Re: News - Picture of Shuttle and ISS Silhouetted against Sun
- From: "William C. Keel" <keel@xxxxxxxxxxxxxxxxxx>
- Date: 2 Oct 2006 08:13:43 -0500
OM <om@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, 2 Oct 2006 04:22:12 GMT, henry@xxxxxxxxxxxxx (Henry Spencer)
wrote:
Yes, even for light-gathering power, despite the smaller aperture, because the CCD needs a lot fewer photons
to form an image than the films the pros were using back then.
...On the other hand, there's two things about CCD that still makes
some astronomers prefer film:
1) The longer you expose a CCD in extreme low light, you get more
noise than you would with film. In addition, colors towards the red
end of the spectrum tend to get overly exaggerated as the exposure
progresses.
2) Cold-packing a CCD does absolutely nothing to improve the light
sensitivity, but freezing that film and then keeping the camera body
cold with an ice bag does improve the emulsion sensitivity by a
significant amount.
Wait a minute - there is a reason that amateur astronomical CCDs systems
usually come with thermoelectric cooling systems, and professionals
are in the habit of using CCDs in vacuum dewars cooled by thermal
contact with liquid nitrogen. Doesn't increase the sensitivity but
decreases thermal noise ("dark current"), which has nearly the
same effect for long exposures. By now, I'm very hard pressed to
think of any astronomical imaging which film does better than the
big CCD systems that, for example, SBIG will gladly sell you. (It
was a major deal when Kodak mass-shipped CCDs as large as a 35mm film frame).
And lots of emulsions were (and remain) notorious for oddball color
shifts during long exposures, as reciprocity failure not only
sets in but differs between emulsion layers.
Bill Keel
.
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