Astronomy: Exploring Time and Space - Week 4


The Tools of Astronomy

Telescopes

  • Gallileoscope 1 inch diameter over 400 years ago, modern telescopes now have diameters of 8 or 10 meters.
  • Telescopes can also be used in invisible light wavelengths, e.g. ultraviolet, infrared, radio.
  • James Webb Space Telescope will bring space astronomy to the level of large ground telescopes, 6.5 meter mirror.
  • Ground based telescopes cost about a billion dollars. Hubble Space Telescope cost about $6-8 billion.
  • Two figures of merit:
    1) aperture/diameter - collecting area is square of diameter,
    2) angular resolution (smallest observable detail).
  • Angular resolution as important as collecting area.
  • The larger the telescope, the smaller the angle that can be resolved.
  • Shorter wavelengths deliver higher angular resolution, more detail.
  • Ground based telescopes limited by blurring effects by Earth’s atmosphere.
  • Two classic types of telescopes:
    1) refractor like Galileoscope with two convex lenses in a tube. Problem is chromatic aberration where images of different wavelengths are not formed in same location. Also limitations in size: to make it larger, the tube has to become extremely long and mass/weight become a problem as well. No telescope larger than 39 inch has been built for research.
    2) reflecting telescope (Newton): Light bounces off a primary mirror, then secondary mirror through a hole in primary mirror - more compact, shorter design.
  • collecting area = square of diameter.
  • resolving power = inverse of diameter.

Observing Limitations

  • Ground based telescopes subject to limitations imposed by atmosphere and terrestrial environment.
  • 1) light pollution, almost continuous urban glow in East of US.
  • 2) blurring or twinkling of starlight caused by turbulent motions in upper atmosphere, above where most of the weather occurs.
  • 3) atmosphere opaque at most wavelengths. Only visible range and few segments of near-infrared are able to penetrate atmosphere, but not large tracks of ultraviolet, x-ray, gamma-ray, millimeter-rays. To observe them, we have to go into space.

Observing Solutions

  • In 1948, Palomar 200-inch telescope was the largest mirror ever built.
  • Dome was the size a cathedral, 10,000 tons of moving glass and steel.
  • New and bigger telescopes are in smaller buildings with compact designs.
  • Telescopes with large mirrors also have large detectors producing huge data rate - a picture with gigabytes size, one night will produce 20 terabytes of data.
  • Giant Magellan Telescope, 7 large mirrors as part of 22 meter area.

Adaptive Optics

  • Allows to essentially cheat the atmosphere and recover the full defraction limited images.
  • Corrections made to secondary mirror, which is 1m and made of very thin 1mm material, usually beryllium, actively supported on actuators on the back side.
  • These can be adjusted many times a second to change the shape of the secondary mirror.
  • Secondary mirror has to be precise to a 10th of the wavelength of the light that’s being imaged (tens of nanometers).
  • Result surpasses images taken from Hubble Space Telescope.
  • Hubble Space Telescope’s maximum angular resolution is about 70 milliarcseconds, or 70 thousandth of an arcsecond.
  • Ground based image with adaptive optics has resolution of 25 milliarcseconds, three times better than Hubble Space Telescope.

Interferometers

  • Method to attain extremely high angular resolution in astronomy.
  • Light from separate telescopes are combined as if they were one large telescope.
  • This does not give you the collecting area equivalent to that one large telescope, but it does give you the angular resolution suitable for the largest separation between the elements.
  • This can be gain of factor of hundreds or thousands over the angular resolution of a single mirror.
  • Better at radio wavelengths than optical wavelengths. Radio waves are billions of times larger than optical waves. In radio astronomy technique used for decades.
  • Very Large Array near Socorro in New Mexico. 27 dishes, each of which is 15 meters across spread on a Y-shape pattern.
  • Signals combined automatically to recreate the effect of a 26 or 27 kilometer wide radio telescope.
  • A frontier facility that’s going to do this at millimeter wavelengths which has never been done before is the Atacama Large Millimeter Array or ALMA.
  • Limit of the angular resolution by this technique is one-ten thousandth of an arc second, incredibly high angular resolution.
  • What is an angle of 0.0001 arcsecond? All the way from Paris to New York City. 0.0001 arch second is the angle of lincoln’s eye on a penny in New York City as seen from Paris.

