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When making an observation using a radio telescope, is it possible to filter out noise coming out from a known source by using another telescope that will focus on that noise source?
By filtering, I mean processing the observation on a computer and calculating the filtered image using the second telescope source. Assume that both of the telescopes are connected and can easily sync on time and location and exchange other information.

Coherent interfering signals are usually referred to as RFI (or Radio Frequency Interference). One could certainly digitally filter out a coherent interfering signal from a telescope by observing from an additional telescope focused on the source of the interfering signal, given the proper geometry. Here is a research paper on Radio Frequency Interference Mitigation that outlines some of these very techniques in section 3.4: Post correlation studies.

Incoherent interference is typically referred to as noise, and won't be the same at the two telescopes. This means an astronomer can't use one telescope to filter out the noise at the other. Some examples of noise sources are:

1. Noise that comes from thermal from the telescope's electronics
2. Ground backscatter into the antenna receiver
3. Inaccuracies introduced by analog to digital conversion and other signal processing.

I don't think we can view the CMB as incoherent noise. The CMB is just as much of an extended yet coherent source as any extended object for which radio images exist. Any source that's distant compared to the size of the array is going to have coherence, since all antennas receive roughy the same waves, just delayed differently by their path lengths1.

Just for example:

The only problem is that CMB is not localized so it will be present no matter where one points a radio telescope. In other words, it's not "noise" because it's really there!

If you could build a second antenna so far away that parallax moved your intended object substantially (if for example you were making a radio image of Jupiter using an array of radio telescopes in a big halo orbit around a Sun-Earth Lagrange point) then in that case you might be able to

• Characterize it as noise, since the contribution would be different in each receiver
• Remove it somewhat from your image of the foreground object since it would be more local interference or incoherent thermal noise in each receiver2.

But of course Jupiter is pretty loud, so it might not be necessary.

1Although I struggle with the concept, we can think of each radio photon as entering all telescopes and interfering with itself, even if that happens six months later after the hard drives have all been shipped to a central location for processing!

2Actually it would be almost exactly like thermal noise, about 2.72548±0.00057 K in fact. (Slightly related: Why doesn't thermal radio emission from a DSN “hot dish” completely swamp the benefits of a cold LNA?)

Well, i didn’t build it myself, but I’m working at the university of Amsterdam on software for LOFAR.

too bad the software is not open source (yet).

Does it monitor 10-250 MHz signal?

Secondly, most government agencies around the world ban (or at least don’t look too kindly upon) transmitting on 1420 MHz. As would probably most of aliens’ governing agencies )

I don’t mean to be a hater, but i don’t think you know how a radio telescope work. when you use an optical telescope you don’t “broadcast” light into space. With a radio telescope you passively analyze the radio waves from a very very small section of the sky. Most of the time it is useful for scanning for new planets and stars. (e.g. a neutron star spins super fast, the EM profile of this is pretty unique.) So the government really wouldn’t care.

THAT BEING SAID: most of the time Radio Telescopes are pretty broad band.

Transmitting is not allowed so that the band is clear for radio astronomers to listen on.

My take on his comment was that it may also be rare for aliens to be broadcasting on that frequency and therefore there would be no little green men to listen to. I don’t think he was implying that anyone, alien or otherwise, would get upset at setting up a radio telescope.

Wim Apon (Dutch) is building an amateur radio astronomic observatory.

Yes Gijs,
Thanks: see my site at
http://home.kpn.nl/apon001/huis.htm
i did experiments with a rotatable beam antenna at 35 MHz, adding interferometry at 31 MHz, phasing interferometry at 31 MHz and made a map of the sky at this frequencies, and a 12.5 GHz disk experiment. See also my weblog… so you can see what i am doing day by day.
Wim

By the way: now i am using my own SETI-receiving station at home.
I published all software on my site, so everybody can make his own
SETI-receiving station now. You can use a TV-disk and a usb-receiver. or
a short-wave-communication receiver or whatever you have.
Every frequency without terrestrial signals will be okay.
An old xp computer does the job.
Just for the fun
Wim

This days i am experimenting with a phasing interferometry radio telescope on
146 MHz. In this stage i am building the electronics en getting the feeling of the
signals of the stars on this frequency. I see now nice fringes, so i am going on.

