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Dual Channel Photometer for KBO search by occultations

Dual Channel Photometer for KBO search by occultations. Varun Bhalerao Advisor: Shri Kulkarni 24 Sep 2007. Motivation. Kuiper Belt: 30 – 50 AU Remnants of protostellar disc Size distribution gives information about formation history Low mass end not well studied

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Dual Channel Photometer for KBO search by occultations

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  1. Dual Channel Photometerfor KBO search by occultations Varun Bhalerao Advisor: Shri Kulkarni 24 Sep 2007 Varun Bhalerao

  2. Motivation • Kuiper Belt: 30 – 50 AU • Remnants of protostellar disc • Size distribution gives information about formation history • Low mass end not well studied • Aim: To build an instrument to detect small Kuiper Belt Objects Varun Bhalerao Slide 1 of 15

  3. r -3 rb ~ 1 km rb ~ 10 km rb ~ 50 km dN(r)/dr r -4 r / [km] Kuiper Belt • Broken power law distribution • Exact break radius rb unknown: 1~50 km • Probe low mass end to place constraints on population Bernstein et al. 2004, Cooray & Farmer 2003, Trujillo et al. 2001, Pan & Sari 2005 Varun Bhalerao Slide 2 of 15

  4. Detecting small KBOs • Too faint for direct imaging: R≈30 • Detect by occultations • KBO size: ~1 km • Angular size at 40 AU = 35 μas • Stellar angular diameter: • A0V star, mv=12, angular size = 14 μas • Projected diameter at 40 AU = 0.4 km • Star may not be fully occulted Varun Bhalerao Slide 3 of 15

  5. Occultation Characteristics • Diffraction ! (Fresnel scale= 1.5 km) Varun Bhalerao Slide 4 of 15

  6. Other effects • Wavelength band of observation • Background star: size, spectral type • Omnipresent noise ! • Rate: • Opposition / quadrature • 1 event per night ~ 1 event per month Varun Bhalerao Slide 5 of 15

  7. Current Surveys • Current surveys: CCD based • TAOS • Roques & Moncuquet • Other surveys: • Georgevits: Fiber fed spectrograph • Chang et al.: RXTE data • Limitations: Sampling rate, Sensitivity Varun Bhalerao Slide 6 of 15

  8. Instrument Design Varun Bhalerao

  9. Reliable detection • Good signal to noise ratio (aim: 5) • High speed sampling: • Ensure multiple data points on lightcurve • Sample at 100 Hz • Rejection of false positives • Monitor two nearby stars simultaneously • Relative Photometry • Detect > 7% variation at R=6 • Local effects show up in both lightcurves Varun Bhalerao Slide 7 of 15

  10. Detector • APD – Avalanche Photodiode • CCD: readout noise, cost • PMT: quantum efficiency, ease of handling • Noise characteristics: • NEP: 2 fW/Hz½ • Detection limit: • 0.2 pW R.M.S. • Limiting magnitude R≈8 (1 meter telescope) Varun Bhalerao Slide 8 of 15

  11. APD spectral response • Peak 800 nm, FWHM 340 nm • Peak quantum efficiency: 0.4 Varun Bhalerao Slide 9 of 15

  12. Telescope • Table Mountain Observatory • Pomona college • NASA • Telescope: • 1 meter aperture • F/9.5 • Useful FOV 11’ • Plate scale 21”/mm Varun Bhalerao Slide 10 of 15

  13. Optical Design • Direct focus not possible • Pickoff mirrors to accommodate 2 modules • Field lens to correct for tracking errors Varun Bhalerao Slide 11 of 15

  14. Mechanical Design Blue indicates light path Light tight box Adjusting screw Diagonal mirror Moving stage APD / PMT Micrometer Stage motion Varun Bhalerao Slide 12 of 15

  15. Testing • Asteroid occultations • None at site in next year • Optical pulsar: Crab • mv≈16 • Lunar occultations • Earthshine, star magnitude… • Other rapid transients Varun Bhalerao Slide 13 of 15

  16. Current Status • One APD module delivered, one awaited • Optical design • Code-V simulations • Observing time: • Second half of October • Second half of November Varun Bhalerao Slide 14 of 15

  17. Questions Motivation Occultation Lightcurves Detectors Optical Design Mechanical Design Testing Varun Bhalerao Slide 15 of 15

  18. Extra Slides Varun Bhalerao

  19. Making a broadband lightcurve KBO diameter: 0.6 km Star diameter: 1.0 km Varun Bhalerao

  20. Broadband lightcurves (KBO radii) Background source diameter: 1.0 km Varun Bhalerao

  21. Monochrome point source Varun Bhalerao

  22. Monochrome extended source Varun Bhalerao

  23. Geometric regime Varun Bhalerao

  24. Non-central occultations Varun Bhalerao

  25. Size-distance degeneracy Varun Bhalerao

  26. ORT: expected pattern Varun Bhalerao

  27. ORT: Observations Varun Bhalerao

  28. Fourier transform Varun Bhalerao

  29. ORT: Reduced lightcurve Varun Bhalerao

  30. Bessel and Lommel functions Varun Bhalerao

  31. Lightcurve Distance of KBO center from line of sight Fresnel scale atdistance a from observer KBO Radius Varun Bhalerao

  32. Projected star sizes Varun Bhalerao

  33. Varun Bhalerao

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