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Astrophoto of the Week: Star Trails

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  • Astrophoto of the Week: Star Trails

    Star trail photos are a classic in astrophotography. In the past with film cameras all that was needed was a camera with a "bulb" setting and a lockable shutter release. Use some slow film and leave the shutter open for as long as you like. The rotation of the Earth then causes the apparent motion of the stars to form circular trails in the image.

    This is one type of photo that a digital camera is ill suited to take. Digital cameras have electronically actuated shutters and are quite limited as to the length of time the shutter can be held open. Holding the shutter open places a heavy demand on the battery. With my Canon Rebel and a good battery it is able to hold the shutter open for at most 2 hours. I tried a photo taken in the traditional way and while it turned out it wasn't as good as I would have liked. Not only does the shutter pose a limitation but the much higher sensitivity and lack of reciprocity failure of the digital sensor means that even at the slowest settings the sensor will fully saturate in less than 2 hours.

    Here is an example of a traditional star trail image taken with the Rebel. It also illustrates the amount of light pollution that is now produced by the town of Williams Lake. Faint crepuscular rays can be seen emanating from town as the skylight passes through the trees.




    For this second photo I tried an entirely different approach. Instead of holding the shutter open I took a 30 second exposure once every 5 minutes or so and then "stacked" the images afterward in the computer. This combines all the images into a single composite image and avoids the issue of sensor saturation. In this photo the high brightness area in the middle is the accumulated light of the Milky Way as seen in the summer.

    Free software for calculating bolt circles and similar: Click Here

  • #2
    Evan,

    This is the first time I've seen a stacked star trail. The technique shows promise.

    My Panasonic LX3 has a Multiple Exposure mode. This records up to three exposures on the same "frame" or file, similar to what we used to be able to do with a film camera. I wonder if other cameras might have this feature, but with an unlimited number of exposures. If so, that would obviate image stacking of separate frames.

    Your continuous star trails are thinner than the diameters of the star "dots" in the stacked image. Perhaps the stacked image is overexposed, with consequent spillover to adjacent pixels. I don't see much image color, which might be another indication of overexposure. But I can see why you might not want to make (say) five-second exposures, and more of them -- that would involve stacking a lot more separate images.

    [edit] oops, overexposure can't be an explanation, since that is only a function of f-stop, which would have been the same in both photos.
    Last edited by aostling; 04-10-2009, 04:50 PM.
    Allan Ostling

    Phoenix, Arizona

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    • #3
      Wow, what timing! Last night I was trying to stack some night time images (136) that I took in Costa Rica. It's my first attempt at this - pictures taken with a really cheap canon camera and chdk. Chdk has been mentioned here before, it has a really nice built in intervalometer. Anyway having some software probs - computer running out of memory. I will post my picture if I can make it work, but don't expect too much!
      Paul L

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      • #4
        I manually stacked that image in Paint Shop Pro since the stacking software didn't want to do what I wanted. The exposure and saturation have been jacked around by the stacking process since my primary concern was to equalize the apparent brightness of each image regardless of when it was added to the stack. As with so many of these types of things I should write my own stacking program for stacking images you don't need or want to align in the customary manner.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Ok, here is what I have so far.

          Click for full size - Uploaded with plasq's Skitch

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          • #6
            Hi Evan,

            This was not stacked, rather, I shot this with conventional film (E 100 vs) using a Sinar F2 (4 X 5 inch format) and a NIKKOR 210mm f5.6 lens. Exposure time was nearing 1 hour.

            Harold

            For those having fought for it, Freedom has a flavor the protected will never know.
            Freedom is only one generation away from extinction.

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            • #7
              Nice photos guys. Film is still the best way to capture some things but it will soon be nearly impossible to have it developed. One of these evenings I am going to try a trail sequence using very short and frequent exposures.

              BTW, if you do a trail image with it slightly defocused you can really get a nice rendition of the different colors of the stars.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Originally posted by Evan
                Film is still the best way to capture some things but it will soon be nearly impossible to have it developed.
                That's one reason I have my own "wet lab". Being able to process films and print B+W or color takes away some of the fear when shooting film. That which I fear is being unable to purchase film for my SF, MF, and LF cameras. Another fear is continuation of photographic chemicals. Perhaps someday they will make affordable digital backs for my 4x5 and 8x10 Sinars and Shen Hao.

                Harold
                For those having fought for it, Freedom has a flavor the protected will never know.
                Freedom is only one generation away from extinction.

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                • #9
                  Nice shots.. And it shows the northern position you are in, tight radius. JR
                  My old yahoo group. Bridgeport Mill Group

                  https://groups.yahoo.com/neo/groups/...port_mill/info

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