CHERENKOV TELESCOPE ARRAY FOR INTENSITY INTERFEROMETRY Dainis - - PowerPoint PPT Presentation

Intensity Interferometry workshop – Salt Lake City, January 2009

Dainis Dravins

Lund Observatory, Sweden

www.astro.lu.se/~dainis

OPTIMIZING THE CHERENKOV TELESCOPE ARRAY FOR INTENSITY INTERFEROMETRY

CTA, Cherenkov Telescope Array

PRIORITIES IN EUROPEAN ASTRONOMY 2010-2020

ASTRONET Infrastructure Roadmap http://www.astronet-eu.org/ For the section on High-Energy Astrophysics, Astroparticle Physics and Gravitational Waves, highest-priority near-term (−2015) project is CTA; in overall list is 2nd highest priority among medium-scale ground-based projects (following the European Solar Telescope). ESFPRI , European Strategy Forum on Research Infrastructures ftp://ftp.cordis.europa.eu/pub/esfri/docs/esfri_roadmap_update_2008.pdf Eight prioritized projects within Physical Sciences and Engineering, include CTA ASPERA network on astroparticle physics http://www.aspera-eu.org/ The priority project for VHE gamma astrophysics is the Cherenkov Telescope Array, CTA.

Cherenkov Telescope Array

www.cta-observatory.org

The CTA Design Study is to optimize the planned observatory

Primary targets of are to constrain design and technology options; Optimize the cost/performance ratio; Define how CTA is to be constructed and operated; Build and test prototype telescope(s)

CTA CONSORTIUM DESIGN STUDY

Approximately four years, 2008-2011 11 work packages

PHYS

Astrophysics and astroparticle physics

MC

Optimization of array layout, performance studies and analysis algorithms [MonteCarlo]

SITE

Site evaluation and site infrastructure

MIR

Design of telescope optics and mirror

TEL

Design of telescope structure, drive and control systems

FPI

Focal Plane Instrumentation

ELEC

Readout electronics and trigger

ATAC

Atmospheric monitoring, associated science and instrument calibration

OBS

Observatory operation and access

DATA

Data handling, processing, management and data access

QA

Risk assessment and quality assurance

PHYSICS WORK PACKAGE

Work Package Coordinator: Diego F. Torres

ICREA & Institut de Ciencies de l'Espai (IEEC-CSIC), Barcelona, Spain

Physics WP topics & Task Leaders Dark matter / Fundamental physics — Jan Conrad Extragalactic background light / Cosmology — Daniel Mazin AGNs — Helene Sol, Catherine Boisson, Andreas Zech Cosmic rays / Clusters / Starbursts — Olaf Reimer Microquasars / Binaries — Josep M. Paredes Cosmic rays / SNRs / Molecular clouds — Stefano Gabici Pulsar-wind nebulae — Okkie de Jager Pulsars / Globular clusters — Bronek Rudak Galactic center — Stefan Funk Multi-wavelength / Transients / GRBs — Sera Markoff Timing — Dimitri Emmanoulopoulos Surveys / Sub-arrays — Guillaume Dubus Extended / Diffuse Sources — Sabrina Casanova Intensity Interferometry — Dainis Dravins Direct-Cherenkov light / CR composition — Rolf Bühler

Cherenkov Telescope Array

CTA General meetings

May 4-5, 2006, Berlin, Germany March 1-2, 2007, Paris, France January 24-25, 2008, Barcelona, Spain November 3-5, 2008, Padua, Italy May 11-13, 2009, Cracow, Poland

CTA desiderata

Isochronous telescope design ?

Parabolic or Schmidt better than Davies-Cotton for ∆t < few ns

Cherenkov telescopes are usually Davies–Cotton or parabolic In a Davies–Cotton layout, all reflector facets have same focal length f, arranged on a sphere of radius f. In a parabolic layout, mirrors are arranged on a paraboloid, and the focal length of the (usually spherical) mirror facets varies with the distance from the optical axis. Both have significant aberrations off the optical axis, the parabolic slightly worse than Davies–Cotton. Time dispersion introduced by the reflector should not exceed the intrinsic spread of the Cherenkov wavefront of a few ns. Parabolic reflectors are isochronal – apart from minute effects caused by individual mirror facets being spherical rather than parabolic. Davies–Cotton layout causes a spread of photon arrival times at the camera; a plane incident wavefront results in photons spread over ∆t ≈ 5 ns, with an rms width ≈ 1.4 ns.