Sure radio telescopes are cheap now but the black hole machines will still cost you.

Most C-Band satellite dishes are 10 feet in diameter, although ones as small as 8 feet (central of the U.S.) and large as 15 foot (capable of handling trans ocean satellites or Alaka and Hawaii residents) are available.

While some folks consider satellite dishes to be eyesores, techies tend to see them as very beautiful tools.

As a general rule you don’t want to hook up a microwave antenna (including 1420 Mhz.) directly to a receiver, there’s too much loss over any distance. It’s generally better to use a downverter mounted at the antenna which reduces the frequency to something more manageable with reasonable coax cables. Of course one of the key advantages of an SDR is its wide frequency range.

I’ve got an SDR on order and I’m certainly interested in using it for radio astronomy. I am concerned about the gain and signal-to-noise ratio which will probably be pretty bad, but an external preamp and downverter will improve the performance.

Its just a USB TV Tuner can’t you put that out at the Dish and use the USB connection (since its digital) to connect to the computer

People used to do this with wi-fi dongles to increase the range so I would think yes, you could do the same with a radio telescope. You’d have to figure out some way of running such a long USB cable though, unless you had something like a Raspberry Pi sitting nearby to convert the USB into Ethernet.

USB is very short regardless of cable quality. You would have to sit out by the dish with a laptop. “malvineous” has a good idea with the conversion to Ethernet. Would a Raspberry Pi have enough power to do this with sufficient bandwidth though?

Another thought.. so long as some sort of computing device is being placed near the dish like that.. what if the backend sdr code could just run right there and only use the network for the display and/or playback? Could a Raspberry Pi handle that? I know the Raspberry Pi is very low spec compared to GNU Radio recommendations but I assume the recommendations are including processing power for the display, window manager, etc… If what was running out at the dish was completely cut down, just a minimalist OS with the SDR backend on top might it work?

gnuradio allows you to send the stream from a computer to another, so you can digitize it near the dish with the Rpi, send it through ethernet and on a beefy PC do the rest of the heavy signal analysis. I assume the Rpi is more than enough for this I guess I’ll know when I receive my Rpi soon and try it with my rtlsdr dongle :)

Or even more simpler, get an active USB cable or a USB over ethernet cable extender (dx has them)

Use [whatever] to downsample the Rf at the antenna. Then serialise the signal – and light up a LED. Fibre optic cable, photodetector at computer end.

If you are only listening to one band, you don’t need to send “change channel” signals. You get isolation with optic fibre, which is missing from standard ethernet solutions.

Contact, what a great movie! I’m still a fan after all these years.
It surely has something to do with Jodie :)

Oops, sorry. Please disregard that “Report Comment” as I hit that instead of the “Reply” button. -_-‘

Anyhoo, yeah I still tear up at the end. Carl died during film, didn’t live to see the final cut.

inflatable parabolic antenna dude

This is very interesting. I haven’t built a hardware radio telescope myself, but I’ve used Arecibo and the VLA for pulsar astronomy. Most relevantly, there is MIT’s “Small Radio Telescope” project. We actually have one on the roof of the physics building, and I worked with some undergraduate students to get it working.

What you can do with such a setup is principally limited by sensitivity. Practical dishes are really small by radio astronomy standards. Nevertheless, with the SRT you can, for example, map the rotation curve of the galaxy (combine this with a map of the visible matter and you can show that there must be dark matter!). Pulsar observations, though dear to my heart, are going to be a real challenge. A little easier if you’re far enough south to see the Vela pulsar (by far the brightest radio pulsar), but you’re still talking about hour-long integration times, folded with a known period for the pulsar, to see anything.

