Please help, dots in my stars!

Discussion in 'Canon EOS' started by burger, Mar 19, 2010.

  1. I've taken a shot of the South African stars and the Milky Way with my 50D and 24-105L. The details are as follow: f 7.1, ISO 100, 24mm and shutter set to BULB at 2414 sec open. It took the same tame for the camera to process the photo and the result was very disappointing, a lot of black dots in the shot. See 100% crop of the photo. When I did this last with my 300D, the photo was much better. O-yes, the camera was mounted to a tripod and I did forget to put the IS off.
    00W2cm-230551684.jpg
     
  2. I realize why you've shown it this way, but you're displaying it so there isn't any black in it at all. Any picture will break up at such settings. I don't, however, know what the black specks are, but surely they're an artifact of the overprocessing. I wonder if you wouldn't get better results with the in-camera noise reduction off, and then do post processing?
    If you set the black sky to black, you don't get any Milky Way, but you do get star trails that don't look too bad.
    00W2dI-230557884.jpg
     
  3. Dennis,
    did you shoot RAW? You should! What were the noise reduction (NR) settings of your camera? There are two custom functions concerning NR in your camera: "Long exposure NR" and "High ISO NR".
    Long exposure NR takes a "dark frame" directly after the real exposure. This is a frame created with the same settings as the real frame, but with the shutter closed. Obviously you had this setting enabled -- that is why the camera took so long time to process the frame. And you should leave this setting enabled, because it is much more effective than trying to get rid of the long exposure noise in post. But be sure the temperature of the camera does not change too much during the process -- that would change the noise pattern. Maybe the IS gyros produced heat? The camera turns off the gyros for the dark frame because they are useless, and the camera might cool down -- just a guess :)
    Then there is High ISO NR. It does also apply to ISO 100, inspite of the name. You should leave this setting off, shoot RAW and do the noise reduction in post.
    -- Sven
     
  4. Dennis,
    I’ve yet to attempt any star trail photography, but my understanding is that the preferred technique with digital is to combine multiple shorter exposures taken in rapid succession. It’s supposed to result in much less noise and much higher image quality. It also offers a safety margin: if a plane crosses the field of view, you can edit it out of just that frame before combining it with the rest. And, if some idiot shines a flashlight at the camera, it’s only that one frame that gets ruined.
    Google should be able to answer lots of your questions.
    Cheers,
    b&
     
  5. The black dots are from the long exposure NR (the dark frame subtracted from the light image). In this situation, and the really long exposure to disable this feature.
    You could try the image without dark subtraction, and see how it goes. It would probably be better to remove the noise with a dark frame in post production. You can then use multiple dark frames stacked and averaged to subtract from your light frame.
    Deepskystacker is a freeware program that can really help you with this. I use it all the time for astrophotography.
    Tom
     
  6. A good primer:
    http://www.astropix.com/HTML/I_ASTROP/TRIPOD/TRIPOD2.HTM
     

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