Astronomical Seeing

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Astronomical Seeing
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The Project

• Students will be introduced to the concept of
  astronomical seeing and how it affects the
  quality of astronomical images.
• The causes of seeing are discussed.
• Students are presented with a selection of images
  taken under different conditions and are asked to
  chose which images have been taken under the best
  seeing conditions.
• A discussion is then initiated on how best to
  identify the effects of seeing in an image and
  how scientists take seeing into account.

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Setting Up the Experiment 1

• Students are presented the data twice.
• Distribute copies of the image sets (.jpg format)
  for each object.
• Images can then be analysed on computers by
  loading the images in your default image viewer
  or viewed collectively via projection.
• Distribute a copy of the worksheet to each of the
  students.
• Allow the students to quickly rank the quality of
  the astronomical image.
• If the students cannot distinguish between an
  image, note this on the worksheet.

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Setting Up the Experiment 2

• Deliver the lecture on seeing (concept
  introduction folder) , and allow the students to
  have a more detailed look at the images.
• Using what they have learnt from the lecture. See
  if they can now distinguish between some of the
  better images.
• Prompt the students to look for areas where two
  stars are close together the better the seeing
  the more resolvable (distinguishable) the two
  stars will be.
• Look out for stars which are much dimmer in some
  images due diffusion through seeing effects.
• Look for small features e.g. Craters on the Moon,
  the rings on Saturn or dust in galaxy and nebulae
  images. The sharper and better resolved the
  detail on these features, the better the seeing.

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Measuring and recording 1

• Rank the images in order of their quality on the
  work sheet.
• 1 Best and 6 Worst.
• If you are unsure, place a joint ranking with the
  image you think is the closest.

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Astronomical Seeing

• Even the best ground based optical telescopes are
  restricted by the presence of the Earths
  atmosphere.
• Light from distant objects must pass through the
  Earths atmosphere before we can observe it.
• The atmosphere contains a layer of turbulent air.
• As the light passes through this turbulent layer
  the light waves are perturbed, altering how they
  are detected on the ground.
• This effect is called seeing.

Image created by NSO
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Seeing and Meteorology

• Variation in temperature, humidity and wind speed
  make the atmosphere very turbulent.
• Turbulent air contains pockets or cells which
  have differing density to the region of air
  surrounding it.
• It is cells such as these which cause clear-air
  turbulence, which is often experienced when
  flying in aircraft.
• These cells will vary in size and shape and tend
  to drift around in the atmosphere.
• Due to their differing density, each cell will
  have a slightly different refractive index.

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Refractive Index

• Light in a vacuum travels at a constant velocity,
  c. (3x108 ms-1)
• When light travels in a medium, the velocity
  changes by a factor of 1/n , where n is the
  refractive index of the medium.
• The refractive index depends on the
  characteristics of the medium.
• Changing from one medium to another will cause
  the angle of the incident light to change.
• This occurs as the light travels from cell to
  cell.

Image created by NSO
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Seeing and Meteorology (2)

• Large temperature gradients cause turbulence.
  These arise if air masses of different
  temperatures mix.
• This will occur when hot air rises from the
  ground and meets the colder air at higher
  altitudes or when the wind drives in weather
  fronts from surrounding areas.
• This means that seeing is better when
• Observations are recorded at high altitude. i.e.
  The light passes through less of the turbulent
  air.
• Observations are recorded during a period of high
  pressure, when wind speeds are low at all
  altitudes.
• Observations are taken close to the zenith, where
  there is less atmosphere for the light to pass
  through.

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Scintillation

• If the cells of varying refractive index are far
  above the telescope, scintillation occurs.
• Scintillation is observed as irregular changes in
  the brightness of the observed objects.
• This is what makes stars twinkle at night.
• This will make dim objects and stars invisible on
  images taken during bad seeing.

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Oscillation

• Light travelling through areas of differing
  refractive index will also change position in the
  focal plane.
• This causes distortions in the recorded image.
• This effect is called oscillation.
• The distortion rate is very high, typically more
  than a 100 times a second.
• Since exposure times are normally much longer
  than the distortion rate. Distortion is averaged
  over the time of exposure resulting in a blurry
  image.

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Seeing and Stars

• The resolution limit of a telescope , or how well
  a telescope can see objects, is determined by the
  size of its main mirror.
• However, telescope resolution is also limited by
  the diffraction of light.
• The result of this is that distant point source
  objects, such as stars, spread out to a small
  spot known as the Airy disk.
• Astronomical seeing causes this disk pattern to
  be disrupted into a speckle pattern.
• This will cause stars next to each other to merge
  into a single object.
• On larger telescopes the diffraction effects are
  very small due to the large size of the mirror.

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Pickering Scale

• Due to oscillation, a point source such as a star
  will spread out and become speckled.
• The Pickering scale is a method of quantifying
  how good or bad seeing is.
• 1 Perfect seeing 10 Very bad seeing.

1 2 3 4
5
6 7 8 9
10
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Measuring and recording 2

• Rank the images in order of the quality of seeing
  on the work sheet.
• 1 Best Seeing to 6 Worst Seeing.
• Record the reasons for choosing the rank of each
  image.
• Record which part of the image has been used to
  identify bad seeing.
• Compare the best and worst image. What are the
  differences?

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Discussion After the Experiment

• Is bad seeing easier to identify on some objects
  more than others?
• What are the best methods for identifying bad
  seeing on an astronomical image?
• Pick the image with the worst seeing, what kind
  of weather might you have expected on that day?
• Are there any images which have artifacts that
  cannot be attributed to seeing?

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Questions, Exercises and Tasks

• What methods are there for overcoming
  astronomical seeing?
• Are some parts of the world affected more than
  others? Where is the best place to locate a
  telescope?
• Are small telescopes affected more than large
  telescopes?
• Will observations of more distance objects such
  as galaxies be more prone to seeing effects?

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