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Nautilus: New Lens Concept for Space-based Array

As we’ve been talking about the limitations of giant telescopes in recent days — and a possible solution in David Kipping’s idea of a ‘terrascope’ — it pays to keep in mind how our ability to collect light has changed over the years. Thus the figure below, which is drawn from a new paper from Daniel Apai and Tom Milster (both at the University of Arizona) and colleagues. Here we see four centuries of evolution for light-collecting power through refracting and reflecting telescopes, with the introduction of segmented mirrors making larger apertures possible.

Image: This is Figure 1 from the paper (click to enlarge). Caption: Evolution of light-collecting area of ground-based (blue, green) and space-based (red) telescopes. The evolution is characterized by alternating stages of slow growth (when existing technology is scalable) and pauses (when existing technology cannot be scaled up). The data points represent the installation of the largest telescopes in their era and are connected to highlight general trends. Gray regions mark the approximate stages in the evolution when lenses, monolithic mirrors, and then segmented mirrors become to massive to be viable with existing technology. Telescopes used multiple different technological solutions to collect light. Large jumps in diameter are due to change in technology due to technological breakthroughs. Credit: Daniel Apai / Tom Milster / UA.

Anticipated designs for space telescopes take us from Hubble’s 2.4-meter mirror up to the segmented mirrors of the James Webb Space Telescope (6.5 meters) and future concepts like the Large UV/Optical/IR Surveyor (LUVOIR), with a 15-m primary telescope. The gradual growth of aperture sizes is, Apai and Milster argue in their paper, a bottleneck in the design of astronomical telescopes, and one we’ll have to overcome to study Earth-like planets and their potential biosignatures.

What the scientists are offering is a telescope concept in which the primary mirror is replaced by what the authors describe as multiorder diffractive engineered (MODE) material lens technology. Here we move from refraction, which involves light changing direction as it moves from one medium to another, to diffraction, where the waves of light change direction as they encounter barriers and openings in their path — think Fresnel lenses, where waves can interact constructively or destructively depending on wavelength. The MODE lens developed at UA by the authors is in effect a hybrid between refractive and diffractive lens technologies.

In their paper, Apai and Milster propose a space telescope called Nautilus, which would operate as a fleet of 35 14-meter wide spherical telescopes, each of them more powerful than the Hubble instrument. Within each Nautilus unit, an 8.5-meter diameter MODE lens would be used for high-precision transit spectroscopy, along with a 2.5 meter lens optimized for wide-field imaging and transit searches. The combined array would be powerful enough to characterize 1,000 exoplanets from a distance as far as 1,000 light years. The light-collecting power Nautilus achieves, by the authors’ calculations, is the equivalent of a 50-meter diameter telescope.

Image: Each individual Nautilus lens is 8.5 meters in diameter, larger than the mirrors of the Hubble Space Telescope and James Webb Space Telescope. Credit: Daniel Apai.

The authors argue that “…the concept described here offers a pathway to break away from the cost and risk growth curves defined currently by mirror technology and has the potential to enable very large and very lightweight, replicable technology for space telescopes.” Such lenses are less sensitive to misalignments and deformations than conventional refractive lenses and are readily replicated through processes of optical molding. Says Apai:

“Telescope mirrors collect light – the larger the surface, the more starlight they can catch. But no one can build a 50-meter mirror. So we came up with Nautilus, which relies on lenses, and instead of building an impossibly huge 50-meter mirror, we plan on building a whole bunch of identical smaller lenses to collect the same amount of light.”

These lenses work via a design the authors have patented, one that allows them to be both large and far lighter than monolithic mirrors. Large aperture lenses using diffractive methods can be produced without the mass and volume of material required by refractive designs. They are much thinner, formed around a series of separate sections mounted in a frame. The easiest analogy is with lighthouse lenses, which are built around concentric annular sections.

That makes them less expensive to launch, but the design also takes into account the practicalities of the space launch business, with the capability of stacking individual modules on top of each other. The lenses are 10 times lighter in areal density and 100 times less sensitive to misalignments. Thus the potential is here to sharply reduce launch costs as well as the cost of initial fabrication through a series of instruments using replicated components and identical telescopes — with a fleet of 35 in play, Nautilus collects light efficiently and distributes risk among multiple instruments. That latter point resonates given how much rides on a successful launch and deployment of missions like the James Webb Space Telescope.

