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How to convert the intensity of light of a star

  1. Apr 23, 2015 #1
    If you take the spectra of a star you usually have a big primary mirror (lets say 1m) to get a lot of light, and use an exposure time usually several tens of minutes.

    Can one use an astronomical spectrograph for a led/arclamp light source? How does the light intensity of a star compares to conventional light sources? Will you need lower exposure times?
     
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  3. Apr 23, 2015 #2

    Drakkith

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    I don't see why not.

    Probably. Most light sources are MUCH brighter than a star.
     
  4. Apr 23, 2015 #3

    SteamKing

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    You use a big mirror and long exposure times to collect enough photons traveling from the distant star to make a noticeable change in the photographic plate, CCD, or whatever device or medium you are using to make a photograph or a spectrograph or whatever. One or two or a handful of photons don't make a noticeable enough difference to these media, so you must collect bunches of them from the star being observed, and that takes time.

    If you use the same equipment on a nearby light source, like a candle or LED light, there are many more photons to capture; thus exposure times can be greatly reduced, lest the image become totally washed out, like an overexposed photograph.
     
  5. Apr 23, 2015 #4
    Thanks for the replies Drakkith and SteamKing,


    So if this is true, I just wondered why dont they use echelle spectrographs in optical sciences, where you have 2 dimensional spectrum (orders on top of each others). When astronomers need high resolution they use these kind of instruments.

    They use an echelle grating, and a cross disperser element like this:
    http://www2.keck.hawaii.edu/inst/hires/lightpath.jpg

    and the result is a spectrum with orders on top of each others:
    http://www.obs-hp.fr/guide/sophie/spec_echelle_labo.jpg
     
    Last edited: Apr 23, 2015
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