To get a working SRT-like setup you’d need, approximately:
* A dish antenna
* An altitude-azimuth mount for the above
* A low-noise amplifier for 1420 MHz
* A hardware band-pass filter
* A noise diode and maybe some flaps to close the feed, for calibration
* The RTLSDR gizmo
* Control electronics
* Software

Unfortunately, the biggest limitation you’ll run into is the dish. The SRT uses commercial

2 m dishes, which are already big and heavy enough to require a fairly expensive alt-az mount. If you could get your hands on one of the bigger (4m?) dishes, you’d gain dramatically in sensitivity, but the mount would be correspondingly heavier and more expensive.

Once past that hurdle, though, the rest is fairly straightforward (at least by comparison). You can buy LNAs off the shelf you might want to combine yours with some kind of cooling system, even just a Peltier unit. A band-pass filter you can probably also buy off the shelf the point of it is that there’s *so* much interference these days that it’s best if you can keep it out of the ADC. The control electronics for the alt-az mount and anything else are easily built using an Arduino.

For the software, if you have something like GNU Radio, you’re pretty much golden. Almost all radio astronomy measurements amount to simply computing the power in a fixed bandpass over a fixed time. For “continuum” observations (measuring the Sun’s brightness, for example) your bandpass is as wide as you can manage and your time period is usually seconds long. Spectral-line work uses narrower bandpasses but similarly long times. All this is easily implemented within GNU Radio.

If you want to do interferometry, you’re going to want at least very stable VFO/clocks, and ideally shared VFOs between multiple receiver modules. Probably doable but it might require some hacking on the dongles.

On the other hand, since the dongle is *so* cheap, you might have an interesting time building an array of crossed dipoles, with an RTLSDR and a Raspberry Pi on each one, to serve as a LOFAR-style interferometer. Using lower frequencies also gives you higher fractional bandwidths and almost all sources are brighter, so this might be a good way to go.

TLDR: Perfectly doable, but I don’t know that you’re going to save much compared to the SRT’s

## Radio-burst discovery deepens astrophysics mystery

The Arecibo Observatory is shown. Credit: NAIC - Arecibo Observatory, a facility of the NSF

The discovery of a split-second burst of radio waves by scientists using the Arecibo radio telescope in Puerto Rico provides important new evidence of mysterious pulses that appear to come from deep in outer space.

The finding by an international team of astronomers, published July 10 in The Astrophysical Journal, marks the first time that a so-called "fast radio burst" has been detected using an instrument other than the Parkes radio telescope in Australia. Scientists using the Parkes Observatory have recorded a handful of such events, but the lack of any similar findings by other facilities had led to speculation that the Australian instrument might have been picking up signals originating from sources on or near Earth.

"Our result is important because it eliminates any doubt that these radio bursts are truly of cosmic origin," said Victoria Kaspi, an astrophysics professor at McGill University in Montreal and Principal Investigator for the pulsar-survey project that detected this fast radio burst. "The radio waves show every sign of having come from far outside our galaxy – a really exciting prospect."

Exactly what may be causing such radio bursts represents a major new enigma for astrophysicists. Possibilities include a range of exotic astrophysical objects, such as evaporating black holes, mergers of neutron stars, or flares from magnetars—a type of neutron star with extremely powerful magnetic fields.

Optical sky image of the area in the constellation Auriga where the fast radio burst FRB 121102 has been detected. The position of the burst, between the old supernova remnant S147 (left) and the star formation region IC 410 (right) is marked with a green circle. The burst appears to originate from much deeper in space, far beyond our galaxy. Credit: © Rogelio Bernal Andreo (DeepSkyColors.com)

"Another possibility is that they are bursts much brighter than the giant pulses seen from some pulsars," notes James Cordes, a professor of astronomy at Cornell University and co-author of the new study.

The unusual pulse was detected on Nov. 2, 2012, at the Arecibo Observatory, a National Science Foundation-sponsored facility that boasts the world's largest and most sensitive radio telescope, with a radio-mirror dish spanning 305 metres and covering about 20 acres.

