Interline and frame transfer CCDs

in the overall extrinsic quantum efficiency by a factor of two. The electrode and gate structures of the active pixels are a bit different from those in a normal CCD, causing a further reduction in quantum efficiency and a somewhat different overall spectral response. Due to these two factors and the fast readout times of modern CCDs, interline devices are generally not seen in astronomy much anymore.

Frame transfer devices work in a manner similar to that of having two separate CCDs butted together. One half of the device is active (exposed to incoming light) and records the image while the other half is shielded and used as a storage device. The end of the integration sees the image data quickly transferred or shifted to the identical storage half, where it can be readout slowly as a new integration proceeds on the active side. Frame transfer CCDs are used in most commercial video and television cameras for which the active image is readout at video rates (30 frames per second). Astronomical imaging generally can not occur at such high rates because of photon starvation, but frame transfer devices have been used in a number of interesting astronomical instruments. For example, ULTRACAM at the 4.2-m William Herschel telescope (Dhillon & Marsh, 2001) and a CCD time series photometer at the University of Texas (Nather & Mukadam, 2004). More on high-speed CCD observations will be presented in Chapter 5.

Figure 2.5 provides an illustration of these two types of CCD. We note here that in both of these special purpose CCD types, the movement of the accumulated charge within each active area to the shielded, inactive area can be accomplished at very high rates. These rates are much higher than standard CCD readout rates because no sampling of the pixel charge or A/D conversion to an output number occurs until the final readout from the inactive areas. As we will see, the A/D process takes a finite time and is one cause of noise added to the output signal. Therefore, any on-chip shifting or summing of charge will introduce essentially no additional noise into the data.

Before we leave this section, we are compelled to discuss a non-astronomical use of CCDs that is likely to be of interest to some readers of this book. Megapixel multiple-frame interline transfer CCDs (MFIT CCDs) produced with alternating sets of three pixels covered with red, green, and blue filters and which are readout at 30 frames/second have become common. Have not heard of them you say? Well, these are the imaging devices used to bring you high definition TV (HDTV). So the next time you watch an HDTV program (this author has yet to see one) sit back and smile realizing that the event is brought to you by your old friend, the CCD.

Imaging cells

Imaging cells

Output register Output amplifier

Storage cells

Storage cells

Fig. 2.5. Cartoon view of (top) a frame transfer CCD and (bottom) an interline CCD. From Eccles, Sim, & Tritton (1983).

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