Varun Bhalerao Dual Channel Photometer for KBO search by occultations Varun Bhalerao Advisor: Shri Kulkarni 24 Sep 2007
Dec 20, 2015
Varun Bhalerao
Dual Channel Photometerfor
KBO search by occultations
Varun Bhalerao
Advisor: Shri Kulkarni
24 Sep 2007
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
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Varun Bhalerao
Kuiper Belt• Broken power law
distribution
• Exact break radius rb unknown: 1~50 km
• Probe low mass end to place constraints on population
rb ~ 1 km
rb ~ 50 km
rb ~ 10 km
r -3
r -4
r / [km]
rq
dr
dN
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Bernstein et al. 2004, Cooray & Farmer 2003, Trujillo et al. 2001, Pan & Sari 2005
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, mv=12, angular size = 14 μas
– Projected diameter at 40 AU = 0.4 km
• Star may not be fully occulted
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Other effects
• Wavelength band of observation
• Background star: size, spectral type
• Omnipresent noise !
• Rate:– Opposition / quadrature – 1 event per night ~ 1 event per month
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Current Surveys
• Current surveys: CCD based– TAOS– Roques & Moncuquet
• Other surveys:– Georgevits: Fiber fed spectrograph– Chang et al.: RXTE data
• Limitations: Sampling rate, Sensitivity
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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
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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)
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APD spectral response
• Peak 800 nm, FWHM 340 nm
• Peak quantum efficiency: 0.4
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Telescope• Table Mountain
Observatory– Pomona college– NASA
• Telescope:– 1 meter aperture– F/9.5– Useful FOV 11’– Plate scale 21”/mm
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Optical Design
• Direct focus not possible
• Pickoff mirrors to accommodate 2 modules
• Field lens to correct for tracking errors
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Mechanical Design
Adjusting screw
Diagonal mirror
Moving stage
APD / PMT
Light tight box
Blue indicates light path
Stage motion
Micrometer
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Testing
• Asteroid occultations– None at site in next year
• Optical pulsar: Crab– mv≈16
• Lunar occultations– Earthshine, star magnitude…
• Other rapid transients
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Current Status
• One APD module delivered, one awaited
• Optical design– Code-V simulations
• Observing time:– Second half of October– Second half of November
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Questions
Occultation Lightcurves
Motivation Detectors
Optical Design
TestingMechanical
Design
Slide 15 of 15
Varun Bhalerao
Bessel and Lommel functions
m
m
m x
mmxJ
2
0 2)!(!
)1()(
)()1(),( 2
2
0
kn
kn
k
kn JU
Varun Bhalerao
Lightcurve
)(2
cos),(2
)(2
sin),(2
),(),(1
)(
2222
2221
22
21
U
U
UU
I
),(),()( 21
20 UUI )(
)(
Fr
Fx Distance of KBO center from line of sight
KBO Radius2
aF
Fresnel scale atdistance a from observer