The optical system of the H.E.S.S. imaging atmospheric Cherenkov

• telescopes. Part I: Layout and components of the system

K.Bernlöhr, O.Carrol, R.Cornils, S.Elfahem P.Espigat, S.Gillessen, G.Heinzelmann, G.Hermann, W.Hofmann, D.Horns. I.Jung, R.Kankanyan, A.Katona, B.Khelifi, H.Krawczynski, M.Panter, M.Punch, S.Rayner, G.Rowell, M.Tluczykont, R.van Staa Astropart.Phys. 20, 111 (2003)

The Davies–Cotton configuration forms a focal surface at the center

• f curvature of the optical support,

7.3 m from the mirror surface. A Davies–Cotton layout gives smaller aberrations off the optical axis compared to a parabolic design. A disadvantage is that the structure is not isochronous.: Rays striking mirrors at different distances from the optic-axis have different transit times to the focal plane. For the 10 m Whipple telescope the spread of transit times is 6.5 ns. The Whipple Observatory 10 m Gamma-Ray Telescope, 1997–2006

• J. Kildea et al. , Astropart.Phys. 28, 182 (2007)

DAVIES-COTTON SPHERICAL REFLECTOR DESIGN

Parabolic reflector of MAGIC, Roque de los Muchachos, La Palma

MAGIC has isochronous parabolic reflectors with an intrinsic time spread of 400 ps, sufficient to resolve the time structure of the cosmic showers

Top: Spherical (Davies–Cotton)

A spherical reflector substantially widens the photon pulse. At detecting 10 GeV γ-showers, the pulse width on the spherical telescope's focal plane may reach 15– 20 ns instead of the inherent 5–8 ns. Angles of incidence = 2°

Bottom: Parabolic

Performance of a 20 m diameter Cherenkov imaging telescope A.Akhperjanian & V.Sahakian Astropart.Phys. 21, 149 (2004)

INTRINSIC TIME SPREAD IN 20 m ∅ CHERENKOV TELESCOPES

INTRINSIC TIME SPREAD IN 20 m ∅ CHERENKOV TELESCOPES

Performance of a 20 m diameter Cherenkov imaging telescope A.Akhperjanian & V.Sahakian Astropart.Phys. 21, 149 (2004)

Top: Spherical (Davies–Cotton)

Dish arrival time and camera arrival times of photoelectrons initiated by the photons from 10 GeV γ-showers. The observation height is 5 km a.s.l., and the showers impact distances are: 50 m (solid), 100 m (dashed), 150 m (dotted) and 200 m (dash-dotted).

Bottom: Parabolic

• R. Mirzoyan, M.I. Andersen: A 15 deg Wide Field of View Imaging Air Cherenkov Telescope

Astropart.Phys. (2009) = astro-ph 0806.0297

IACT Schmidt telescope

Diameter 7.0 m F-ratio 0.8 Focal length 5.6 m Field of View 15° Resolution (RMS) < 1′ Non-isochronicity ≤ 0.03 ns

The mirror and the focal plane have their centre of curvature at the centre of the corrector plate. The Schmidt corrector is shown with the aspheric shape magnified by a factor of 20. Both the nominal corrector and a Fresnel version is shown.

