History of the Observatory:

Stebbins and the Birth of Photoelectric Photometry (1903-1922)

Being too ambitious to stay at Mount Hamilton or Berkeley, Dr. Joel Stebbins (1878-1966) decided Illinois provided promising prospects, so he accepted the directorship of the Observatory in 1903. Stebbins, born in Nebraska in 1878, graduated from the University of Nebraska in 1899. Performing his graduate work at the Washburn Observatory in Madison Wisconsin, he moved on to the Lick Observatory. There, in 1903, he became the third man to earn his doctorate in astronomy from the University of California and Lick Observatory where he is still considered one of the greatest researchers produced by Lick.

The early years of his administration brought many changes. The Observatory was incorporated into the Mathematics Department and had no operating budget. Stebbins taught both math and astronomy classes. After spending eight dollars out of his own pocket, he recognized the need for proper funding and proposed a department of astronomy be started. The Board of Trustees agreed and the Division of Astronomy in the Department of Mathematics was initiated. Its first budget was $750 over the 1905-1906 school year. The curriculum was expanded from three classes taught by the first director George Myers to nine, including Astronomy for Engineers, Observational Astronomy, Seminar and Thesis. Most of the classes, taught by Stebbins and an instructor, required weekly use of the Observatory's facilitates.

Beginnings of photometry

Stebbins, an avid observational astronomer, began an observing program using the Pickering visual program surveying stars in search of undetected eclipsing binary stars. Later in 1904, he began the estimates of the relative magnitudes of 107 double stars, duplicating the work of astronomer E.C. Pickering in 1878. This project leads to a revolution in astronomical photometry.

Prior to 1907, there were several different methods to estimate a star's magnitude. The oldest and most inaccurate were visual estimates. Accurate to, at the very best, 0.2 magnitudes, it was similar to judging the difference in weight of two rocks by holding them in your hands. William Bond at Harvard started photographic photometry in 1850’s. By comparing the image size and density of the stellar images, it was possible to achieve and accuracy of 0.02 magnitudes. A commonly used photometer was the Pickering polarizing visual photometers invented by Edward C. Pickering at Harvard in the 1870's. Restricted to mostly double stars, the photometers incorporated doubly refractive crystals, whose separation was variable, and an analyzer of polarized light. By varying the distance between crystals and by measuring the angles of polarization, the difference in magnitude could be determined.

It was during Stebbins' photometric project when he first realized the need for a new method of photometry. In the summer of 1905, Stebbins was married and he soon found a source of inspiration for a new photometer. He provided the following account at a dinner of the American Astronomical Society in 1957.

The photometric program went along well enough for a couple of years until we got a bride in our household, and then things began to happen. Not enjoying home alone, she (May Stebbins) found that if she came to the observatory and acted as recorder, she could get me home earlier. She wrote down the numbers as the observer called them, but after some nights of recording a hundred readings to get just one magnitude, she said it was pretty slow business. I responded that someday we would do this by electricity. That was a fatal remark. Thereafter she would often prod me with the question: "When are you going to change to electricity?" It happened that within a two or three months the department of physics gave an open house, and one of the exhibits was in charge of a young instructor F.C. Brown. He showed how when he turned on a lamp to illuminate a selenium cell, a bell would ring; when the lamp was off, the bell would stop. Here was the idea; why not turn a star on to a cell on a telescope and measure the current? (Stebbins, Early Photometry, 507)

Selenium cell photometry

Stebbins and Brown soon became friends and began some preliminary lab work. By June 1907, the photometer, with a selenium cell manufactured by Elster and Geitel at its heart, was ready for testing. On the first night Stebbins operated the telescope and photometer while Brown watched the batteries and galvanometer in the west-central transit room. They first tried Jupiter, but achieved no deflection or response; after several more tries, still no results. Refusing to be beat, Stebbins noticed the light of the nearly full Moon shining though the dome slit. He removed the photometer from the telescope, and using a piece of stovepipe as a shield, pointed the photometer at the Moon. The first deflection was achieved.

