Radio Astrophotography

We have been making interesting radio pictures using both Green Bank Observatory’s manually controlled 40-foot diameter telescope and their Skynet-controlled 20-meter diameter telescope. The older pictures below were made with the former, and preliminary radio image-processing algorithms and software that I developed in the 1990s. The newer pictures were made with the latter, and updated algorithms and software that we have been developing for Skynet.

Optical, H-alpha data (black-and-white layer) taken by Steven Christensen and processed by Steven and Tim Christensen. Radio data (false-color layer) taken with Green Bank Observatory’s 20-meter diameter radio telescope, using Skynet (Dan Reichart, John Martin, Dylan Dutton, Michael Maples, Travis Berger, Joshua Haislip, Frank Ghigo).

Optical, H-alpha data (black-and-white layer) taken by Steven Christensen and processed by Steven and Tim Christensen. Radio data (false-color layer) taken with Green Bank Observatory’s 20-meter diameter radio telescope, using Skynet (Dan Reichart, John Martin, Dylan Dutton, Michael Maples, Travis Berger, Joshua Haislip, Frank Ghigo).

GBO 20-meter, L band (1.4 GHz)

Orion Complex

Text from submission to NASA’s Astronomy Picture of the Day

At 1.4 GHz, Green Bank Observatory’s 20-meter diameter telescope has a resolution of only 0.75°, and consequently can resolve only very large structures, making the Orion Complex an ideal target.

However, such large fields of view can present a challenge for optical telescopes, which typically image far smaller portions of the sky.

This optical — radio combination was achieved by tiling 15 130-minute exposures in H-alpha, to which we added four 170-minute 1.4-GHz maps that we made with GBO’s 20-meter.

The H-alpha mosaic was made by fellow UNC-Chapel Hill professor Steven Christensen and his son Tim Christensen.

The four radio maps do not form a mosaic, but overlap completely. This was done to beat down noise, but also to better identify and eliminate interference-contaminated data, which is an ever-increasing problem in the radio.

The optical, H-alpha mosaic is presented in black and white, to which we add color using the lower-resolution radio data.

The 20-meter is effectively a single-pixel camera.  To make a picture with it, you have to move it back and forth, and across, in a raster pattern, collecting data while its moving.  We then use software that we have been developing at UNC-Chapel Hill to fill in the gaps between the data points (which it does without additionally blurring the picture).

The two bright radio sources are Orion A, which is coincident with the Trapezium, and Orion B, which is near the Horsehead Nebula. Both are sources of bremsstrahlung radiation, emitted from hot, ionized, and dense gas surrounding hot, young stars. Recombination of protons and electrons in this gas, into hydrogen atoms, releases H-alpha photons as well — which is why these optical and radio pictures pair so well.

Barnard’s Loop is seen in blue, as are clouds of cold hydrogen gas, which also emit at this radio frequency.

The 20-meter is the first radio telescope in the Skynet Robotic Telescope Network, which otherwise includes nearly two dozen optical telescopes spanning four continents and five countries.  All Skynet telescopes are controllable through a simple web interface, and serve both professional astronomers and students, of all ages.  To date, nearly 50,000 students have observed with Skynet telescopes.

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GBO 20-meter, X band (9 GHz)

Solar Eclipse of 2017

Text from submission to NASA’s Astronomy Picture of the Day

We observed the Great American Eclipse of 2017 at 9 GHz (3 cm) using Skynet and Green Bank Observatory's 20-meter diameter radio telescope.  

The 20-meter is effectively a single-pixel camera.  To make a picture with it, you have to move it back and forth, and across, in a raster pattern, collecting data while its moving.  We then use software that we have been developing at the University of North Carolina at Chapel Hill to fill in the gaps between the data points (which it does without additionally blurring the picture).

At 9 GHz, the 20-meter can resolve astronomical structures to 0.12 degrees.  This is less than the half-degree sizes of the sun and moon, making these pictures possible.

Two cool science points:

1.  Notice that it is not completely dark where the moon blocks the sun.  This is because the moon emits its own radio waves, due to its temperature.  In fact, these data can be used to measure the moon's temperature, just below its surface.

2.  If one looks carefully, or if one adjusts the brightness and contrast, as we have done in the pictures to the right, one can see bright spots on the sun.  These are sunspot groups, which are not dark, but bright at radio wavelengths.  This is because electrons spiraling around the strong magnetic fields associated with these regions emit radio waves, called "synchrotron" radiation.

