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74361Re: Relative Exposure times

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  • stan_ccd
    Jul 21, 2014
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      #1: take sub-exps at least 20 minutes.

      As you know, narrow band filters block most of the light and pass a weak signal. So 60 seconds is much too short to accumulate enough signal to overcome camera noise (readout).  That camera (STF) should easily be able to take 20-30 min exps, which is highly beneficial for narrow band (especially, but also good for most broad band imaging as well).

      #2:  If you want to feature the non-H-a then take even longer  O-III and S-II

      Of the 3 bands in the "Hubble  palette", H-a is by far the strongest signal.  If you were to balance the images according to actual strength then in many cases you might as well not even bother to take O-III or S-II because they are drowned out by H-a (except for some regions in some planetary nebula). Essentially all of the garish false color narrow band images you see are highly distorted in order to show non-H-a features.

      #3: concentrate on H-1 (color is not mandatory or even desirable)

      Many narrow band practitioners put most of their energy into the H-a because that is the strongest signal and often the most interesting.  In M57, H-a has many fine details with high contrast, whereas O-II is diffuse and nearly featureless and S-II is weak and not very interesting.


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