phyzguy said:
The six diffraction spikes are more caused by the three supports that support the secondary mirror. As said above, the Hubble has four secondary supports, so you get four diffraction spikes. The three Webb supports lead to six diffraction spikes, because you get one opposite to the support as well. I think the segmented mirrors also contribute to the diffraction spikes, but I think the secondary supports are the main cause.
That isn't
quite right.
If a telescope's secondary mirror was attached with a single rod/pole holding there would still be two diffraction spikes; each spike being perpendicular to the direction of the pole/rod/support.
In other words, consider a vertical pole that extends from the bottom of view to the center, where it holds the secondary mirror, the result would be two
horizontal diffraction spikes: one in each direction. Now you can add another support, going from top to the center, without increasing the number of diffraction spikes, since the diffraction spikes from the two supports simply overlap.
The bulk of the diffraction spikes on the James Webb Space Telescope (JWST) come from the hexagonal pattern of the mirrors. Each mirror has 6 sides, and each side produces two diffraction spikes, one in each perpendicular direction. So there are really 12 spikes, but due to the symmetry of a hexagon there's overlap, so the end result is 6 observed spikes caused by the hexagonal shape.
Now we consider the 3 rods from the spider vanes holding the secondary in place. Two of those rods are aligned with the sides of hexagonal shape of mirror, so no
new diffraction spikes are created by these rods (since they overlap with other spikes). That leaves the third rod that is not in line with any of the sides of the hexagonal shape of the mirrors. This rod leaves a new set of diffraction spikes -- and those additional two spikes are perpendicular to the rod.
So in the end, there's 8 diffraction spikes on JWST's optics: 6 prominent ones and 2 minor ones in the diffraction pattern.
[Edit: Technically, of the 8 diffraction spikes, there's 4
really prominent ones caused by both the hexagonal mirrors plus two of the secondary support rods, 2 somewhat prominent ones caused by the hexagonal mirrors without any secondary mirror support diffraction, and finally 2 subtle spikes caused by one of the support rods.]
It's worthwhile to note that this this diffraction pattern (which is wavelength dependent; I haven't discussed that here) doesn't just affect the resultant appearances of bright stars, but also the dim stuff too. Every light ray, every pixel in the image is affected by the convolution of the diffraction pattern across the entire image. Sure, the pattern is only obvious to bright stars due their higher contrast, but the diffraction pattern actually affects everything.
Edit: Conceptually, this is like trying to paint a picture on a canvas with the limitation that your brush is the shape of the diffraction pattern. It's not possible to place a dot on the canvas smaller than the brush/diffraction pattern; it's all or nothing. You can change the brightness, but not the size (for a given color). Also different colors will have slightly different sized brushes, thus slightly different sized diffraction patterns.
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