Re: synthetic clock drive ccd
Bratislav Curcic (epabcc@epa.ericsson.se)
Fri, 24 Mar 95 09:22:22 DST
"Synthetic clock drive" CCD matches the readout time to the diurnal
movement of the sky. Say, your pixel/focal length gives you 3 arcsec
per pixel - you can expose your CCD for 0.2s, read it out (it will take
say another second), and then expose for another 0.2s. But, you'll have
to adjust for diurnal motion, because sky has moved some 15 arc sec
westwards while you've been reading out the image. So your software
will have to align (overlap) the exposures by shifting second image 5
pixels to the "east", before adding. You continue this until you run
out of overlap. Of course, signal/noise suffers - the effect is
similar to what you get by combining shorter exposures in autoguiding
("track and accumulate") CCDs, but of course much shorter exposures (in
order of 0.1-0.2s) result in far noisier images. Another possibility is
scanning large areas of sky where no long exposure is needed (i.e.
Moon).
Bratislav