SALT Seeing the Spectrum

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SALT Seeing the Spectrum
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A rainbow over Sutherland, South Africa home of
the Southern African Large Telescope (SALT).
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Rainbows reveal the nature of light a sliver of
the vast electromagnetic spectrum that stretches
from the energetic high frequencies of gamma rays
to the low energy waves of radio.
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• The atmosphere is opaque to most frequencies of
  the EM spectrum. However it is transparent to
  wavelengths of the visible part of the spectrum.

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• The eyes of animals are adapted to be sensitive
  to this part of the spectrum illuminates the
  surface of the Earth.

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UV
The atmosphere absorbs most of the UV part of the
spectrum. But the huge eye of SALT has been
designed to capture UV and reveal information
that other telescopes are blind to.
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Your skin is sensitive to ultraviolet.
Ultraviolet wavelengths are responsible for
creating the multitude of skin colours in the
human race.
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UV has two main effects on the skin it forms
vitamin D and causes cancer. These two selection
pressures push the gene pool of a population in
opposite directions.
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In latitudes towards the poles a lack of UV has
selected for paler skins that allow the waves to
penetrate and create the vitamin. Closer to the
equator, too much UV has selected for darker
skins that absorb UV and protect the skin from
damage.
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• SALT can image objects in the UV down to 320nm.
• That means all of the band that is responsible
  for skin darkening- UVA (380315 nm) and a bit
  of the sunburn band - UVB (315280 nm)

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• This image of the Lagoon Nebula was created from
  three exposures taken by SALT.
• 120 seconds in the UV, 20 seconds in visible
  wavelengths and 40 seconds in infrared.

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Focusing plane
SAC
Main mirror
This UV capability is created by SALTs 5
reflective surfaces. These are coated so that
they reflect UV rather than absorb it. The
spherical aberration corrector (SAC) is needed
because the primary mirror is spherical and
cannot focus a sharp image without correction.
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This graph shows how good SALTs surfaces are at
reflecting different wavelengths. The
aluminium coating of the primary mirror (black)
and the silver and aluminium coatings in the SAC
combine to create a total reflectance (red).
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So we now have electromagnetic radiation with a
broad spectrum, collected from a large area and
sharply focussed. We can put detecting
instruments into the light path to analyse the
radiation and gain information about the objects
that emitted it.
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Instrument Payload
These are arranged on the tracker, above the SAC.
SAC
Primary mirror
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Here is the instrument payload in place. It has
a mass of 100kg. Lets look inside the casing.
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SALT Prime Focus Payload
There are currently three scientific instruments
but there is room for more.

• Prime Focus Payload (1000 kg)
• Science instruments
• Prime Focus Imaging Spectrograph (PFIS)
• Fibre Instrument Feed (FIF)
• SALTICAM (optical imager)
• Facility instruments
• Acquisition camera (SALTICAM)
• Guidance focus system
• PFIS slit-viewing optics
• Fold mirrors (to 3 focii)
• Moving pupil baffle
• Atmospheric Dispersion Compensator (ADC)
• Payload alignment system (collimator and
• M-Z intereferometer
• SAC structure

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SALTICAM is what it says SALTs camera. This
galaxy, 30 million light years away in the
constellation Pavo, was captured by just three 10
second exposures each in UV, blue and infrared
wavelengths.
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SALTICAM can image an area of the sky about 1/7th
the width of your little finger at arms
length! It can record detail about 20,000 times
smaller than the width of its view.
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SALTICAM has a jukebox full of filters that it
can put into the light path and filter the
wavelengths given out by different elements.
It can also measure the brightness of very
distant objects up to 20 times a second.
Tracking the changes of objects over such a small
time scale is a speciality of SALT.
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Here is a spectrum revealed in the reflection of
sunlight from a CD. It is created when the light
waves bounce off the closely packed rows of pits
that encode the music. Waves of the same length
interfere. At each angle you view at, a
different frequency of waves boost each other
making a colour appear.
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Spectrographs are instruments that separate
incoming light according to wavelength and record
the resulting spectrum in a detector. Here is a
high resolution image of the Suns spectrum. Each
dark band is created by a specific element in the
Suns atmosphere.
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Hydrogen
Helium
Potassium
SALT can monitor the spectrum of 100 objects at a
time using a special laser cut mask. It can be
used to study the largest scales in the Universe
and work out the detailed movements of
galaxies. Look at these spectra. Try to
identify the element on the next slide
Sodium
Oxygen
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Click for the answer
Sodium
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Here is an image of a candle through a homemade
spectroscope made with a piece of CD. The
bright yellow line shows that there is sodium in
the candle.
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What can SALTs spectrographs do?
They can use etalon filters to take images
using a slice of the spectrum. Because of the
light gathering capability of SALT, they can work
at high speed (10 observations a second). They
can use optical fibre feeds, sampling the
spectrum from many objects in the field of view
at the same time.
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Here is the light path to the Prime Focus Imaging
Spectrograph
Grating Jukebox
Collimator Optics
Camera
Detector
Etalons
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PFIS works with either etalons or gratings
inserted here
And here it is in detail
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There is a similar Fabry-Perot etalon filter in
the North and South solar telescopes. This
filter is tuned to the emissions of hydrogen
atoms at 656.3nm, revealing details of the outer
layer of the Sun.Schools in South Africa and
Norfolk can borrow the telescopes see the
website for details.
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Finally, what are some of the things SALT be
using its spectroscopes for
Optical fibre feeds - searching for planets
around other stars. Etalon filters - studying
the mass distributions in galaxies and the
interactions between stars. High speed
spectrometry studying black holes and neutron
stars.