‘Aragoscope’ Offers High Resolution Optics in Space

by Paul Gilster on August 12, 2014

Our recent discussions of the latest awards from the NASA Innovative Advanced Concepts office remind me that you can easily browse through the older NIAC awards online. But first a word about the organization’s history. NIAC operated as the NASA Institute for Advanced Concepts until 2007 under the capable leadership of Robert Cassanova, who shepherded through numerous studies of interest to the interstellar-minded, from James Bickford’s work on antimatter extraction in planetary magnetic fields to Geoffrey Landis’ study of advanced solar and laser lightsail concepts. The NIAC Funded Studies page is a gold mine of ideas.

NIAC has been the NASA Innovative Advanced Concepts office ever since 2011, when the program re-emerged under a modified name. NASA’s return to NIAC in whatever form was a welcome development. Remember that we had lost the Breakthrough Propulsion Physics project in 2002, and there was a time there when the encouragement of ideas from outside the agency seemed moribund. Now we’re seeing opportunities for new space concepts that have ramifications on how NASA conducts operations, a welcome platform for experimentation and discovery.

Over the years I’ve written a number of times about Webster Cash’s ideas on ‘starshades,’ which came together under the New Worlds concept that has itself been through various levels of NASA funding. Starshades are large occulters that are used to block out the light of a central star to reveal the planets orbiting around it. Properly shaped, a starshade placed in front of a space telescope can overcome the diffraction of light (where light bends around the edges, reducing the occulter’s effectiveness). Cash’s New Worlds pages provide an overview of his starshade concepts and link to NASA study documents presenting the idea in detail.

With this background, it’s interesting to see that NIAC awarded Cash a new NIAC grant in June to study what he calls an Aragoscope, named after Dominique-François-Jean Arago, who carried out a key experiment demonstrating the wave-like nature of light in 1818. Rather than overcoming the diffraction of light, as the starshade is designed to do, the Aragoscope would take advantage of it, blocking the front of the telescope with a large disk but allowing the diffracted light to converge to form an image behind the disk. This ‘Arago Spot’ (also called the ‘Poisson Spot’) was what Arago had demonstrated, a bright point that appears at the center of a circular object’s shadow.

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Image: Arago spot experiment. A point source illuminates a circular object, casting a shadow on a screen. At the shadow’s center a bright spot appears due to diffraction, contradicting the prediction of geometric optics. Credit: Wikimedia Commons.

How to put this effect to work in the design of a space telescope? Unlike a starshade, the Aragoscope would be circular in shape, an opaque disk whose diffracted light is directed toward a pinhole camera at its center, then to a telescope that provides extremely high resolution views of stellar objects. Cash sees the method as a way to lower the cost of large optical systems limited by diffraction effects in a dramatic way. Rather than being overcome by such effects, his instrument would gather diffracted light and refocus it. From the NASA announcement last June:

The diagram in the summary chart shows a conventional telescope pointed at an opaque disk along an axis to a distant target. Rather than block the view, the disk boosts the resolution of the system with no loss of collecting area. This architecture, dubbed the “Aragoscope” in honor of the scientist who first detected the diffracted waves, can be used to achieve the diffraction limit based on the size of the low cost disk, rather than the high cost telescope mirror. One can envision affordable telescopes that could provide 7cm resolution of the ground from geosynchronous orbit or images of the sky with one thousand times the resolution of the Hubble Space Telescope.

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Image: A view of the Aragoscope, an opaque disk with associated telescope. Credit: Webster Cash.

An article in Popular Mechanics this July notes that a 1000-kilometer Aragoscope could study the event horizon of black holes in the X-ray spectrum, making for highly detailed views of interesting galactic nuclei like that of M87. But Cash also talks about picking out features like sunspots and plasma ejections on nearby stars, and says an early target, once a space-based version of the Aragoscope could be launched, would be Alpha Centauri A and B. It’s intriguing that the Aragoscope, in some ways, turns Cash’s earlier starshade concepts on their head, aiming for high resolution rather than high contrast. “I spent a lot of time understanding the physics of destroying diffractive waves very efficiently,” he told the magazine. “In the process, it’s not hard to see that you can use those detractive waves to create images.”

