Transcript VLTI @ Paranal 60th STC meeting

VLTI @ Paranal
60th STC meeting
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)
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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:
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Essential improvements:
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Tilt and Strehl induced intensity fluctuations･
Phase unwrapping errors in FINITO.
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:
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–
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Improve FINITO behavior at stroke transitions.
Make central fringe identification robust.
Improve the large-signal servo bandwidth of the delay line control.
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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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•
•
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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.
Injection drop
outs?
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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 fans
an 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 1.8m telescope at 10 microns).
J moon in IRIS
Fwhm = 7.6 arcsec
AT2 pupil on ARAL
through DL6
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Delay Lines : DELIRIUM
- two 2D capacitive
sensors (+water pots for
earth horizon referencing)
DELIRIUM linearity
DELIRIUM raw data (DL5).
Diff between 2 scans with wire
sags of resp. 9 and 6 mm
- One inclinometer for roll
measurement
- Metrology (laser or
coarse)
- Complete (6 DOF)
trajectory reconstruction
Weird, but goes away with
laser metrology (TBC)
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DL rail shape control
Closed 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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DL5, Aug 22nd, 22:44
Residual errors
wheel frequency: wobble
PSDs
1.7 x wheel frequency (210cm):
not understood
?
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Pupil position (ARAL) vs. OPL
DL rail drift
DL6 was aligned in Dec 2004 (summer time)
Concrete
junctions
DL6, June 10th, before alignment
Missing coils
DL6, June 17th, after alignment
DL6, Aug 2nd, after 6 wks and T drop of 2degC in tunnel
DL5, Aug 5th, before alignment
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Iquique Earthquake
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June 13th, 22:44 UT (during rail alignment campaign)
Richter 7.9
Distance 500 km north (i.e. wf // to rail) Blue: DL6 at 08:38 UT
Red: DL6 at 23:23 UT
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 notso-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.
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