Limits of Spectroscopy

What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

Ta.

Abdul Ahad

What is the faintest "source" that can be spectroscopically analysed

3 photons, one for each color

In sci.astro.amateur, Abdul Ahad wrote:
What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

How big is the telescope? What is the throughput and resolution of the
spectrograph, the QE of the detector, and how long are you willing to
integrate on the target? What is the FWHM of the seeing, and how bad
is the light pollution or sky background in your wavelength range?

It's not a simple question.

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

In most cases you can't resolve individual stars, so a spectrum of
the galaxy consists of the blended contribution from various types of
stars. That's not "noise" though.

Quasar spectra are quite different from stars, and it is often simple
to distinguish the two sources and subtract out the stellar contribution
to the spectra.

Dave

David Whysong
DWhysong (at) physics (dot) ucsb (dot) edu

On Thu, 04 Mar 2004 07:16:29 GMT, David Whysong
wrote:

In sci.astro.amateur, Abdul Ahad wrote:
What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

How big is the telescope? What is the throughput and resolution of the
spectrograph, the QE of the detector, and how long are you willing to
integrate on the target? What is the FWHM of the seeing, and how bad
is the light pollution or sky background in your wavelength range?


QE= Quantum Efficiency the energy needed to split off measurable
electrons. with perfect efficiency, one photon would split off one
electron. there's inherent inefficiency in this transfer based on
work function and black body properties of the detector material,
amongst other reasons.

FWHM = Full Width at Half Maximum it applies to a well shaped peak
on a graph. its the point halfway up to the maximum of the peak at
the theoretical fattest part of that peak. its the value of the
weighted average of the peak.


It's not a simple question.

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

In most cases you can't resolve individual stars, so a spectrum of
the galaxy consists of the blended contribution from various types of
stars. That's not "noise" though.

Quasar spectra are quite different from stars, and it is often simple
to distinguish the two sources and subtract out the stellar contribution
to the spectra.

Dave

David Whysong
DWhysong (at) physics (dot) ucsb (dot) edu

(Abdul Ahad) wrote in message . com...

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

Not at all! First of all, most of the light from quasars is *not*
from multiple stars, but from the active core itself -- a single
source. Second, it doesn't matter if the light of a galaxy comes
from multiple stars, as long as those stars are all doing the same
thing. When you are measuring red shift, you are looking for certain
spectral lines. Assuming no radial motion to or away from Earth,
all stars in the universe will put those lines in exactly the
same place, so the spectrum of a galaxy would look more or less
like the spectrum of a single star multiplied by a few billion.

Now in fact, galaxies rotate, so that even if the galaxy as a whole
is at rest w.r.t. the Earth, the spectral lines are "smeared" due
to the fact that the stars at one edge are moving towards Earth
and the stars at the other edge away. But when you are looking
at cosmologically significant distances, galaxies as a whole are
moving away from Earth at a large fraction of the speed of light,
which is *far* greater than the rotational speed of a galaxy.

As for other people's comments, barring the ability to measure the
energy of individual photons -- which is *not* currently possible
in the visible spectrum -- of course you need more light to do
spectroscopy than to do simple photography. How much more depends
on how finely you want to resolve those spectral lines.

- Tony Flannders

In message , Tony
Flanders writes

As for other people's comments, barring the ability to measure the
energy of individual photons -- which is *not* currently possible
in the visible spectrum -- of course you need more light to do
spectroscopy than to do simple photography. How much more depends
on how finely you want to resolve those spectral lines.

Measuring the energy of individual visible light photons was once a
popular technique in the late 70's combining a large scope, spectrometer
and Boksenberg's Image Photon Counting System to do exactly that task.
It opened up the possibility of obtaining spectra from very much fainter
objects than was possible with conventional film emulsions.

Wavelength dispersion determines the energy, and the imaging system is
sensitive to single photons with good QE at suitably low intensities.
There are noise problems with it, but for a while it was the method of
choice.

Now largely supplanted by CCDs, but I think it is still used for certain
jobs.

Regards,
--
Martin Brown

(Abdul Ahad) wrote in message . com...
What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

That would depend, if you float in space with a big aperture telescope
and 'stare' at a distant object for several days then the telescope is
capturing quite a lot of light which can be used to learn from.

Like amatuers who use several minute (or more) exposure cameras to
produce the excellent images occasionally posted on their websites.

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

I can imagine a rough estimate of the general proportions of materials
in the distant galaxy can be made, though not of individual stars.

(Abdul Ahad) wrote in message . com...
What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

Ta.

Abdul Ahad

The distance to galaxies is determined by spectroscopic analysis.
I think the record is right at 30M; Keck verified primevil galaxies
found in one of the hubble deep fields. The big telescopes are
routinely doing spectro work at M27. This spectro work is concerned
mainly with the Lyman Alpha emissions or other ultraviolet emissions
that have been red shifted down to optical wavelengths. The detection
of the pattern is all that is necessary to determine red shift.
These are typically done on meduim resolution spectragraphs.

High resolution spectro work to determine elemental composition is
being done down to M26. Individual stars in M31 and other rather
near galaxies has been acomplished. These are done on high resolution
(see Echelle) spectrographs. Resolutions in the 0.1 Angstrom range
are routine.

