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Thread: Astronomical spectroscopy

  1. #1 Astronomical spectroscopy 
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    Are there any setbacks/disadvantages to astronomical spectroscopy? What are the down points? What impediments or problems are encountered? (Physical/visual obstacles, cost, time taken to conduct a study etc.). What are the outstanding problems- are there any factors which delay research (for example)?

    I hope this question is fairly accessible. (Sorry if it seems vague, I know this is a vast area of study).

    Thank you.


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  3. #2  
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    Compared to what? It all depends on what you want to achieve or measure. Could you be a bit more specific?


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  4. #3  
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    Quote Originally Posted by Dishmaster
    Compared to what? It all depends on what you want to achieve or measure. Could you be a bit more specific?
    This can cover any area of astronomical spectroscopy; it is a general question about its down points. Any information is appreciated with interest.

    Thank you
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  5. #4  
    Moderator Moderator Dishmaster's Avatar
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    You could fill a complete lecture series with that topic. So I will just highlight a few issues:

    There are restrictions for technical and natural reasons. One frequent application of spectroscopy is the measurement of velocities by the different flavours of radial velocity shifts. The accuracy of the velocity is restricted by the accuracy of the spectral calibration. In order to measure the velocity, you must know the rest frequency of a spectral line with a high certainty. For optical spectroscopy, this can be achieved with gas or vapour cells containing a gaseous medium whose spectrum is well known. A high number of lines increases the accuracy by cross referencing. These cells are put inside the optical path of the spectrograph that measures both the astronomical target and the known spectrum of the gas cell. The involved uncertainties put a limit to the minimum velocities that can be detected. E.g. in exoplanet search, this produces a selection effect towards the fast planets. The same problem arises, if the resolution power of a spectrograph is not high enough. If you cannot sample or resolve a line well enough, you only know the frequency of the line centre with a low certainty.

    The same example also has an intrinsic limit: You can only measure the radial velocity, i.e. the velocity component directed towards earth. If the planetary orbit is tilted, the true velocity can be much higher. This also affects the mass of the supposed planet, i.e. the mass is always only a lower limit, as long as the inclination angle is unknown.

    Another problem is blending of lines along a line of sight. If the material is transparent, you average the measured velocities along the line of sight producing a line broadening. Although such a measurement still bears some information about the kinetics, you will never get a line narrow enough to achieve a high accuracy on the velocities involved.

    Let me know, whether these answers head into the direction you intended.
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  6. #5 Re: Astronomical spectroscopy 
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    Quote Originally Posted by emetzner
    Are there any setbacks/disadvantages to astronomical spectroscopy? What are the down points? What impediments or problems are encountered? (Physical/visual obstacles, cost, time taken to conduct a study etc.). What are the outstanding problems- are there any factors which delay research (for example)?

    I hope this question is fairly accessible. (Sorry if it seems vague, I know this is a vast area of study).

    Thank you.
    Ive been doing amateur astronomical spectroscopy for years. You should join
    one of our Yahoo spectroscopy groups; the SA group (Star Analyser made by Paton Hawksley) or the more advanced Spectroscopy Group where Christian Buil and
    Robin Leadbeater moderate things. Its fun!

    What do you want to know?

    Example: Right now there are a ton of great amateur spectroscopy websites active on the net. Just Google /amateur spectroscopy/ or /astro spectroscopy/ ...
    anything close will work and you will have a year's reading and enjoyment ahead
    of you.

    Right now Im using several different gratings and spectroscopes. At the low
    end I prefer the Baader "Blaze Gitter" but they are pricey and a bit hard to get.
    200 lpm and very good bandpass below 300nm and well above 1000nm.

    Im really passionate about this hobby so I will stop and hopfully we can
    share and discuss a few things. Im glad I found this! Thanks, you made my evening.

    Georger
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