The VORTEX Coronagraph in VISIR 2.0: Coronagraphy and Mid-Infrared Imaging?!? 1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO co-authors: VISIR commissioning and upgrade teams, especially Daniel Asmus, Pedro Baksai, Derek Ives, Gerd Jakob, Florian Kerber, Jean-Paul Kirchbaumer, Eric Pantin (CEA), Eszter Pozna, Miguel Riquelme, Alain Smette, Joerg Stegmeier, Konrad Tristam Background Noise Limit: "Observing at 10μm has been likened to observing visually through a telescope lined with luminescent panels and surrounded by flickering light as though the telescope were on fire" Low & Rieke, 1974 ph/pix/s for Nyquist sampling and standard conditions in ground based astronomy wavelength interval [μm] 1.9-2.1 2.1-2.3 2.2-2.4 3.3-3.7 4.5-5.5 9.0-11.0 19.0-21.0 photons per pixel per second 60 440 1050 1.2 10 6 1.2 10 8 8.4 10 9 5.5 10 10
Transcript
The VORTEX Coronagraph in VISIR 2.0: Coronagraphy and Mid-Infrared Imaging?!? 1st International Vortex Workshop, CalTech, Aug. 18, 2016Ulli Käufl, ESOco-authors: VISIR commissioning and upgrade teams, especiallyDaniel Asmus, Pedro Baksai, Derek Ives, Gerd Jakob, Florian Kerber, Jean-Paul Kirchbaumer, Eric Pantin (CEA), Eszter Pozna, Miguel Riquelme, Alain Smette, Joerg Stegmeier, Konrad Tristam
Background Noise Limit:"Observing at 10μm has been likened to observing visually through a telescope lined with luminescent panels and surrounded by flickering light as though the telescope were on fire" Low & Rieke, 1974
ph/pix/s for Nyquist sampling and standard conditions in ground based astronomy
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 5
VISIR upgrade pitfalls, or finding a dead rat is never a good sign:
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 6
VISIR upgrade
in May 2012 VISIR was taken out of operation for the upgrade
the entire upgrade proceeded with “few excursions” as planned and as scheduled
VISIR was back on the telescope in July 2012
the action item list was basically empty
everything appeared to work fantastic, but
in spite of a remarkable cosmetic quality, the sensitivity of VISIR was lousy
the spectrometer throughput appeared to be three times that of the imager
VISIR was thus brought back to the lab to understand issues
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 7
VISIR upgrade resurrected(Nov. 2015 … )
to recover detector performance fast M2 chopping (5 Hz) needed implementation
problems of the astatic support (2-3 ”/h “drift” of VISIR) solved
cryogenic issues: once wrongly assembled & bad connectors ...
ground loops in detector
first set of SAM masks wrongly designed
issues with the telescope
….
Found later a horse shoe, small, thin and beaten up, so the recovery from the fat dead rat took some time
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 8
VISIR upgrade, status image quality potential on bright objects
left linear, right logarithmic (burst mode, shift and add)green bar 1 arcsec
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 9
VISIR upgrade, status image quality pit falls
star falls between detector stripes, logarithmic “bar” <5%does not affect photometrycan be diluted by dithering and field rotationthreshold behaviour under investigationcoronagraph helps ...
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 10
Sparse Aperture Masking (SAM) masks in cold pupil for dedicated filters
multi-baseline Fizeau interferometer
now commissioned and fully operational
BLIP-regime, throughput only enters with √
tricky data flow
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 11
SAM masks: first results from VISIR early SV in Feb. 2016
Credit: Joël Sanchez Bermudez, MPIA
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 12
Coronagraphythe phase masks
the phase masks on the VISIR aperture wheel
annular groove (AGPM) and 4 quadrant (4QPM) phase masks1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 13
Location of critical elements in optical path
Schematics VISIR imager 0.045”/pix configuration
field stop with AGPM and 4-quadrant (4QPM) phase masks
filter wheel in pupil plane with Lyot stops
cold baffle limits solid angle at focal plane to ~400 deg2
Illumination of focal plane with ~10% of a 290 K black body
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 14
Lyot stop at filter wheel
Lyot Stop in VISIR cold pupil combined with dedicated filters
individually mounted and centered for each dedicated filter
centering a bit tricky
geometry not yet optimized
interesting filters may also be used without stop
with standard digital camera focused to ∞ in focal plane alignement of Lyot stop with instrumetn cold pupil can conveniently be checked
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 15
lab testing of VISIR annular groove phase mask
REM image of device in VISIR
throughput outside sweet spot was measured to be 95.4%to be compared to uncoated diamond: ~ 65%
on sweet spot, attenuation ~100
mid-IR pupil image on sky:pupil alignment VISIR-Telescope and Lyot stop vs cold stop leave room for improvements(also pupil imaging lens out of focus)
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 16
“lab testing” of VISIR annular groove phase mask
VISIR star simulator: left sweet spot touches Airy ringright, star on sweet spot, attenuation ~1001st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 17
Coronagraphy vs PSF subtraction
