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Old May 31, 2004, 2:04 PM   #1
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Does anyone know what kind of digicam I can use to take pictures of the planets, and stars? I heard that the camera has to have the ability to have It's shutter open as long as it takes to capture a picture.



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Old May 31, 2004, 4:04 PM   #2
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Most digicams only can do 8 second or so exposures. Even top of the line DSLR's seem to be limited to around 30 seconds.

None of those time spans seems long enough for certain types of night photography. Is this an inherant shortcoming of digital cameras relative to film?

Also what exactly is technically going on inside a digicam while it's capturing the long exposure image both during and after? On my Panasonic FZ10, an 8 second exposure causes the camera to go busy for a quite a while once the shutter has closed. What exactly is it doing that's so compute intensive?
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Old May 31, 2004, 7:40 PM   #3
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Quote:
On my Panasonic FZ10, an 8 second exposure causes the camera to go busy for a quite a while once the shutter has closed. What exactly is it doing that's so compute intensive?
It's a form of noise reduction called 'dark frame' subtraction. The camera makes a second exposure with the shutter closed equal in length the the one that captured the scene. The dark frame image is captured with the shutter closed and will consist (mostly) of the hot pixels--a form of fixed pattern noise--which the camera processor then subtracts from the image of the scene. The idea being that the hot pixels registered in the scene will show up in equal numner and position in the dark frame. When the hot pixels are removed, the resultant image will be only of the scene and 'random' noise.
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Old May 31, 2004, 10:39 PM   #4
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This style is called astrophotography. If it interests you, it may be helpful to plug that into a search engine.
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Old Jun 1, 2004, 10:35 AM   #5
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Thanks I'll do that.
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Old Jun 1, 2004, 6:46 PM   #6
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I'm not sure about which digicams to look for in particular, but there's a nice forum over at http://www.cloudynights.com/ubbthrea...eads.php?Cat=0 where you can check up and learn a great deal about astrophotography. Most of the people there use stuff from Nikon, with shutter delays of 1-5 minutes.

I use a simple Canon A70 which offers 15 seconds of open shutter time which is more than enough to capture planets or any other bright objects (of magnitude < 4: you'll learn this if you get into astrophotography). If you want to get into nebula and deep space objects however, you're going to need a minimum of 30sec – 1 minute shutter delay. There's a lot of other variables as well, but this is something to aim for in the camera. There's also a neat technique you can apply later which superimposes multiple (dark) 1 minute exposures to generate a 5 minute (bright) exposure. So you can "cheat" within reason.
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Old Jun 1, 2004, 8:23 PM   #7
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Thanks for all your input!

I'll check out that cloudynights.com forum.




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Old Jun 2, 2004, 4:43 AM   #8
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I don't know if this is common knowledge but Venus is due to cross the sun on Tuesday 8th Juneaccording to the news this morning. I think they called it the Venus Transit, don't know if it's possible to photograph it though. :-)
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Old Jun 2, 2004, 8:04 AM   #9
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All you need is a good telescope and any old webcam. That is what The Sky At Night has led me to believe. They had a competition a while back where viewers were asked to take pictures of space and lots of people did it that way. There was one guy who had spent thousands on a telescope in his back garden and all he used to take pictures was a £20 webcam.
I've looked at their website but can't find information of that competition but they do have a gallery.
http://www.bbc.co.uk/science/space/s...de/skyatnight/
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Old Jun 2, 2004, 1:38 PM   #10
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Most dslr's are limited to 30 sec normally but several have a feature where if you turn the dial one more notch is 'bulb' mode. It will hold the shutter open as long as you hold the button down. A cable release and a camera with mirror lockup are bet for this. Some cameras even have a remote where you don't need to hold the button down; you press once to start and again to stop. Also, most dslr's now do the dark frame subtraction simultaneously with the exposure so you don't have to wait.

I would suggest a dslr because the large sensor will perform much better and capture more light. The photoreceptors of each pixel are bigger and therefore more sensitive to light, the same way that a 14" telescope has more light-gathering power than a 6". Also you should get a lens with the largest possible maximum aperture and the largest possible front element - these will increase your light-gathering power. I also suggest a heavy duty tripod, not a wimpy plastic one.
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