OK it is really late here, but today's reveal of the first JWST image of the galaxy cluster SMACS-0723 was amazing, and I am still digesting it. There is so much stuff going on, it is absolutely breathtaking!

Just to get an idea of what an improvement this is over Hubble, over at The Robot Report, they show this beautiful side-by-side view of the previous best image taken with Hubble, and tonight's result. It's wild.

There is a high-resolution (around 30 MB) .png version of the image over here.

My God... It's full or arcs!

On top of the better contrast ("depth", as astronomers call it) of the JWST image, it definitely also shows better detail than HST does. Look at all the star clusters you can see, both in the lensed galaxy and (I guess) in the lensing galaxy cluster.

For comparison, here is approximately the same region as seen by Hubble Space Telescope, until tonight the sharpest image ever taken of it. The difference is breathtaking.

Honestly, I am almost a little intimidated and scared by the sheer amount of information JWST will bring us. How can anyone process all of that? This image alone looks like the data behind it could be the basis of at least one entire Ph.D. But most of all, it is extremely exciting. We are going to see a flurry of amazing image releases over the next months.


Well, probably.

How many papers were based on the HST deep field image?

@thriveth I came looking for a reference to 2001, and here it is!

The features (galaxies?) that appear stretched out and bent, is that gravity lensing, or mirror distortion, or is “bent elliptical” a regular galaxy shape, or…? (or is that a question we hope JWST can help answer?)
I see some of the effect in the Hubble image (so probably not a mirror thing), but it’s so much more apparent in the new one, there seems to be an overall pattern to it?

@silvermoon82 What you see is (mostly) strong gravitational lensing, and there is a lot of that going on here. As you say yourself, there is already quite a bit of it in the Hubble image, but we see so much more of it with JWST. We kind of suspected this would be the case beforehand - the better contrast simply means we can see fainter galaxies, so many more things such as faint arcs like these are popping out.

@silvermoon82 But one thing is suspecting it, another thing is seeing it. The more you look at the JWST image, the more you realize that they are just everywhere.

@silvermoon82 It is also the hope that deep observations of these lensing clusters with JWST will help us find some of the very earliest galaxies in the history of the Universe. Many of these are so small and faint they are on the edge of what even JWST can see. But if one of them happens to lie between one of these arcs, we could be in luck and get a direct view of the completely new-born Universe.

Oh, wow - that’s exciting!
So is that the same sort of technique as the Hubble Deep Field image, but we’d point the telescope at the middle of these arcs?

@silvermoon82 Something like that. The same idea has been tested with Hubble, but JWST will just have way more muscle to do the job.

Funny enough, the most distant galaxies we know today were actually not found this way, but without lensing. That is because only a small fraction of the sky is behind a lensing cluster, so it's a numbers game. If there happens to be one of the first galaxies in the right place, the lensing will be immensely helpful, but if we don't have this kind of luck, it will not help much.


So, I guess the left is a Hubble visible spectrum image, whereas the JWST image is infrared, mapped to RGB color bands for display.

Reading about this, it looks like NIR only images 2-bands, roughly centered at 2μm & 4μm, not three, so they've remapped the colors in some way to generate the 3-band color image we see.

Not sure exactly how they do that, but I'd naively assume something like red = B, green=(A+B)/2, blue=A, where A is the 2μm band and B is the 4μm.

@TerryHancock My guess is that the HST imaging is mainly in infrared - Hubble's sharpness in UV and Optical is comparable to JWSTs in the near-infrared. But Hubble's sharpness is quite dramatically worse in the IR, as is also clearly visible here.

@TerryHancock No, RELICS is much newer, the IR images are taken with the WFC3/IR channel, in the filters F105W, F125W, F140W, and F160W. My guess is the image is some combination of (some of ) those.

@TerryHancock ...You can find the HST image of the cluster on the RELICS website and it does indeed look like it is the ACS-WFC3/IR image on the upper left that is shown in the comparison.

@TerryHancock ...And according to STScI, the JWST image is actually taken in all six broad NIRCam filters, with each color representing a stack of two filters as seen in this image:


These details should be in the press release. This is the entirety of what the damned thing says about wavelength:

"This deep field, taken by Webb’s Near-Infrared Camera (NIRCam), is a composite made from images at different wavelengths, totaling 12.5 hours – achieving depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks."

I'm not mad. I'm just _disappointed_. 🤨

@TerryHancock I agree, but we should remember that today's preview briefing for the US president was just that, a preview briefing, seemingly planned quite hastily, so all that information might come out with the planned image release tomorrow.


Thanks for all the background info! I was digging through the project website and not turning up much (and maybe contradictory info?)

@thriveth I'm told one of the selling points of the JWST is that, since it can capture a lot of light, is also a lot faster. The new image took just a day of observing, do you happen to know how long of an exposure that side-by-side from Hubble took?

@Timo_Micro According to the RELICS website, HST image is 5 orbits which corresponds to around 7.5 hours, so actually shorter than the JWST imaging. So HST could have gotten it better than what we see here. I guess tomorrow's images of Stephan's Quintet will be a better comparison of JWST vs. HST because that field is really well studied with HST already.

@thriveth Oh thanks! That's interesting. Looking forward to seeing even more images (and more importantly, their expert interpretation and scientific analysis of the data). 🌃

@thriveth @dpthorngren There’s a lot of… gravitational lensing(?) going on. It’s amazing how clear the JWST photo is!

Link to Bird Site Post 

@tsturm @thriveth It's so great! I don't want to steal it but this post highlighting how you can see multiple copies of the same galaxy due to the gravitational lensing.

Link to Bird Site Post 

@dpthorngren @thriveth Thanks for pointing me to that post!

@tsturm @dpthorngren Yes, a lot. It was kind of expected, it's one of the most massive galaxy clusters known - but actually seeing it is still breathtaking.

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