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Old Jun 20, 2005, 1:08 AM   #1
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I'm currently using a dslr, and I love taking night shots few lights.* Then I heard from somewhere that that is bad for the CCD.* Is this true?* I leave the shutter open for about 15 to 20 seconds each time.
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Old Jun 20, 2005, 3:13 AM   #2
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I'm not sure. Hopefully someone can shed some light on the subject, as I sometimes at night like to set my Canon S1 IS to F/2.8, ISO400, and 15 seconds, to get night exposures that look like they're taken in the daytime (and even then they're often underexposed. And, there's been a couple times where I had some distant light sources in the picture - those areas completely blew out. If I was able to set a much longer shutter speed and stop down the aperture, would I be able to properly expose the sky and normal ground stuff (to make it look like it was high noon) without blowing out the little (due to the distance or size) lights that might find their way into the frame?
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Old Jun 20, 2005, 3:46 AM   #3
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Unless you have the sun in the picture, you should not be harming the sensor. The only negative aspect is potential heat buildup, which may, hypothetically, shorten the sensor life. I suspect that the difference would be less than the normal statistical variations, though.

Pianoplayer;

In order to do what you are attempting, you would have to take two exposures; one to get the dark detail, and another to get the lights properly exposed, and blend them in editing software. The dynamic range of the sensor is not wide enough to get both.

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Old Jun 20, 2005, 2:06 PM   #4
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Shouldn't cause a problem at all as far as I can see. The light won't damage it, as there'd be less light hitting the sensor than when the shutter is open for a 10th of a second on a bright day.

Don't forget with non-slr digital cameras there's light hitting the sensor the whole time they are switched on and in photo taking mode, and they cope fine.
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Old Jun 20, 2005, 2:28 PM   #5
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People doing Astrophotography with DSLR's have the shutter open repeatedly (5, 10 or more times) for 2-10 or more minutes at a time, until they have enough stackable exposures to produce the image they want. During that time they also take a couple of dark frames to be able to subtract the noise from the real exposrues.

Does not seem to hurt the DSLR's :-). Having an external power supply is good for this, and the winter time would help keep the camera cool and reduce noise.

Peter.
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Old Jun 20, 2005, 5:58 PM   #6
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So how do I do "stacked" exposures with my Canon S1 IS? I'd want to stop down the aperture to maybe F/8.0. I am limited to a shutter speed of 15 seconds per exposure, and can go up to ISO 400. Would I get less noise by using the equivalent of 120 seconds at ISO 50 versus using 15 seconds at ISO 400?


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Old Jun 20, 2005, 6:57 PM   #7
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pianoplayer, you take a bunch of exposures of the same object. then you use a stacking software like registax (free) or astrostack. these programs let you input and allign multiple frames which gives you more details than just one frame. most use these softwares to input .avi files from video cameras and i don't know if they will allow stacking of frames greater than 640X480. i know they help choose (sometimes automatically) the best images of a set of frames. as far as still frames bigger than vga, i don't know if there are any software packages that will do that. maybe you have stack them in photoshop by using the layer merge option?? not sure. but, for sure, you will have to manually allign the different frames.

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Old Jun 20, 2005, 10:07 PM   #8
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Yes, there are programs that will do the stacking and automatic dark frame subrtraction for DSLR JPG's not sure about RAW files. I have not seen/tried any free ones, it can also be manually done with PS CS and layers but that is major tedious.

Stacking is not usable for teresterial shooting.
The images all need to be exactly aligned and no motion in them, hence the polar alignable tracking mounts like the the very expensive Losmandy, or AstroPhysics, and not so expensive Vixen Super Polaris or Atlas and the relatively inexpensive (Chinese Vixen clones = CG5, EQ5) or (Atlas clone= HEQ6). I paid over 1000$cdn for my cheap Chinese clone mount with dual axis drives and no computer control, that is another 400$ option. Then of course you need the actual telescope that sits on the mount, and an pigiback guide scope with an autogider on it.

Or you can go the Meade route with a fork mount, still over 2500$ by the time you have a relatively small 8 inch working scope setup.


Here are some very good Astro images done with Canon DSLR's by some people in Denmark I believe. http://www.schweifstern.de/astroimages.htm
I have not managed to do any good ones yet, it is very difficult to get reasonable results.

Peter.
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Old Jun 21, 2005, 3:06 AM   #9
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Most of my super-long-exposure shots would be of objects that will stay still for a VERY long time, and the camera will be mounted on a tripod or other stable surface, with the self timer and intervalometer in use.

With long exposures (assuming I'm targeting the same exposure in both of the following cases), would ISO 50 at 8 minutes be cleaner than ISO 400 at 1 minute, or would the longer exposure time used at ISO 50 balance the noise out, or is long exposure noise worse than high ISO noise when the combined exposure times for the multiple frames are sufficient?

That said, with 15 seconds, ISO 50, and F/8.0 (to get as much of a depth of field as possible), on a moonlit or starlit night I cannot usually see ANYTHING that isn't near a light source unless it IS a light source. Would this having very dark frames be a cause for concern or would sufficient exposures make a nice daylight-like picture of a dark object?

Also, how do you do the layer stacking in Photoshop 5.5? (for this one, we'll assume that since the camera is securely stabled and the object is unmoving, the subject will be in EXACTLY (to the PIXEL) the SAME place in every frame. Would you open a new blank photoshop image with the same pixel dimensions as your shots, then open your shots, paste them as layers into your .PSD, change the opacity on the layers, combine them, and / or what?
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Old Jun 21, 2005, 8:27 AM   #10
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Just where did You ever hear that information? I looked in My Nikon manual and they do not say anything about it. Every Nikon Digital camera I have ever owned all had a BULB setting to record hours if needed. I think someone was pulling your leg on that one.
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