The 20-meter is the first radio telescope in the Skynet Robotic Telescope Network, which otherwise includes nearly two dozen visible-light telescopes spanning four continents and five countries.  All Skynet telescopes are controllable through a simple web interface, and serve both professional astronomers and students, of all ages.  To date, nearly 50,000 students have observed with Skynet telescopes.

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GBO 40-Foot, L band (1.4 GHz)

Polarization of the North Polar Spur

A red-blue combination of two perpendicularly polarized images of the North Polar Spur made by undergraduate students using Green Bank Observatory's 40-foot diameter telescope and the original version of our image-processing software, revealing its extreme polarization. 

Unpolarized objects, such as the two active galaxies near the center of the image — Hercules A (3C 348, above center) and 3C 348 (below center) — and the plane of the Milky Way itself, have equal amounts of red and blue and hence appear purple.

The North Polar Spur however varies from red to purple to blue, indicating high levels of polarization. We have confirmed this structure using GBO’s 20-meter when its receiver is oriented the same as the 40-foot’s.

We have dimmed the plane of the Milky Way in this image, to better highlight the North Polar Spur, 3C 348, and 3C 353.

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GBO 40-Foot, L BAnd (1.4GHz)

Cassiopeia A and Cygnus A

Broadband images made by undergraduates using Green Bank Observatory’ 40-foot diameter telescope and the original version of our image-processing software.

The plane of the Milky Way runs through the two images. The bright source in the first image is Cassiopeia A, one of the last supernovae to explode in our galaxy, around 1680. The bright source in the second image is Cygnus A, an active galaxy, and the primary calibration source for the radio sky.

Altogether, these data took 7.5 hours to collect, where the students manually reversed the mapping direction of the telescope every two minutes.

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Using both the 40-foot (blue points) and more recently GBO’s 20-meter (red points), participants of our ERIRA program have measured the brightness of Cas A and Cyg A each year since the early 1990s. Using these and archival measurements (black and green points), we showed that Cas A brightened (left panel), or at least faded more slowly (right panel), between the early 1970s and the early 1990s.

We also used these and other data to produce new and improved spectral and temporal models for the four primary calibrators of the radio sky (Cas A, Cyg A, Tau A, and Vir A).

This student-led project was published in the Monthly Notices of the Royal Astronomical Society.

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GBO 40-Foot, 21-cm Line

Separating Spiral Arms and Measuring the Galactic Warp

An RGB combination of three narrowband images made by undergraduates using Green Bank Observatory’s 40-foot diameter telescope and the original version of our image-processing software. 

The narrowband frequencies were chosen such that the red image captures neutral-hydrogen emission from our arm of the Galaxy; the green image captures Doppler-shifted neutral-hydrogen emission from the next arm out; and the blue image captures even higher velocity neutral-hydrogen emission from the arm beyond that. 

The red arm is broad and diffuse because we are in it.  The blue arm sits above the green arm because the Galaxy is warped in this direction – something that our students discovered independently…even if 28 years after the fact. 

Cygnus A is a broadband synchrotron source, and consequently appears white.

We are looking forward to remaking this picture with GBO’s 20-meter.

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GBO 40-FooT, 21-cm Line

Visualizing Andromeda’s Rotation

An RGB combination of three narrowband images of Andromeda made by undergraduates using Green Bank Observatory’s 40-foot diameter telescope and the original version of our image-processing software. 

The narrowband frequencies were chosen such that the red, green, and blue images capture Doppler-shifted neutral hydrogen emission from the receding side, center, and approaching side of Andromeda, respectively.  We are literally seeing Andromeda’s rotation in color!

Students use these data to estimate the mass of Andromeda.  In a related project, students use the 40-foot’s neutral hydrogen spectrometer to measure the maximum Doppler shift of gas at different longitudes along the Galactic plane and calculate the rotation curve and radial mass distribution of the Milky Way.

We are looking forward to remaking this picture with GBO’s 20-meter.

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GBO 40-FooT, L Band (1.4 GHz)

Survey of the Galactic Plane (and Extragalactic Regions of Interest)

Broadband images made by undergraduates using Green Bank Observatory’s 40-foot diameter telescope and the original version of our image-processing software.

The plane of the Milky Way runs through these images (a curved line in this coordinate system). From left to right, the brightest sources are the sun, Orion A and B, Taurus A, the moon, Cassiopeia A, Cygnus A, many sources near the Galactic center, the North Polar Spur, Virgo A, and Quasar 3C 273. Many fainter sources are also detected.

Altogether, these data took between 25 and 30 hours to collect, spread out over a few days, where the students manually reversed the mapping direction of the telescope every two minutes.

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The Mighty Forty!!!

No, it’s not really this big…