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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?

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  • $\begingroup$ By the way, "CCD" would be a great tag for this and other Astronomy questions, but it doesn't yet exist. If someone with sufficient reputation could create and tag this, I'd be grateful. $\endgroup$ – Daniel Griscom Nov 26 '15 at 11:55
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    $\begingroup$ I think the "astrophotography" tag would be appropriate. $\endgroup$ – James K Nov 26 '15 at 17:30
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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.

Dither patter

Image from havard.edu

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

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