From Waxlight to Moonlight: 21st Century Standard Candles at NIST | NIST – NIST | Mega Mediakw

NIST lunar telescope on Mauna Loa volcano, part of our first measurements of lunar radiation from this site.

Recognition:

J.Woodward/NIST

One of the unexpected rewards of working at NIST was the opportunity to see other disciplines through the NIST prism of metrology and standards. Working with NASA scientists, astronomers, oceanographers, and geologists, I have had the opportunity to witness the lives of scientists in a variety of fields.

My way of interacting with these researchers is often to calibrate the sensors on their instruments. These calibrations help the instruments accurately measure light and other electromagnetic emissions from objects the scientists are studying, whether it’s the Pacific Ocean, a forest fire, or a distant galaxy. To properly calibrate these researchers’ sensors, we need reliable methods to measure the light itself. My NIST colleagues and I are currently engaged in some state-of-the-art efforts to make these measurements better than ever. But before I tell you about the NASA high-altitude planes we’re using and the lunar observatory we’re building, let’s talk about the earliest standard for measuring light output: the humble candle.

Among America’s first candle makers were Native American peoples of the Northwest. From the first century AD, they made candles from a fish called eulachon, or “candlefish,” a type of smelt, by placing the dried fish on a forked stick and then lighting it. Candle evolution evolved over time. With the arrival of English settlers in the 16th century, colonial women began making sweet-scented wax candles infused with laurel berries. Other candle types were developed over time, but none were consistent enough to serve as a standard. Then, in the late 18th century, people invented the first “standard candle,” which burned brighter than any candle before it, by using spermaceti wax, a pearly-white, waxy material found in the sperm whale’s head cavity. The standard or international candlestick became a measure of the intensity of the light source. In 1860, the English adopted the definition of candlepower as a one-sixth pound candle made of sperm wax burning at a rate of 120 grains per hour. Incandescent lamps replaced candles to provide a standard for light intensity in 1921; Developments continue to this day.

At the same time, the definition of the brightness of a light source changed from the candle to the candela, a scientific unit of light. Today, all of the world’s basic units of measurement, including the candela, can be expressed in terms of physical constants. NIST was involved in this redefinition of the International System of Units (SI). Today’s measurements, including light measurements, are more accurate than ever because they can be linked to the new SI.

In the last decade, NIST has returned to the “candle” business, but with all the modern knowledge and skills that the last century has brought us. Instead of the earthly candles of old, we turned to the stars and moon as our luminous objects of choice. Not only do these modern candles shed new light on the cosmos, they can also increase the accuracy of satellite measurements of everything from the weather to crop yields on farms.

Our foray into modern candle making was spearheaded by the late Keith Lykke, a research chemist, my former group leader, and amateur astronomer. We based our approach on the fact that 20th-century astronomers realized that stable, well-known stars can act as “candles” in the sky. The brightness of these “standard candles” can be used as a reference for important astronomical and cosmological measurements, from the distance to other galaxies to the age of the universe.

For what was then known as the NISStars program, we identified a set of stable stars as our standard candles. From mountaintop observatories, we began taking measurements of an important quantity in the stars known as irradiance, essentially the amount of energy they emit each second, hitting every square meter of a detector on Earth. As a metrology institution, we then did the NISTy thing of tying our irradiance measurements to the SI. These calibrated stars, with known spectral irradiances associated with primary standards in our laboratory, are standard candles for the 21st century with a wide range of potential applications.

Side-by-side images of the night sky show stars of the Big Dipper and NISTAR labeled with their names.

The NIST artificial calibration star (NIST Star or NISTAR) with the Big Dipper stars in the background. The amount of light emitted by our artificial star is very similar to the amount of light emitted by the stars of the Big Dipper. The image on the right was taken later in the evening with the NIST star more clearly visible.

