Exploring the Solar System with the Voyager Spacecraft’s Cameras, Polarimeters, and Magnetometers


As we continue our celebration of Voyager's fourth decade in space, let's take a look at some of the instruments the Voyager spacecraft use: cameras, polarimeters, and triaxial fluxgate magnetometers.
As the Voyager spacecraft are approaching their 40th year in space, All About Circuits is celebrating the incredible accomplishments of gifted engineers with a weekly series of articles that reminds readers about the electrical engineering accomplishments of a past generation.
Over the past several weeks, we've looked at different aspects of the Voyager mission. Catch up by reading these articles:

This week, series coordinator Mark Hughes brings us information about the Voyager cameras, polarimeters, and magnetometers.

Voyager's Imaging Science Subsystem

Regrettably, the camera systems on the Voyager spacecraft were disabled decades ago. This is because they traveled so far out into space that they no longer receive enough light to function as intended. NASA turned off the camera system for Voyager 1 after it took the "Solar System Family Portrait" in 1990, while Voyager 2 took its last photo at its Neptune encounter in 1989.


Diagram of the "Solar System Family Portrait" taken by Voyager 1. Image courtesy of NASA

Voyager has two digital video cameras with 800x800-14 µm pixel resolution mounted at the end of its adjustable scan platform. With 8-bits per pixel, each frame required 5,120,000 bits that could be recorded on a magnetic tape with 536 million bits capacity at 115,200 bps or transmitted back to the Deep Space Network receivers on Earth at 8400 or 14,400 bps. The tape backup was necessary to store images during times of occlusion (when a planet or satellite blocked the radio transmission path) and was played back at a lower data rate.
Each camera has focusing and filtering optics that allow scientists to construct color images from the 8-bit grayscale captures.

This artistic interpretation of Jupiter's atmosphere illustrates the type of image recorded through various filter wheels. Photo by Nasa, interpretation by Mark Hughes

One is a wide-angle 200 mm f.l., 60mm diameter, (f/4.17 effective aperture with obscuration and transmission losses) camera that is sensitive to a spectral range of 400 nm to 620 nm and has a field of view of 3.2°. It passes light through a filter wheel with notch filters in the range of 425nm to 600 nm that contain clear, violet, blue, green, orange, a 0.7 nm sodium filter centered at 589 nm, and two methane filters centered at 541 nm and 619 nm.

A reconstruction of the wide-angle camera sideways diagram found here

The other is a narrow-angle 1500 mm f.l., 176mm aperture,  (f/11.8 effective aperture with obscuration and transmission losses) camera that is sensitive to a spectral range of 420 nm to 620 nm that has a field of view of 0.4°. A filter wheel with notch filters in the range of 345 nm to 590 nm filter has two clear filters, two green, and one violet, blue, orange, and ultraviolet filters.

The Vidicon Videocamera Tube

After light passes through the focusing optics, the filter wheel, and a shutter, it arrives at the faceplate of an improved Vidicon camera tube.


Image from Olympus Scientific

The faceplate of the Vidicon tube has a signal plate composed of a conductive layer of optically-transparent and electrically-conductive tin-oxide (SnO₂) and a semiconducting target plate made of antimony trisulphide (Sb₂S₃).
When no photons strike the faceplate, the tin-oxide layer acts as an excellent insulator. As photons strike the tin-oxide, they cause electrons from lower energy bands to move to conducting bands in the tin-oxide. There, the free electrons move to the signal plate and are drawn away from the faceplate by a positive potential difference maintained between the signal plate and the rear of the tube.
At the same time, a cathode emits electrons at the back of the tube and a potential difference that exists between two grids slowly accelerates the electrons towards the faceplate while electromagnetic deflection coils guide them to different locations on the screen. As the electrons approach the screen, they enter a region where an electric field slows them down to almost zero velocity. They then fill the holes left by the electrons that were liberated by the photoelectric effect, resetting that portion of the target.
A greater number of incident photons would free a larger number of freed electrons in those areas of the target and would register as higher current flow as electrons fill holes at that moment in the scan.
The camera system weighs 84.15 lbs (38.17 kg) and uses 41.9 watts of power.

Photopolarimeter System (PPS)

Let's take a look at the photopolarimeter systems on the Voyager spacecraft. They, like the cameras, were disabled due to lack of available light.
Light is composed of propagating electromagnetic waves. The electric and magnetic field components oscillate in planes that are orthogonal to the direction of travel. When the electric fields of all light waves in a sample are aligned, the light is said to be polarized. Polarizing filters allow light with parallel polarization to pass while blocking light of a perpendicular polarization.


