Two questions on astro photography

Discussion in 'Beyond the Basics' started by battletone, Nov 16, 2009.

  1. battletone

    battletone TPF Noob!

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    Its supposed to be a clear night tonight, and I don't think the moon is out. I am excited....two nights in a row.

    So I am going to go take a few pics here in a bit, but I am too anxious to google this (I will try if time permits), plus I have some other stuff I need to try to get done soon.

    What is a good exposure guideline for stacking multiple images? (high iso? low iso?, wide open?) I plan on shooting at 18mm. I have read about it in the past as being a great way to even use webcams to get much more light than is possible without one of those tracking machines.

    Also, is this cloudy area across the middle (which is the second time I have captured it) the Milky Way? This is what I would like to do the stacking process on.

    [​IMG]
     
  2. battletone

    battletone TPF Noob!

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    Would this be effective in taking multiple exposures of 20 seconds each? I have no tracking device.
     
  3. ecnal

    ecnal TPF Noob!

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    I have no tips for this type of photography, but yes, that is the milky way.

    Back in high school we used to watch the sky at night on the beach, and after about twenty minutes your eyes adjust and you can see it, sometimes pretty well.
     
  4. chyidean

    chyidean TPF Noob!

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    Ugh, I wish we could see the stars as well where I live! Here in Southern California, we'd be lucky if we could see at least thirty points of light in the sky.
     
  5. astrostu

    astrostu I shoot for the stars

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    Well, I wasn't going to respond, but since that quote is from my website ... I guess I should. :p

    Of course it's a great way to do it! It's on my website!!

    Okay, but seriously, I think with an 18 mm lens you could go for 30 seconds without seeing trails (do a test). If you want to have the sharpest possible photograph, then you're somewhat limited to around an f/8 aperture. I think on your first try, though, you can get by with something smaller, perhaps 1 stop smaller than your widest aperture (3 f/settings smaller as each is 1/3 of a stop).

    Then, use the lowest ISO you can while still getting as many stars as you want. With a wider aperture, you'll get more faint stars. With a smaller aperture, you'll get less. Compensate by boosting the ISO.

    If you do end up stacking, then the background noise of the sky (not the stars) will decrease because the little specs of color noise (from the higher ISO ... or just regular camera noise (and I'm not being very specific here because I don't want to get too technical)) will average out because they won't be the same in every photograph.

    I wrote that guide more for deep-sky objects where your object isn't moving, so you can't actually average the images together for star trails (if you do, the trails will be very dark). What I would suggest is actually doing a 2-step processing, where you average all your photos together and use that as the "background" sky. Then use an equivalent MAX() function on the same photos and what will come through is just the stars. Layer that on top of the background one and you'll come out with something reasonable.

    I realize that I may not have explained that all very well, but to go into more detail would require many many paragraphs. Perhaps another forum member who's done this can post it more concisely.
     
  6. battletone

    battletone TPF Noob!

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    Thanks. Is the Milky Way considered deep space ant not really moving?

    The image I posted was:

    Camera Make: Canon
    Camera Model: Canon EOS DIGITAL REBEL XSi
    Image Date: 2009:11:15 22:44:25
    Flash Used: No
    Focal Length: 18.0mm
    CCD Width: 5.35mm
    Exposure Time: 30.000 s
    Aperture: f/3.5
    ISO equiv: 1600
    White Balance: Auto
    Metering Mode: Spot
    Exposure: Manual
    Exposure Mode: Manual


    I realize 1600 might be considered too noisy, but I really like how many faint stars I am able to get. I think anything lower than 800 and the cloudy MilkyWay doesn't really show up well. But maybe stacking will fix that.

    I will get a handful of shots from different apertures and iso settings.
     
  7. Garbz

    Garbz No longer a newbie, moving up!

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    Yes definitely. You do not want to shoot at High-ISOs. Stack multiple ISO100-200 shots together to get much better results.

    I am confused as to the topic of deep space objects. The milky way will move in relation to the ground, so if you have something in the foreground then you'd need to layer two images on top of each other as said above, however for just a shot of the sky a lot of stacking software will do automatic alignment of images to compensate for the fact that the stars move as the earth turns.
     
  8. astrostu

    astrostu I shoot for the stars

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    Oh ... Garbz makes a good point (and I was hoping he'd post here). I was thinking that the original poster wanted to do star trails. That's what most people want to do when photographing stars, so I kinda went into auto-pilot there.

    If you just want effectively a long exposure of the sky but don't want to see the trails, then you either need that correcting mount, or you have to stack. Automated software that will also rotate the images will be required because the sky does rotate relative to Earth.

    And you can also ignore what I was saying about doing the two-part stacking, one for sky and one for trails, if you're just looking for the stars to look like stars and not trails.

    I left it out but now it makes sense to say it -- the Milky Way, in my opinion, looks pretty bad when shown in a star trail ... it just comes out as this blurry streak and not as a recognizable, sharp structure.
     
  9. MBasile

    MBasile TPF Noob!

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    And those are just the planes you can see through the smog!
     
  10. battletone

    battletone TPF Noob!

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    Well I couldn't bring myself to shoot anything lower than 800. Its a hard habit to break I supposed, but everything looks so dark at 800 with any aperture smaller than f3.5....but I think I averaged about 8 exposures at 30 seconds each with various apertures going from f3.5, f5.6, f6.7, and f8.

    Yes, no star trails. I have been enjoying doing those with 20 minute exposures lately, and did one last night at the end. (didn't stay up for the meteors, I know...:thumbdown:)
    My goal is two fold, either get the maximum amount of stars in my image, or get the Milky Way to show up as best as I can...or both.
    I run CS4, so I was planning on using the auto align for all my images, after that I am lost. I would assume it might be one of the blending options that I will be using, but I can't think of one that will really do what sounds like is needed.

    To both of you, if a lower ISO will really make the difference even at those smaller apertures (like 5.6 and 8), if it is clear tonight (like it says it is) and the moon stays away, I will give it another go. Its just really discouraging to see so few stars that make it into the image when using 30 second exposures at those low ISOs. ...and I probably should have taken more than 8-10 images of each, but I didn't know how far to go.
     
    Last edited: Nov 17, 2009
  11. astrostu

    astrostu I shoot for the stars

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    Look up "photoshop stacks." This seems to give you a basic idea.
     
  12. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Everything is moving and the concept of deep space is notional, though space is aptly named. There is lots of space out there.

    Our galaxy is called the Milky Way. It's a flattened disk that rotates once every 250 million years or so out here where our Sun is. It's home to a few hundred billion stars. Those bright lanes you see in the sky (the Milky Way) is the combined light from a few billion of those stars, to far away to be resolved as discrete points of light without some pretty serious equipment.

    It wasn't know until the 1940's (Edwin Hubble) but, The Milky Way is just one of several hundred billion other galaxies (each containing several hundred billion stars) in this universe. There are a couple other galaxies that can be seen with the naked eye because they are close, relatively speaking. The few that are close are known as "The Local Group". Andromeda (seen in Northern hemisphere) and the Large and Small Magellenic Clouds (seen in the Southern hemisphere) are members of The Local Group.

    So, what is deep space. It's not likely that the human race will ever venture beyond our own solar system, which is much larger than most people realize. Our solar sytem extends way beyond the orbit of Pluto.

    So for human purposes anything beyond our own solar system could easily be considered 'deep space'.

    Edit: Astrostu has pointed out that Hubble made his discovery in 1925, no the 1940's.
     
    Last edited: Nov 18, 2009

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