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Why a Digicam Can't Take Long Exposures
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The reason a normal digital camera won�t work for astrophotography (other than bright objects like the moon or sun) is as follows: all CCD chips produce �dark current� - after a short period of time the CCD starts to create �spots� of light in the image because the CCD is at room temperature, at which internal heat-generated activity in the CCD causes these spots to occur.  You don�t see these spots when you use a typical digital camera under normal (terrestrial) use for two reasons: the exposure time for normal pictures is very short (fractions of a second), and there is usually a lot of light entering the camera which overwhelms these small spots.  But astrophotography requires longer exposures than normal photography and the dark current in the CCD chip of digicams does mess up your image.  Astronomical CCD cameras include a cooling mechanism coupled to the CCD, that dramatically reduces the dark current (see the Thermoelectric Cooling page).  The lack of such a cooling system is really the primary barrier to using a digicam for deep-sky objects, or for long-exposure terrestrial photography as well.  (Of course, the cooling system adds cost so astronomy CCD cameras are more expensive than digicams.)

You can see this dark current for yourself - if your digicam allows time exposures then take a few with the lens cap on. You will find that with exposures longer than a half-second or so, your pictures will show a false star field that couldn�t really be there since the lens cap was on. You can still use your digicam for images of the moon or sun since they are bright enough to require short exposures. (Technically, the moon is a sun-lit object that requires - more or less - the same exposure that any daylit terrestrial object needs.)

Note that it�s no accident that the images Scopetronix posts on their site, taken with digicams, are primarily of the moon. Digicams will work for the moon or sun, but not for deep sky objects.

The manufacturers of professional digicams (e.g. Canon, Nikon, and Minolta) are working hard on this problem, presumably so that professional photographers can use their high-end digicams for night photography. I do expect that at some point the technology will evolve and affordable digicams will be able to take somewhat long exposures.  But for now, they can�t.  In the meantime, some amateur astronomers have taken astrophotos of bright nebulae with consumer digicams by coupling them to very large-aperture (and thus very expensive) scopes. But they�ve made a very different price investment than you and I.

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