A new, thin-lensed telescope design could far surpass James Webb | Hacker News Reader

A new, thin-lensed telescope design could far surpass James Webb

theconversation.com

86 pointsxoa36 comments

They seem to be essentially talking about Fresnel lenses or am I misunderstanding something? Why are they calling them diffractive lenses, I always thought that Fresnel lenses are also refractive with some unwanted diffraction at the edges of the ridges? Or do those proposed lenses work differently, does diffraction contribute intentionally and significantly because of the finer structure?

I am confused at the beginning, but it seems to me the author did not know the difference between diffractive optics and Fresnel lens. That significant reduce the credibility of the author. It is embarrassing for someone in academia to confuse these two different things.

Perhaps they should have started at the Wikipedia entry for “Zone Plate” instead

>It is embarrassing for someone in academia to confuse these two different things.

I've been digging through the academic publications for a completely different field and it appears that they have very little or outdated knowledge of parallel fields, almost siloed.

I believe the author showed up in the comments on the Ars version of the article [0] and touched on some of these ambiguities.

[0] https://arstechnica.com/space/2023/07/a-new-thin-lensed-tele...

One of the comments (from user Wickwick) explains the difference concisely:

> I was going to make a small adjustment to the post on this topic. The lenses Fresnel designed were refractive (and reflective) and not diffractive. In fact, it's the diffraction of the transitions that ruins the image quality of these sorts of lenses.

This is what I understand is the drawback of a Fresnel lense. The article is written in first person, but I can't find the name of the author anywhere around the article.

I was expecting an article about negative index of refraction metamaterial lens and then read Fresnel and basically stopped reading.

The main advantage of a reflective telescope is lack of chromatic aberrations, because light never disperses. The other, related advantage is that you don't need glass that's transparent over a large range of wavelengths. JWT ranges from the end of the visible spectrum to like 30 µm.

Correct. Almost all large telescopes (read: ~50 cm aperture and wider) are reflectors. Glass is used almost exclusively on camera lenses only, and that's because a 'reflector camera' would be un-ergonomic and impractical. Not much of a concern for a bunch of mirrors stationary in a building or rocketed up to space.

There are some reflector lenses available for SLR cameras, not everyone likes their different optical properties though.

I have one of these- a 500mm "equivalent" (https://www.amazon.com/gp/product/B004WLEEHI) and it's definitely got weird optical properties.

Sadly sensors are all bundled up with the rest of the camera, because a Schmidt camera (corrector plate like your objective, adjustable primary mirror, sensor directly in the primary focus with it's back facing the scene) can easily get a very wide aperture (e.g. nicely matching a CMV12000 with a fairly tame single glass up front and a tame mirror in the back, plus a slab and a plano-convex lens right at the sensor to flatten the field/focus, yielding 400mm f/2.0, though that's with the focal point dead center in the sightly over 800mm tube; still around f/2.4~f/2.8 brightness to the full f/2.0 sharpness (near center, but even at the corner you need a monochrome sensor to feel the mushiness)).

Celeste's prosumer 8~14" (aperture) Schmidt-Cassagrain typically support replacing the secondary mirror with a camera mount to get that much shorter focal length (IIRC around f/5).

Might be nice with one of those very narrow studio video camera bodies (e.g. Black magic has some); the catadioptric nature means you can carry even the very big ones on your shoulder (I'd highly recommend a gimbal if used for shooting from the shoulder).

I'd like to understand what you're saying better. I mainly purchased that "lens" because it was the most affordable way to upgrade to a higher magnification on a DSLR. But for another purpose (solar imaging), I'm actually moree interested in using machine vision cameras with C-mount. Aperture isn't super important because the sun has lots of photons (so many I have to use a filter that fits to the lens) but a flat field is dfinitely desired.

I like the sensor you mentioned (CMV12000) but I assume it's obscenely expensive... $20K

No it's 2k$ last I checked.

A drawback is that you either have a spider construction blocking parts of its view (which in itself causes diffraction and reduces contrast), or you have an off-axis telescope, but this requires asymmetrical surfaces which are much harder to create than spherical and parabolic ones.

Fun Fact: Diffractive optics exhibit chromatic aberration in the opposite direction to Refractive optics, so the two can be used together to cancel each other out!

