Star Arcs

Discussion in 'Beyond the Basics' started by rmh159, Mar 7, 2007.

  1. rmh159

    rmh159 TPF Noob!

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    I thought I read a formula once for figuring out how long of an exposure you would need to see a certain degree of arcing in stars when taking nite shots. Does anyone know this? I want to say 10 minutes = 15 degrees.

    I'm interested in getting a long exposure shot on a clear sky and I really want to nail this affect. There's a lake nearby that I want to use and hopefully if it's a calm nite I'll get a nice reflection off of the lake as well.

    If anyone has any other advice for this type of shot please feel free.
     
  2. Patrice

    Patrice No longer a newbie, moving up!

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    The earth rotates 15 degrees in one hour. So, six hour exposure would show any particular star tracing a 90 degree arc. This is very noticeable when the camera is aimed at Polaris.
     
  3. rmh159

    rmh159 TPF Noob!

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    Damn... my math was a bit off and not for the better.

    Also along this topic... what type of conditions would you need for that long of an exposure? No moon, no wind? Assuming the conditions are the same, aside from the arcing would you notice much difference between a 1 hour vs a 6 hour exposure?

    I'd like to get out an just experiment with this but when each exposure takes a few hours, going into it the shoot with as much knowledge as possible could save a ton of time and effort.

    Thanks
     
  4. RacePhoto

    RacePhoto TPF Noob!

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    Will this help or make things worse? :lol:

    360 degrees, one revolution divided by 24 hours (which is 1440 minutes)

    360/24 = 15 degrees Each minute is .25 degrees

    Yes you will notice a difference because film becomes less sensitive as the time becomes longer. Reciprocity is the word you want to look up. That's why some astro photographs have cooled film. For us, it shouldn't matter that much. Just figure that the second hour the film responds less to light than it did in the first hour. I don't know how much less.

    You can use a ND "neutral density" filter and shoot at f/22 for longer exposures that don't get too much light too fast for your desired time. Slower film also helps a little, but when you are talking an hour or two, the difference between 64 ASA film and 400 film isn't much. (or is that DIN or ISO?) You get the idea. But remember slower film has finer grain.

    A rock solid tripod is good. You can put a bean bag, across the cross support of the tripod for better stability. Hang a weight from the center. A rock will do just fine. You want that tripod solid.

    Some will use mirror lock up. I don't find it much of a problem when the picture takes 5 minutes to an hour and the mirror flaps just once at the start.

    Since I don't know what you are taking a picture of, I can't answer the Moon question. You can use the Moon for the light, or possibly want to avoid it. Also the sky near any city will have flare and light pollution which will produce an orange glow.

    Unlike years ago, you will now get satellite streaks along with the star arcs. I plan on capturing some of those, intentionally this Summer. But my older time exposures, don't have them. New ones of the same subjects will.

    Warm weather you will get more waves in the atmosphere. Higher up, you will have less atmosphere, say on a mountain for example. OK we can't go up on a mountain, but telescopes can. :lol:

    You are correct to think about wind. If there's anything that can move, even the camera, it will become less sharp. After the contest is over I can post an example of "long 2am exposure" and explain some more. It took me many tries to get the color to look natural, and it still looks strange.

    From my personal experience, the longer the exposure, the stranger the colors get. If there's any light from a man made source it can make for some interesting rays of colors. Expect color shifts.

    I've never tried time exposures with digital. (yet?) Film seems so much easier and doesn't take special electrical connections to keep the sensor activated. Plus you don't have to have an expensive camera, shutter release attachments and equipment, to do film time exposures.

    Just remember to tell them when you take the film in, that you want all the prints, they aren't mistakes. Sometimes they will just get rejected and pulled as "bad photos" because they are mostly black. On the bonus side, I've had some places mark them "free" because the pictures didn't turn out like the rest of the roll of film. They only charged for the "good ones". That was an red aurora in the Summer, that looked like red streaks and blobs in the dark.


    Besides your camera:
    Cable release
    Tripod

    optional:
    ND filter
    cooking timer to measure exposures, and be a reminder
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    The most important thing not mentioned is light pollution. In Brisbane 4km from the Central Business District I can take maximum about 90minute exposure before the sky starts going reasonably light orange. 12km from the CBD 5 years ago, I took a 3 hours exposure with my film camera and the sky started going ocean blue. Conversly in Canberra a friend of mine took a 5 hour exposure of the sky and it was still almost dark black thanks to the almost complete lack of light pollution (no lights in Canberra were allowed to point to the sky because of the observatory)
     

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