignment is insufficiently robust to allow the operation of the VCMs that control the pupil.

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FINITO tiger team report (June 2005)

• The primary limitation to fringe tracking using the UTs is OPD vibrations in the telescope and beam transport; with the ATs it is flux drop-outs due to poor coupling onto the FINITO fibers.

• Although it needs improvements, fringe tracking with FINITO essentially has the potential to deliver stable fringes to instruments slaved to it.

• In our review of the existing documentation, we conclude that known vibrations dominate the OPD fluctuations and constitute the primary limitation to fringe tracking at VLTI. In addition to mitigation of the vibrations, we believe a global approach is needed to identify and monitor the interferometer vibration sources. Beyond the primary limitations, secondary limitations exist and are identified; these include:

– Tilt and Strehl induced intensity fluctuations ･– Phase unwrapping errors in FINITO.

• Essential improvements: – Vibration mitigation, particularly in the 20 to 150 Hz frequency range. – Tunnel seeing mitigation to allow stable coupling and improving the bandwidth of the proposed tip-tilt

control system to 1 Hz or higher. – Increase the priority of commissioning the variable curvature mirrors.

• Key additional recommendations: – Improve FINITO behavior at stroke transitions. – Make central fringe identification robust. – Improve the large-signal servo bandwidth of the delay line control.

– Investigate the nature of coupling fluctuations on ATs/UTs using IRIS.

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Beam injection

• Beacon from Nasmyth A to IRIS

QuickTime™ and a decompressor

are needed to see this picture.

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Beam injection

• Star to IRIS

QuickTime™ and a decompressor

are needed to see this picture.

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Injection drop outs?

• MACAO suffers from ‘saturation’ events. The mirror does not have enough voltage to correct the request and the loop opens. The cause is fairly well understood and is a result of cross talk between aberrations which results in high order modes carrying small amounts of tip-tilt.

• The original ‘fix’ that permits operation was to manipulate the interaction matrix to reduce the voltage requests. This reduces the duration of the open loop events but does not make them go away. Good for an AO system bad for injecting to a fringe tracker.

• Current state of affairs: problem understood and present in the simulations. Fix tested in simulation (saturation management rather than fixing the IM). First test deployment possibly next week.

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OPD variations

• The telescopes have been designed taking great care to minimize vibrations. They were also tested during commissioning (one of the many certifications) to comply with the requirements. Changes were made on M4 and the M3 tower following those tests and improvements made to the cooling pumps.

• Another campaign to improve the pumps on UT1 is planned. The other UTs appear quiet(ish).

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MACAO cabinet fansan ongoing battle

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Hang’em low

• Acoustic propagation?

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Cooling pumps of the telescope

• 96 Hz:– Passive damper (December)– Replace alt. platform cooling hoses

(December)

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Non-atmospheric OPD

• Current state: OPD variations are still higher than specified. BFQ critical until the planned improvements.

• Long term solution is probably active tracking of vibrations either with accelerometers ala Keck or with lasers going off M4 and back to the tunnel.

• Strategy: Wait, measure and think.

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Delay lines and VCMs• Delay lines is a simplification for a complex

system. Rails, carriages, metrology altogether get called delay lines.

• From the review: to maintain the alignment of the delay lines required much expert (or even better than expert) manpower and even then is unpredictable.

• Importance: Without aligned delay lines we cannot inflate the VCMs which in turn implies negligible field in the ATs (also a cause for flux drop outs) and also poor pupil location (in piston) for MIDI and AMBER.

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Delay lines…

• We took DL5 and 6 out of ‘operation’ and experimented on them.

• The other 4 continue in operations

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VCMs

• Without the VCMs functioning the field of view of the ATs is limited to an arcsecond or so (approx the diffraction limit of a 1.8-m telescope at 10 microns). J moon in IRIS

Fwhm = 7.6 arcsecAT2 pupil on ARAL through DL6

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Delay Lines : DELIRIUM- two 2D capacitive

sensors (+water pots for earth horizon referencing)

- One inclinometer for roll measurement

- Metrology (laser or coarse)

- Complete (6 DOF) trajectory reconstruction

DELIRIUM raw data (DL5).Diff between 2 scans with wire sags of resp. 9 and 6 mm

DELIRIUM linearity

Weird, but goes away with laser metrology (TBC)

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DL rail shape controlClosed loop control based on global

reconstruction– Influence function measurements (on

one support)– Construction of IM / CM (assuming same

response on all supports)– Scan results multiplied by CM to produce

correction sequence (control gain = 1)– Corrections clipped to 10% of max error

(or 5 microns ~ limiting accuracy)

Noise : replace by 0s

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Residual errorsDL5, Aug 22nd, 22:44

1.7 x wheel frequency (210cm): not understood

wheel frequency: wobble

PSDs

?

Pupil position (ARAL) vs. OPL

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DL rail driftDL6 was aligned in Dec 2004 (summer time)

DL6, June 10th, before alignment

DL6, June 17th, after alignment

DL6, Aug 2nd, after 6 wks and T drop of 2degC in tunnel

DL5, Aug 5th, before alignment

Concrete junctions

Missing coils

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Iquique Earthquake

Blue: DL6 at 08:38 UTRed: DL6 at 23:23 UT

• June 13th, 22:44 UT (during rail alignment campaign)• Richter 7.9• Distance 500 km north (i.e. wf // to rail)• The rail did not deform at all

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So where are we?• Delay line alignment may be close to being solved for the not-

so-far ends of the carriage travel. We have tested and now are deploying. Surprises may come but we like where we are.

• Hysterisis of the carriage is under study. • Activation of the VCM mount (in the original plan for the

recovery and suspended during the investigations) may still be required to overcome the wobble at the far end of the delay lines.

• Vibrations are being addressed but slower than we would like. However, no show stopper as yet identified for H and K. J is a bit harder.

• We think we understand the flux drop outs both for ATs and UTs. For UTs the solution may be at hand, for ATs it may require low order AO. We need to walk first… this may come sooner or later.