How are the gaps filled in images captured by CCD arrays? Large telescopes don't use a single CCD; they use arrays of them. Since CCDs can't be seamlessly joined, this means that every captured image must have gaps. But, the resulting images almost never have gaps when they are presented to the public.

How are these gaps filled? Does the telescope capture a number of overlapping images and then merge the results? Wouldn't this result in differing exposure levels for differing areas of the image, which, even though it could be compensated for in the processing, would give different noise characteristics to the different areas?
In general, the common practice is to dither exposures and fill in the gaps. The dither serves multiple purposes:


provides some exposure in the gaps between the CCDs
smooths out pixel-to-pixel variations in the response.
prevents one bad pixel from ruining an entire observation


The dither pattern varies from telescope to telescope. For example, the following image shows the dither pattern of Chandra telescope.



Image from havard.edu

The effect of dither is removed during high-level ground processing of the data.