06 ashish mahabal bse1

BigSkyEarth I: Sensors in Astronomy

Ashish Mahabal Center for Data-Driven Discovery, Caltech

Apr 5, 2016

Ashish Mahabal - BSE I

Sundial and friends

Samrat Yantra at IUCAA

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Ignoring Galileo

to embracing Gravitational-Waves

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The EM spectrum

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Photographic plates

• Silver salt emulsions - light sensitive

• Same principle as older (pre-digital cameras) but on glass plates [palomar story]

• large collections exist (Harvard computers) [great for archival searches of variables - DASCH]

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Carina Nebula on a 1951 plate by Karl Heinze held by Thurburn Baker (ADAP). 38cm x 38cm

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Photomultiplier tubes• 1% photometry (0.01 mag accuracy)

• variable stars

• set of electrodes multiply number of electrons through higher voltages: gain of a million

• resolution not great

• used extensively in cosmic-ray and neutrino detectors [super-kamiokande has 11200 50cm tubes looking for Cerenkov radiation]

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Charge-Coupled Devices (CCDs)

• Workhorse of modern-day astronomy

• Typically a 2D set of light-sensitive pixels (picture elements)

• photons -> electrons

• CSS 10k x 10k chip

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features• Great dynamic range (twice that of CMOS) [but is it good

enough? problems with bright objects]

• Great quantum efficiency (can reach 95%)

• compare solar cells

• Good spectral range

• Needs to be cooled for astronomical purposes (LN2) [Orion challenge]

• a bit slow

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PQ CCDs

112-CCD, 161 Mpix camera

• multiple amplifiers for readouts (either faster on same chip, or multiple chips): crosstalk [PQ]

• gaps between adjacent CCDs to accomodate attendant electronics: leads to more complex cadence

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CRTS-II

•  Same telescopes

•  Bigger cameras and FOVs:

MLS 1.5m, 1.2 -> 5 sq. deg

CSS 0.7m, 8.2 -> 19 sq. deg

Spectral Instruments camera with Imager Labs thinned 10.5K2 CCD.

Upgrades funded and underway.

8/27/14 51 Ashish Mahabal

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CCD pipeline

• Bias subtraction (ambient noise)

• Flat fielding (detector response - per pixel)

• cosmic ray removal (grazing incidences)

Big Picture at Griffith

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spectroscopes• filters are low-res spectrographs

• to capture emission and absorption lines, we need real spectroscopes

• series of mirrors - fraction refracts; different angle depending on wavelength

• limited resolution

• gelatin and Bragg’s diffraction can be used for better

• not below 3000 Angstrom

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DBSP at palomar

• two arms

• wider wavelength coverage at better resolution

• separate gratings

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Spectroscopy pipeline

• 1d response through polynomial fits

• arcs for wavelength calibration

• spectrophotometric standards for flux calibration

• selection of appropriate wavlength range for best science (e.g H-alpha or L-alpha)

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integral field units

• SDSS example

• hundreds of thousands of quasar spectra 17


NIR spectroscopy

• more challenging - water vapour and CO2 absorption

• keck spectrographs (NIRSPEC, MOSFIRE)

• above the atmosphere18


radio detectors• large wavelengths - so larger

receivers (lambda/d)

• can have bigger gaps (mesh)

• arrays for Fourier domain coverage

• SKA and path-finders NEP uv coverage

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subMM detectors• molecular clouds

• Star formation: H2, CO

• dark clouds: Frozen CO, N

• water vapor limitation (dry/cool/stable site needed)

• Mauna Kea, Chile, Hanle, South Pole, (Greenland)

Balloons (BLAST), Stratosphere (SOFIA)

LDN 1768

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X-ray detectors

• focussing: higher resolution/sensitivity • collimating: coded masks, large FOV

Swift: GRB finder

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Polarimeters• To measure polarized light

BL Lac with RoboPol

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Gravitational-wave detectors

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Summary

• Wide variety of sensors

• Immense variety of science

• Many recent/ongoing innovations

• Better/vaster data accumulating

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Science

06 ashish mahabal bse1