Life in the Universe

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
• 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
lmean =
W0 =
ò l× R(l)dl
ò R(l)dl
R(l )d l
Effective Wavelength
leffective =
ò l× f l × R(l)dl
ò f l × R(l)dl
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
Isophotal wavelength
Bandpass measurement equivalent to a
measurement of the monochromatic flux
at a certain wavelength times the
W 0 × f liso
ò f l × R(l)dl
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”
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
Important Terms
• Color correction
• filter response function
• Kind of
• effective wavelength
• isophotal wavelength
• filter bandpass
Chapter/sections covered in this lecture : 10.1-10.3

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