While fast radio bursts last just a few thousandths of a second and have rarely been detected, the international team of scientists reporting the Arecibo finding confirm previous estimates that these strange cosmic bursts occur roughly 10,000 times a day over the whole sky. This astonishingly large number is inferred by calculating how much sky was observed, and for how long, in order to make the few detections that have so far been reported.

"The brightness and duration of this event, and the inferred rate at which these bursts occur, are all consistent with the properties of the bursts previously detected by the Parkes telescope in Australia," said Laura Spitler, lead author of the new paper. Dr. Spitler, now a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Bonn, Germany, was a PhD student at Cornell when the research work began.

The bursts appear to be coming from beyond the Milky Way galaxy based on measurement of an effect known as plasma dispersion. Pulses that travel through the cosmos are distinguished from man-made interference by the effect of interstellar electrons, which cause radio waves to travel more slowly at lower radio frequencies. The burst detected by the Arecibo telescope has three times the maximum dispersion measure that would be expected from a source within the galaxy, the scientists report.

The discovery was made as part of the Pulsar Arecibo L-Band Feed Array (PALFA) survey, which aims to find a large sample of pulsars and to discover rare objects useful for probing fundamental aspects of neutron star physics and testing theories of gravitational physics.

Efforts are now under way to detect radio bursts using radio telescopes that can observe broad swaths of the sky to help identify them. Telescopes under construction in Australia and South Africa as well as the CHIME telescope in Canada have the potential to detect fast radio bursts astronomers say these and other new facilities could pave the way for many more discoveries and a better understanding of this mysterious cosmic phenomenon.

• Post published: April 27, 2020
• Post category: Technology

Electromagnetic waves generally called the “Radio waves” are waves whose wavelength is a little longer than that of white light and with a much lower frequency than light. These waves are generated naturally by generating pulses of electricity. They have their wavelengths and frequencies lying in a broad electromagnetic spectrum. They contain the least energy and are the longest of any electromagnetic wave. Radio waves vary from about a few centimetres about the length of a pencil, to waves the length of a vehicle, all the way up to gigantic waves longer than the diameter of our planet Earth. Heinrich Hertz discovered them in 1888. They are as pacy as visible light radiations and are widely used in modern technology. Do you wish to know about its applications? If affirmative, then follow me as I will highlight the 12 significant uses of Radio waves.

Radio waves have their use in studying the “Radio Astronomy,” in which the astronomers have to map the radio waves being emitted from the objects in space, by using the radio telescope.

2. Extraterrestrial research

They can provide information about the extraterrestrial materials in the universe because they can travel long distances, which is impossible through an optical telescope.

Radio and TV broadcasting is the transmission of audio and visual via radio waves from a transmitter to the audience through antennas.

4. Remote control car and toys

Ever wondered how cars and toys are controlled remotely? The signals are transmitted via Radio waves.

Remote control cars are one of the greatest technological breakthroughs. Today, Cars can be controlled from afar by simply pressing buttons of the remote controller.

Radio waves are also used for the transmission of Data and Wi-Fi from one place to another through QAM (Quadrature Amplitude Modulation).

RADAR (Radio Detection and Ranging System) is a defencing system, comprised of radio waves, which are used by Naval and Air Forces, in order to detect the location and distance of an intruder from the point where the radar is placed.

Radio waves are also the carrier of GPS signals, being transmitted from the satellites.

Radio waves are also used in a technology called “Radio Tomography.” It involves imaging the people and objects in a geographical region using invisible radio waves by using a receiver and a sender (low powered devices). For instance (Adaptive Bayesian Radio Tomography).

9. In Magnetic Resonance Imaging

Radio waves are harnessed in Medical imaging of the internal body organs through MRI (Magnetic Resonance Imaging). Nowadays, these Radio waves are also used to treat a number of medical conditions like diathermy.

10. In beam-forming and massive MIMO

The latest 5G technologies also use Radio waves to maximize the signals that the connected device receives. This is also called beamforming and massive MIMO.

11. In Security systems

Security systems that are present in the doors of shopping malls and in homes work by emitting radio waves on the object that passes in front of it and thus detect its movement. It comprises of a device-hub system that is plugged in the wall.

