Photometric System

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Photometric System ASTR 3010 Lecture 14 Textbook Ch.10

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Photometric System. ASTR 3010 Lecture 14 Textbook Ch.10. History of Photometry. Ptolemy & Tycho : by eyes, ~0.5mag 19 th century: naked eyes, ~0.3mag late 19 th century: photometer, ~0.25mag early 20 th century: photograph, ~0.03mag 1940s: photomultiplier tube, ~0.005mag - PowerPoint PPT Presentation

Transcript of Photometric System

Page 1: Photometric System

Photometric System

ASTR 3010

Lecture 14

Textbook Ch.10

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History of Photometry• Ptolemy & Tycho: by eyes, ~0.5mag• 19th century: naked eyes, ~0.3mag• late 19th century: photometer, ~0.25mag• early 20th century: photograph, ~0.03mag• 1940s: photomultiplier tube, ~0.005mag• late 20th century: CCD, ~0.01mag• present: Kepler, ~0.00001mag transiting planets.

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Response Function• Bandpass: a range of wavelengths where an instrument is sensitive

• single-band photometry: e.g., Kepler. Time series• broadband multi-color photometry: shape of the spectrum, ultra low resolution

spectroscopy. “broad” = Δλ/λ > 10%. “Color”=color index, brightness and color.• narrow- & intermediate band photometry: to isolate a specific line, molecular

band, etc., E.g., Balmer discontinuity, TiO band, Hα, etc.

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Features relevant to narrowband photometry

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Filters• determined by the combined spectral response of various sources (filter,

detector, atmosphere, & telescope). E.g., Johnson-U

• bandpass filter• high-pass filter• low-pass filter• neutral density filter

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Response Function

• bandwidth (W0)• λpeak

• λcen

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Effective Wavelength

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Color correction• Large survey with several passbands (e.g., all-sky survey)• Measurements with the same filter result in different λeff

• Assume a certain source spectrum shape.

λλcat

color correction

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Isophotal wavelength

Bandpass measurement equivalent to a measurement of the monochromatic flux at a certain wavelength times the bandpass

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Color indices

Difference b/w two magnitudes ≈ slope of spectra ≈ Blackbody temperaturecolor index = m (shorter λ) - m (longer λ)• all indices should be zero for Vega “Vega system” or “Vega magnitudes”

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Discontinuity, line strength, etc.

Usually, two band magnitudes are sufficient to quantify these…

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Line index

line index = mnarrow – mwide

Good example = Hα young star survey

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Curvature index

(X-C) > 0 for emission(X-C) < 0 for absorption

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Planet imaging filter

• Spectral Differencing Imaging (SDI)

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In summary…

Important Concepts• Color correction• filter response function• Kind of

Important Terms• effective wavelength• isophotal wavelength• filter bandpass

Chapter/sections covered in this lecture : 10.1-10.3