Hubble in trouble

To clarify the above a bit more, the hexagonal mirror pattern is what causes just the number of spikes in that stacked star image. The phase mismatch between those primary mirror sections -- some are too high compared to the desired optical surface, some are too low, likely be several wavelengths -- determines the amount of optical energy thrown out of the central spot and into those bars, and their visible length. When the phasing adjustment steps finally get all those mirrors to the same phase / same height, those bars should shrink down to just 1 or 2 diffraction rings around the central core, depending on phasing accuracy and on how over-exposed the image is.

A perfect star image on a perfectly configured and focused telescope with a round aperture, with no obstructing mirror supports, would look like this, with some visible diffraction rings around the central spot.


Theoretically there are very many rings, as shown in this laser image, but in practice only 1 or 2 are bright enough be visible in common astronomy, and only on very bright point sources (i.e. individual stars):

Note that lasers operate in a very narrow band, just a single wavelength, making the rings more distinct. Wideband optical imaging will smear the rings together, helping hide those beyond the first. See the Airy disk article for more detail.

Non-circular aperatures will produce slightly different patterns, and straight edges will add spikes.

Separate from the above, typical reflecting telescopes have a 'spider' support to hold the secondary mirror, blocking a small amount of light. This inherently also causes 'spikes' in the the final image. Hubble's spider has 4 vanes, causing 4 spikes on very bright point sources (nearby stars) but not visible on dimmer and defuse objects, such as the distant galaxies in this snippet from the Hubble Ultra Deep Field:



Wikipedia indicates that 3-vane mirror supports, such as on JWST, will make 6 spikes:
https://en.wikipedia.org/wiki/Diffraction_spike

 

Airy disk gets first mention on PriusChat! Now we're having fun.

 

Looking at a higher resolution of the individual mirror images, before stacking, here are some of the best ones. While the central spot looks circular, the first 'ring' looks like the same hexagon shape as the mirror. The second ring seems segmented, with bright portions along the flats of the first ring, and dark at the corners.


Others are not as clean, and some show portions of the third ring, turned to various orientations. Hopefully the fully adjusted final stacked image will have a decent resemblance to the ones above, but significantly smaller (larger total aperture -->> finer resolution) and brighter.

In the ideal case, the central spot contains almost 84% of the total optical energy, the first ring contains 7%. Ideal is not within reach, but I don't yet know how much they aim to get into the final central spot(s). Focus accuracy will vary across the field of view of each instrument, and there are multiple instruments mounted side-by-side across a wider focal plane.

 

According to JWST's calendar, they ought to have entered Step5, Fine Phasing stage of mirror calibration, but we haven't yet heard any results from Step4, Coarse Phasing. Considering that the earlier steps seemed to have happened early, this delay seems sort of like an indicator of trouble. I sure hope it isn't.

 

Seems important to mention this an infrared image shifted (not the right word?) into visible orange-ish light. It was imaged after a 2 micron filter, described as broadband, but I don't know how broad.

Two microns is 2000 nanometers. Orange light is about 600 nanometers, so that's the shift.

 

Despite being vastly better than the original stacked image, this low-res image (800x506 pixels, 0.27 MB) published by Ars Technica just doesn't do justice to the fine-phasing results. For a far more eye-popping image, go look at the higher-res version (5437x3438 pixels, 5.47 MB) in the NASA press release:
https://www.nasa.gov/press-release/nasa-s-webb-reaches-alignment-milestone-optics-working-successfully

Click on the thumbnail to blow it up to full screen, for whatever screen size you have, then click again to see portions of it in its full glory. Or go directly here:
https://www.nasa.gov/sites/default/files/thumbnails/image/telescope_alignment_evaluation_image_labeled.png

The central star is seriously overexposed, hiding the inner rings around the Airy disk. But browsing around at other, dimmer stars, I see several good examples of a hexagonal inner ring, and hex fragments of additional rings.

And boatloads of wonderful galaxies.

 

?

 

Hubble persists and deserves its own thread. If we agree that JWST is past jinxing, let whomever would bear that burden start a new JWST thread.

We are a few months away from its data flow beginning, so no rush.

 

Boatloads @127. Gonna need a bigger boat. Here we have a very large photon bucket looking at light stretched into infrared from yonder receding* galaxies. Presumption is that no JWST image can avoid seeing them. Question is, what can 100-ish pixels from yonder galaxies teach?

receding* not their fault. One doubts that they might even care...

 

They think we're receding.

 

I'm guessing we won't need the really big boats until they start moving up from short test exposures to long real observations, revealing the faint stuff.

In this instance, both sides are correct.

 

I see that Bob has done that.

 

Hubble orbital boost plans:

There's a New Plan to Relocate Hubble Space Telescope and Extend Its Lifespan