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Photometric Analysis of Asteroids

Photometric Analysis of Asteroids. Sara Barber Acknowledgements: Dr. Bill and Erin Cooper. Project Evolution. Old Project : Opposition Effect of Dark Asteroids Goal: Make photometric observations of asteroids with low reflectivity near opposition Create lightcurves for these asteroids

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Photometric Analysis of Asteroids

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  1. Photometric Analysis of Asteroids Sara Barber Acknowledgements: Dr. Bill and Erin Cooper

  2. Project Evolution • Old Project: Opposition Effect of Dark Asteroids • Goal: • Make photometric observations of asteroids with low reflectivity near opposition • Create lightcurves for these asteroids • Problem: • CCD malfunction • New Project: Photometric Analysis of Trojan Asteroids • Goal: • Analyze previously obtained images of Trojan asteroids • Create lightcurves for these asteroids

  3. Motivation • My Goal: Create lightcurves for Trojan asteroids • Future Goal: Combine lightcurves throughout asteroid’s orbit to determine 3-D shape • Shape & Spin Rate  Density • The density could put a limit on the asteroid’s composition. • Trojan Composition v.s. Main-Belt Composition • Different origins within the solar system? • Better understanding of solar system’s evolution

  4. Outline • Trojan Asteroids • Lightcurves • Photometry Steps • CCD Photometry • Image Processing • Complications • Measuring • Calibration • Lightcurves • Results Preparing for a night of observing.

  5. Trojan Asteroids Trojan Asteroids Main Belt Asteroids http://epsc.wustl.edu http://cseligman.com • Asteroids in orbit around Jupiter’s 4th and 5th Lagrange points

  6. Lightcurves Lightcurve Lightcurve Lightcurve Asteroid Orbit • Lightcurve: change in brightness throughout rotation • More illuminated surface area  brighter • Less illuminated surface area  dimmer

  7. Photometry • Photometry: technique for measuring an object’s brightness • Steps • Take exposures • Process images • Measure object’s brightness • Calibrate measurements • Create lightcurve

  8. STEP 1: CCD Photometry • Charged Coupled Device (CCD) • Photon hits Si substrate & photoexcites e- • 1 photon = 1 e- • Electrons trapped in “pixels” by electrodes w/ applied voltage • Get series of numbers that are reconstructed to make image CCD Electrodes Conduction Band CCD:Top View Valence Band

  9. STEP 2: Image Processing • Want uniform background • Sources of Background Inhomogeneity: • Thermal Signal Thermal energy is enough to kick electrons into conduction band (CCD not cooled uniformly  have gradient of thermal signal) • Dark Frame • Pixel-to-Pixel Variations  Flaws on CCD chip, dust shadows • Flat Frame

  10. STEP 2: Image Processing RAW DARK FLAT - ÷ FINAL =

  11. Dark Frame Flat Frame

  12. Dark Subtracted Reduced Image Raw Image Flat Divided

  13. Images

  14. Complications Asteroid

  15. STEP 3: Measuring Aperture Annulus • Measure electron count within aperture • Only want electron count from source • Need to subtract count from background (scattered moonlight, city lights, etc.) • Aperture  Source + Background • Annulus  Background Source = Aperture Count - Annulus Count Star Field

  16. STEP 3: Calibration • We have electron counts, want physical magnitudes • Observe flux standard stars (stars of well known magnitude) • Measure e- counts for these stars • Use linearity of CCD (double e- count = double flux) to calibrate source • Source e- count  Source magnitude

  17. STEP 4: Lightcurve • Plot brightness vs. exposure time

  18. STEP 4: Lightcurve • Phase Lightcurve • Use previously published rotation periods to plot brightness vs. phase

  19. Results

  20. Questions?

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