Compositeman

Hi All - I've just taken up astro again after 30 years away doing physics. I hope I will be able to contribute to the lounge when I've had more experience in future. I'm mainly interested in equipment and astrophotography. Currently I have a Meade 4504 but am using the goto mount with a Canon 500D and various lenses to start. I have a beginner's question: All stars are point objects irrespective of their brightness. I thought the image size of a point object is the Airy Disc size and does not depend on the point object's brightness? Why then are brighter stars' images so much bigger in diameter than faint stars'? It's not just my camera because Hubble images are the same, as are the old photos on film.