CCD CHARGE SHUFFLING

Central to almost all modes of TTF use is charge shuffling. Charge shuffling is movement of charge along the CCD between multiple exposures of the same frame, before the image is read out (Clemens & Leach 1987, Cuillandre et al. 1994). An aperture mask ensures only a section of the CCD frame is exposed at a time. For each exposure, the tunable filter is systematically moved to different gap spacings in a process called frequency-switching. This way, a region of sky can be captured at several different wavelengths on the one image (Fig. 1). Alternatively, the TTF can be kept at fixed frequency and charge shuffling performed to produce time-series exposures. Each of these modes is described in the following section.

The TTF plates can be switched anywhere over the physical range 2 to 12 m at rates in excess of 100 Hz, although in most applications, these rates rarely exceed 0.1 Hz. If a shutter is used, this limits the switching rate to about 1 Hz. Charge on a large format CCD can be moved over the full area at rates approaching 10 Hz: it is only when the charge is read out through the amplifiers that this rate is greatly slowed down. The TTF exploits the ability of certain large format CCDs to move charge up and down many times before significant signal degradation occurs (Yang et al. 1989). In this way, it is possible to form discrete images taken at different frequencies where each area of the detector may have been shuffled into view many times to average out temporal effects.

The field of view available in shuffle mode depends on the number of frequencies being observed. When we move the charge upwards, say, information in pixels at the top of the field is rolled off the top and lost. For example, two frequencies requires that we divide the CCD into three vertical partitions where information in one of the outer partitions is lost in the shuffle process. In the limit of n frequencies, where n is large, only half the available detector area is used to store information. The new MIT-LL (rows columns) format CCDs with 15m pixels will increase the detector area available for shuffling by threefold compared to the present Tek CCD (24m pixels). This is because the instrument field of view projects to an aperture 1024 pixels in diameter and shuffling is only possible in the vertical direction.

One application of charge shuffling that does not sacrifice detector area is time-series imaging (Section 3.3, below). This is because the imaging region is relatively small and able to be read out as the time-series progresses.

© Copyright Astronomical Society of Australia 1997