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Advanced CCD Workshop

Advanced CCD Workshop. Arne A. Henden Arne@aavso.org. Time. Accurate time under your control Most computers -> use NTP Necessary for simultaneous observations (satellite, flare) Exposure time has error (open/close/syst) Exposure length is “error”. Flat Fields.

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Advanced CCD Workshop

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  1. Advanced CCD Workshop Arne A. Henden Arne@aavso.org

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

  3. 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 10000*sqrt(100/10) (or 30000) electrons minimum. • For mmag photometry, need even more, plus lamp color match.

  4. Accurate Photometry • Transformations • Scintillation • Differential airmass • Signal/noise

  5. 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

  6. Scintillation • Caused by earth’s atmosphere • Important for small aperture and/or short exposure • Reduce effect by working close to zenith

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

  8. Signal/noise considerations • For precise work, must consider other factors besides Poisson noise • Different noise factors important in different regimes

  9. 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

  10. 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)

  11. 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

  12. 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

  13. 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

  14. Gamma-Ray Burst Afterglows • Early time observations require cookbook procedure (you can’t 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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