annular groove masks produce a point-symmetric λ/2 phase shift around the sweet spot
extinction requires matched wavelength and full spatial coherence: => thermal background is not attenuated=> compared to a classic occulting disc no “black spot” in the center: such a disk would produce 63 ghost images and would in general severly affect the image cosmetics
relatively easy to operate and to keep star aligned
extinction ratio AGPM: ~ 100 ; 4QPM: >20
non-negative process, contrary to PSF subtraction:naïvely comparing performance of both methods I used to argue that in the extreme BLIP a coronagraph has not advantage over PSF-subtraction …
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 18
VISIR AGPM results on sky
examples from1st early science verification withVISIR AGPM (Feb. 2016)
data taken under ratheradverse conditions(mediocre seeing, high humidity and telescope problems)
intermediate images stored every ~100millisec, frame selection
tracking issues of the VLT not yet corrected for axis and pupil
please note WR104 is a new challenge foralgorithms like QACITS
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 19
VISIR SV-test Sirius B with AGPM (program M. Sterzik)
@ VLT 1.2 λ/D ~ 0.3” still detection of Sirius B notnecessarily trivial
data were taken in fieldstabilized mode, but for twodifferent position angles
Right: encircled energy vs aperture radius (“) for Sirius A in chopping-off position, indicating some residual image deterioation
data reduction here and the following slides courtesy A. Kaufer & M. Sterzik 1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 20
VISIR SV-test Sirius B with AGPM (program M. Sterzik)
top: Sirius A bleedingthrough AGPM(125 Jy @ 12.4μm)
left: combined data set reduced, Sirius B well detected(5mJy @ 12.4μm)
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 21
VISIR SV-test Sirius B with AGPM (program M. Sterzik)
plot of efficiency of AGPM during this program vs. radius of aperture size
contributing factors for deterioration:- seeing - pointing problems on all time scales
however, sensitivity better than advertised, while Sirius B fainter than assumed
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 22
AGPM: the pale red dot
Image of AGPM vs Sky
image of 30K AGPM masksagainst telescope thermal background and sky
sweet spot emits like astar
not obvious in noise maps
but ..
is it a really not a problem?
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 23
AGPM: the pale red dot with more thermal background
trace through images
black: FOV clear to telescope and sky
red: ~ 20% of area of field mask obscured with 290K black body=>increase of intensity
of spot is marginal
pale red: <80% increaseof shot noise in AGPM centerbut is chopping away nicely
conclusion: this artifact is not limiting performance or observations
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 24
imaging planets around α
1,2 – Cen with VISIR
challenges:
VISIR as “visiting instrument” to VLT-UT4 as soon as adaptive secondary is in place
improve VISIR, VLT and operations - use AO and active tracking to increase Strehl- use dedicated filter and clean freshly coated telescope- improve Lyot stop and generally pupil alignment- use different chopping/nodding scheme- use the telescope with normal PWV (2.5mm) rather than the extreme conditions of Feb. 2016 (7.5 mm)
sensitivity: signal from planet with r = 2 x r⊕: 80μJy @ λ~12μmto be compared with ~ 2.9mJy, 10 σ in 1h achieved for Sirius B
contrast: Cen A: ~ 6x10 -7 at 2.8 – 3.8 λ/DCen B: ~1.5x10 -6 at ~2 λ/D
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 25
VISIRAO Phase-A
Schematic VFM
VISIRAO Phase-A
Vacuum Housing
Dicke-Switch: D-shape mirror Stepper drive
commanded by NGC TTL pulses (optional encoder to record phase)
Black-body unit (Hindel Sphere)
PT500; current drive provide “heater”; voltage provides T
imaging planets, extrapolating from SPHERE
sources of non-common path aberrations: compared to SPHERE:few and well understood “training” of AO with very bright stars
usage of ADI
residual speckle intensities~ 1/λ2
conclusion: team at ESO, M.Kasper (lead), R. Arsenault, G.Jakob, H.U.Käufl and R.Siebenmorgen, is convinced detection of a 2-earth is feasible
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 28
beyond coronagraphy:photon angular momentum
shining light through a vortex with a phase step of n*λ produces light with photon angular momentum equal n * ħ !
in astronomy light passing a rotating black hole can acquire such photon angular momentum (e.g. D. Hetharia et al. 2014)
left, from G. Anzolin et al. (2008),shows how starlight after aphase shifter – here a fork -focuses differently
1st International Vortex Workshop, CalTech, Aug. 18, 2016 Ulli Käufl, ESO slide # 29
CONCLUSIONS VISIR upgrade has substantiallyimproved high definition and high contrast imaging
the AGPM in VISIR is a close to perfect device
VISIR with AO – as visiting instrument to the ESO AOF – has the potential to image Earth-like planets in the α-Cen system
while exo-planets will largely be the domain of future E-ELTs the VISIR AGPM will allow for in depth unique investigations of matter very close to bright stars:
disks, ejecta, accretion phenomena ….
wishlist for VISIR
an AGPM with 2-4 sweet spots for normal chopping/nodding
a solution for the tracking problems including the implementation of algorithms like QACITS, however, “biased”
