A-SETI Defocus Example

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 made. 

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 defocus. 

Here's an example of what we're looking for:

Simulation of a "flare star" (zoom of previous image's upper-left corner).

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WebMaster: B. Gary.  This site opened:  2013.02.06 Last Update:  2013.02.07