DIGITAL PHOTON CORRELATORS @ Lund Observatory 2008/09: 700 MHz clock rate (1.4 ns time resolution) 200 MHz maximum photon count rates per channel (pulse-pair resolution 5 ns) Photon pulses at TTL voltages High-speed correlators may be limited by telescope non-isochronicity

CTA desiderata

Sharper PSF gives less background

Sky brightness: (a) Dark sky; mV ≈ 21.5 mag / arcsec2 (b) Full Moon; mV ≈ 18 mag / arcsec2 ⇒ mV ≈ 9.4 (a) and 5.9 (b) for 5 arcmin ∅ ⇒ mV ≈ 12.9 (a) and 9.4 (b) for 1 arcmin ∅

R.H.Garstang: Night-sky brightness at observatories and sites, Publ.Astron.Soc.Pacific 101, 306 (1989)

Determination of the night sky background around the Crab pulsar using its optical pulsation E.Oña-Wilhelmi, J.Cortina, O.C.de Jager, V.Fonseca Astropart.Phys. 22, 95 (2004)

SKY BACKGROUND COUNT RATES

Expected count rates in HEGRA CT1 (4.2 m ∅, FoV 15 arcmin) and MAGIC I (17 m ∅, FoV 6 arcmin) ”Background” = Crab nebula background + Light Of the Night Sky during dark-sky conditions

Intensity interferometry “should” be possible to carry

• ut in full moonlight

when Cherenkov observations are not feasible

? ? ?

CTA desiderata

Detectors for huge photon fluxes?

Photon counting @ 100 MHz – 10 GHz? Silicon detector arrays?

CTA desiderata

Handling high data rates ?

Can photon time-tagging to 1 ns-100 ps be preserved until a computing location?

CTA desiderata

Detectors – only central pixel(s) ?

• r should one have separate detectors?

Capability of Cherenkov Telescopes to Observe Ultra-fast Optical Flares C.Deil, W.Domainko, G.Hermann, A.-C.Clapson, A.Förster, C.van Eldik, W.Hofmann Astropart.Phys., in press (2009) = astro-ph 0812.3966

7-pixel camera on the lid of the H.E.S.S. Cherenkov camera

A 7-pixel camera was custom-built and mounted on the lid of the Cherenkov camera of a H.E.S.S. telescope using a plane secondary mirror to put it into focus. Its central pixel was used to continuously record the light curve of the target, while a ring of six ‘outer’ pixels was used both to monitor the sky background level and as a veto system to reject background events

• ccurring in the atmosphere

Capability of Cherenkov Telescopes to Observe Ultra-fast Optical Flares C.Deil, W.Domainko, G.Hermann, A.-C.Clapson, A.Förster, C.van Eldik, W.Hofmann Astropart.Phys., in press (2009) = astro-ph 0812.3966

Observations with a 7-pixel camera mounted on the lid of the Cherenkov camera of a H.E.S.S. telescope Examples of background events most likely caused by a meteor (left) and lightning (right). Airplanes, satellites and meteors passing through the field of view produce time-shifted flares. Lightning at the horizon is scattered in the atmosphere and illuminates all pixels in the same way. Cosmic-ray induced air showers last a few ns, appear as an elliptical light distribution on the sky over ≈ 30 arcmin.

Capability of Cherenkov Telescopes to Observe Ultra-fast Optical Flares C.Deil, W.Domainko, G.Hermann, A.-C.Clapson, A.Förster, C.van Eldik, W.Hofmann Astropart.Phys., in press (2009) = astro-ph 0812.3966

Observations with a 7-pixel camera mounted on the lid of the Cherenkov camera of a H.E.S.S. telescope An event likely caused by Sun-illuminated space debris, plotted before and after the event in the central pixel. Assuming h=1000 km, a deduced angular speed of 0.6°/s corresponds to 10 km/s, typical for space debris orbits.

The Central Pixel of the MAGIC Telescope for Optical Observations F.Lucarelli, J.A.Barrio, P.Antoranz, M.Asensio, M.Camara, J.L.Contreras, M.V.Fonseca, M.Lopez, J.M.Miranda, I.Oya, R.De los Reyes, R.Firpo, N.Sidro, F.Goebel, E.Lorenz, N.Otte Nucl.Instr.Meth.Phys.Res.A, 589, 415 (2008)

Support of the central pixel, and a camera rear-side photograph with the PMT installed

The mechanical support holding the PMT at the central aperture position, consists of two parts: * One part is fixed to the metal support plate (dubbed “Swiss cheese” because of its many holes) * The second part, containing the PMT, is screwed into the central aperture of the “Swiss cheese” plate

CENTRAL PIXEL IN THE MAGIC I TELESCOPE

CTA desiderata

Provision for inserting optical elements in front of detector

such as focusing lenses or color filters

CTA desiderata

Provision for focusing at infinity

rather than on Cherenkov light in the upper atmosphere

CTA desiderata

Telescopes in optimal pattern to cover interferometric (u,v)-plane ?