The selenium photometer used by Stebbins and Brown consisted of the selenium cell and a galvanometer. A selenium cell, which changes resistance when exposed to light, consisted of "...two wires wound in parallel in a double spiral around a flat insulator and the area between them on one side is smeared with amorphous selenium, which, when heated and applied properly, takes the crystalline form that changes electrical resistance when exposed to light" (Stebbins,Early Photometry, 507). Brown did manufacture some of the cells but they were also obtained from J.W. Giltay of Holland and E. Ruhmer of Germany. The cell was housed in a light-tight box with a glass window that admitted the light from the telescope. Asbestos covered the exterior of the box. The cell was connected as one arm of a Wheatstone bridge with the second and third arms being 10 and 100 ohms respectively and the fourth arm having a variable resistance one tenth of the cell, which was typically over 3 million ohms. The galvanometer, made by Leeds & Northrup, was connected to the bridge. Two dry batteries provided the needed 6 to 10 volts.

Soon after their successes, Stebbins and Brown began a program to measure the variation of the Moon's light with phase. The first night of the project, 23 June 1907, provides a good example of their early method of observations. Stebbins would make a set of four ten second exposures by pointing the cell at the Moon through a window. Brown, at the galvanometer, notes the deflection and the time. After each set, the photometer was brought downstairs where measurements were made at various distances from a standard Kohl candle. A second set of measurements of the Moon would follow. In between each exposure, one minute was allowed for the cells to recover. The end result was the best light curve for the Moon since Zöllner's work in the 1860's. The curve showed the east hemisphere of the Moon to be darker than the west.

They continued to work with and improve the cells. Stebbins noticed on a clear cold night after a blizzard that the cell's sensitivity was doubled and the irregularities reduced ten-fold. The cells were now wrapped in insulation and their temperature was maintained near freezing. Another improvement came as a result of an accident. After a presentation, Stebbins had a cell in his pocket wrapped in a handkerchief. Later, he pulled the handkerchief out and the cell fell to the floor, breaking into several pieces. He discovered the smaller pieces had fewer irregularities and were superior to the larger cells. These improvements and a new cell allowed Stebbins and Brown to measure the light of Aldebaran and Betelguese.

By 1908, they were able to measure the brightness of second magnitude stars. In 1909, the new project was begun to measure the changing intensity of the variable star Algol. Instead of comparing the stars brightness to a standard candle, they now compared it to stars whose magnitude was known. The end result was the most accurate light curve obtained for Algol up to that time. Due to the photometers increased sensitivity, approximately an order of magnitude better than any other method, two new features were discovered. The first was a second minimum proving that Algol was an eclipsing binary star system with a large faint star orbiting the primary star. This disproved the popular theory of a large dark body causing the primary eclipse. Stebbins also noted the “reflection effect.” This occurs when the cooler fainter star is heated on the side facing the brighter hotter star. His conclusions were a landmark, scrapping four doctoral thesis and proving the value and abilities of the new electric photometry.

It had taken Brown and Stebbins almost three years to get the photometer under control. In addition to keeping the cell cool, it was found necessary to never break the current to the cell, or else wait an hour for it to recover. The exposures were short and time was allowed for the cell to recover. Selenium had many irregularities that lead to changes in the cell's resistively and produced errors in measurement. In order to reduce the errors, the cells' manipulation was slow and tedious.

Despite the problems, Stebbins began to search for more eclipsing binary star systems between 1910 and 1912. The first star investigated, Beta Aurigae, was found to be an eclipsing binary with twin components. He expanded the project to include 12 known spectroscopic binaries to see if any of them might be eclipsing binaries. By studying such stars, important information on the size, shapes and masses of the stars can be determined. Spica, Alpha Coronae Borealis and Delta Orionis were discovered to be eclipsing systems. The new sensitive photometer was needed to discover these since they all varied by less than a tenth of a magnitude.

