Bob Carlson wrote:


-- Bill Owen, Cassini optical navigation team
Thanks for the good info. Pardon my dumbness in this area but could
the speed of the cosmic ray be determined by using a very short
exposure time? And why would the cosmic ray only affect the ccd
during the exposure? I thought cosmic rays would penetrate to the ccd
at all times regardless of exposure time.

Bob Carlson

Cosmic rays (and other charged particles ... remember, Cassini is inside
Saturn's magnetosphere) are continually impinging on the CCDs. But the
accumulated charge is flushed out immediately before each exposure
begins
(just like any CCD camera on a ground telescope), so whatever happens
before that is gone. During readout, the picture is slowly moved up
the chip into a "serial register", and any cosmic rays which hit the
remaining part of the picture will also get recorded. You do see more
CR hits at the bottom of a Cassini image than at the top. But after
the readout is completed, the numbers are (more or less) safely tucked
away in the solid state recorder, and cosmic ray effects there are much
rarer. Meanwhile, the CCD itself continues accumulating hits until it
gets cleared again before the next picture.

As for determining the speed, no, this is not the kind of instrument
you'd want to use for that.

-- Bill