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  1. #1
    dkordella's Avatar
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    Default can bad seeing cause star trails?



    I'm doing wide field photography on a fixed tripod (which I'm quickly learning is severely limiting what I can do) but Santa Claus and a new mount with GOTO are still a couple months away. I suppose the stars are probably not going away (well, unless betelgeuse goes nova or something)...

    I was shooting last night on my new (to me) 450d with a new ef 50mm f1.8 II lens experimenting with different f-stops keeping exposure length and ISO constant. Then repeating again with a different ISO. I've already learning that shooting at f1.8 gives pretty severe vignetting that I can smooth out with (over) manipulation of curves, so I wanted to (and am following the advice of numerous people here) to stop the lens down once or twice.

    So last night I spend most of the time (about 3 hours) trying to arrive at an "ideal" set of settings for shooting wide field on a fixed tripod. I was trying to image various regions such as orion's belt, heiades and pleiades (not in the same image).

    the declination of orion's belt stars is really low, so a tool I found while searching around indicated I could go as long as 12 seconds before I'd start noticing star trails, so I shot at 10s to err on the conservative side.

    I checked my images once or twice in the camera's LCD and didn't notice any star trails in the first few shots I took, so I began firing away shooting lights, darks, rinse repeat, methodically changing f stop, then ISO, etc.

    When I came back in and started processing, in *every* shot I took the stars were replaced with brilliant little sausages.

    I have taken loads of pictures before up to and exceeding 20 seconds from a fixed tripod (albeit at higher declinations) and gotten every star as a neat and colorful little point.

    Last night's clear sky chart gave the seeing last night a rating of "poor" (but I shot anyway because...well, I don't imagine that needs much explanation in this forum!)

    The star trails were small, it was like looking at bacilli under a microscope (any other biologists here??), but to me really detracts from the image. The bummer to me (as a very new person to the hobby) is that I can see that I definitely captured M42 as well as the flame nebula. Maybe more.

    So back to my question: I had thought that cutting off exposure time at 10 seconds should eliminate the trails, but they did not. Can variable seeing conditions result in trails as an artifact, or is shooting 10s at f2.8 at a low declination the culprit?

    Thanks!

  2. #2
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    Default Re: can bad seeing cause star trails?

    I wouldn't have thought that a 50mm lens would give you bad trails at 12 sec, but any lens at any exposure time will give trails. It's just a question of how bad they are. At the celestial equator, you maximize the length of the trails you will get.

    You can't blame bad seeing. Bad seeing will produce blobby stars, but the motion is random, not linear.

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  3. #3
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    Default Re: can bad seeing cause star trails?

    A randomly selected trail is about 10-11 pixels in length - that's ~5 arcseconds, isn't it? And about 4-5 pixels in width (~2 arcseconds).

    So that's a 2:1 ratio of length to width, so I need to cut my exposure time in half.

    Does this make sense?

    BTW - Keith, I liked your posting about ISO settings. It doesn't apply to me (yet) but a great lesson!

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    Default Re: can bad seeing cause star trails?

    Or just thinking about it differently, at the camera settings I was using, each second of exposure corresponds to 0.5 arcseconds of apparent movement?

    Can I assume from the width of stars that my camera lens's resolution is about 2 arcseconds, so any exposure longer than ~4 seconds will begin to show trailing?

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    Default Re: can bad seeing cause star trails?

    1 second of time corresponds to 15 arcseconds of movement at the equator. Multiply that by the cosine of the declination for higher declinations. Not knowing the pixel scale of your camera, I have no way of knowing how many pixels that would be. You'll have to do that calculation yourself. It varies of course with the choice of lens.

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