Of course, that doesn’t help with the weight on a space-based telescope, but it was being considered quite frequently for VR Headsets a few years ago.

There's much more to a telescope than the primary mirror which the author conveniently neglects. As others point out below, making an IR observatory (JWST) requires keeping everything around the mirror cold so that the mirrors and instruments (not trivial, either) have a hope in heck of working.

Livermore tested a multi-meter thin Fresnel optic back in 2002. They also worked with Robert Lang, physicist and origamist, to figure out how to pack the thing effectively.

https://digital.library.unt.edu/ark:/67531/metadc1412512/m2/...

https://langorigami.com/article/eyeglass-telescope/

This is their paper from 2021: Design aspects of large-aperture MODE lenses https://opg.optica.org/osac/fulltext.cfm?uri=osac-4-1-171&id...

Is this technology really that new? Canon has been selling "Diffractive Optics (DO)" lenses for decades. For example, https://kenrockwell.com/canon/lenses/70-300mm-do-is.htm , https://en.canon-cna.com/pro/infobank/lenses-multi-layer-dif... .

The article acknowledges this and even links to Canon products from 2004.

https://global.canon/en/v-square/34.html

They are not new. The issue is quality and scale.

And launching them into space, of course.

Thing missing from all this, to see into the far infrared things have to be chilled near absolute zero. We do this with JWST. Can these thinner plates handle that cold? Do they still refract correctly?

"far surpass" is interesting when the challenge of the JSWT isn't the lens technology, but everything else.

Hmm. Can you image far infrared with lenses? Wouldn't the imager just "see" the lens?

I don't see any mention of infrared in the article, so they may just be going for visible light

Yes you can, but not from earth. The earth is too warm for that it has to be in space. Typically as long as the lens has a transmission for that wavelength it'll go through. A lot of your astrophography on the earth happens partially in the IR band. Hydrogen Alpha is a common emission spectrum to capture, and about 75% of it falls in the IR range.

Your common cameras (DSLR, etc) have IR filters specifically to prevent IR from reaching the sensor after going through the lens.

If they're only trying to capture in visible, then a better comparison would be the hubble, not the JWST.

Source: am an astrophotographer, see: https://www.jeromehollon.com/

Neat! I bought a 200 mm mirror and other stuff to make a Newtonian telescope, but I only got halfway through putting it together before I got distracted.

Where do you take your images? One of the pictures looks like a back yard.

Yup, I take them from my back yard in suburban kentucky. Bortle 7, so about what you'd expect in the suburbs of any large city.

Building a newt is exciting! It's something I've always wanted to try.

If you only capture visible light, you can't surpass the JWST. The entire point is very distant galaxies and astronomical bodies have red-shifted to the point they're no longer visible and can only be seen by infrared.

Found this thread from nearly a decade ago in cloudynights message board: https://www.cloudynights.com/topic/484005-diffractive-optics...

So far there hasn't been a telescope brought to market with this type of lens that is able to achieve apochromatic status. Apparently there's quite a few issues with off axis chromatic aberrations. That'd be a huge negative for a scientific telescope.

How large do they need to be for (even professional) cameras though? I thought the point of this wasn't the technology, but scaling it up to some absurd size.

Not my area, but the images in the Canon pages seem to depict the lenses as still being relatively thick overall, with several components doing the same job they do in conventional lenses, but with the addition of thin layers with refractive structures to deal with chromatic aberrations, whereas Apai is saying they can do an 8.5m wide lens which is only 0.5cm thick. I don't know the field to say whether that's new, but it sounds different from at least the Canon examples.

Off topic: is this a reputable news source?

It positions itself as "Academic rigor, journalistic flair" and appears to be sponsored by a number of US universities.

But some of the articles on its site seem questionable.

Ars reprinted it

https://arstechnica.com/space/2023/07/a-new-thin-lensed-tele...

> "This article is republished from The Conversation under a Creative Commons license"

This seem more like "free content" for Ars, than them fact checking the source / validity.

Anything in particular you think of? After cursory look at the title nothing stands out.

Seems like it is yes

https://adfontesmedia.com/conversation-bias-and-reliability/

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