Detectors

  • First detector was the eye for the first 200 years.
  • Eye reads out about once every 10th/15th of a second so we can retain the illusion of continuous motion.
  • That limits the amount of light that’s gathered by the eye before it delivers the signals to the brain, and it limits the depth you can see.
  • Beginning in the mid-19th century, photography was used to record astronomical images (e.g. Edwin Hubble).
  • Photography allows data to be gathered for a long period of time. More efficient. But both eye and photography were chemical detectors.
  • Beginning in 50s/60s electronic detectors were developed.
  • Revolution in the mid-late 70s with CCD (charge-coupled devices).
  • CCD’s are small detectors made of solid state materials such as silicon.
  • Usually cooled to liquid nitrogen temperature because this reduces the level of the background noise, which would convert into background light in the detector.
  • Array of small picture elements/pixels, typically 15 or 25 microns each.
  • Charge gets moved along the device, first along the rows and then along the columns.
  • CCD turns incoming photons into electrons and stores them in little potential wells.
  • Potential well can hold a few hundred thousand or a few million electrons before it fills up. So there are limits to how much light CCDs can gather before their potential wells are essentially full.
  • Electrons produce a current and digital signal of intensity.
  • CCDs in cameras work essentially the same way as CCDs in astronomy, but in astronomy they’re of much higher quality.
  • They convert every photon into an electrical signal. Perfectly efficient, no bad pixels. Ability to transfer charge exceptionally good at 99.999% efficiency.
  • CCD for the Large Synoptic Survey Telescope is larger than a dinner plate, 5 gigapixels. Each picture will produce gigabytes of data, 20-30 terabytes in a night - huge challenge for IT and data processing.
  • LSST is a steering telescope that continuously scans the sky and look for things that change.
  • New type of science called celestial cinematography.
  • Willard Boyle and George Smith, the two engineers who developed CCD, got Nobel Prize for their efforts and impact.
  • Data problem is shared by astronomers and high energy physicists at the Large Hadron Collider. Their data rates are similar to the Large Synoptic Survey Telescope’s.
  • Other challenge is to make the data public in realtime since it’s a publicly funded project by tax payer dollars.

Space Astronomy

Hubble Space Telescope

  • Hubble Space Telescope is a NASA facility jointly operated with European Space Agency.
  • 85% of the time on the telescope goes to American or US-based astronomers, 15% to European astronomers.
  • Orbiting telescope, collects light at optical, near-infrared and ultraviolet wavelengths.
  • Launched in April 1990 aboard the space shuttle Discovery.
  • 2.4-meter mirror, weighs about 5 tons, was designed to fit inside the space shuttle bay.
  • In a low-Earth orbit roughly 250 miles straight up.
  • Orbital period is 90 minutes.
  • Passes through a region called the South Atlantic Anomaly where the radiation and magnetic fields in the Earth environment are sufficiently severe that observations are compromised.
  • In a 90-minute orbit, Hubble takes data for about half of that time.
  • Operated out of The Space Telescope Science Institute in Baltimore.
  • Named after Edwin Powell Hubble, the discoverer of the expanding universe and the extra galactic nature of galaxies.
  • When first launched, Hubble was producing blurry images because the mirror was (precisely) wrong.
  • Fixed during first Hubble’s servicing mission Costar, instrument essentially like eyeglasses or corrective lenses to restore perfect vision.
  • Images are produced with a series of exposures, filtered through colored glass (red, green, blue)
  • Separate images for each color are combined afterwards.
  • Colors in Hubble imagery is exact and astronomically correct. In X-ray and radio astronomy you have to assign artificial colors to the data because wavelengths are invisible.
  • Hubble space telescope project chose not to use lurid color tables or false colors.
  • Hubble images are quite ugly in their raw form because detectors suffer many interactions with cosmic rays, high-energy particles.
  • To get rid of the noise multiple, sequential observations with same exposure and filter are taken.
  • 3-5 images are usually sufficient to get rid of 99 or 99.9% of the cosmic rays.
  • Optics distort the geometric grid on the sky. Images are re-mapped to produce perfect 2-dimensional images of angles in the sky.
  • Often the blue channel is associated with an oxygen line, red channel with a hydrogen line

Big Glass

  • Frontier for Space Astronomy will be the James Webb Space Telescope, the successor to Hubble.
  • 6.5 meters in diameter.
  • Extremely challenging design. Mirror will essentially unfold once it reaches it’s location a million miles from the Earth.
  • So far from the Earth, it will be unserviceable by astronauts.
  • It has to work perfectly and correctly, because it can’t be fixed if anything goes wrong.
  • James Webb will not work in the ultraviolet, but only at the reddest of optical wavelengths. Most of its work will be at invisibly long wavelengths just beyond where the eye can see.
  • Work on detecting first light in the Universe, characterizing exoplanets.
  • On the ground, Grand Canary Telescope on the Canary Islands to stare only at one strip of sky that passes overhead.
  • Next very large telescope will be the Giant Magellan Telescope on Alaskan Palace Observatory site.
  • Even larger than the GMT and in a sort of friendly competition with it, is the 30-meter telescope built by the Caltech and University of California Consortium.
  • The TMT will almost certainly built either in Hawaii or also in Chile.
  • The Overwhelmingly-Large Telescope, OWL, and has been renamed the Extremely-Large Telescope, which is a project of the European Southern Observatory.

