Advanced CCD Workshop

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Advanced CCD Workshop

• Arne A. Henden
• Arne_at_aavso.org

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Time

• Accurate time under your control
• Most computers -gt use NTP
• Necessary for simultaneous observations
  (satellite, flare)
• Exposure time has error (open/close/syst)
• Exposure length is error

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Flat Fields

• Adds noise to every science frame
• Goal 10x lower noise contribution from flats
  compared to object
• Typical fwhm area 10pix, so for 0.01mag error,
  want flat pix to have 10000sqrt(100/10) (or
  30000) electrons minimum.
• For mmag photometry, need even more, plus lamp
  color match.

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Accurate Photometry

• Transformations
• Scintillation
• Differential airmass
• Signal/noise

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Transformations

• Necessary to place everyone on same system
  (0.01mag level)
• Due to differences between standard system and
  yours (filters)
• Use set of standard stars and transformation
  equations, least squares
• Only good for non-pathological stars

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Scintillation

• Caused by earths atmosphere
• Important for small aperture and/or short
  exposure
• Reduce effect by working close to zenith

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Differential airmass

• For precise work, need to account for airmass
  difference from top to bottom of frame
• Avoid by never working at high airmass

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Signal/noise considerations

• For precise work, must consider other factors
  besides Poisson noise
• Different noise factors important in different
  regimes

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Aperture selection

• Example WZ Sge good/bad seeing, crowded field
• Curve of growth analysis for maximum signal/noise
• Bright stars - use big aperture, spread light to
  get maximum dynamic range

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Differential photometry

• (V-C), (K-C) common
• Accounts for majority of sky variations
• Ensemble techniques for higher precision
• Uses mean comparison sum(Cmag)/N
• Reduces error by sqrt(N) (9 comps, 3x less noise
  contribution from comp star)

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Stacking images

• Useful to remove cosmic rays, cosmetic errors
• No penalty if sky background limited
• Median worse than straight average/mean by about
  20percent
• Other rejection algorithms

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Faint stars, bright background

• Use smallest possible aperture (psf fitting best)
• Stacking method (average, rejection) makes a
  difference
• Compare all methods against average on clean stars

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Exoplanets

• High precision (millimag level)
• Usually bright stars. Scintillation and finding
  comp stars important
• Use ensemble methods where possible
• High time resolution not important, but
  transformation important if combining datasets

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Gamma-Ray Burst Afterglows

• Early time observations require cookbook
  procedure (you cant be thinking about exposure
  times)
• Rapid fade, so need to get on it fast
• Filters highly important (Rc,Ic,Z)
• Watch stacking techniques to avoid rejecting
  high/low points

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