Theme 6 – Space Telescopes and
Adaptive Optics
ASTR 101
Prof. Dave Hanes
Making Every Photon Count
- and Every Dollar!
1.
2.
3.
4.
5.
6.
Build bigger telescopes (to collect more light).
Find the best observing sites on Earth
Use ultra-efficient detectors. (Waste no light!)
Study big fields of view and/or many objects at once.
Launch telescopes into orbit to get the best images
Use ‘adaptive optics’ to improve ground-based
images (and perhaps even out-perform the Hubble
Space Telescope)
Photographic Plates
Analog (film)
vs Digital
(CCDs)
One Problem with CCDs: Image Scale
- we need huge detectors!
Mosaic Detectors
Plus
Photographic Plates
Digital Detectors
Can be made very large to take
advantage of the image scale of the
telescopes
Some are 100-1000x as efficient as
photographic emulsions
Yields a stable, long-lasting
(permanent?) record
Give instant digital values of intensity
(brightness)
Easily controlled remotely
Data can be transmitted and processed
electronically
Minus
Very inefficient: at most 1% of the
incoming light is recorded, so very
long exposures are needed
Detectors like CCDs can’t be made as big
as photographic plates (but mosaics are
possible)
Plates require additional treatment
(development, etc) and hands-on
intervention
The digital data has to be archived in
a way that we guarantee can be read
in decades to come
The darkness of an image is not
easily converted to a measure of
intensity
Data are not easily shared
CCDs Permit Remote Observing
- including telescopes in space
Next: The Multiplex Advantage
How can we study many objects at once?
Of course, a direct image (i.e. a picture) already
gives you many targets at once, but suppose you
wanted to get a spectrum for each object?
(Remember that spectra give you important
astrophysical information like velocities, chemical
compositions, etc.)
An Exemplary Science Case
[drawn from my own research]
Meet a fossil:
a globular cluster
They contain the
oldest known stars,
dating back to the
origin of the galaxy
In Large Numbers
~150 in our own Milky Way galaxy
~1000 in ‘The Sombrero’
~10,000 in ‘M87’
These galaxies are millions
of light years away!
M87 in Close-up: Many Targets
How do we study them all efficiently?
The goal is to learn
about the mass of this
galaxy (does it contain
‘dark matter’?) and its
formation history.
But for these very faint
targets, it takes perhaps
2 hours of telescope
time to get a good
spectrum!
Multiplex! – Using Fibre Optics
Alternatively…
Hubble Space Telescope:
See https://www.spacetelescope.org/images/
A Rocky Beginning
- the primary mirror was not the right shape!
The Eagle Nebula
Hubble Deep Field
[and later the Ultra Deep Field]
https://www.youtube.com/watch?v=le3ASDvZy_s
Q: Why not study the whole sky this way?
A: It would take about 13 million images!
Coming: the James Webb Space Telescope
The mirror will unfold in several sections
(it’s too big to launch as a unit)
Optimized to study infrared light
Will be very far from Earth – no service missions!
Adaptive Optics
Remember two excellent reasons for putting telescopes
into space:
1.
1.
To work at inaccessible wavelengths: Chandra for
X-rays; Spitzer for Infrared; Swift for gamma-rays;
etc
To get outside the turbulent atmosphere of the
Earth: it blurs the images
But problem number 2 can now be largely overcome!
Why Do Stars Twinkle?
Turbulence in the Earth’s atmosphere, mostly
caused by warm air rising
See the blurry moon at:
http://www.youtube.com/watch?v=DE98X1Bv8h4
With and Without the Atmosphere
Solution: Use a Small Flexible Mirror
‘Flatten the Pringles!’
Note the ‘beamsplitter’:
Some light goes to the
camera, for your final image
(your science!).
Some goes to a sensor, to
measure the blurriness and
control the corrections.
We adjust the adaptive
mirror 100 times a second!
A Bright Reference Star is Needed
- but not every interesting field has one
Solution: Create a Star!
Create an ‘artificial star’ by shooting a laser into the sky!





Fire a yellow laser straight up from the telescope
It encounters the ‘sodium layer’ about 90km overhead
The laser stimulates the sodium ions to glow brightly
Seen from the telescope, that looks like an additional bright
star in the field of view
The distortions and blurriness of that ‘star’ provide the
information you need to correct for the turbulence of the
atmosphere
See a wonderful animation of the whole process at Gemini:
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Gem-Adapt.mp4
The Paradox:
a Light Show!
Great Improvements!
Ground-based telescopes have much bigger mirrors than the HST, so
they can now out-perform it in visible light: they provide better
resolution of details. They can also collect much more light.
But they still can’t work at all wavelengths: there will always be a need
for space telescopes.
One Amazing Discovery
Here, the VLT (Very Large Telescope) in Chile is studying
the centre of the Milky Way, using a laser to permit
adaptive optics.
Details in the Galactic Centre
Here’s a static picture in
1992.
Notice all the stars near the
Galactic Centre, which is
about 25,000 light years
away. Without adaptive
optics, all these images
would be blurred together.
Over a Decade…
Watch this animation, created from the original observations. It
shows the motion of the stars (twice), then zooms in for yet
another more detailed view.
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/MW-SMBH.mp4
The stars are clearly moving in orbits around something invisible
(i.e. giving off no light). We can use our understanding of gravity
to deduce that there is a very massive Black Hole (a few million
times as massive as the Sun) in the centre of the Milky Way. We
would not have discovered this without adaptive optics.
By the way, some galaxies contain billion solar mass black holes
(found in other ways).
At the Center of the Milky Way
!