Photometric System

ASTR 3010

Lecture 14

Textbook 10.4 & 10.5

Photometric System

• Narrow meaning : wavelength response of a band (Rp) instrumental magnitude• Broader meaning: a method for standardizing measurements made with a

photometric band (i.e., with specific Rp and Cp) standard magnitude

Instrumental magnitudes

Standard magnitudes

standardization

How?

Standardization: Can be done by using an object with known spectral shape. But, in practice, a set of “standard” stars are used

Common photometric system• 100s of photometric systems so far• Excellent summary of photometric systems in Annual Review of Astronomy

& Astrophysics by Mike Bessel (2005)

• Visual and photographic systemso Visual (mvis) : naked human eye, peak λ ~ 530nmo International photographic (mpg) : photographic emulsion + atmosphere, peak λ ~

400nmo International photovisual (mpv) : orthochromatic emulsion + yellow filter, peak λ ~

550nm

o 1922 IAU set the zero points (Cp) of both systems such that 6th mag A0V star should have the same magnitudes

color index = mpg – mpv, 0 for A0V, negative for hotter, positive for cooler stars

UBVRI system• Johnson-Cousins system : Most frequently used photometric system • UBV defined first by Johnson & Harris (1954)o RCA 1P21 photomultiplier + several colored glasseso V is similar to mpv

o U-B and B-V zeros for A0V stars• Then, in 60s, R and I bands are added

UBVRCIC system

• Increased sensitivity at R-band and decreased red-edge for I-band

UBVRI Bessell Filter System• Modern CCDs have difficulty replicating the original photomultiplier-based

instrumental system wide variety of CCD spectral response• published a set of combinations of cheap optical glass filters for commonly

used CCDs which would reproduce reasonably well the classic Johnson-Cousins passbands

UBVRI system

UBVRI system color indices

• U-B : Sensitive to Balmer Discontinuity luminosity• B-V, V-R, R-I : sensitive to temperature spectral type• B-V is more affected by metallicity than V-R and R-I• B-V becomes useless for M-type stars (why?)• V-I index is the most purely temperature sensitive index in UBVRI system (V-K) is even better! Why?

A set of suggested standard stars : “Landolt Standards” need for fainter standard stars

JHKLMNQ systems (1-20 microns)• “Broadband Infrared System” : an extension of the UVBRI into IR• Common zero point: colors of an A0V star are zero by definition

• Bandpass definitions can depend critically on atmospheric conditions (mainly water vapor) different observatories with identical hardware will measure differently same observatory will see a bandpass variation due to weather

• In 2000, IAU recommended a new system (Mauna Kea Observatory) for JHK MKO near-IR system

By narrowing the passband (i.e., decreasing FWHM)o Minimize sensitivity to water vaporo Maximize S/N

• A set of standard stars : “Elias Standards”, but still evolving…

Strömgren System (uvbyβ)• In 1950s, Strömgren defined intermediate band system• D. Crawford used it extensively to develop observing techniques in 60s/70s• uvbyβ (six filter system)o (b-y) : temperatureo c1 = (u-v)-(v-b) : luminosityo m1=(v-b)-(b-y) : metallicityo β: better luminosity index for hot *

better temp index for cooler *

• For some reasons, uvbyβ is notbeing used that much nowadays…

Sloan Digital Sky Survey system• A CCD sky survey which measured hundreds of millions objects.• Filters are designed to optimize the silicon CCD sensitivity.• May take over UBVRI system…

SDSS usabilities

Palomar Observatory Sky Survey (POSS)

Others

30 filters

48 filters

Various Color-Color Diagrams

Various color-color diagrams aremade to “isolate” interesting Group of stars

Intrinsic colors of main sequence stars

In summary…

Important Concepts• Photometric system• Color indices and physical

parameters• Color of A0V star

Important Terms• Photographic & photovisual mag• Johnson-Cousin system• Bessel system• Broadband infraredbands

Chapter/sections covered in this lecture : 10.4 & 10.5