The CCD I am most familiar with is an ST-4 by Santa Barbara Instrument Group
(SBIG). With the ST-4, you establish your operating parameters, take a dark
frame, then do the image. The imaging software automatically subtracts the
dark frame from the image. As long as none of the operating parameters are
changed, then the one dark frame will work for all subsequent images.
However, I've read that one should always take a new dark frame immediately
prior to taking the image for critical work. This tells me that the noise
characteristics are not identical every time. I suspect this is the case.
Variables like the cooling temperature, voltage fluctuations and stuff I
just don't understand can change.
In Sabastien's case, I'll bet the symptoms displayed in very long exposures
are not evident in "normal" ones. So yes, a similar dark frame exposure
time for longer exposures would probably be appropriate. But if it is truly
the case that long exposures are the only time it shows, at least there is
an opportunity for a work around. The fact remains in his opinion, and I'm
inclined to agree with him, such an adjustment is absurd for a fairly high
end consumer digital camera. My humble old Oly D300L doesn't do that kinda
foolishness....... <--- obligitory Oly content <g>
John P
______________________________________
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far"
Garth Wood <garth@xxxxxxxxxxxxxxx> wrote:
>
>Interesting. From what you mentioned above, John, I'm assuming that the
noise characteristics of the specific CCD field are always the same? Sure
makes it easy to manipulate, if that's the case. I also assume that
Sebastien would have to make "dark field" exposures that are always the same
elapsed time as the actual subject exposure, in order to do this subtraction
properly...
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