“Currently, mirrors are expensive because it takes years to grind, polish, coat and test,” adds Apai. “Their weight also makes them expensive to launch. But our Nautilus technology starts with a mold, and often it takes just hours to make a lens. We also have more control over the process, so if we make a mistake, we don’t need to start all over again like you may need to with a mirror.”

Image: A labeled illustration of an individual Nautilus unit. It is designed to stack many units on top of each other in a rocket before inflating in space. Credit: Daniel Apai.

The paper takes note of the scalability of the concept:

Due to its relatively low production and launch costs and the identical multispacecraft model that is relatively new to astrophysical space telescopes, the general Nautilus system proposed here provides an easily scalable approach. Such multispacecraft models (multiple identical units) are used commercially (Iridium system) and for geo- and planetary sciences (Voyagers, Mariners, Mars exploration rovers, etc.) to reduce per-unit costs and risks and to extend capabilities. Furthermore, telescopes utilizing similar architecture but increasing in size could demonstrate feasibility and mitigate risks, while producing scientific data.

If the Nautilus model works, the prospect of low-cost space telescopes far more powerful than anything now in space or planned for it opens up, bringing such capabilities to the level of individual universities, who could launch their own small instruments. That would be quite a switch from the current model of having to compete for time on overbooked space observatories like Hubble. And Nautilus itself gives us the potential for observing 1,000 Earth-like planets within the 1,000 light years the light-collecting power of Nautilus is designed to investigate.

The paper is Apai et al., “A Thousand Earths: A Very Large Aperture, Ultralight Space Telescope Array for Atmospheric Biosignature Surveys,” Astronomical Journal Vol. 158, No. 2 (29 July 2019). Abstract.

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{ 30 comments… add one }
  • Bill August 14, 2019, 15:03

    Of all the current proposals for improving the performance of telescopes I find this one the most fascinating. Quantum assisted telescope arrays.

    https://arxiv.org/abs/1809.03396

  • Harry R Ray August 14, 2019, 15:07

    Paul, since this seams to be the week for posts on revolutionary telescope improvements, please check out Microwave Kinetic Inductance Detectors(Mkid for short)currently being calibrated on the now(again)functioning(at least partially) Subaru Telescope. Unlike MODE, which makes it possible to make a lighter lens, Mkid makes it possible to gather light(in the near infrared)MUCH FASTER than before with existing lenses, eliminating extinction issues due to small but quick changes in the weather! If that in itself is not enough incentive to check it out, then consider THIS: Even though the CURRENT Mkid camera is just a prototype, it STILL has an outside chance possibility to successfully image Proxima b should an attempt be made in the near future!

    • Paul Gilster August 14, 2019, 15:25

      This is a new one on me, Harry. Thanks for the tip! I’ll look into it.

      • Harry R Ray August 17, 2019, 13:55

        Paul, if you have not already done so, go to a library and check out the August issue of Smithsonian’s “Air and Space ” magazine and read the article titled “First photograph of Another Earth.”

  • Gary Wilson August 14, 2019, 16:18

    I assume the authors mean dramatically reduce launch costs per area of array? To put up 35 8.5 meter arrays will cost a huge amount I would think but the payoff is large (if it works properly). This seems a very promising approach but I would like to see a realistic estimate of total budget required. And remember what the original JWST budget was just as a caveat.

    • Eric Strobel August 20, 2019, 16:49

      I doubt it’ll cost that much. It’s a mylar balloon. I imagine you could launch several at a time.

      Also, you wouldn’t need too many up to exceed the capability of anything else that’s planned.

  • jonW August 14, 2019, 17:16

    What an inspiring few days of posts–it is terribly exciting to dream about what humans will eventually be able to see (and anxiety-provoking to wonder whether we living now will get to be among those humans).

    Question: why “a thousand” earth-like planets? Surely the concept (if it works as planned) would allow astronomers of the future to characterise *all* planets within its 1000-ly-radius sphere of operation. With somewhere between 10^6 and 10^7 stars in such a volume I would expect more than a thousand earth-like planets, by the loose definition that has prevailed up to now (rocky, orbiting within the HZ).

