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
Analyze previously obtained images of Trojan asteroids
Explain what projects are -- OPPOSITIION TROJAN
TROJAN
LIGHTCURVE

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 Results
CCD Photometry
Image Processing
Complications
Measuring
Calibration
Results
Preparing for a night of observing.

5. Trojan Asteroids
Asteroids in orbit around Jupiter’s 4th and 5th Lagrange points
Trojan Asteroids
Main Belt Asteroids
(green)
(lagrange)

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

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 - ÷ =
Explain CCD here

11. Dark Frame Flat Frame CCD COOLed to decrease noise
DARK -- shutter closed FLAT--uniformly illuminated

12. Dark Subtracted
Reduced Image
Raw Image
Flat Divided

13. Images

14. Complications
Asteroid

15. Source = Aperture Count - Annulus Count
STEP 3: Measuring
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
Aperture
Annulus

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?