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Astronomical filters
ASTR320
Monday October 17, 2016
Astronomical filters
• It is often the case that
we want to understand
the “color” of an
astronomical object,
measured in
magnitudes
• We can divide up the
spectrum of light
produced by an object
into distinct
bandpasses by placing
a filter in the optical
path of the telescope
Examples:
The
electromagnetic
spectrum
Earth’s atmosphere is transparent to visible
light and some microwaves and radio waves.
Color indices
• In astronomy we define the colors of stars quantitatively,
using color indices
• Suppose we measure fluxes in two different filters:
• We can make a color index by subtracting:
• We generally write the color index as letters that denote
filters, e.g.:
• Note that the distance cancels, so the color is the same
for absolute and apparent magnitudes.
Color indices
• By convention, pick cA-cB based on Vega (an A0V type
star) so that for Vega:
• By convention, generally write colors with the shorter
wavelength passband first:
– B-V, U-B, J-K
• So that smaller numbers are always “bluer” and larger
numbers are “redder”
– A-B<0 means “bluer than Vega”
– A-B>0 means “redder than Vega”
What color indices measure
• Most usefully, temperature
• Stars behave largely like blackbodies
– A hot, opaque object produces a continuous blackbody spectrum
of light characterized by its temperature
Blackbody radiation
A blackbody is an object that absorbs all light.
• Absorbs at all wavelengths
• Characterized by its temperature
It is also the perfect radiator:
• Emits at all wavelengths (continuous spectrum)
• Total energy emitted depends on temperature
• Peak wavelength also depends on temperature
Blackbody curves
Stefan-Boltzmann Law
Energy emitted per second per area by a blackbody with
Temperature (T):
E = sT
Where s is Boltzmann's constant.
4
Wien’s law
Wavelength of maximum emission
is inversely related to temperature
max
2,900,000 nm
max 
T
 wavelengt h of maximum emission
T  temperature (in Kelvins)
The color of a star
is related to its
temperature
Betelgeuse: a reddish
star (cooler).
Rigel: a bluish star
(hotter).
Rigel:
• Teff = 11,000K
• B-V = -0.03
Betelgeuse:
• Teff = 3,500K
• B-V = 1.85
Stellar spectra in
order from the hottest
(top) to coolest
(bottom).
Astronomical filters
• Measurements of astronomical objects are made by
different telescopes and instruments all over the world
• When imaging astronomical objects, it is common to
place one of several different filters (conceptually, a
colored piece of glass) in the optical system to measure
the color of an object quantitatively
• Once enough astronomers began to make modern
measurements of astrophysical sources, it became clear
that a standardized system was needed so the
measurements could be compared and confirmed
Astronomical filter systems
We design
photometric
systems to
maximize
information that
can be gleaned
from extremely low
resolution
spectroscopy, i.e.,
photometry.
The Johnson-Morgan UBV System
•
•
•
•
The UBV system was originally
designed by Johnson and Morgan
(1953) to understand stars
(particularly hot stars). The
Johnson-Morgan V band is meant
to simulate and perpetuate
measurements historically made
by the human eye, to which it
approximately matches.
The Johnson-Morgan B band
approximates the blue sensitivity
of the original photographic
emulsions to typical stars.
The B-V color provided a measure
of the temperature of (hotter)
stars.
Johnson and Morgan realized that
much more information was
possible by adding a third filter in
the ultraviolet.
The UBVRI system
• In the 1960s, Johnson (and later others) extended the UBV
system to the red and infrared, with R,I,J,K,L,M,N.... bands.
• In the optical, then, we have the UBVRI broadband system.
• It was found that the UBV system did not work well for very
cool stars, like K and M spectral types, and these very red
stars were easier to study at redder wavelengths. So the V, R
and I bands are often used to study these kinds of stars.
SDSS ugriz system
• The Sloan Digital Sky
Survey used a filter
set based on the
Thuan-Gunn system
• Used in the SDSS
survey, and is now
commonly used by
most astronomers in
order to compare to
the huge dataset of
SDSS
Other filter systems
• Stromgren: stellar
classification,
absolute magnitudes,
surface gravities
• Washington/DDO:
metal abundances
• Tuned narrowband
filters: measure
specific emission line
features in galaxies or
nebulae, redshifts of
galaxies/quasars