Invisible Waves

  • From tiny gamma rays smaller than the size of an atomic nucleus to meter-length radio waves.
  • Radio astronomers have the world’s largest telescope in Puerto Rico, the Arecibo dish, which is 300 meters in diameter, just over 1,000 feet.
  • Arecibo was built in the early 1950s, operated by Cornell University for most of the past half century.
  • Not steerable at all because it sits in a natural depression in limestone country in the central part of the Puerto Rican island, Karst Country.
  • For mid and far infrared with wavelengths of ten microns and longer, you need either to go into space or to very high altitudes.
  • These altitudes can be reached by aircraft and by balloons.
  • NASA has a space observatory using an airplane, a jumbo SP, where the back part of the jumbo is carved out and a three-meter telescope is inserted near the tail of the aircraft.
  • It can point stably at the sky while the aircraft is flying at hundreds of miles an hour.
  • The altitude of these flights are quite high, 45 to 48,000 feet above most of the atmosphere that can absorb infrared radiation.
  • Chandra X-ray Observatory: x-ray telescopes work by grazing incidence.
  • X-rays arrive at a very shallow angle and are reflected at an equal shallow angle. Multiple reflections and nested arrays of mirrors are required to gather x-rays to a focal point.

Beyond Vision

  • Two revolutions in the way we view the universe:
  • 1: invention of the telescope
  • 2: access electromagnetic spectrum with technologies to detect radio, infrared, x-rays, gamma rays, ultraviolet radiation.
  • Perhaps the final frontier in this history would be opening up the universe to gravity waves.
  • Einstein’s General Theory of Relativity predicted that mass would bend space and that light would bent by matter (gravitational lensing).
  • Gravitational lensing effect has been observed now thousands of times.
  • Typical situation is when massive cluster of galaxies containing 10^15 stars, sits between us and more distant galaxies.
  • Huge concentration of matter bends light such that the distant galaxies appear magnified, amplified, and distorted by the foreground cluster.

Gravity Waves

  • Essentially everything we know about the universe comes from electromagnetic radiation of different kinds.
  • But the universe contains “stuff”, and there is a possibility to see it directly.
  • In general relativity, Einstein’s Theory of Gravity, any time a mass distribution changes, it releases ripples into space time that travel outward at the speed of light.
  • These signals propagate through the universe, even passing through matter as if it weren’t there.
  • This happens any time a mass distribution changes, e.g. when a star contracts to a new situation, or explodes at the end of its life, or a black hole forms or swallows some material, or in gravity regime of the early universe.
  • LIGO, the Laser Interferometer Gravitational Observatory experiment designed to detect these minute distortions in space-time.
  • LIGO is twin detector. One is situated in Livingston in the Southeast part of the United States and the other is situated in Hanford in the Northwest.
  • Two detectors are needed to confirm something real was detected. Signals are so subtle and so easy to confuse with noise.
  • European collaborations to build a third detector. With three detectors, you have some possibility of determining the direction of origin of the gravitational waves.
  • They are basically two arms, each five kilometers long, where light travels up and down through a vacuum tube.
  • Light is then combined in something called an interferometer, which is sensitive to tiny changes in the path length of one of the two arms.
  • Interferometry is a fairly standard technique in the lab. But no one has yet executed an interferometer on this huge scale of 5 km.
  • The critical part of the detector is a solid metal mass, extremely accurately known both in its dimension and its mass, which has a mirror attached.
  • Used to confirm what happens when two neutron stars combine or a neutron star and a black hole or two black holes.
  • LIGO is looking for the signal when they coalesce and merge into a single black hole.
  • Then there will be an enormous spike in the intensity and level of the gravity waves.
  • LIGO can also look at other interesting signals, such as the death of a star. LIGO is also hoping to detect signals from the very early universe, when the gravity situation was also changing rapidly, just after the Big Bang.
  • Finally, there are things that LIGO doesn’t know how to detect because we haven’t predicted them.
  • Figure of merit of LIGO is an extraordinary number, a dimensionous number of ten to the minus 22. That’s the level at which LIGO is detecting space time distortions.
  • With a test mass about a meter long, that large block of metal, you’re looking for a variation in its length smaller than the width of a proton.
  • Space mission designed to do things that LIGO cannot do. It’s name is LISA, the Laser Interferometer Space Antenna.
  • LISA is a free floating set of three antennas. They combine their information through space, and form interferometry that way.
  • In space, the stability of the environment allows extremely low frequency gravity waves to be detected.

Frontiers

  • First frontier of observational cosmology comes from optical observing.
  • Earliest landmark looking back in time where we have detailed information of the universe is the cosmic microwave background radiation.
  • This is when the universe became transparent, after having been a hot, dense and opaque gas for the previous time.
  • The microwaves we see with radio telescopes come from a time about 300,000 or 400,000 years after the Big Bang.
  • Curvature of space, dark matter, dark energy in the universe, ruptures in spacetime, cosmic strings ar the current frontier of observational cosmology.
  • Beyond this, there is inflation which seems impossible to observe or get information about.
  • Exotic theories that involve super strings and multi dimensions of spacetime.

Further Reading