Quote from: Steven Pietrobon on 04/19/2016 07:37 amQuote from: ChrisWilson68 on 04/15/2016 07:15 amIn fact, I suspect that this will be the final fate of this project -- it will end up being easier to directly image exoplanets from Earth than to pick up the signal from a tiny spacecraft so far away. So, even if it eventually proves possible to do, I think it will be made obsolete by better uses of the same technology needed to make it possible.They're talking about a 1 km² optical array for receiving the signals from the spacecraft! If you can build that, no need for sending spacecraft, just use the optical array directly. Having optical arrays in various places in the solar system should get you great resolution.To be fair, direct imaging of exoplanets would require gigantic interferometers for high resolutions. See the image attached. For example for 32km pixels at 10 parsecs (32.6 light years) you need 144km2 of mirrors and a 100k km baseline. On the other hand it would take 196 years to get some data from a starshot probe.
Quote from: ChrisWilson68 on 04/15/2016 07:15 amIn fact, I suspect that this will be the final fate of this project -- it will end up being easier to directly image exoplanets from Earth than to pick up the signal from a tiny spacecraft so far away. So, even if it eventually proves possible to do, I think it will be made obsolete by better uses of the same technology needed to make it possible.They're talking about a 1 km² optical array for receiving the signals from the spacecraft! If you can build that, no need for sending spacecraft, just use the optical array directly. Having optical arrays in various places in the solar system should get you great resolution.
In fact, I suspect that this will be the final fate of this project -- it will end up being easier to directly image exoplanets from Earth than to pick up the signal from a tiny spacecraft so far away. So, even if it eventually proves possible to do, I think it will be made obsolete by better uses of the same technology needed to make it possible.
Quote from: Oli on 04/19/2016 10:46 amQuote from: Steven Pietrobon on 04/19/2016 07:37 amQuote from: ChrisWilson68 on 04/15/2016 07:15 amIn fact, I suspect that this will be the final fate of this project -- it will end up being easier to directly image exoplanets from Earth than to pick up the signal from a tiny spacecraft so far away. So, even if it eventually proves possible to do, I think it will be made obsolete by better uses of the same technology needed to make it possible.They're talking about a 1 km² optical array for receiving the signals from the spacecraft! If you can build that, no need for sending spacecraft, just use the optical array directly. Having optical arrays in various places in the solar system should get you great resolution.To be fair, direct imaging of exoplanets would require gigantic interferometers for high resolutions. See the image attached. For example for 32km pixels at 10 parsecs (32.6 light years) you need 144km2 of mirrors and a 100k km baseline. On the other hand it would take 196 years to get some data from a starshot probe.Thanks to spectroscopy an exoplanet the size of Earth at only one pixel could still tell us a good deal. Though the smallest of the interferometers on that picture even at 10 pixels would be great. The IR version seems small enough to be doable but the atmosphere attenuates those wavelengths. So a high enough and wide enough mountain top may not exist.
The larger motivator, I think, is to send pieces of working terrestrial technology to other star systems within a single human lifetime. It's as much about being able to say that there is something of Mankind actively collecting data in situ from other star systems as it is about the data collected, I think.
Yes, getting that kind of resolution at 32.6 light years requires a huge array -- but, even if you do manage to send your 1 gram starship at 0.2c and somehow keep it operational for 163 years, after 196 years you *still* need to have a huge array to pick up the signal from your 1 gram probe 32 light years away -- probably even bigger than 144 km^2.
The biggest issue with all those interstellar probe concepts is how long it takes for the probes to reach their target. Unless we reach a point of technological stagnation making a huge investment in something that will deliver results hundreds of years later is utterly pointless. In particular when there are competing concepts like telescopes. Until we want to look at flora and fauna on a nearby exoplanet in detail I think interstellar probes make no sense.
My scant understanding of optical inferometers is that the processors have to be wildly fast to synch the signals, and that the problem gets worse the larger the distance between collectors. Radio inferometers are easier because the wavelengths are so much longer than visible light.I guess the good news here is that non-related advances in computing speed will naturally overcome the processing hurdles specific to optical interferometry, and in time, any telescope could be linked to create an array of any size.More to the point of this thread though, photos of rotating planets would need to be very short duration photos to avoid smearing. Otherwise, all you can do is measure average albedo, temps, atmospheric composition, etc....
Because of the short wavelengths, optical interferometers have to combine light from each telescope live. That's a complicated optical process.Radio interferometers with much longer wavelengths can record signals and combine them later. As long as the observations are synchronized via atomic clocks.The CHARA Array at Mount Wilson has a maximum baseline of 330 m, but that's with six 1 m telescopes for a total of less than 5 m2. Multiple km2 arrays are an incredible concept.
