Photometric System
ASTR 3010
Lecture 14
Textbook 10.4 & 10.5
Photometric System
( )
m p = -2.5log Fp + C p
¥
= -2.5log ò Rp ( l) f l dl + C p
0
• 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
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 systems
o Visual (mvis) : naked human eye, peak λ ~ 530nm
o International photographic (mpg) : photographic emulsion + atmosphere, peak λ ~
400nm
o 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 glasses
o 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 JohnsonCousins passbands
UBVRI system
UBVRI system color indices
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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 vapor
o 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
o
o
o
(b-y) : temperature
c1 = (u-v)-(v-b) : luminosity
m1=(v-b)-(b-y) : metallicity
β: better luminosity index for hot *
better temp index for cooler *
• For some reasons, uvbyβ is not
being 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 are
made to “isolate”
interesting
Group of stars
Intrinsic colors of main sequence stars
In summary…
Important Concepts
Important Terms
• Photometric system
• Color indices and physical
parameters
• Color of A0V star
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•
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Photographic & photovisual mag
Johnson-Cousin system
Bessel system
Broadband infraredbands
Chapter/sections covered in this lecture : 10.4 & 10.5