High resolution work to determine shifting spectral content due to
large planets is not at this time photon limited as all the stars
are quite bright reletive to the redshift analysis stuff. The problem
for this analysis is stability of the receiving equiptment {scope
plus spectrograph} and the stability of precision reference line
sources. Resolution in the 20 Hz range allows detection of Jupiter
sized objects to several thousand light years. The stability of
the reference line source must be significantly smaller than
the spectral resolution for years at a time. Several planets have
been confirmed with amateur spectra on 20" sized telescopes.

As to noise: To the spectrograph, if photons are allowed to pass
the 'slit' the spectrograph images the spectra. So if the slit is
set at 1 arc second, the spectrograph creates a spectrum of
everything that passes the slit. Sometimes it becomes aparent
that spectral lines from several sources have been imaged at the
same time. In the early days of spectroscopy (turn of previous
century) many binary stars were determined to be binary by this
technique.

(Mitch Alsup) wrote in message . com...
(Abdul Ahad) wrote in message . com...
What is the faintest "source" that can be spectroscopically analysed
via a telescope for fraunhofer lines and elemental composition?

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

Ta.

Abdul Ahad

The distance to galaxies is determined by spectroscopic analysis.
I think the record is right at 30M; Keck verified primevil galaxies
found in one of the hubble deep fields. The big telescopes are
routinely doing spectro work at M27. This spectro work is concerned
mainly with the Lyman Alpha emissions or other ultraviolet emissions
that have been red shifted down to optical wavelengths. The detection
of the pattern is all that is necessary to determine red shift.
These are typically done on meduim resolution spectragraphs.

High resolution spectro work to determine elemental composition is
being done down to M26. Individual stars in M31 and other rather
near galaxies has been acomplished. These are done on high resolution
(see Echelle) spectrographs. Resolutions in the 0.1 Angstrom range
are routine.

High resolution work to determine shifting spectral content due to
large planets is not at this time photon limited as all the stars
are quite bright reletive to the redshift analysis stuff. The problem
for this analysis is stability of the receiving equiptment {scope
plus spectrograph} and the stability of precision reference line
sources. Resolution in the 20 Hz range allows detection of Jupiter
sized objects to several thousand light years. The stability of
the reference line source must be significantly smaller than
the spectral resolution for years at a time. Several planets have
been confirmed with amateur spectra on 20" sized telescopes.

As to noise: To the spectrograph, if photons are allowed to pass
the 'slit' the spectrograph images the spectra. So if the slit is
set at 1 arc second, the spectrograph creates a spectrum of
everything that passes the slit. Sometimes it becomes aparent
that spectral lines from several sources have been imaged at the
same time. In the early days of spectroscopy (turn of previous
century) many binary stars were determined to be binary by this
technique.

O.K. I understand the *technical* limits on spectroscopy imposed by
resolution, photon counts, etc and I suspect that spectroscopy may not
be a suitable method for analysing elemental make up of the early
universe (really far away galaxies).

Ultimately, I guess my question is: how do we deduce the fact that a
galaxy or other cosmological object beyond, say a few billion light
years distant, is lacking in heavier elements? If we cannot observe
individual 'stars' separately from their galaxies in order to deduce
their elemental composition using spectroscopy... what are the
alternatives?

My question moves more into the spehere of Astrophysics...
Specifically, if a star starts from Hydrogen/Helium fusion, going up
the scale...through Carbon burning... all the way up to Iron (for
stars of great initial mass) at the end of its evolutionary cycle,
then how do we get elements further up the scale on the Periodic Table
such as Rubidium, Strontium, Lead... all the way to Uranium? What are
the theories for their formation?

Is there a distance threshold (so many billions of years back in
time/out in space) beyond which the heaviest naturally ocurring
elements are *proved* never to have existed?

thanks.

(Abdul Ahad) wrote in message . com...
Ultimately, I guess my question is: how do we deduce the fact that a
galaxy or other cosmological object beyond, say a few billion light
years distant, is lacking in heavier elements? If we cannot observe
individual 'stars' separately from their galaxies in order to deduce
their elemental composition using spectroscopy... what are the
alternatives?

Good question: we can deduce that an element is present in a star
by resolving an emission line in the star's spectrum. We can
deduce that an element is present in a gas cloud by resolving
absorption line when the gas cloud is between us and the star(s).

My question moves more into the spehere of Astrophysics...
Specifically, if a star starts from Hydrogen/Helium fusion, going up
the scale...through Carbon burning... all the way up to Iron (for
stars of great initial mass) at the end of its evolutionary cycle,
then how do we get elements further up the scale on the Periodic Table
such as Rubidium, Strontium, Lead... all the way to Uranium? What are
the theories for their formation?

The heavier elements are cooked in the milliseconds between the
colapse of the supernovaing star and the rebound of the supernovaing
star. The nuclear density goes up many times, and allows reactions
that do not occur in a normal fusion buring star. Pressure and
temperatures are extreme during this colapse through rebound phase
of the supernova, providing the environment to cook up the heavy
elements.

Is there a distance threshold (so many billions of years back in
time/out in space) beyond which the heaviest naturally ocurring
elements are *proved* never to have existed?

Another good question, and the answer you got two years ago would
not be the same answer you get today! A couple of years ago we
thought the first generation stars would have to have lifetimes of
1-2B years before supernovaing and dispersing heavy elements. We
now have evidence of the first generation stars being heavier
and thereby having shorter lives (as short as 200 M years) before
spewing heavy elements into their galaxies.

thanks.

Mtich