Another application includes RFID (Radio Frequency Identification), which is basically a fast tag reading technique but using radio waves instead of light (used by barcode reader). It can read 1000 tags at a single time and is much faster than the barcode reader, which requires light to transmit and receive data.

Radio waves are low-frequency electromagnetic waves that convey electrical impulses from one media to another remotely and with no need for physical contact between the two media. It has extensive uses in broadcasting and radar systems.

## When was the Very Large Array built?

Beginning in the 1960s, scientists conceived of a gigantic radio dish array that could complement the work of single-dish facilities, according to a history from the National Radio Astronomy Observatory (NRAO), which oversees the VLA. Because of the principles of optics, several telescopes can work in tandem and combine their data to act as one telescope with a collecting area the size of the distance between the individual dishes — a technique known as interferometry.

Congress approved funding for the VLA in August 1972 and construction started the next year. The facility was completed and formally dedicated in 1980, costing a total of $78 million in 1972 (the equivalent of$485 million today), or approximately $1 per taxpayer, according to the New Mexico tourism department. The VLA sits on the Plains of San Agustin, a flat stretch of empty desert around 50 miles (80 km) northwest of Socorro. Because it is far from major cities and surrounded by mountains on every side, which act a natural fortress to keep out radio interference, the site is ideal for radio astronomy, according to the NRAO. The dry climate is another key to the VLA's success. Radio waves are absorbed by water molecules in Earth's atmosphere and so being in a place with extremely low humidity allows the facility to have a clear window into events in the night sky. ## Is it possible to filter radio-waves using another radio telescope? - Astronomy Radio astronomers have directly measured the distance to a faraway galaxy, providing a valuable "yardstick" for calibrating large astronomical distances and demonstrating a vital method that could help determine the elusive nature of the mysterious Dark Energy that pervades the Universe. Galaxy UGC 3789 Visible-light image of UGC 3789 CREDIT: STScI "We measured a direct, geometric distance to the galaxy, independent of the complications and assumptions inherent in other techniques. The measurement highlights a valuable method that can be used to determine the local expansion rate of the Universe, which is essential in our quest to find the nature of Dark Energy," said James Braatz, of the National Radio Astronomy Observatory (NRAO), who presented the work to the American Astronomical Society's meeting in Pasadena, California. Braatz and his colleagues used the National Science Foundation's Very Long Baseline Array (VLBA) and Robert C. Byrd Green Bank Telescope (GBT), and the Effelsberg Radio Telescope of the Max Planck Institute for Radioastronomy (MPIfR) in Germany to determine that a galaxy dubbed UGC 3789 is 160 million light-years from Earth. To do this, they precisely measured both the linear and angular size of a disk of material orbiting the galaxy's central black hole. Water molecules in the disk act as masers to amplify, or strengthen, radio waves the way lasers amplify light waves. The observation is a key element of a major effort to measure the expansion rate of the Universe, known as the Hubble Constant, with greatly improved precision. That effort, cosmologists say, is the best way to narrow down possible explanations for the nature of Dark Energy. "The new measurement is important because it demonstrates a one-step, geometric technique for measuring distances to galaxies far enough to infer the expansion rate of the Universe," said Braatz. The GBT Robert C. Byrd Green Bank Telescope CREDIT: NRAO/AUI/NSF The VLBA Very Long Baseline Array CREDIT: NRAO/AUI/NSF Dark Energy was discovered in 1998 with the observation that the expansion of the Universe is accelerating. It constitutes 70 percent of the matter and energy in the Universe, but its nature remains unknown. Determining its nature is one of the most important problems in astrophysics. "Measuring precise distances is one of the oldest problems in astronomy, and applying a relatively new radio-astronomy technique to this old problem is vital to solving one of the greatest challenges of 21st Century astrophysics," said team member Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA). The work on UGC 3789 follows a landmark measurement done with the VLBA in 1999, in which the distance to the galaxy NGC 4258 -- 23 million light-years -- was directly measured by observing water masers in a disk of material orbiting its central black hole. That measurement allowed refinement of other, indirect distance-measuring techniques using variable stars as "standard candles." The measurement to UGC 3789 adds a new milepost seven times more distant than NGC 4258, which itself is too close to measure the Hubble Constant directly. The speed at which NGC 4258 is receding from the Milky Way can be influenced by local effects. "UGC 3789 is far enough that the speed at which it is moving away from the Milky Way is more indicative of the expansion of the Universe," said team member Elizabeth Humphreys of the CfA. Following the achievement with NGC 4258, astronomers used the highly-sensitive GBT to search for other galaxies with similar water-molecule masers in disks orbiting their central black holes. Once candidates were found, astronomers then used the VLBA and the GBT together with the Effelsberg telescope to make images of the disks and measure their detailed rotational structure, needed for the distance measurements. This effort requires multi-year observations of each galaxy. UGC 3789 is the first galaxy in the program to yield such a precise distance. Team member Cheng-Yu Kuo of the University of Virginia presented an image of the maser disk in NGC 6323, a galaxy even more distant than UGC 3789. This is a step toward using this galaxy to provide another valuable cosmic milepost. "The very high sensitivity of the telescopes allows making such images of galaxies even beyond 300 million light years," said Kuo. Braatz works on the project with Reid and Humphreys of the CfA Jim Condon of the NRAO Cheng-Yu Kuo of the University of Virginia Christian Henkel of the MPIfR Fred Lo and Violette Impellizzeri of the NRAO Ingyin Zaw of New York University Avanti Tilak of the CfA and Lei Hao of the University of Texas. ## Is it aliens? Scientists detect more mysterious radio signals from distant galaxy For the past decade, scientists have been puzzling over powerful, millisecond-long flashes of energy from deep space. Some scientists think these “fast radio bursts,” or FRBs, come from natural sources, such as newborn neutron stars or black holes. Others think they could be signals from alien civilizations. One thing’s for sure: FRBs are more common than we realized. In the latest discovery, scientists working as part of a$100-million initiative known as Breakthrough Listen used artificial intelligence to detect dozens of additional FRBs coming from FRB 121102, an as-yet-uncharacterized source in a galaxy 3 billion light-years from Earth.