Important only for larger telescopes,

• r will the plane be filled in anyway ?

Aperture configuration optimality criterion for phased arrays of optical telescopes L.M.Mugnier, G.Rousset, F.Cassaing J.Opt.Soc.Am. A 13, 2367 (1996)

Examples of optimization using different criteria: Top: Noise at the spatial frequency that is most attenuated by the optical system Bottom: Average noise at all relevant frequencies

”OPTIMAL” TELESCOPE PLACEMENTS FOR INTERFEROMETRY ?

ESO Paranal

ESO Cerro Paranal

Aperture Rotation Synthesis: Optimization of the (u,v )- Plane Coverage for a Rotating Phased Array of Telescopes O.Guyon, F.Roddier Publ.Astron.Soc.Pacific 113, 98 (2001)

Array configurations optimized for Earth-rotation aperture synthesis: The pupil configuration is at left, and its autocorrelation function at right graph. Apertures are scaled to provide a full (u, v)-plane coverage during half a rotation. Circles show the path of the autocorrelation domains during the rotation. Their regular spacing demonstrates a very good (u, v) coverage.

”OPTIMAL” TELESCOPE PLACEMENTS FOR INTERFEROMETRY ?

Aperture Rotation Synthesis: Optimization of the (u,v )- Plane Coverage for a Rotating Phased Array of Telescopes O.Guyon, F.Roddier Publ.Astron.Soc.Pacific 113, 98 (2001)

Array configurations optimized for Earth-rotation aperture synthesis: The pupil configuration is at left, and its autocorrelation function at right graph. Apertures are scaled to provide a full (u, v)-plane coverage during half a rotation. Circles show the path of the autocorrelation domains during the rotation. Their regular spacing demonstrates a very good (u, v) coverage.

”OPTIMAL” TELESCOPE PLACEMENTS FOR INTERFEROMETRY ?

Aperture Rotation Synthesis: Optimization of the (u,v ) - Plane Coverage for a Rotating Phased Array of Telescopes O.Guyon, F.Roddier Publ.Astron.Soc.Pacific 113, 98 (2001)

IMAGE RECONSTRUCTION ALGORITHM FOR EARTH-ROTATION APERTURE SYNTHESIS For each snapshot exposure (no rotation), the entrance pupil (a) samples a domain (b) of the Fourier transform of the image. The image’s Fourier transform information is extracted by Fourier transform (d) of the snapshot image (c). This process is repeated for each rotation step until the Fourier transform of the image is completely known up to a cutoff frequency. An inverse Fourier transform is then performed to recover an image (e) which can be Fourier filtered to reduce ringing effects.

S.LeBohec, M.Daniel, W.J.de Wit, J.Hinton, E.Jose, J.A.Holder, J.Smith, R.J.White: Stellar Intensity Interfero- metry with Air Cherenkov Telescope Arrays, AIP Conf. 984 (2008); D.Dravins & S.LeBohec: Towards a diffraction- limited square-kilometer optical telescope: Digital revival of intensity interferometry, SPIE Proc. 6986 (2008) Left: Distribution of interferometer baselines in one possible large-scale array of 81 telescopes placed in a 1 km2 square grid with 125 m spacing. The upper scale indicates the baseline for the first interferometric minimum for a uniform stellar disk observed at 350 nm. Right: The two-dimensional baseline distribution, with scales in meters.

