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Photometric System

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

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Photometric System

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  1. Photometric System ASTR 3010 Lecture 14 Textbook Ch.10

  2. 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.

  3. 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.

  4. Features relevant to narrowband photometry

  5. 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

  6. Response Function • bandwidth (W0) • λpeak • λcen

  7. Effective Wavelength

  8. 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. color correction λ λcat

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

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

  11. Discontinuity, line strength, etc. Usually, two band magnitudes are sufficient to quantify these…

  12. Line index line index = mnarrow – mwide Good example = Hα young star survey

  13. Curvature index (X-C) > 0 for emission (X-C) < 0 for absorption

  14. Planet imaging filter • Spectral Differencing Imaging (SDI)

  15. In summary… Important Concepts Important Terms effective wavelength isophotal wavelength filter bandpass • Color correction • filter response function • Kind of • Chapter/sections covered in this lecture : 10.1-10.3

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