Defocusing can be used to distinguish between a star-like object
(similar PSF) and a cosmic ray defect. Here's an example of
defocusing just enough to enable that distinguishing to be
Galactic anti-center image (05:45:39 +28:57.1). Example of
just enough defocusing for distinguishing between a cosmic ray
artifact and a "flare star" present in just one of many images.
FOV = 27 x 18 'arc, north up, east left. Filter is Cb (clear with
blue-blocking), exposure time is 60 seconds, limiting
magnitude ~ 19 or 20.
Notice that the stars that aren't saturated in this image show the
donut PSF that all Cassegrain telescope produce when defocsued. My
CCD is a dual chip (autoguider second chip, an SBIG patented
configuration), and when I focus the autoguider chip the main chip
is slightly out of focus. To produce slightly more defocus for the
main chip is it better to move "in" (instead of "out") in order to
preserve good autoguider focus while achieving main chip
Here's an example of what we're looking for:
Simulation of a "flare star" (zoom of previous image's
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WebMaster: B. Gary. This site opened: 2013.02.06. Last Update: 2013.02.07