Tentative recommendations

Desire also smaller telescopes to be isochronous (< 1 ns) Desire Schmidt-type small telescopes for image quality Provision to mechanically refocus on infinity Use standard CTA detectors in central pixel(s) Provision to mount optics in front of central pixel(s) Provision to mount equipment in front of camera cover Limit photon count rates with wavelength filters Photon pulse train precise to < 1 ns to computing station Avoid placing the large telescopes on a regular grid

“Homework” outside CTA

Prototype instrumentation & test observations Real-time or off-line correlation ? Understanding detector noise sources Understanding noise sources in the sky Software tracking stars across the sky (LOFAR, MeerKAT ?) Astrophysical targets and their spectral features? Simultaneous observations in different wavelengths? Understanding information content in correlation functions Demonstrate possible ”full” image reconstruction ?

The competition…

Visions towards kilometric-scale

• ptical imagers

Seeing limited HST 8m + AO E-ELT

(diffraction limit a few milliarcsec in the near-IR)

Spatial Resolution

ESO Paranal: Auxiliary telescopes of the Very Large Telescope Interferometer

SHAPE OF ACHERNAR

Image of the rapidly rotating ( Vsin i ≈ 250 km/s ) star Achernar (α Eri, B3 Vpe), from VLTI observations. Axis ratio = 1.56, the most flattened star seen so far. Because of the projection effect this ratio is a minimal value; the star could be even flatter. Individual diameter measurements are shown by points with error bars.

The spinning-top Be star Achernar from VLTI-VINCI A.Domiciano de Souza, P.Kervella, S.Jankov, L.Abe, F.Vakili, E.di Folco, F.Paresce Astron.Astrophys. 407, L47 (2003)

VLBI maps at two epochs of SiO maser emission around TX Cam. The color bar gives flux in Jy/beam

J.Yi, R.S.Booth, J.E.Conway, P.J.Diamond SiO masers in TX Cam. Simultaneous VLBA observations of two 43 GHz masers at four epochs Astron.Astrophys. 432, 531

COOL SUPERGIANT SIMULATION

Bernd Freytag (Uppsala)

Models for nonspherical stellar models with differential rotation. From the surface inward, the surfaces enclose mass fractions of 1.0, 0.995, 0.95, and 0.5. Radiative portions of the interior are white; convective regions are gray.

K.B.MacGregor, S.Jackson, A.Skumanich,T.S.Metcalfe On the Structure and Properties of Differentially Rotating, Main-Sequence Stars in the 1–2 Mo Range, Astrophys.J. 663, 560

DONUT-SHAPED STARS?

The Luciola flotilla of many small collector mirrors operates like one giant diluted mirror. Focal beam-combiners independently exploit the sky image formed at the focal surface.

A.Labeyrie, H.Le Coroller, J.Dejonghe, O.Lardière, C.Aime, K.Dohlen, D.Mourard. R.Lyon, K.G.Carpenter Luciola hypertelescope space observatory, Exp.Astron., in press (2008) & ESA Cosmic Vision 2015-2025 proposal

Luciola* Hypertelescope

* genus of fireflies

Concordia Station

http://www.concordiastation.org/

F.Vakili et al.: KEOPS: Kiloparsec Explorer for Optical Planet Search

Imaging synthesis optical array proposed at Dome C in Antarctica. KEOPS individual telescopes are grouped around the optical recombiner. Concordia station is visible in the distance.

CTA, Cherenkov Telescope Array

Which will come first ?

The science cases for kilometric-scale

• ptical imaging are overwhelming

“Our local Universe is teeming with stars, but despite 400 years

• f telescopic observations, astronomy is still basically

incapable of observing stars as such! Although we can observe the light radiated by them, we do not (with few exceptions) have the capability to observe the stars themselves, i.e., resolving their disks or viewing structures across and outside their surfaces (except for the Sun, of course!). One can just speculate what new worlds will be revealed once stars no longer will be seen as mere point sources but as extended and irregular objects with magnetic or thermal spots, flattened or distorted by rapid rotation, and with mass ejections monitored in different spectral features as they flow towards their binary companions. It is not long ago that the satellites of the outer planets passed from being mere point sources to a plethora of different worlds, and one might speculate what meager state extragalactic astronomy would be in, were galaxies observed as point sources only.”

(Dravins & LeBohec, SPIE Proc. 6986, 2008) In 2009, we celebrate 400 years of telescopic astronomy