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Long exposure night sky.

Forums Forums Get Technical Farktography tech talk Long exposure night sky.

Viewing 15 posts - 1 through 15 (of 17 total)
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  • #1392
    U-Man
    Participant

    This past week I was out in the sticks away from light pollution and decided to try some long exposure pics. I wanted a bunch of sky so I used a 10-22 mm at its widest and pointed it Polaris. I had no clue about settings so I just put the f-stop in the middle (f11) clicked on the shutter release and left. 1360 seconds later (22+ minutes) I came back.

    This is what I got.

    Magnified version (auto levels in PS)

    What are all of the colored dots? How do I avoid them? What settings have you guys used?

    #18476
    sleeping
    Participant

    What are all of the colored dots?

    Sensor noise

    How do I avoid them? What settings have you guys used?

    Look for a long exposure noise reduction setting for your camera, but note that this generally doubles the exposure time (it involves the camera generating a second exposure with the shutter closed to map the noise).

    Or you can shoot film, which doesn’t have this problem (but you have to deal with reciprocity failure instead…)

    #18477
    Flavivirus
    Participant

    Thats your CMOS sensor showing its weak spots. These dots are actually going to be static – every time you take the exposure, the same dots will be “hot”. One of the ways to negate this is to take a second picture with the lens cap ON for the same length of time, overlay it into photoshop and “subtract” 🙂

    FV

    #18478
    jpatten
    Participant

    What ISO were you at? a lower ISO will have less noise.

    #18479
    Elsinore
    Keymaster

    You shoot with a 20D, right? See if you have dark frame subtraction as an option (might be a custom setting) for noise reduction. That’s what sleeping is referring to (though it can also be done “manually” like Flavivirus mentioned.) It definitely becomes time intensive once you go over 7 or 10 minute exposures and need another 7-10 minutes to do the dark frame subtraction. Definitely keep to lower ISO, but unless you have an a cooled CCD specifically for astrophotography, you’ll probably find that exposures beyond 15-20 minutes on a DSLR are really pushing the bounds of what the camera is capable of in a single exposure. I think the longest I’ve done was about 20-22 minutes, and I doubt I’d bother with anything more than that.

    One thing to consider (though it wouldn’t be allowed for Farktography) is stacking exposures. A lot of the best astrophotographs are actually stacks of exposures because it helps increase the signal-to-noise ratio (literally). You’d probably have to turn off dark frame subtraction for something like that since it renders your camera un-usable for the dark frame exposure and you’d lose the continuity of your trails.

    #18480
    U-Man
    Participant

    You guys are smart. Thanks alot.

    ISO setting? I don’t know what I was at. I’ll have to check the file when I get home. As I said, I was just taking a swing at it to see how it worked. I may try a 22 minute exposure with the cap on to map the noise and then subtract in PS.

    #18481
    jpatten
    Participant

    I have even seen stacking done with Web cams attached to a telescope. The person who did it took literally Hundreds of frames I think before stacking them but it was incredible what he came up with. I cant find the link right now for it, but it should be googleable (is that a word?)

    #18482
    U-Man
    Participant

    I enjoy this stuff. Thanks for responding.

    I haven’t found the time to break out the Canon booklet. But I will. In trying to answer jpatten’s question about ISO setting, I find that I can’t. I don’t know where to look. I have this little Adobe Photoshop Album program that I bought 6 or 7 years ago (and don’t use anymore) that will display some properties like f-stop, shutter speed and camera name. But it doesn’t have all of the settings.

    How can I find that other stuff?

    #18483
    SilverStag
    Participant

    if you put the pic on Flickr, the EXIF data will show up in the ‘more properties’ item on the photo page. Otherwise, google for ‘opanda iExif’, which is a good EXIF viewing proggie.

    #18484
    Killerclaw
    Participant

    I used my FF exif add on and it appears whatever you upload with strips the exif.

    #18485
    orionid
    Participant

    I’ve been playing with this the last couple of nights, and am driving up to the north shore (oahu) where there is less light pollution tonight to try some more. I have noticed that with a Nikon D50, the longer exposures, approaching 20 minutes, tend to drain the battery quickly. I might have to try the stcking idea, that might help save my batteries.

    Also, if the trails are your primary goal, the closer your sky subject is to the equatorial plane, the more star motion you get for a given amount of time. This can let you get more magnificent trails with shorter exposures, which should reduce noise. Also, increasing your focal length will amplify the speed with which the stars will cross your sensor, making more impressive trails, but will also lower the overall amount of light that reaches the sensor, so you will need to open your arperture to compensate.
    As of right now, I’m still doing mostly trial and error, and I’m convinced that it all comes down to “gut feel” based on subject and ambient light levels. I’ll post some of tonights results, whether good, bad, or both for critism and comments.

