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

2. Varun Bhalerao 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 Slide 1 of 15

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

4. Varun Bhalerao 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, m v =12, angular size = 14 μas –Projected diameter at 40 AU = 0.4 km Star may not be fully occulted Slide 3 of 15

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

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

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

8. Varun Bhalerao Instrument Design

9. Varun Bhalerao 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 Slide 7 of 15

10. Varun Bhalerao Detector APD – Avalanche Photodiode –CCD: readout noise, cost –PMT: quantum efficiency, ease of handling Noise characteristics: –NEP: 2 fW/Hz ½ Detection limit: –0.2 pW R.M.S. –Limiting magnitude R≈8 (1 meter telescope) Slide 8 of 15

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

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

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

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

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

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

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

18. Varun Bhalerao Extra Slides

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

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

21. Varun Bhalerao Monochrome point source

22. Varun Bhalerao Monochrome extended source

23. Varun Bhalerao Geometric regime

24. Varun Bhalerao Non-central occultations

25. Varun Bhalerao Size-distance degeneracy

26. Varun Bhalerao ORT: expected pattern

27. Varun Bhalerao ORT: Observations

28. Varun Bhalerao Fourier transform

29. Varun Bhalerao ORT: Reduced lightcurve

30. Varun Bhalerao Bessel and Lommel functions

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

32. Varun Bhalerao Projected star sizes

33. Varun Bhalerao