  • Christopher Phoenix August 14, 2019, 17:21

    MODE is a very exciting potential improvement to space telescope technology, and doubly exciting since it offers the possibility of observing a much wider sample of exoplanets. It is quite interesting that these researchers propose circumventing existing limits of telescope aperture by switching to a fundamentally different technology (in this case, a new way to focus light via diffraction).

    This pattern occurs throughout history. Once an existing technology (sailing ships, refracting telescopes, microchips…) is invented, engineers improve it for a while. At some point, however, we reach the limits. Sailing ships cannot sail faster than the wind and current. The lenses used in refracting telescopes begin to sag due to gravity, and run into other problems as well (the practical limit is about 1 meter diameter). At that point, we need new approaches, like steam power and the reflecting telescope.

    I’ll be watching this space for sure. The far limits of telescope technology have always fascinated me.

    • Paul Gilster August 15, 2019, 7:19

      Really good to see you back on the site, Christopher. Thanks for your thoughts on this.

      • Christopher Phoenix August 15, 2019, 16:21

        Thanks for the welcome back, Paul. I’m glad to take part in the conversation again!

    • Ron S. August 15, 2019, 8:45

      “Sailing ships cannot sail faster than the wind and current.”

      Yes they can. This is a common error that spans a wide range of technologies where a cursory inspection of the problem parameters can be deceiving.

      • Christopher Phoenix August 15, 2019, 16:18

        Yes they can. This is a common error that spans a wide range of technologies where a cursory inspection of the problem parameters can be deceiving.

        Yes, that is a good catch, but I did not intend to make a precise statement about the relationship between wind speed and the speed of a sailing ship. My point is that sailing craft depend on changeable factors beyond our control for propulsion, a massive disadvantage compared to engine driven craft. Furthermore, we can (and have) design propulsion systems that vastly exceed the potential of sails even under the best conditions.

        It would have been far better for me to phrase it “Sailing ships cannot travel faster than the changing winds and currents allow.” Honestly, though, this doesn’t affect my main point!

        • Ron S. August 15, 2019, 21:54

          Please also note that I was not engaging in pedantry. The error is minor. That is why I elaborated further to explain myself. We are making different and, I believe, valid points.

  • Geoffrey Hillend August 14, 2019, 17:31

    I would like to see some more details of how the diffraction telescope works using light rays and a focal point since it looks like the Nautilus designers are trying to use the diffraction lens the same way a reflector mirror would work. Reflectors reflect the light in the opposite direction with the focal point some distance in front of the mirror. Diffraction means the wave bends around the corner of an object like the double slit experiment in quantum mechanics. I don’t see how diffraction can be used to make a main objective. Fresnel lenses use light collimation, reflection and refraction but not diffraction. https://en.wikipedia.org/wiki/Fresnel_lens .

    I don’t think a telescope lens can be constructed from using diffraction since only the corners are used and not the total diameter and light gathering power of the main aperture. A Fresnel lens is designed for light collimation and since it uses refraction it would be limited to the size of refractor type telescopes. It would work the same as a refractor with a long focal point, long main tube length and be limited how large a refraction type main objective lens could be made which is much smaller with much less light gathering power than any reflector like James Webb, etc.

  • Thomas R Mazanec August 14, 2019, 17:57

    Are you going to add this one to your projects list? :-)

    • Paul Gilster August 15, 2019, 7:19

      Soon, as we see it evolve and it develops some resources on the Net.

      • Eric Strobel August 20, 2019, 17:05

        I’d love to see them raise some $$ by implementing this on a smaller scale for amateur astronomy. Would certainly help prove the concept. Can you imagine a 24″ telescope for under $2k? I’d certainly look at buying one. At these smaller scales they could probably do additive manufacturing. At the same size range, do space qualification by putting them on cubesats… send them up collapsed & have a small bottle of gas in the satellite to inflate.

  • Patrick Underwood August 14, 2019, 20:47

    Note that the diameter of the SpaceX Starship is 9 meters.

    • Eric Strobel August 20, 2019, 16:45

      It’s not clear these need to be in their fully inflated configuration for launch.

  • Ronald August 15, 2019, 5:18

    First of all, I hope you had a very refreshing summer vacation, Paul, so well-deserved.
    And, if I am not mistaken, CD just had its 15th (!) birthday yesterday or so. Congratulations, Paul, on your awesome website, and you tireless efforts to keep us informed about all interesting new studies, developments and discoveries with regard to stellar planetary systems and interstellar travel.
    Through the years CD has been my pied-a-terre for this topic.
    Thank you and keep up the good work!

