long exposure for astronomy (or what ever)

Discussion in 'Digital SLR' started by Dimitri, Jan 16, 2005.

  1. Dimitri

    Dimitri Guest

    went out last night with the astronomers, and i thought i can make
    pictures...

    so you can set the d70 to 30 sec. that's it? what if i want to make an
    1 hour exposure? I found out that when i set the d70 to bulb, i can
    start it with the remote and turn it off with the remote.

    but it stays only open for about 30min. and then it takes another 30
    min before i can touch it again.

    with any film slr you could do that easy, is this where we meet the
    limit?
    i also noticed, that this is very hard on the batteries. normally i
    don't have to charge them for 1000's of images, and with long exposure
    i can make 2-3 images and the battery is flat.

    cheers
     
    Dimitri, Jan 16, 2005
    #1
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  2. Dimitri

    mc Guest

    I'm not sure why it stops at 30 minutes...

    That second 30 minutes is a dark frame subtraction (Mode 2). Go to Mode 1
    instead. Or, better yet, simply power the camera off after the shutter
    closes (right at the end of the first 30 minutes). At that point, the
    camera has saved a truly raw image, with no despeckling or dark frame
    subtraction, which will be surprisingly sharp. Then turn the camera on and
    take the next one. Take *one* dark frame this way, manually, and subtract
    it using software.

    Disclaimer: This is what I've been told about the D70. I have not yet used
    one.
    To make a 1-hour exposure, you should take 10 6-minute exposures and add
    them. This results in far less noise than a single 1-hour exposure. For
    some techniques to do the adding with Photoshop, see
    www.covingtoninnovations.com/dslr.
    You can use software like Images Plus, which is specifically for astronomy,
    to do it more conveniently.
    You should be able to go a couple of hours with one battery charge. I
    always bring a spare battery with me. I don't know about Nikon, but Canon
    also makes an external AC power supply, and people are working out ways to
    run it from a 12-volt storage battery.

    I hope to have my hands on a D70 in the next month or two, and then I'll
    greatly expand my coverage of it on my web site.

    --
    Clear skies,

    Michael A. Covington
    Author, Astrophotography for the Amateur
    www.covingtoninnovations.com/astromenu.html
     
    mc, Jan 16, 2005
    #2
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  3. Dimitri

    Ed Ruf Guest

    On 16 Jan 2005 04:30:56 -0800, in rec.photo.digital.slr-systems

    Sounds like you had noise reduction enabled, which takes another shot using
    the same exposure values without the shutter being opened.
     
    Ed Ruf, Jan 16, 2005
    #3
  4. Dimitri

    Roger Guest

    You might want to look into a program such as "stacker which lets you
    "stack" shorter exposure images on top of each other. It also gives
    much better resolution than a single long exposure.

    Dark frame subtraction or noise reduction (NR) mode works well, but
    virtually all digital cameras use it for exposures longer than a few
    seconds. You will gain a lot of noise beyond ASA 800 for long
    exposures although I have done 10 minute ones with and without NR that
    came out well. However at ASA 1600 the noise was quite plain when the
    image was blown up to full screen size. NR took most of that out, but
    I'd stick with about ASA 400 or even 200 for the long stuff and shoot
    a bunch of 5 minute exposures.

    I use the remote to trigger the camera and it works quite well
    reflecting off the back of the 10" telescope.
    In the D-70 I can shoot like that all night without the batter giving
    out. OTOH if I try long exposures with my old E-20N the 2,000 mah AAs
    go flat in an hour or so.

    Roger Halstead (K8RI & ARRL life member)
    (N833R, S# CD-2 Worlds oldest Debonair)
    www.rogerhalstead.com
     
    Roger, Jan 17, 2005
    #4
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