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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
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Photometric System
ASTR 3010
Lecture 14
Textbook Ch.10
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.
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.
Features relevant to narrowband photometry
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
Response Function
• bandwidth (W0)• λpeak
• λcen
Effective Wavelength
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
Isophotal wavelength
Bandpass measurement equivalent to a measurement of the monochromatic flux at a certain wavelength times the bandpass
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”
Discontinuity, line strength, etc.
Usually, two band magnitudes are sufficient to quantify these…
Line index
line index = mnarrow – mwide
Good example = Hα young star survey
Curvature index
(X-C) > 0 for emission(X-C) < 0 for absorption
Planet imaging filter
• Spectral Differencing Imaging (SDI)
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