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Stopped down Celestron 9.25 vs Orion ED80

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#26 sixela

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Posted 19 October 2005 - 02:09 PM

For stars, it's complex, but at sizes for which the Airy disc gets mapped to one pixel on a CCD, the exposure time needed for a given brightness depends entirely on the aperture.

Once your image scale is so large that stars are really Airy discs (i.e. expose an area on the sensor), they behave like extended objects.

#27 sixela

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Posted 19 October 2005 - 02:14 PM

Example: 80mm f/10 vs 80mm f/5; the f/5 has shorter exposure time because the f/10 gets illuminated less.


For extended objects, each given area of the sensor gets four times less light on the f/10 because the f/10 spreads the same light over four times the area (given its image scale that is twice as large).

For point objects, both scopes map all the light coming from that 80mm aperture onto a single point, so brightness is identical at identical exposure time.

#28 jrcrilly

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Posted 19 October 2005 - 02:23 PM

For stars, it's complex, but at sizes for which the Airy disc gets mapped to one pixel on a CCD, the exposure time needed for a given brightness depends entirely on the aperture.

Once your image scale is so large that stars are really Airy discs (i.e. expose an area on the sensor), they behave like extended objects.


That's a better explanation than mine, which was oversimplified. Thanks!


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