The work is the first step in the initiative’s grander plans for using AI to find hidden patterns in the bigger sea of cosmic signals that come our way — research that could finally provide an answer to that eternal question: Are we alone in the universe?

"It's a great way of developing the kinds of techniques that we ultimately want to use to find other types of signals that might come from extraterrestrial intelligence," says Andrew Siemion, principal investigator for Breakthrough Listen and director of the Search for Extraterrestrial Intelligence (SETI) Research Center at the University of California, Berkeley.

In August 2017, the Breakthrough Listen team discovered 21 fast radio bursts from FRB 121102 during five hours of observations made by a radio telescope in Green Bank, West Virginia. In their latest study, which will be published in an upcoming issue of the Astrophysical Journal, the researchers deployed a specialized AI technique known as deep learning to see if any signals had been overlooked in their initial research.

Siemion gave Yunfan "Gerry" Zhang, a doctoral student at Berkeley, the job of training a deep learning algorithm to hunt for the additional bursts. The trained AI was turned loose to sort through 400 terabytes of observational data — a huge trove containing about as much data as is contained in 40,000 hours of 4K video.

After a month of work, Zhang strolled into Siemion's office and told his stunned mentor that he had discovered about 100 previously undetected bursts. To be sure Zhang was right, the researchers used standard computer software to clean up the messy signals — and confirmed the existence of at least 72 additional bursts.

The same AI approach could help astronomers find new repeating sources of fast radio bursts closer to Earth than FRB 121102. If closer repeater sources do exist, astronomers might be able to get a better look at them using optical and X-ray telescopes, says Harvard astrophysicist Avi Loeb, the science theory director for all initiatives funded by the Breakthrough Prize Foundation.

