[irwjager (Ivo)]

Originally Posted by Merlin66

Leinad,
That's the nub of the problem - at least for me...
If the PSF of the Airy Disk contains ALL the energy/ photons available from the star, then the maximum size of the star image can NEVER exceed this size....hence my laser pointer analogy.... there's never any illumination beyond a 10 x 10 box...so whatever the exposure the size should always be less than 10 x 10.

The Airy Disk *is* a Point Spread Function. It describes how a single point of light (a star) is spread over (in this case) the CCD when it is shone through a circular opening (in this case the aperture of a telescope).
The sum of all the energy 'scattered' throughout the image by the PSF is indeed equal to the amount of energy received from the point light. Again, the PSF, in this case the Airy Disk, spreads the energy out over a huge area.

You seem to be confused by the 'maximum size of a star'? In a perfect world every star in your image would be a point light. But we run into two problems;

1. Your CCD wells saturate. They start clipping the signal and can't register a star that's brighter than 'maximum brightness', nor can your computer display display such a brightness. Therefore the signal is capped to 'pure white'. The star itself may in fact be much brighter than that but we can't record that, nor can we display that, so we cap it to the maximum brightness we can represent with our system(s).
2. The starlight is necessarily diffracted by the aperture of your telescope. This leads to the light being spread over a larger area than just a single point.

Now, combining the two 'problems', more starlight can be captured by your wells because the starlight is spread by the PSF over a larger area, hitting more CCD wells. No longer is the point concentrated into a single well, instead it is spread over all the wells in your CCD. Some may still saturate and be completely white, but others may not.


Again, your laser pointer analogy is flawed as the laser light is not diffracted.

If you look at the profile data say for Star#1 and Star #2 (one is saturated the other is well exposed). The "max" diameter recorded for #1 is 20 pixel and for #2, 12 pixel, and the FWHM is 6 pixel v's 3.5 pixel.
Why, when the brighter star is over-exposed more does the "size" increase by 166%....Aah...the longer exposure collects the light from the outer reaches of the PSF curve...Hmmmm don't think so....

Again, Gaussian Curve fitting to overexposed stars and then comparing FWHM readings is next to meaningless.

IVO,
The second comment you refer to - wasn't the Airy disk being drowned out, but the light in the extreme "wings" ie that last 6% of total energy being drowned in the background noise....

I understand that, but you can't 'drown out' a PSF. It is merely a function by which you multiply the energy of a star. There are very bright stars (ex. Alnitak) which, when you multiply them by your scope's PSF will readily show up (and ruin your image) even though their core is way off the CCD.