[Merlin66 (Ken)]

Using a CCD camera, each pixel records the photons and displays an ADU - the relationship between them being the gain of the camera.
Every star image produced by a telescope is basically an Airy Disk.
Due to atmospherics etc (ignore guiding errors and aberrations for a moment) a PSF (Point Spread function) curve is seen, basically the same Gaussian shape as the Airy disk but much larger in diameter.
The usual measure of this curve is the FWHM ( the width/diameter of the curve at half the peak intensity)
Still with me?
I do a 3 min exposure.....
The fainter stars appear smaller than the brighter stars ( get's even worse if the star intensity causes saturation...)
Why is this so??
The full extent of the PSF curve doesn't change.
Why doesn't the intensity just build up in each pixel within the PSF curve? Why does it appear to enlarge the PSF curve??
The reasons given say that as the exposure increases the CCD can record "fainter outer regions of the curve" and these show up as an increased diameter.....Sorry, I can't see that!
If ALL the light from the star is within a defined PSF curve and that curve, for a faint star is say 5 pixel diameter, then surely for a brighter star the total size should still be 5 pixel allbeit with a higher peak intensity.
Add 10 x 30 sec star images.....compare the size with a 5min star image????
I'm finding that if the star image is NOT saturated, then the PSF curve remains close to constant...when the star becomes a saturated image it starts to grow outwards ie 5 pixel max profile for a non-saturated star image v's 15-20 pixel for a saturated star!!!
( Ron Wodaski in his "The New CCD Astronomy", p 50-53 demonstrates this issue but gives no reasoning or answer to the possible cause.)
Can anyone add to my confusion?????