### Space A mysterious particle from a distant galaxy made its way to an observatory in Antarctica

"We still have no new clue on whether the origin is artificial or natural," said Loeb, who was not directly involved in the Breakthrough Listen work.

Loeb had previously examined the possibility that fast radio bursts come from radio transmitters constructed by an advanced alien civilization — perhaps evidence of powerful energy beams used to propel alien starships. He also theorized that repeater sources such as FRB 121102 are more likely to be such alien signals because natural origin explanations would most likely produce only a single burst.

But the Breakthrough Listen team is already looking beyond fast radio bursts. It's developing AI to analyze a wider range of similarly interesting signals coming from nearby stars and galaxies. In this case, AI's broad pattern recognition abilities could prove especially helpful when nobody knows exactly what an alien signal might look like.

"If AI could flag things that don’t look right or don’t look natural, that might be an interesting thing to do," says Seth Shostak, a senior astronomer at the SETI Institute in California who was not involved in the Breakthrough Listen study.

Eventually, AI could do much more than just filter radio signals. Shostak speculates that AI — already enabling Internet searches for cat and dog videos — could someday automatically search telescope images for unusual visual features that might represent huge alien megastructures from either existing or bygone civilizations. As he puts it, "AI can find cats on the Internet, but maybe it could also find Klingons in space.”

## Is it possible to filter radio-waves using another radio telescope? - Astronomy

Another important power of a telescope is its ability to make us see really small details and see sharp images. This is its resolving power. Objects that are so close together in the sky that they blur together into a single blob are easily seen as separate objects with a good telescope. The resolving power is measured in the absolute smallest angle that can be resolved. The absolute minimum resolvable angle (smallest visible detail) in arc seconds = 252,000 × (observation wavelength) / (objective diameter). The wavelength and diameter must be measured in the same length units (i.e., both wavelength and objective diameter given in meters or both in nanometers). A telescope with one arc second resolution would be able to see a dime from about 3.7 kilometers (2.3 miles) away. Modern telescopes are able to count the number of lines in President Roosevelt's hair on a dime at that distance.

The desire is to make as small as possible. This can be done by making the observation wavelength small (e.g., use UV instead of visible light) or by making the objective diameter large. Another way to understand it is the more waves that can be packed on the objective, the more information the telescope detects and, therefore, the more detailed the image is. A 40-centimeter telescope has two times the resolution of a 20-centimeter telescope at the same observing wavelength ( for the 40-centimeter telescope is one-half the for the 20-centimeter telescope). However, fluctuations in the atmosphere will usually smear images into a fuzzy blob about one arc second or more across so the resolution is usually limited to the resolution from a 12.5-centimeter telescope on the ground. I will discuss the atmosphere's effect on images further in the another section and ways you can compensate for it.

The Five-hundred-meter Aperture Spherical Telescope (FAST)

A spectacular example of such a system is the Very Large Array shown here. This telescope is made of 27 radio dishes, each 25 meters in diameter, on a Y-shaped track. Fully extended, the Very Large Array is 36 kilometers across and has a resolution of around one arc second (depending on the radio wavelength). It has the light-gathering power of a 130-meter telescope. Aerial views are shown below.

Another example is the Australia Telescope Compact Array outside of Narrabri. Six 22-meter dishes can be placed in an array 6 kilometers across. A photo tour of the site is available here.

The Very Long Baseline Array is a huge interferometer that uses ten telescopes placed in sites from Hawaii to the Virgin Islands (see map below). This telescope is 8,600 kilometers across and has a resolution as good as 0.0002 arc second! With a resolution about 50 times better than the Hubble Space Telescope, it is able to detect features as small as the inner solar system at the center of our galaxy, about 27,000 light years away. A similarly-sized array of radio telescopes, called the Event Horizon Telescope, is being used to image the supermassive black holes at the centers of galaxies. Astronomers are constructing radio telescopes out in space that work in conjunction with ground-based radio telescopes to make interferometers much larger than the Earth (see also the Orbiting VLBI web site).

