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    sk66's Avatar
    sk66 is offline Lovable Contrarian
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    Default Astrophotography, how do you get the colors?

    With the 800mm, 2x tc, and 1.5 crop I've got an effective focal length of 2400mm....I'm thinking I should be able to get some cool shots....but if I take a picture of Jupiter and her moons, or Saturn, they still just look like white dots. I tried reducing the exposure, but then they just look like underexposed white dots.

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    What kind of shutter speeds are you getting? If they're long enough you could be getting motion blur
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    sk66's Avatar
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    I was shooting pretty fast....I think around 1/500...
    It's hard for me to focus in live view, put into manual focus and switch to mirror lockup without loosing the composition.

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    Doug Sundseth's Avatar
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    Well, back when cameras still used film I was a university astrophysics student. We had a 16" (aperture) telescope, probably about a 5m lens. With that I got a fairly nice photo of Jupiter and the Galilean moons, but hardly any detail, even though we had a mount compensating for the planet's rotation. No color either, but we were shooting in B&W, which I'm told affects that.

    Note that this was also from about 7300' in an arid climate in winter (from which you might be able to identify the university), so possibly better atmospheric conditions than you're dealing with in Pa.

    Planets are hard.

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    ravncat is offline Friendly Astrophysicist
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    Quote Originally Posted by sk66 View Post
    With the 800mm, 2x tc, and 1.5 crop I've got an effective focal length of 2400mm....I'm thinking I should be able to get some cool shots....but if I take a picture of Jupiter and her moons, or Saturn, they still just look like white dots. I tried reducing the exposure, but then they just look like underexposed white dots.
    well jupiter subtends an angle of about 30 - 50 arc seconds... (depending on where it is in it's orbit relative to earth) and you should have a field of view that covers about 1 degree- there's 360 arc seconds in a degree... so , Jupiter should be about 1/12th of the size of your frame, which is pretty large. Since the earth rotates through 15 degrees in an hour, (At 2400 you can expect stellar motion from more than 1/4th of a second - You'll want to be slightly faster than that for planets...

    You should be looking at iso 100 at f16 at 1/15th of a second for a good exposure of Jupiter. (I don't know what the aperture of your lens is - but assuming your 800mm is at f8 , and then loses 2 stops to the 2x TC. That means you're shooting wide open... (I recommend this Free Astrophotography Software for a good calculator for exposure)

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    sk66's Avatar
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    I don't think it's the "exposure" that's the problem. I can shoot fast enough to prevent motion blur (but camera shake is a bitch!) and I can over/under expose.....but I just can't get any detail/color regardless of the exposure.

    I know when viewing the moon thru a telescope there's a filter you should/can use....am I missing a filter or something? And what about those nebulas? I can't even find them.

    I guess maybe even a small refractive telescope that can see those things is still more lens than 2400mm?

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    ravncat is offline Friendly Astrophysicist
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    Quote Originally Posted by sk66 View Post
    I don't think it's the "exposure" that's the problem. I can shoot fast enough to prevent motion blur (but camera shake is a bitch!) and I can over/under expose.....but I just can't get any detail/color regardless of the exposure.

    I know when viewing the moon thru a telescope there's a filter you should/can use....am I missing a filter or something? And what about those nebulas? I can't even find them.

    I guess maybe even a small refractive telescope that can see those things is still more lens than 2400mm?
    It really does become about light gathering with telescopes over "magnification power" You need to be able to collect as much of the faint red light as you can to get faint red color. I can only imagine that the 2x tc isn't gonna do you any favors optically. Still, the difference is that the camera is able to collect light over time. You're gonna want to minimize your abberations and freeze motion if you can. You'll lose color depth by upping your ISO. The other giant thing, is of course the quality of your atmosphere, which changes quite a bit.

    For planetary, alot of people go with a webcam on a telescope so they can use stacking and take a bunch of images to find the sharpest ones, so they can deal with the atmospheric disturbances.

    Practical Formulae for the astrophotographer may have more information - I suggest stacking images - How to Process Planetary Images - Astrophotography - SkyandTelescope.com has a good quick guide. One day, I will live somewhere where it will be dark and I can buy a telescope so that I can use prime focus (telescope to camera directly) with a nice large dobsonian with a tracking mount....
    Last edited by ravncat; 10-25-2011 at 11:26 PM.

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