photometric system astr 3010 lecture 14 textbook ch.10

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