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{ 11 comments… read them below or add one }

Wojciech J August 12, 2014 at 17:32

Amazing, why not useful for detecting life bearing worlds as I understand, it could provide interesting option for observation of possible techno-signatures.

superkuh August 12, 2014 at 17:32

An opaque disk certainly sounds cheaper than a large mirror but I wonder about the precision required. Does anyone have any idea of the disk edge radius tolerances needed for a 7m design?

David A. Czuba August 12, 2014 at 18:41

What an amazing principle of diffraction Cash is using to develop technology. It immediately made me think, as it may have bought to mind in others, about detecting Poisson Spots from eclipses and occultations, and what they may indicate about the light from the object beyond them.

Alex Tolley August 12, 2014 at 19:15

Conceptually similar to the FOCAL mission, but using diffraction instead of gravitational focusing? I assume that because diffraction depends on wavelength, this makes it even easier to do spectrographic analysis by simply moving the focus?

If the technique works as advertised, it should be extraordinarily cheap to make the disk, pack and deploy it. So what are the problems that need to be addressed?

Alex Tolley August 12, 2014 at 19:56

So what are the problems that need to be addressed

Some further reading suggests that the circular disk needs to be circular with a tolerance of less than the wavelength of light. This suggests that the surface temperature will need to be extremely even to prevent even the slightest differential expansion of the disc that would deform it near perfect manufacture. The disc will also need to be positioned well away from any gravitational sources. So not easy to construct or deploy.

I just wonder whether this might even be usable as a crude gravitational wave detector if it was large enough.

Ronald August 13, 2014 at 0:10

Hi Paul, I am presently in Florida, enjoying a vacation with my two sons (Erik 17, Robert 19).
We visited Kennedy Space Center today, though very touristically exploited, also a place of vision and future.
This reminded me that around this time, early August if I am not mistaken, it is 10 years ago that you started your website Centauri-Dreams.
That means 10 years of vision and awesomeness, the hope and courage to dream of a Big Future.
Paul, your website has been the pied-a-terre for me and many visionaries and interstellar dreamers for all those years now.
All the best of success for you for many years and decades to come.
Keep the dream and the vision alive!

Ole Burde August 13, 2014 at 6:15

The new idea of an Aragoscope might have a great potential in the future , and NIAC is exactly the right framework to support it . On the other hand , it would also be a great thing if NASA’s efforts actually led to ANY kind of starshade actualy being BUILD . The New Worlds starshade-telescope is a cheaper version of another program that never got of the ground , and in 2015 it will itself have to compete with several others in order to get ANY funding . I dont think you have to be paranoid in order to be afraid of the scenario where the ”new promising ideas of an Aragoscope” becomes the perfect excuse for ”waiting a little longer until the best aproach becomes clear” , in short killing the New Worlds programe ..

Paul Gilster August 13, 2014 at 8:51

Ronald writes:

This reminded me that around this time, early August if I am not mistaken, it is 10 years ago that you started your website Centauri-Dreams.

Ten years indeed, as of August 4. You have a great memory, my friend! Thanks very much for your kind words above, the kind of thing that keeps me going. It’s been a wonderful pleasure writing for this audience, and I thank you for your many comments over the years. Enjoy Florida!

Alex Tolley August 13, 2014 at 12:38

the circular disk needs to be circular with a tolerance of less than the wavelength of light.

I’m wondering if the Arago spot can be adjusted computationally for defects. If known beams are used, the resulting spots could be computationally adjusted to retrieve the correct image. This would then be applied to the actual image. It may be possible to do this in almost real time, to correct for distortions. Conceptually similar to adaptive optics, but using a different approach.

NS August 13, 2014 at 12:38

I’m unclear if this concept could be demonstrated with a ground-based telescope, or if it inherently can only work in space e.g. with the constraints Alex Tolley describes.

WM Morgan August 21, 2014 at 12:19

Could such a disk be used with the currently-existing Hubble?

That might involve an ad-hoc but well-thought-out maintenance mission (or two), then boost the Hubble to geosynchronous or to a Lagrange point. But I’m sure that would all still amount to a tremendous bargain.

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