So I, like most other astronomy enthusiasts, have, at some point, not been able to see something clearly, such as a star, and have had to squint to be able to see it better. When this happens, the object seems to become clearer, but the light seems to gain a cross, or sometimes a hexagon, or an irregular line shape when doing so. I've noticed a similar effect on different models of telescopes, especially the newtonian reflector series. More on this here: (http://www.youtube.com/watch?v=ipe3NN1yPzM). Is this a similar effect on the eye? But more importantly how does squinting improve the quality of the picture?
Squinting works the same way as a pinhole camera.
Ideally, light from a single point source entering your eye anywhere on your pupil will be focused on a single spot on your retina. But this works perfectly only if you have perfect vision; otherwise light entering near the top of your pupil may be directed to a slightly different spot on your retina than light entering near the bottom.
By squinting, you block out some of the light from the edges, effectively making your pupil narrower, creating a sharper but dimmer image on your retina. (You may find that it improves the vertical resolution more than the horizontal resolution.)
If you happen to be nearsighted (as I am), you can see a similar effect by looking through a small pinhole, or through a small aperture made with your fingers. If the light is bright enough, you'll see a dimmer but sharper image.
The irregular shapes may be interference from your eyelashes.