1 / 52

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.

keiji
Download Presentation

Advanced CCD Workshop

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related