    • Paul Gilster August 15, 2019, 7:17

      Ronald, kind words indeed. Let me also point out that you’ve been around Centauri Dreams for almost its entire run, and you’re right, we just hit the 15th anniversary. The site has always relied on reader insights and suggestions and I continue to appreciate yours.

  • Ronald August 15, 2019, 5:40

    And then, in fact quite related to the study of “Earth-like planets and their potential biosignatures”, a very relevant new study by Penn State Univ., the most accurate estimate of the frequency of Earth-like planets in the HZ around solar-type (broadly: FGK) stars:
    “Occurrence Rates of Planets Orbiting FGK Stars: Combining Kepler DR25, Gaia DR2, and Bayesian Inference”, Hsu et al. 14 August 2019;
    https://iopscience.iop.org/article/10.3847/1538-3881/ab31ab

    And Science Daily article:
    https://www.sciencedaily.com/releases/2019/08/190814111903.htm

    Quote:
    “Based on their simulations, the researchers estimate that planets (…) from 0.75 to 1.5 Re, with orbital periods ranging from 237 to 500 days, occur around approximately one in four stars.”

    ArXiv preprint:
    https://arxiv.org/abs/1902.01417

    In particular, see Figure 2 and Table 2. And Table 3 for a summary of findings.

  • Jim Early August 15, 2019, 10:59

    The illustration appears to be a fast lens with a focal length close to the lens diameter. Such a low mass Fresnel lens will give a light gathering power of a large optic without the higher resolution of a large optic. This is similar to the lower mass optics of a lighthouse (or for older readers, the lens stuck to the back window of a Volkswagen bus). Such a system would work for spectroscopy changes in starlight during a planet transient. In this type of Fresnel lens the adjacent ring sections are not coherent.
    To separate the planet light from the starlight would require a coronagraph or starshade.
    A coherent Fresnel lens as used in laser fusion optics requires tenth wave precision in fabrication and usually have large f numbers (~100). These systems require separate spacecraft because of the kilometer scale focal lengths.
    Does this design use a separate spacecraft for the final optics?

    • Daniel Apai August 18, 2019, 18:34

      Hi Jim, this concept looks for temporal modulations of the starlight during planetary transit, so it does not need to spatially separate light from the planet and the star. There is no separate spacecraft with the detector or a starshade. Note, MODE lenses are not Fresnel lenses, but much more advanced optical elements. The light passing through the different zones is coherent. MODE lenses have much shorter focal lengths and greatly reduced chromatic dispersion with respect to regular Fresnel lenses.

  • Ronald August 16, 2019, 16:23

    I was just wondering: has there been any recent news about the Kepler Earth analog KIC 12266812, as mentioned in https://www.centauri-dreams.org/2019/05/08/planetary-interiors-a-key-to-habitability/comment-page-1/#comment-196855

    • Harry R Ray August 17, 2019, 10:11

      Not that I know of, but while we are waiting for news on that one, check out KIC9084589! This WAS the most similar planet candidate to Earth until KIC12266812 came along. The only difference is that THIS one ALSO has a Kepler Objects of Interest designation, KOI 7923.01. I do not know if KIC12266812 will EVER get a KOI designation or not, because the Kepler team has CEASED designating KOI’s after the Kepler mission ended, probably due to government regulations.

      • Harry R Ray August 17, 2019, 13:48

        The only way either of these intreguing candidates can be confirmed would be by follow-up observations with PLATO, which may take a decade or so to show results, unless, of course, we get REAL LUCKY and develop a means to observe them with modest ground based telescopes able to conduct dedicated observations because they were left behind in useful science enterprises due to larger and more advanced telescopes coming online. Such a means is starting to show SOME progress with already fungtioning prototypes. These prototypes are called diffractive diffusers(Paul: you should check these out,too) and the real McCoys, when they are perfected, will be called beam shaping diffusers(Where do they come up with these names?). If things all go as planned, this will allow smaller than one meter telescopes to achieve precise transit curves of Earth-sized planets orbiting sun-like stars out to 1,ooo parsecs or so, so that dozens of Kepler candidates can be proved or disproved.

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