The imager has four standard resolution modes: no binning, 2x2 binning, 3x3 binning, and 9x9 binning. It can also be used in Nx1, Nx2, and Nx3 binning modes where N is an integer between 1 and 256. This is very useful when using the Spectrograph.
The SBIG-8E uses a Kodak KAF-1602E CCD. It has a spectral sensitivity range of 360 nm to 880 nm (>20%), peaking at 570 nm. See Fig. 1.
Fig. 1 Spectral response of the KAF-1602E CCD.
Note, though the dynamic range of the chip is 65 536, the chip becomes non-linear above about 40 000 counts (see Fig. 2) when used in the high-resolution mode (no binning). The 2x2 and 3x3 binning truncate the dynamic range to only the linear regime. i.e., in 2x2 or 3x3 binning, a pixel is linear until staturation at 65 536 counts.
Fig. 2 The number of counts recorded as a function of the exposure duration, plotted for a large number of chip pixels, randomly selected. The data were generated using a flat field, so only the relative exposure time is important. The images (taken at a variety of exposure lengths) were unbinned and no dark current was subtracted. A small fraction of the pixels start to go non-linear at 40,000 counts. Many are linear to 45,000 counts. We also see that saturation occurs at a variety of levels between 50,000 and 60,000 counts.
Fig. 3 The mean count rate versus mean count for the CCD.
The data are the same as in Fig. 2.
For each of 36 frames, with exposures of 1 to 36 seconds,
the mean counts and the mean counts/exposure give the points in the
above figure.
Again, we see the CCD registers consistently at about 4850 cts/s
until about 40 000 counts. The count rate itself is not important--it
is a function of the brightness of the flat field. What is
important is the linear response fails above 40 000 counts.
The dark counts are composed of two components: the bias (which is essentially the null-exposure readout noise) plus the charge built up during the exposure which is called the dark current even though it doesn't move.
The bias is not a function of exposure time, the current is.
Both components are functions of temperature.
So:
Dark(T,exp) = Bias(T) + Current(T,exp)
The current increases linearly with the exposure time (Figure 1).
The rate of increase with exposure-duration is a function of temperature (Figure 2).
For every 5C or so, the rate doubles.
Indeed, the rate is well fit by:
<dcts/dt> = (0.223±0.001) cts s-1 10T/(23.9±0.5 C)
which doubles every 4.58±0.03 C
Note that this is always less than the 1e-1/s advertised for the ST-8E, so long as it is cooled.
This rate is a mean rate, averaged for the base-line (not inclusing hot) pixels on the chip. Pixel to pixel variations are significant so the equation should only be used as a guideline for determining minimum cooling temperatures for a given exposure.
A mean bias frame appropriate for the temperature must be subtracted to properly reduce the contribution of the dark current.
These values were determined over the range of temperature of -20C to 10C.
The mean cts/pixel in the bias is a function of temperature (Figure 3), going as
<cts/pixel> = 97.8±0.7 + (11.2±0.5) 10T/(21±2 C)
