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Astronomy with cm – Mpc lenses Phil Marshall KIPAC – SLAC – Stanford University

Astronomy with cm – Mpc lenses Phil Marshall KIPAC – SLAC – Stanford University February 28 th 2004. The Human Eye has an aperture of 7mm or so when dark-adapted provides an image updated every eighth of a second

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Astronomy with cm – Mpc lenses Phil Marshall KIPAC – SLAC – Stanford University

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  1. Astronomy with cm – Mpc lenses Phil Marshall KIPAC – SLAC – Stanford University February 28th 2004

  2. The Human Eye • has an aperture of 7mm or so when dark-adapted • provides an image updated every eighth of a second • has a logarithmic response to brightness, which has led astronomers to measure observed flux in magnitudes: m = -2.5 log10(flux) + constant • gives an angular resolution of about 1arcmin Faintest star visible by eye from a dark site has magnitude 6 In Palo Alto one can sometimes see the Big Dipper – mag 2

  3. Collecting photons Use CCDs (charge coupled devices) to detect photons Amount of charge built up in pixel ≈ no. of photons Images manipulated as arrays of numbers

  4. Astronomy with a digital camera Exposure time = 16 secs Aperture diameter = 30mm ⇒ see to magnitude 10.4?

  5. Wide field! 48x36 degrees...

  6. Zoom in (after the exposure!):

  7. The pleiades star cluster: Resolution limited by camera optics, ~3arcmin Human eye does 3 times better!

  8. Comparison with Palomar digitized sky survey (1949) http://www.astro.caltech.edu/observatories/palomar/

  9. Comparison with Palomar digitized sky survey http://archive.stsci.edu/dss/ Magnitude limits: Naked eye in Palo Alto: ~2 Camera image: ~5 (predicted 10.4) DSS: ~21

  10. Telescopes: Faintest star visible by eye from a dark site has magnitude 6 Ron got comparable results in Palo Alto by storing photons An 8.4m lens would collect (8.4m/7mm)2 times more light than a dark-adapted eye ⇒ 15 magnitudes fainter (bit less for inefficency) Integrate for an hour: ⇒ another 10 magnitudes (bit less for inefficency) Resolution is (8.4m/7mm) times higher: 0.05 arcsec? ( = 1.22/D when “diffraction-limited”)

  11. Refracting Reflecting

  12. Parabolic mirrors

  13. Making an 8.4m parabolic mirror: Melt glass – rotate furnace – cool carefully – polish. Do not drop. cf. Palomar 200inch http://medusa.as.arizona.edu/mlab/mlab.html http://wood.phy.ulaval.ca/english/intro/what.htm

  14. Example images – nearby galaxies cf. Digicam http://www.astro.princeton.edu/~frei/catalog.htm Filters used to make separate red and blue images Then combine to make colour picture + = Spiral Elliptical

  15. Spectroscopy Diffraction grating: d sin() = m  Best to use reflection grating:

  16. A stellar spectrum: No prizes for guessing which star... Continuum with absorption lines – temperature and composition Continuum is a 5700K black body

  17. A typical galaxy spectrum: Absorption and emission lines Positions known from atomic physics http://www.sdss.org/

  18. Redshift: Galaxies appear to be receding from us: spectral lines are redshifted Doppler shift is not quite right – the wavelengths are stretched by the expansion of the Universe Redshift z Universe scale size R = 1/(1+z) Ned Wright's cosmology tutorial http://www.astro.ucla.edu/~wright/

  19. Limits to image quality Night sky is bright (even on Mountain tops!) Scattered light from moon, cities Airglow (chemiluminescence) Faint objects are lost in noise Atmosphere is turbulent Twinkling of stars = blurring of images (“seeing”) Resolution ≤ 1 arcsec at good site Solution – get above atmosphere!

  20. http://hubblesite.org

  21. Hyperbolic orbit r(t) Deflection angle: Does this happen? Deflection of light by massive bodies http://www.theory.caltech.edu/people/patricia/lclens.html http://www.mathpages.com/rr/s6-03/6-03.htm

  22. Deflection of light by massive bodies GR – light is deflected by, and travels slower in, a gravitational field (latter accounts for missing 2) Refractive index is given by Index is greater than 1, and gravity is an attractive force: massive bodies focus light, acting as “gravitational lenses” Effect is greatest for rays passing close to point mass, or through regions of high density Index varies over field of view: a highly aberrated system!

  23. Lens geometry On axis source S produces ring image when c Off axis: partial ring, or “arcs” Magnification: image sizes increase roughly as 1/(1-c)2

  24. Demonstrating gravitational lensing http://vela.astro.ulg.ac.be/themes/extragal/gravlens/bibdat/engl/DE/didac.html

  25. Numbers c = 1 g cm-2 (Dd / 700 Mpc)-1 (1 Mpc = 3 x 1022 m) c = 2x1025 g cm-2 (Dd / 0.5m)-1 (nuclear ~ 1015 g cm-3) 700 Mpc is a cosmological distance (z=0.35) 1 g cm-2 = 1011 Mo / (0.3 kpc)2 Galaxies make good gravitational lenses!

  26. Gravitational lensing by galaxies Galaxy lens lying in front of small light source Yellow ring marks “critical curve”, cross is optical axis Lens demo by Jim Lovell http://www-ra.phys.utas.edu.au/~jlovell/simlens/

  27. RXJ0911+0551 2 lens galaxies, 1 source quasar Lens galaxies are different colour 4 images of quasar Many more lens images at http://cfa-www.harvard.edu/castles/

  28. RXJ0911+0551 Refractive index is independent of wavelength This is an X-ray image! No visible lens galaxy – we are not seeing stars...

  29. X-ray Astronomy Ionising radiation, absorbed by most things – including the atmosphere All X-ray telescopes are satellites

  30. X-ray Telescopes Particle behaviour makes focusing tricky: absorption not reflection Refractive index is <1 for most materials esp. metals Total external reflection occurs at grazing incidence X-ray telescopes are long! http://www.chandra.harvard.edu http://xmm.vilspa.esa.es/

  31. X-ray Detectors Band gap in silicon is a few eV One optical photon excites one electron in the CCD pixel No energy information X-ray photons deposit all their energy: charge proportional to energy. Dependent on frequent readout X-ray images are colour! Reflection grating spectrometers can be used too: problem is always getting enough photons...

  32. Cosmic telescope design Wide field to catch chance alignments – try a few hundred times bigger angular size: expect strong lensing in dense central regions Stay at cosmological distance: c = 1 g cm-2 = 1015 Mo / (30 kpc)2 Clusters of galaxies contain typically: 100 galaxies at 1011 Mo each 3 x 1014 Mo hot (transparent) plasma 7 x 1014 Mo cold (transparent) dark matter Clusters make good gravitational lenses!

  33. A wide field cosmic telescope: Abell 2218

  34. Abell 2218: Many muliply-imaged galaxies are visible Mass distribution of lens can be precisely modelled Lensing geometry is an important constraint on galaxy redshift, as well as (faint) spectrum Galaxy appears to have magnitude 28 – but has been magnified 25x by the lens... z=7 would make it the most distant galaxy known to date (last week). Universe was 1/8 its current scale and a very different place... http://xxx.arxiv.org/abs/astro-ph/0402319

  35. 21st Century Astronomy Has grown out of our frustration at being stuck on Earth combined with the usual thirst for more information Uses large telescopes with sensitive detectors at dark sites or in space Involves collecting EM radiation over the whole spectrum, measuring its intensity, colour and polarisation; particles arrive from the sky as well Makes extensive use of basic physics, and some cunning and guile!

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