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By reprogramming the CCD controller it should be possible to reduce dead-times still further. For example, at present the CCDs are cleared prior to all exposures. The charge is transferred to the output but not digitised. This operation takes approximately 10 s per pixel, and hence 15 seconds for a full EEV chip. There are alternatives. It is possible to flush the charge out almost instantaneously by using all of the CCD clocks simultaneously instead of one-by-one, as in the normal horizontal clocking process. Alternatively, since each full-frame read-out essentially clears the CCD anyway, we could avoid clearing the chip altogether, assuming that the previous frame was not over-exposed and there is not much light leakage or dark current since the last read-out. In addition, since the read-out noise is known to be a function of read-out speed, it should be possible to offer a greater variety of read-out speeds and noise levels.
For even greater reductions in dead-time, it should be possible to store a series of short exposures on-chip. The interval between exposures can be very short, especially if the image-window is almost one-dimensional, ie. spectroscopic. A small box, defined by a dekker, consisting of rows at the top of the CCD is illuminated. On completion, the box is moved quickly down the chip and the clear area can be exposed again. This can be repeated until the chip is full. The whole frame can then be read-out from the bottom of the CCD. The exposure time per box can be in the range ms. For an EEV chip it takes about 0.04 ms to shift down one row of data; thus the total time per box ms. For a box of size 10 rows, and a 60 s exposure, the total time interval between boxes would be 60.0004 s. Even if such a system were available now, it would still be impaired by the relatively slow DMS-VAX link. However, work is in progress to replace this next year with a system which saves the data directly onto the observing SPARCstation. This should provide an order of magnitude improvement in the dead-time due to archiving data on disk.
It should be noted that most of the above enhancements are still at the drawing-board stage and it is by no means clear when, if ever, they will become available to the community; this will depend on demand from users and the availability of engineering effort.