Astronomy%20with

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
(No Transcript)
12
Refracting
Reflecting
13
Parabolic mirrors
14
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
15
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
16
Spectroscopy Diffraction grating d sin(?)
m ? Best to use reflection grating
17
A stellar spectrum

No prizes for guessing which star...
Continuum with absorption lines temperature and
composition
Continuum is a 5700K black body
18
A typical galaxy spectrum
Absorption and emission lines Positions known
from atomic physics
http//www.sdss.org/
19
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/(1z)

Ned Wright's cosmology tutorial
http//www.astro.ucla.edu/wright/
20
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!
21
http//hubblesite.org
22
(No Transcript)
23
Deflection of light by massive bodies
http//www.theory.caltech.edu/people/patricia/lcl
ens.html http//www.mathpages.com/rr/s6-03/6-03.h
tm
24
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!
25
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
26
Demonstrating gravitational lensing
http//vela.astro.ulg.ac.be/themes/extragal/gravle
ns/bibdat/engl/DE/didac.html
27
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 (z0.35) 1 g cm-2 1011
Mo / (0.3 kpc)2 Galaxies make good
gravitational lenses!
28
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.ed
u.au/jlovell/simlens/
29
RXJ09110551
2 lens galaxies, 1 source quasar Lens galaxies
are different colour 4 images of quasar
Many more lens images at http//cfa-www.harvard.e
du/castles/
30
RXJ09110551
Refractive index is independent of
wavelength This is an X-ray image! No
visible lens galaxy we are not seeing stars...
31
X-ray Astronomy
Ionising radiation, absorbed by most things
including the atmosphere All X-ray telescopes
are satellites
32
X-ray Telescopes
Particle behaviour makes focusing tricky
absorption not reflection Refractive index is
lt1 for most materials esp. metals Total
external reflection occurs at grazing
incidence X-ray telescopes are
long! http//www.chandra.harvard.edu http//xmm.v
ilspa.esa.es/
33
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...
34
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!
35
A wide field cosmic telescope Abell 2218
36
(No Transcript)
37
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... z7 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
38
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!