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Limits of Spectroscopy



 
 
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  #1  
Old March 3rd 04, 08:59 AM
Abdul Ahad
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Default 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
  #2  
Old March 3rd 04, 02:31 PM
HAVRILIAK
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Default Limits of Spectroscopy

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

3 photons, one for each color
  #3  
Old March 3rd 04, 03:33 PM
Andrew Urquhart
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Default Limits of Spectroscopy

Ioannis wrote:
? "Abdul Ahad" ?????? ??? ??????
om...

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


I see no reason why it shouldn't be precisely the same magnitude
limit that exists for the larger scopes.

If it can be photographed, it can be spectrographed.


You're going to need a good few photons to be able to remove the
uncertainty as to peaks and troughs in the wavelength profile. In this
recent story:
http://www.astronomynow.com/news/040...t_galaxy.shtml
I imagine they had enough photons to calculate an approximate blackbody
curve to determine the redshift, but I very much doubt they could tell
you what the composition of the galaxy was!
--
Andrew Urquhart
Reply: www.andrewu.co.uk/about/contact/


  #4  
Old March 4th 04, 08:16 AM
David Whysong
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Default Limits of Spectroscopy

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
  #5  
Old March 4th 04, 08:19 AM
David Whysong
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Default Limits of Spectroscopy

In sci.astro.amateur, Ioannis wrote:

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


I see no reason why it shouldn't be precisely the same magnitude limit that
exists for the larger scopes.


Spectrographs usually have much less throughput than cameras; there are
losses due to the grating (or grism) and re-imaging optics. Furthermore,
the light is spread over many pixels, each of which has read noise and
dark current.

So you do lose substantial SNR with a spectrograph, compared to imaging.

Dave

David Whysong
DWhysong (at) physics (dot) ucsb (dot) edu
  #6  
Old March 4th 04, 09:25 AM
Martin Brown
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Default Limits of Spectroscopy

In message , Ioannis
writes

? "Abdul Ahad" ?????? ??? μ???μ?
. com...

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


A devious answer to this is that we can analyse clouds of hydrogen gas
that happen to be in the line of sight between us and a distant quasar
no matter how faint the cloud itself may be. The light from the quasar
allows us to see the composition and redshift of the intervening gas
cloud.

I see no reason why it shouldn't be precisely the same magnitude limit that
exists for the larger scopes.

If it can be photographed, it can be spectrographed.


You have it exactly backwards. Anything that a spectrograph will work on
will be plenty bright enough to photograph. The main reason the biggest
scopes are in demand is to feed enough light into spectrographs.

Imaging a star requires obtaining enough signal to noise on a handful of
pixels. Imaging a spectrum can require a few thousand times more SNR.

Try to make a spectrum with 0.1nm resolution across the visible to look
in detail at Fraunhofer lines and that same amount of starlight is
spread across 4000 pixels with consequent loss of limiting magnitude
(roughly about 9 mags ignoring other secondary factors).

Fortunately you don't always need very high resolution spectra to see
interesting things - some of the wilder active objects have bright
emission lines that stand out even at relatively low dispersion.

There are amateur spectra of some brighter quasars and Seyferts about.

Regards,
--
Martin Brown
  #7  
Old March 4th 04, 11:44 AM
Tony Flanders
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Default Limits of Spectroscopy

(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
  #8  
Old March 4th 04, 02:58 PM
beavith
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Default Limits of Spectroscopy

On Thu, 04 Mar 2004 07:19:13 GMT, David Whysong
wrote:

In sci.astro.amateur, Ioannis wrote:

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


I see no reason why it shouldn't be precisely the same magnitude limit that
exists for the larger scopes.


Spectrographs usually have much less throughput than cameras; there are
losses due to the grating (or grism) and re-imaging optics. Furthermore,
the light is spread over many pixels, each of which has read noise and
dark current.

So you do lose substantial SNR with a spectrograph, compared to imaging.


SNR = signal to noise ratio

Dave

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


  #9  
Old March 4th 04, 03:03 PM
beavith
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Default Limits of Spectroscopy

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


 




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