Optical Astronomy Imaging Chain: Telescopes

1
Optical Astronomy Imaging Chain Telescopes CCDs
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Reflector telescopes basic principles

• reflection angle in angle out
• as a result, spherical mirrors would suffer from
  spherical aberration
• the virtues of parabolas
• parallel incident rays are brought to common
  focus
• gt primary mirrors are ground to paraboloid shape

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Optical Reflecting Telescopes

• Basic optical designs
• Prime focus light is brought to focus by primary
  mirror, without further deflection
• Newtonian use flat, diagonal secondary mirror to
  deflect light out side of tube
• Cassegrain use convex secondary mirror to
  reflect light back through hole in primary
• Nasmyth focus use tertiary mirror to redirect
  light to external instruments

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Telescope f-ratio

• f F/D where F is focal length and D is diameter
• must consider focal length of primary secondary
  mirrors combined
• Determines plate scale
• plate scale is measured in e.g. arcsec per mm at
  the focal plane
• can be estimated from our friend, the small-angle
  relation thetaS/F
• plate scale theta/S 1/fD
• for an f/16 10 telescope, plate scale 50
  arcsec per mm

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CCDs pixel scale and field of view

• Example CCD pixel scale
• take a plate scale of 50 arcsec per mm
• CCD pixels are about 25 microns
• gt pixel scale would be 1.25 arcsec per pixel
• Example CCD field of view
• For a 1000x1000 CCD with 1.25 arcsec pixels,
  field of view is 1250 or about 21 arcmin (could
  image most of Moons surface)

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CCDs pixel scale and field of view

• Want to match CCD pixel scale to image smear
  point spread function
• remember main sources of image smear
• telescope angular resolution
• atmosphere
• ideally, arrange pixel scale such that 2 CCD
  pixels cover width of PSF
• image field of view then limited by format
  (number of pixels) of CCD
• the bigger the better, but bigger means more
  expensive

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CCDs noise sources

• dark current
• can be removed by subtracting image obtained
  without exposing CCD
• leave CCD covered dark frame
• read noise
• detector electronics subject to uncertainty in
  reading out the number of electrons in each pixel
• photon counting
• Poisson statistics if I detect N photons, the
  uncertainty in my photon count is root(N)

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CCDs artifacts and defects

• bad pixels
• dead, hot, flickering
• methods for correcting
• replace bad pixel with average value of the
  pixels neighbors
• dithering telescope take a series of images,
  move telescope slightly to ensure image falls on
  good pixels
• pixel-to-pixel differences in QE
• can construct and divide images by the flat field
• flat field is what CCD would detect if uniformly
  illuminated
• saturation
• each pixel can only hold so much charge (limited
  well depth)
• at saturation, pixel stops detecting new photons
  (like overexposure)
• charge loss occurs during pixel charge transfer
  readout

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Spectral Response (sensitivity) of a typical CCD
UV
Visible Light
IR
Relative Response
300
400
500
600
700
900
800
1000
Incident Wavelength nm

• Response is large in visible region, falls off
  for ultraviolet (UV) and infrared (IR)

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Filters

• Because CCDs have broad spectral response, need
  to use filters to determine e.g. star colors in
  visible
• broad-band filter width is about 10 of filters
  central wavelength
• example V filter at 550 nm will allow light from
  500 to 600 nm to pass through
• astronomers use BVRI blue, visible, red, IR
• narrow-band filter width is lt1
• example H-alpha covers 650 to 660 nm