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Forum: Photographic Technique
11-11-2009, 03:32 PM
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Thanks. =) I always wanted to be an astronomer, but ended up doing aerospace research instead. It's still a good hobby, though.
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Forum: Photographic Technique
11-09-2009, 02:46 PM
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Just bumping this up to see if the OP ever got his star trail problems resolved. I'd love to see the final result.
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Forum: Photographic Technique
10-28-2009, 09:20 PM
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Now that I've investigated it more closely, I'm convinced it's not a case of changing zoom. It was definitely shifted down partway through the exposure quickly, all at once, and by about 20 degrees. How do I know this?
I took the liberty of identifying a couple recognizable constellations: the Big Dipper and Little Dipper, and showing them in the attachments. The first picture has a black line that "connects the dots" and forms the Little Dipper, with the end of the handle at Polaris. The connected points are the ends of each star's respective trail. Notice that these are bright stars sharing a common center of curvature...all belonging to the same half of the exposure, which I will call "Exposure A".
The second picture shows the same thing, connecting bright stars in Exposure B to show the Big Dipper. Note that it appears in the same part of the frame and actually overlaps the Little Dipper, which doesn't happen in reality. It also has a different center of curvature, so you know it was a different part of the exposure after the camera moved. At the time the picture was taken (based on EXIF data in the original picture) the Big Dipper would have been below (toward the horizon) Polaris by about 20 degrees. Therefore, that's how far the camera moved on the tripod. I know it was moved suddenly instead of gradually because there are no streaks connecting the two sets of trails.
Besides moving by itself, there is one other possibility... (No, not polar bears again!) Maybe the camera was set to do multiple exposures, and the OP tried to take two separate images but actually took one double exposure pointed at slightly different parts of the sky.
As a bonus, in the Little Dipper picture, I drew a line showing two stars that are 15 degrees apart in the sky. Using that line as a scale for the whole frame, I concluded that the diagonal field of view of the frame is about 59 degrees. This angle of view corresponds to a focal length of about 25mm on an APS-C sensor, which validates the OP's description of his shooting setup. Exposure B, where the Big Dipper is in the middle of the frame, appears to be at the same zoom setting as Exposure A. Notice the the white and blue stars at the end of the bowl of the Big Dipper (the pointer stars, Dubhe and Merak, for you astronomy buffs). They appear twice in the image: once where I have marked them in green for Exposure B, and they also appear the same distance apart at the bottom right of the frame as part of Exposure A. I know those stars at the bottom are the same because they clearly belong to Exposure A, as does Polaris, and they're in the right place relative to the Little Dipper. Since they're the same distance apart in both halves of the exposure, you can safely say the zoom ring didn't move.
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Forum: Photographic Technique
10-28-2009, 08:36 PM
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Not true. If the center of rotation (Polaris) isn't in the center of the frame, it will move to a different part of the frame when the zoom changes, or maybe even out of the frame entirely. If it's near the top when zoomed out, it could disappear from the frame altogether when zoomed in. And then you'd end up with two sets of trails, each with a different center point. That's pretty much exactly what it looks like is going on in this picture
Notice that three critical points—the center of the frame and the apparent center of rotation of each of the two sets of star trails—lie in a single line. That's just what would happen if you changed the lens focal length without changing the position of the camera. If that change in zoom happens rapidly enough, there would be not connection between stars (the radial ramps you're talking about).
The lack of a double image in the roof just means it only showed up in the wider half of the exposure, and the longer-focal-length portion didn't have the roof in the frame.
So the question is: what woudl have changed the zoom setting of the lens? Sudden, and quick zoom creep seems unlikely. Maybe someone did it accidentally. Polar bear?
If I get bored, I can try to figure out what the angle of view (and thus the focal length) for each of the two apparent zoom settings is. I don't know what that would tell us, but it would be interesting.
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