A couple of weeks ago, I posted a question in the
Euclid Update thread, regarding several of the first pictures released:
... I like to pay attention to the diffraction-spike patterns produced by new telescopes. I know that this is an update thread and not a discussion thread, but - briefly - does anyone know why Euclid has that specific pattern? Since the "spider" which mounts the secondary mirror has only 3 struts, (and the primary is circular) I was expecting it to look a little different. In that very last hi-res picture, it appears that the brightest stars have 6 bright spikes in sort of a regular hexagon shape, as well as a faint spike halfway between each of the bright ones, for a total of 12. Hubble produces 4, corresponding with each of it's four struts mounting the secondary. JWST produces 6 corresponding with the mirror's hexagonal shape, and 2 sets of 3 faint spikes corresponding with its 3 struts.
FutureSpaceTourist posted an excellent picture of a top view Euclid's mirror assembly in Reply #71 upthread. I found a side-view diagram of that (though I'm not sure if it's up to date):
https://spie.org/images/Graphics/Newsroom/Imported-2012/004459/004459_10_fig1.jpg
Comga sent me a message answering my question...
Hi
This is my thing
The “streaky starburst fuzz” between the diffraction spikes is a manifestation of polychromatic speckle, on which I did my thesis oh do many decades ago. IMO this is why we traditionally draw stars with things sticking out, like five, six, or seven “points”, despite them really being infinitesimal dots.
Every diffraction spike extends in both directions so three spiders produce a six pointed diffraction pattern.
I see no other consistent structure in the speckle pattern.
Some modest sized amateur telescopes avoid diffraction spikes by mounting the secondary on a ring. This increases the diffracted energy by a factor of Pi, but spreads it in all directions, like the fuzz near the overexposed point in the Euclid image, so it’s not a spurious “feature”.
... and also recommending that I start a new Discussion thread for Euclid.
So that's what I'm doing right now.
While I'm at it, this video came out recently that gives a good overview of the science goals of the mission. A number of similar videos have been posted in the Update Thread.