Recognition:

NIST

The original NISStars measurements were made at the Fred Lawrence Whipple Observatory on Mount Hopkins in southern Arizona. We focused on measuring two stars: α-Lyr (Vega), 25 light-years away, the primary astronomical calibration star on which all stellar radiation is based; and Sirius, which is 8.6 light-years away and the brightest star in the night sky. Sirius is almost twice as bright as Canopus, the second brightest star. To calibrate our telescope, an artificial star was created at the Mount Hopkins summit facility of the Multiple Mirror Telescope (MMT) by placing an incandescent bulb in an optical component known as an integrating sphere. By adjusting the electrical current to the lamp, we can vary the amount of light from our artificial star. We then measured the amount of light emitted by our star from a location further down the mountain, which served as a calibration source for NIStars telescopes. As an example, we matched the intensity of our star to the intensity of the stars in the Big Dipper. The images above show NIST’s artificial star along with stars forming the Big Dipper in the background. The figure at left shows the shadow of Mount Hopkins with the MMT at the summit; The image on the right was taken later in the evening, with the NIST star being observed more clearly.

Keith passed away suddenly in the spring of 2016, but the project continues. We are deploying the NISStars system to the Paranal Observatory in northern Chile. Located in the Atacama Desert, the driest non-polar desert on earth, the observatory offers some of the best astronomical observations in the world.

Although not used in the typical astronomical sense, the moon also functions as the standard celestial candle. The proportion of sunlight that is reflected from the lunar surface, known as the reflectance, is extremely stable. Because the amount of light reflected from the moon depends on the area illuminated by the sun, models are used to predict the amount of light illuminating a sensor for a given measurement configuration and time of year.

Measurements of the moon are regularly taken from a variety of satellite sensors to assess how their respective systems are changing over time. As such, the moon’s brightness serves as an important reference, helping to calibrate satellite imagery and ensure the satellites are providing accurate information on everything from cloud cover to wildfire intensity. Unfortunately, the uncertainties in current lunar models are too great for future satellites to rely on the moon as the only reference object they need to calibrate their sensors.

To address this issue, two complementary projects are underway at NIST to help fully establish the Moon as an absolute SI-traceable calibration target for use by satellite sensors measuring Earth properties. One project is the Airborne-Lunar Spectral Irradiance (air-LUSI) project. Our multi-institutional team built an instrument that measures lunar radiation from an ER-2, a high-altitude NASA aircraft operating out of their Armstrong Flight Research Center in Palmdale, California. During the flights, the Air LUSI instrument measures the lunar radiation over 95% of the earth’s atmosphere.

Six men are standing in front of an airplane in a hangar, equipment on carts in front of them.

Air-LUSI team (left to right): Steven Grantham (NIST), Andrew Newton (McMaster University), Kevin Turpie (University of Maryland), John Woodward (NIST), Thomas Larason (NIST), Stephen Maxwell (NIST) . Missing from the picture are Thomas Stone (USGS), Andrew Gadsden (McMaster University) and myself. The ER-2 aircraft is directly behind, and the center of the wing pod where the instrument is installed can be seen to Grantham’s left.

Recognition:

NIST

In case of problems during take-off or landing, each take-off and landing is recorded by a chase car. Speeds can exceed 160 km/h (100 mph) as the car follows the plane across the runway. We were invited to sit in the chase car when our instrument was on the ER-2. It was very exciting to follow the starts; even more exciting during landing when the plane flew just over the pursuit car on landing.

A man stands to the right of a metal arch with a telescope and other equipment inside.

The airborne LUSI telescope mounted in the tail of the ER-2 superpod.

Recognition:

Ken Ulrich/NASA

Two men peer into the open fuselage of a small plane with wires and other connections inside.

John Woodward looks a little concerned as he loads air LUSI cables into the rear fuselage under the telescope mount.

Recognition:

NIST

For the second project, a small NIST team (Steve Maxwell and John Woodward) is building a lunar observation system known as the Mauna Loa Observatory Lunar Spectral Irradiance (MLO-LUSI) on the Hawaiian volcano Mauna Loa. MLO-LUSI will perform nighttime measurements of the moon’s spectral irradiance, with airborne LUSI measurements providing complementary data less affected by the atmosphere.

It is planned to merge our measurements with those of other US and European agencies to produce a combined data set supporting a new low-uncertainty moon model. Once the new model is developed, we believe it will stand the test of time and help the moon become a standard candle used to calibrate satellite sensors for centuries to come.

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