    #18486
    jpatten
    Participant

    Id like to try some very LONG exposure stuff, but need to get an AC adapter first. I have noise reduction turned off on my camera, and haven’t done more than a couple of minutes of exposure time, usually at ISO 100. But i have Terrible light pollution here so I haven’t even attempted star trails. Some days I feel lucky that a full moon isn’t washed out

    #18487
    orionid
    Participant

    Okay, so after running a couple test shots, and explaning to Honolulu PD (who so rudely woke me up 20 minutes into a 60 minute shot – also ruining the shot with their flashy blue lights) that I was niether a hobo nor making out with somone on the rocks, I got these. All three of these were 18mm focal length, ISO 200, f/5.6, 20 minute exposure plus or minus eye-wiping time, pointed south. Also in all three you can see the milky way (vertical, milk-like trail), and a weather phenomenom indigenous to hawaii known as “mountains pulling random clouds out of their arses”


    In this one, you can see the same hot pixels as in U-man’s first post, though not as intense. Also the rocks have an almost hellish red glow, thanks in part to an incadescent streetlamp at the parking lot outside the cove I was at.


    For this one, I turned on the in-camera long exposure noise reduction, as well as set the white balance for incadescent lighting. Where the hot pixels are now gone, a new side effect has surfaced in the form of broken trails and black dots where noise was removed. Also, I had to sit around for another 20 minutes while the lcd screen on my camera flashed “JOB NR.”


    In this one, I left the WB set for incadescent lighting, but turned off noise reduction. The noise is now in the green and blue range, which I’m assuming is an effect of the WB adjusting the red noise. For the most part, I’m pleased with how this turned out for online or low-res applications, but if I were going to print, I would definately stick to a 400 or 800 ISO setting and stack 2-5 min exposures.

    Also, heres one more that is somewhat showing what I meant by the equatorial plane.

    Unfortunately, this exposure wasn’t long enough to really show the differences in trail length, but ther are some other good lessons here.
    This is pointed upward facing just south of west (I’m in hawaii, the further north you are, the further south of due west your equatorial plane will be. As you can see, the straight streaks tilt at approximately 21 degress off vertical, due to my location at 21 degress lattitude. To the either side you can start to see the star trails curve towards that respective pole (North, to the right, is more intense due to being in the northern hemisphere) You can use this to your advantage when composing your shot in that it is entirely possible to decide in advance whether you want trails that curve up, down, left, right, full circle, or even a combination of those (hello wide-angle) before you set up the tripod and walk away for however long.

    #18488
    soosh
    Participant

    I’ve never gotten that sort of discontinuation in trails from noise reduction software. I have gotten it in one very long exposure, but it was also windy as hell that night and I’ve chalked it up to the camera maybe being jostled on the tripod by the wind.

    On heavily moonlight nights, you’re going to get a lot more light in the sky. When possible, you’ll get the best darkness to the background when the moon is a sliver or less.

    Also consider stopping down a bit, to like f/8 or so. You’ll get less stars overall, but that can be good.

    I always use noise reduction, and rarely shoot at anything above ISO 100. I just don’t like the noise my camera generates at higher ISO values.

    #18489
    orionid
    Participant

    I’ve never gotten that sort of discontinuation in trails from noise reduction software. I have gotten it in one very long exposure, but it was also windy as hell that night and I’ve chalked it up to the camera maybe being jostled on the tripod by the wind.

    On heavily moonlight nights, you’re going to get a lot more light in the sky. When possible, you’ll get the best darkness to the background when the moon is a sliver or less.

    Also consider stopping down a bit, to like f/8 or so. You’ll get less stars overall, but that can be good.

    I always use noise reduction, and rarely shoot at anything above ISO 100. I just don’t like the noise my camera generates at higher ISO values.

    Now that I think about it, the discontinuation is evendent in all three, and is mor prominent on the left side, where clouds were randomly appearing and re-vaporizing. My guess now that I’m awake, is that the stars were temporarily blocked by a cloud for that brief point in time.

    All of these shots were before the moon came up, but yeah, I did have some problems with the streetlamps. There’s not really anyplace out here that doesn’t have streetlamps, except Dole’s pineapple and coffee plantation, and I’m not about to tresspass on a multi-billion dollar fruit company’s land.

    I have tried a couple stops down, the 60 minute one that got ruined was at f/14, and I didn’t feel like trying it again. I’ll be in the mountains of VA in a couple of weeks, so I’m going to try again when I get there. I’m even thinking of doing one at f/22 and leaving it for a few hours to see what happens. I just wish I had thought of this about a month ago when I was on Mauna Kea. That was dark and clear.

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