Quote from: RonM on 04/21/2016 01:10 amBecause of the short wavelengths, optical interferometers have to combine light from each telescope live. That's a complicated optical process.Radio interferometers with much longer wavelengths can record signals and combine them later. As long as the observations are synchronized via atomic clocks.The CHARA Array at Mount Wilson has a maximum baseline of 330 m, but that's with six 1 m telescopes for a total of less than 5 m2. Multiple km2 arrays are an incredible concept.CHARA array has optical links between elements, which up until now has been the main techincal constraint for all optical interferometers.According to the paper up thread, seems that this restriction is going awaybsoon and electronically linked array elements will become a real possibility soon, with the promise to extend baselines to kilometer ranges and beyond
JWST with a good starshade (which is in itself a big investment, actually) would allow you to do study of a nearby exoplanet. Even though you'd just see a single pixel, you could get light curves, could even get rough images of continents, detailed spectroscopy, etc, if you looked long enough and with the right instruments.
According to the paper up thread, seems that this restriction is going away soon and electronically linked array elements will become a real possibility soon, with the promise to extend baselines to kilometer ranges and beyond
Quote from: savuporo on 04/21/2016 01:31 amAccording to the paper up thread, seems that this restriction is going away soon and electronically linked array elements will become a real possibility soon, with the promise to extend baselines to kilometer ranges and beyondNote that is an intensity interferometer. It may be useful, but AFAIK it doesn't have the same capabilities as traditional optically linked interferometers or radio VLBI.
Quote from: Oli on 04/20/2016 07:55 amThe biggest issue with all those interstellar probe concepts is how long it takes for the probes to reach their target. Unless we reach a point of technological stagnation making a huge investment in something that will deliver results hundreds of years later is utterly pointless. In particular when there are competing concepts like telescopes. Until we want to look at flora and fauna on a nearby exoplanet in detail I think interstellar probes make no sense. Nah, that's basically saying that Voyagers and Pioneers weren't worth doing, because we were going to have NERVA powered starships any day now that could get there faster.
The Starshot sail would fly edge-on to minimize the cross-section exposed to matter in the interstellar medium. Here we’re dealing with a lot of unknowns because we’ve only gotten one mission out beyond the heliopause, and it — Voyager — wasn’t designed to do the kind of measurements we’d like to have about the Local Interstellar Medium (LISM). But based on what we do know about local ‘bubbles’ in the medium and our Sun’s position in them, a fast mission to Alpha Centauri seems survivable at least by some of the craft thrown at it. Redundancy thus becomes crucial, which is why the plan is to send a large number of sails.And here we arrive at yet another challenge, or ‘miracle’ if you will. We’ll look at getting a signal back to Earth on Monday, but the plan is to use the sail itself as an optical element, turning it into a phased receiver as well as a transmitter. The tolerances needed in doing this, and the technologies required to shape the sail at its destination, remain unexplored territory. We have to ensure that this element is not the showstopper. As you might expect, data reception back on Earth is to be handled through the enormous laser array that sent the craft.That array also serves as a kilometer-class telescope, meaning it would have a useful future of continuing astronomical observation. And as a beamer, says Worden, the laser array is multi-purpose. A successful beamer could make possible any number of missions within the Solar System and beyond, including the gravitational lens FOCAL mission. We have to remember we’re not just targeting Alpha Centauri. “We’re convinced we can contemplate in this century, and perhaps in a single generation, expanding the human reach to the stars.” Note the plural.
Quote from: hop on 04/21/2016 01:59 amQuote from: savuporo on 04/21/2016 01:31 amAccording to the paper up thread, seems that this restriction is going away soon and electronically linked array elements will become a real possibility soon, with the promise to extend baselines to kilometer ranges and beyondNote that is an intensity interferometer. It may be useful, but AFAIK it doesn't have the same capabilities as traditional optically linked interferometers or radio VLBI.That Wikipedia article seems out of date. It seems that even though direct phase information is lost, aperture synthesis images actually are still possible. But the math involved is beyond me
3.4. Image reconstruction from second-order coherenceWhile intensity interferometry possesses the advantage of not being sensitive to phase errors in the optical light path, ordinary two-telescope correlations also do not permit such phases of the complex coherence to be measured. These correlations provide the absolute magnitudes of the respective Fourier transform components of the source image, while the phases are not directly obtained. Such quantitites can be used well by themselves to fit model parameters such as stellar diameters, stellar limb darkening, binary separations, and circumstellar disk thicknesses, but actual images cannot be directly computed through a simple inverse Fourier transform.While a two-component interferometer (such as the classical one at Narrabri) offers only very limited coverage of the Fourier (u, v)-plane, a multicomponent system provides numerous baselines and an extensive coverage of the interferometric plane. Already intuitively, it is clear that the information contained there must place stringent constraints on the source image. For instance, viewing the familiar Airy diffraction pattern (cf. Fig. 1 left), one immediately recognizes it as originating in a circular aperture, although only intensities are seen. However, it is also obvious that a reasonably complete coverage of the diffraction image is required to convincingly identify a circular aperture as the source.Various techniques (most unrelated to astronomy) have been developed for recovering the phase of a complex function when only its magnitude is known. Methods specifically for intensity interferometry were worked out by Holmes et al. (2004, 2010, 2013) for one and two dimensions, respectively. Once a sufficient coverage of the Fourier plane is available, phase recovery and imaging indeed become possible. Nuñez et al. (2012a,b) applied this phase recovery to reconstruct images from simulated intensity interferometry observations, demonstrating that also rather complex images can be reconstructed. (However, a limitation that still remains is the non-uniqueness between the image and its mirrored reflection.)