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#25
12-12-2011, 11:33 PM
 Merlin66 (Ken) Spectroscopy Wizard Join Date: Oct 2005 Location: St Leonards, Vic Posts: 7,102
"Gannet on a stick...Gannet on a stick" - The cinema scene from Monty Python...
To quote:
True, the central blob will be the first to completely saturate, but it doesn't stop there.

83% of all starlight falls in that central blob. But what if you got so much starlight that that 83% blob gets saturated? The other 17% all of a sudden becomes very important - that's where things start to saturate instead - the star starts to grow beyond the blob.
Humour me for a moment...
Let's call the "Blob" (which contains 83% of all the starlight) an Airy Disk, and the ongoing (decreasing) waves of light surrounding it an "Airy Pattern" In this Airy Pattern there are definite brighter/ darker rings which decrease in intensity ( #1 = 8% of the total, #2 = 3% etc etc)
By definition the image of the Airy disk for any star (not just the faint ones or the bright ones - every star, big or small) extends -
Airy disk = 1.342 x focal ratio
For a f7.5 scope this gives a size of 10 micron.
Outside this disk there's a brief gap (first dark diffraction ring) before the 1st ring....
( When you observe a star at high magnification you can always see the diffraction/ Airy disk - it never changes size between stars)

The peak intensity (for a non-saturated star image) increases with exposure..ie Brighter (or more exposed stars) have a higher intensity peak than fainter (less exposed stars)
So, if we had a peak of say 10000ADU for a bright star, the Airy disk diameter would be a constant and the sum of all the pixels in the disk would represent 83% of the total energy/photon count. Let's say the total ADU count within the Airy disk is 300,000ADU - then only 29,000 ADU would be spread into the 1st ring, and 10,800 ADU in the larger 2nd ring. Thats a very large decrease in intensity (ADU/pixel)!!!

So, back to my conclusions...
1. The Airy disk for every unsaturated star is constant
2. The peak intensity increases with brightness/ exposure.
3. You have to fill the whole Airy disk before the lower intensity of the Airy Pattern begins to contribute to the increasing area of the star image.

I couldn't get your computer generated image to show a distribution similar to that of a real star...

"Part 3 - the story so far...Mary, the dark haired divorcee ran of with Guy, the one with the bushy eyebrows......."