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Advanced CCD Workshop Arne A. Henden

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1 Advanced CCD Workshop Arne A. Henden

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6 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

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10 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.

11 Accurate Photometry Transformations Scintillation Differential airmass Signal/noise

12 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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21 Scintillation Caused by earths atmosphere Important for small aperture and/or short exposure Reduce effect by working close to zenith

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23 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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25 Signal/noise considerations For precise work, must consider other factors besides Poisson noise Different noise factors important in different regimes

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27 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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33 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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35 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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44 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

45 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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49 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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