Reference wanted: spectrum shift due to earth's orbit
Tom Roberts
variations in the spectra of distant stars due to the earth's orbit.
Does anyone have a reference for a measurement of this? Or a reference
describing how to account for it? Or a textbook discussing this? (Google
found some lab homework on this, but I want a reference to a measurement
or a discussion.)
BTW how is this handled? I would guess that spectra are conventionally
corrected to the solar system barycenter, but I know next to nothing
about the details of modern astronomy, and would like to know how real
astronomers handle this.
(Why do I want this? Because I am updating a FAQ page on
experimental tests of Special Relativity, and I just realized
that this would be a good, clear demonstration that
wavelength is observer dependent.)
Another question: is any spectroscopy done using frequency? If so, can
one combine with a wavelength measurement of the same line and show
f*lambda=c? A reference to that would be most welcome.
Tom Roberts
Marshall Perrin
> Current astronomical spectrometers are sensitive enough to detect the
> variations in the spectra of distant stars due to the earth's orbit.
> Does anyone have a reference for a measurement of this? Or a reference
> describing how to account for it? Or a textbook discussing this? (Google
> found some lab homework on this, but I want a reference to a measurement
> or a discussion.)
>
> BTW how is this handled? I would guess that spectra are conventionally
> corrected to the solar system barycenter, but I know next to nothing
> about the details of modern astronomy, and would like to know how real
> astronomers handle this.
Yes, precisely: radial velocities are converted to the solar system
barycenter. Such conversions are essential to any of the precision
Doppler radial velocity projects that find extrasolar planets. I don't know
a reference off the top of my head, though I suspect you can find one
in the radial velocity literature by Marcy et al. or Fischer et al.
(search on ADS: adswww.harvard.edu). In the meantime, though, I can point
you at the source code for a commonly-used barycentric correction routine:
http://idlastro.gsfc.nasa.gov/ftp/pro/astro/baryvel.pro
It's well commented and should give you an idea of the algorithms
involved.
> Another question: is any spectroscopy done using frequency? If so, can
> one combine with a wavelength measurement of the same line and show
> f*lambda=c? A reference to that would be most welcome.
That's such a fundamental result that it will be difficult to find a
specific reference to it. But the design of the vast majority of
telescopes and astronomical instruments would allow such a
measurement, either directly or implicitly: Most directly via radio
telescopes, since those measure frequency directly in their receivers
while wavelength is used implicitly to design the antennae and
waveguides that funnel light to the receivers; less directly via
something like an optical spectrograph, which uses wavelength to
direct light to different regions of a detector, while providing a
crude measurement of energy (related to frequency of course) via the
detection band gap of the detector semiconductor. Come to think of it,
X-ray spectrographs (such as those on Chandra or XMM) with their
energy-resolved detector readouts provide a better example than
optical spectrographs.
- Marshall
David M. Palmer
<tjrobe...@sbcglobal.net> wrote:
> Another question: is any spectroscopy done using frequency? If so, can
> one combine with a wavelength measurement of the same line and show
> f*lambda=c? A reference to that would be most welcome.
Marshall Perrin mentioned some of this, but in radio astronomy
interferometric imaging (e.g. VLA, VLBI):
1) Frequencies are measured directly by the receiver, so you get the
redshifts of lines
2) At a given frequency, image reconstruction depends on wavelength, to
and accuracy of a few millimeters over the diameter of Earth.
If f*lambda were not equal to c, nothing would work. Different
frequencies would produce images of the same source at different
locations.
But now that I think of it, f*lambda != c, due to the fact that the
interplanetary, interstellar, and intergalactic media are not perfect
vacua. (This does have measured effects such as scintillation and
dispersion.) I don't know if this is corrected for explicitly, or if
it is just taken out by calibration against nearby sources.
--
David M. Palmer dmpa...@email.com (formerly @clark.net, @ematic.com)
Ken S. Tucker
My favorite relation on how "Doppler" was used was
in this red shift experiment...
http://en.wikipedia.org/wiki/Pound-Rebka_experiment
That merges SR and GR pretty nicely.
You can also see how "Doppler" is used here...
http://www.einstein-online.info/en/spotlights/redshift_white_dwarfs/index.html
though somewhat misnomed to the so-called
Einstein "red-shift", by considering spectral lines.
Regards
Ken S. Tucker