Simple explanation of polarizing filters. Image courtesy of Bigshot.

The light that leaves the sun is not polarized—an essentially infinite number of atoms and molecules in the photosphere of the sun send light traveling outwards in all directions and all polarizations at once. This creates a random source of light that illuminates the planets of the solar system.
Different frequencies of light interact with different molecules in a planet's atmosphere and on the planet's surface to become polarized. The intensity of light that passes through various color filters and then different polarizing filters can allow scientists to determine the relative abundance of and types of molecules.
The PPS on Voyager consists of a 15 cm telescope that focused light into the path of three overlapping filter wheels and then passed to a photomultiplier tube. The polarizing filter wheel contains linear polarizing filters aligned at 0, 60, and 120 degrees, as well as a blank.
A color filter wheel contains notch filters centered at 590 nm, 490 nm, 390 nm, 310 nm, 265 nm, 235 nm, 750 nm, and 727 nm that were used to identify sodium, hydrogen, helium, calcium, carbon monoxide, oxygen, magnesium, silicon, potassium, and methane.

Image of photopolarimeter system from C.F. Lillie and R.S. Polidan, Northrop Grumman Cooperation 

After passing through the filters, the incoming light would arrive at a photomultiplier tube. There, one or many incoming photons would arrive at a photocathode and (through the photoelectric effect) cause one or many electrons to be ejected from their parent atom. Those electrons would accelerate due to a potential difference along the length of the tube to another surface called a dynode.
Each electron's charge and added kinetic energy would increase the number of electrons that leave the surface of the dynode and accelerate through an electric field to another dynode. The chain reaction continued until the electrons reached the end of the tube where a now very large number of electrons travel through the anode to a current meter.

Image of photomultiplier tube from Florida State University

The photomultiplier tube acts as a very sensitive one-pixel analog camera with an adjustable field of view. As Voyager traveled around a planet or satellite of interest, scientists would carefully track the spacecraft position and orientation while recording data. One PPS dataset describes "The 'East-West' map is a raster scan of the 0.11 deg field of view across Saturn's northern hemisphere at a phase angle of 10 deg. These were limb-to-terminator scans at 5 latitude bands."
The photopolarimeter weighs 4.4 kg (9.7 lb.) and uses 2.4 watts average power.

Magnetic Field Instruments

Still in service (at least, for now), are the Voyager magnetometers.
Each Voyager spacecraft carries four magnetic field instruments. Two high-field instruments that are sensitive to
 to
are mounted on the body of the spacecraft. Two low-field instruments that are sensitive to
to
are mounted on an extendible epoxy-glass boom.

Artistic Interpretation of the triaxial ring core fluxgate magnetometer with a pickup coil (tan→blue) wrapped around the outside of an excitation coil (red) that is helically wound around a ferromagnetic toroidal core (orange). Image by Mark Hughes

The ring-core fluxgate magnetometers used on Voyager work by applying an alternating current to an excitation coil that is wound around a ferromagnetic core and monitoring changes in the magnetic hysteresis excitation and relaxation curves with an externally wrapped pick-up coil.
Each instrument is composed of three single-axis, ring-core, fluxgate magnetometers. As the spacecraft moves through the solar system and around planets, the magnetometers provide three-dimensional magnetic field strength that scientists on the ground can use to construct magnetic field models of the planets, moons, and solar system.

http://lasp.colorado.edu/home/mop/files/2015/02/FIELD_LINES_TRAJECTORIES.png
Voyager magnetic field data from the University of Colorado Boulder. Learn more on their site for the Laboratory for Atmospheric and Space Physics

The total weight of the magnetic fields experiment is 5.5 kg (12 lb.). The experiment uses 3.2 watts of power.

Conclusion

While the cameras and photopolarimeter systems were turned off decades ago due to lack of available light, the magnetometers continue to measure the feeble magnetic fields that exist in the interstellar medium.
The low power requirements of the magnetometer system would allow it to detect magnetic field information about the interstellar medium for many decades to come—however, sometime within the next several years, power levels will fall to the point that there isn't enough to run the computers and transmitter.


Featured image is an image Voyager took of Jupiter, courtesy of NASA.
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