Sites for the Very Long Baseline Array---an array 8600 km across!

Other huge radio telescope arrays include Australian Square Kilometre Array Pathfinder (ASKAP) made of 36 identical antennae, each 12 meters in diameter, in western Australia and the Atacama Large Millimeter/submillimeter Array (ALMA) at over 16,500 foot (5000 meters) elevation in the Atacama Desert in Chile. ALMA is made of 66 total antennae with 54 of them 12 meters in diameter and 12 of them 7 meters in diameter in an array 16 kilometers across. Both ALMA and ASKAP are large international projects.

Central Cluster of antennae of the Atacama Large Millimeter/submillimeter Array (ALMA)

Astronomers are also now connecting optical telescopes to increase their resolving power. Two nice examples are the Large Binocular Telescope Interferometer on Mt Graham in Arizona (USA) and the Very Large Telescope Interferometer of Paranal Observatory on Cerro Paranal in the Atacama Desert, northern Chile.

The EHT is composed of radio telescopes, which only detect radio waves. Light and X-rays just hit it like they hit another giant structure of steel and aluminum.

One way to increase the sensitivity of the EHT is to capture more energy from the black hole targets at each EHT site. Since black holes emit radiation at many frequencies, we can do this by increasing the range of frequencies that are recorded during EHT observations. This, in turn, requires electronic systems and recording systems that operate at higher speeds. Industry trends that allow faster personal computers and higher capacity hard disk drives have enabled the EHT to leap forward to recording rates that are more than a factor of 10 faster than for any other global array. This is embodied in “Moore’s Law”, a heuristic coined in 1965 by Intel co-founder Gordon Moore, has predicted the exponentially increasing power of integrated circuits for the subsequent decades.

The effect of Moore’s Law has enabled the EHT to gather, record, and process much larger bandwidths at a fraction of the cost of earlier pioneering VLBI systems. The resulting increase in observing sensitivity has helped extend the EHT’s reach to longer baselines, and resulted in higher quality data sets with much better “signal-to-noise” ratio, or SNR.

The EHT equips each single dish site with specialized electronics designed and supplied by the collaboration. Though historically, analog VLBI equipment was used, in the modern era digital electronics is prevalent and has been the mainstay of the EHT. For single dish telescopes, the primary unit is called the VLBI “Digital Back End”, or DBE, which samples analog data from a radio receiver and feeds the formatted digital data to a data recorder.

Several different types of digital backend have been used in EHT observations, including the first-generation DBE1 system, the Digital Base Band Converter (DBBC) system, developed in Europe, and the ROACH Digital Backend (RDBE). The most recent incarnation is called the “R2DBE” or “ROACH2 DBE”, and has been deployed at all EHT sites. The R2DBE samples and processes data at a rate of 16 gigasamples-per-second, perfectly matched to the recording data rate of the Mark6 digital recorder, the latest generation of EHT VLBI Data Recorder. ROACH stands for “Reconfigurable Open Architecture Computing Hardware” and is shared by an open source astronomical instrument collaboration called “CASPER” the Collaboration for Astronomy Signal Processing and Electronics Research”.

Each Mark6 recorder receives digital data at a rate of 16 Gigabits/sec from the R2DBE and distributes it among a total of 32 hard disk drives grouped into 4 modules of 8 disks each. The EHT is scheduled to record an aggregate rate at each site of 64 Gigabits/sec by using 4 Mark6 units in tandem. This rate is matched to the maximum bandwidth current available from the key ALMA site (Atacama Large Millimeter/Submillimeter Array) that has the largest collecting area of all the EHT sites.

Recorded disk packs from each site are shipped back to two central locations, the Max Planck Institute in Bonn, Germany, and the MIT-Haystack Observatory in Westford, Massachusetts, for correlation. The DiFX, or “distributed F-X” software correlator is now used for EHT correlation. Among other advantages, software correlation clusters are scalable and the programs are easily customized. CPU-based processors are commodity products so in the processing domain as well as the recording the EHT take’s advantage of Moore's Law advances in processing power.