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Imager Specifications

  • SBIG ST-8E Dual CCD Self-Guiding Cooled (made by the Santa Barbara Instruments Group)
  • 1530×1020 pixels (Med. Res.: 765×510, Low Res.:510×340)
  • 9 micron pixels => 0.286″/pixel
  • 13.8×9.2 mm detector surface => 7.3’x4.9′ coverage
  • 16 bits/pixel => dynamic range of 65 536
  • 3 Mb image files with no binning
  • Anti-Blooming Gate
  • ~50 000 e Full Well Capacity (ABG)
  • ~100 000 e Full Well Capacity (NABG)
  • Dark Current: 1 e / pixel / s
  • Exposures of 0.11 to 3600 seconds (0.01 s resolution)
  • Gain: 2.3 e / ADU
  • Read Noise: 15 e RMS
  • Cooling: Thermoelectric, -25 C from ambient minimum

The imager has four standard resolution modes: no binning, 2×2 binning, 3×3 binning, and 9×9 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.

Spectral Response

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.

Figure 1

Fig. 1 Spectral response of the KAF-1602E CCD.

Dynamic Range

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 2×2 and 3×3 binning truncate the dynamic range to only the linear regime. i.e., in 2×2 or 3×3 binning, a pixel is linear until staturation at 65 536 counts.

Figure 2

Analysis and Figure: Chris Frye

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.

Figure 3

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.

Dark frame characteristics

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.

Dark(T,exp) = Bias(T) + Current(T,exp)

The current increases linearly with the exposure time (Figure 1).

Figure 4

The rate of increase with exposure-duration is a function of temperature (Figure 2).

Figure 5

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)

Figure 6