Perplexed by the Airy

I probably won't understand the answer to this, but here goes....

When you push a four inch refractor (but this applies to any scope design) to around 50x per inch you get for sure the Airy disk and it's plain and easy to see. So there is a point source of light and something happens to make that effect. It is more readily viewed on bright stars.

If you look at TWO adjacent stars, very close, you will see TWO Airy disks.

When you put the same scope at the same power on something like Jupiter the view is peachy keen. One would think that Jupiter can be thought of as a mass of adjacent points. If two stars give you two Airy disks, one would think, that this point over here on Jove would give an Airy, and so would this one over here, and that one, and the result would be an indistinct mass with no contrast. Or perhaps an Impressionist assemblage of pointillism. There's plenty of brightness, just as bright stars show an Airy more readily.

I've never seen anyone ask this question before so I'll put it out.

thanks

Greg N

Greg Nowell said:
When you put the same scope at the same power on something like Jupiter the view is peachy keen. .... on Jove would give an Airy, and so would this one over here, and that one, and the result would be an indistinct mass with no contrast.

I don't think there is any mystery here. The resolution for
a single star, double star, or Jupiter is all the same.
Around 50x per inch Jupiter starts to look fuzzy.
At 100x per inch it looks like your "indistinct mass with
no contrast."

I've never seen anyone ask this question before so I'll put it out.

thanks

Greg N

This is frequently discussed, just not quite in these terms. I think your model is a reasonable one, the view of a planet can be treated as the summation of a number of airy disks and diffraction rings.

This is the fundamental reason larger scopes provide better resolution and that larger central obstructions reduce the contrast/high frequency resolution. Larger scopes provide smaller pinpoints as well as tighter diffraction rings. This means that pixels are smaller and the smaller diffraction rings the affect the nearby points less, thus sharper more well defined images of extended objects like planets.

Larger central obstructions transfer more energy into the diffraction rings, making them brighter and thus smearing nearby points more.

So this summing of airy disks and diffraction rings at some level does become a unresolved mess but this limit is well understood both practically and theoretically and so that is why we use the sorts of magnifications we do. In a 4 inch refraction the reduction in sharpness/crispness is quite apparent as one pushes from say 125x to 200x to 250x and beyond. This is the effect you are wondering about.

Jon Isaacs