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RGB stars-exposure and quantity

gregory miller
in PixInsight
Hi Adam and everyone else:

When looking at other's work, I often see that when combining RGB stars into a narrowband image for natural star color, the imager will expose the RGB filters for the standard 300 seconds, and shoot dozens and dozens, and sometimes more, of exposures. You're only exposing for star color, and the background will be removed in StarXTerminator. I shoot no more than ten (and that's probably more than necessary) of each RGB filter, exposing for 120 seconds to capture plenty of stars and their color, and I don't I bother with flats. Again, this is just to combine RGB star color with narrowband. 

Why would you shoot so many exposures at so long an exposure time just to capture star color? This seems like a huge waste of time that could be better spent shooting more narrowband images! Am I missing something?

Gregory B. Miller

Comments

  • BlockHeadBlockHead
    Maybe. Your comment that this "captures plenty of stars and their color" is not an objective one. 
    There are cases where the background removal is aided by the better S/N of stars. In addition, sometimes stars are embedded in nebulosity and the extraction will be affected if the S/N is low... then when you blend them in to your NB...it doesn't look good.

    You can tell me.."it looks fine to me"... but again, this is a subjective point.

    But how a about a little secret? The background of NB images (where there is no nebulosity) is quite dark and noisy! Did you know you can blend the RGB stars *with* its background at a small level and  "even" out the NB image? (This may let a little RGB nebulosity in... but it will not hurt). This is a good example of needing extra high quality RGB data. So I think it is just a matter of usage and subjective consideration of what is enough.

    -the Blockhead
  • gregory miller
    Adam (and everyone!):

    THIS is why I am a member of Adam Block Studios. Things I had not considered are brought to the fore and suddenly makes perfect sense. Actually, up to now I have been shooting my RGB stars for the full 300 seconds, and with good RGB stars/narrowband post-processing results. Now I see the wisdom in continuing to do so.

    As for your tip about blending a little RGB background into the narrowband to smooth things out, would this be something to do if you were not using NoiseXTerminator towards the end of processing, or to use both? 

    Thank you so much Adam.

    Gregory B. Miller 
  • BlockHeadBlockHead
    The blending of RGB background is a trick I use if the background of the NB is too "weird" (dominated by noise of one color... artifacts of star removal... etc). It is something that is a tiny subtle thing to do...but it can help in sticky situations.

    -the Blockhead
  • starfield
    For what it's worth Bill Blanshen has published a RGB Stars to NB combination script that seems to work really well.   He uses a starless background of the RGB image so I wonder if he's using a similar approach.  
  • Jeffrey Cymmer
    Wouldn't 5 minute subs for RGB stars blow out the stars?
  • BlockHeadBlockHead
    Not for 16-bit sensors.
    (in addition, there are also OSC vs mono considerations. Mono tends to allow for better color "distinctness" in my opinion.)

    -the Blockhead
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