Enter the photocell

Although the photometer was used to discover five new eclipsing binary stars, construct a light curve for the Moon, determine the mid-eclipse time of the 24 July 1907 lunar eclipse and determine the magnitude of Comet Halley in May, 1910, Stebbins was not completely satisfied with the selenium photometer. The cells were not very sensitive, only stars brighter than the third magnitude could be studied. They also had a narrow spectral response, were not readily available, were difficult to work with and the characteristics varied from one cell to another. The solution to Stebbins' problems was found in physicist Jakob Kunz. Kunz was born and educated in Switzerland and, after advanced studies in England, emigrated to the United States. Kunz was an excellent theoretical physicist who enjoyed experimentation and felt the best things in physics were in the sky.

Kunz had worked with and manufactured photoelectric cells since 1909. A photoelectric cell produces a current when light falls on the metal surface and ejects an electron. In the autumn of 1911, Kunz met Stebbins and suggested he try a photometer based on photo emissive cells instead of selenium. Kunz half promised to have a cell ready in a week or so.

It was not until December 1912 before Kunz and another physicists, W. F. Schulz were ready to place a photometer with a potassium hydride photo emissive cell at the focus of the 12-inch refractor. They successfully measured the current produced by the light of Capella falling on the cell in December and the light of Acturus in April 1913. At this time, Stebbins was on sabbatical in Europe. While there he discovered two other groups had also been working with photoelectric cells, paralleling the work at Illinois. He visited the Berlin-Babelsburg Observatory where Paul Guthnick had been developing a photometer since 1912. He used the photometer, attached to the 12-inch Zeiss-Respold refractor, to study Beta Cephei in 1913. Edgar Meyer and Hans Rosenberg, working independently of Guthnick, were also applying photoelectric cells at a private observatory near Tübingen Germany. Stebbins attended the meeting of the Astronomische Gesellshaft in 1913 where Rosenberg spoke on his work. Rosenberg continued to work with photocells until 1925 while Guthnick used them until his death in 1947.

Schulz (until 1914), Kunz and Stebbins began lab experiments in the manufacture of sensitive cells. The manufacturing of the cells was described by astronomer A. Whitford as "an art dependent on Kunz's instinct and skill" (Whitford, 17). The photocell was a hand blown bulb, originally of glass, which was covered on the inside by an alkali metal, usually lithium, sodium or potassium. The pure metal is used to silver the bulb and act as a cathode. A well-insulated wire in the form of a ring centered in the bulb acted as an anode. The cell was filled with an inert gas, typically helium, neon, or argon, and then sealed. Kunz found the methods of Elster & Geitel was best to make the cells. Stebbins and Kunz discovered several important steps that needed to be added in order to reduce the dark current and to provide a linear relationship between light intensity and the current produced by the cell. By covering the entire interior of the cell, except for a small hole to allow light to enter, with an alkali metal, the dark current was reduced. In addition, Kunz was able to achieve a linear relationship by using a platinum ring electrode inside the cell and by crossing the electrode with fine wires. Others had attempted to manufacture photoelectric cells; by they were unable to achieve the linear relationship as Kunz had done. Another important breakthrough occurred in 1916 when the first photocell made of quartz was produced. Quartz glass was a better insulator than normal glass, so the dark current was further reduced providing more accurate data. These early breakthroughs allowed the photocells to be practical and useful in astronomical photometry.

The photocells were mounted in a light-tight wood box attached to the drawtube of the telescope by four support rods. The photocells could be changed if desired, but most of the work done after 1916 used the first quartz photocell, numbered QK99. Inside of the cell box was the grounding key that controlled the current to the cell. The switch was thrown by pulling a cord. Light from the star could be viewed by using a movable prism. A glass plate covered the entrance pupil. Different neutral shade glasses could be placed in the light path to reduce the intensity of the light and different diagrams also added. The box was continually pumped with dry air to prevent moisture from damaging the cells. The air bubbled through two sulfuric acid washes bottles, located in the east closet next to the Equatorial room, and then was brought over to the telescope.

From the bottom of the cell box hung a gimbal arrangement that maintained the electrometer in a vertical position. Weights hung under the electrometer to help maintain position. A fine wire from the negative terminal of the cell was carried through the joint on amber bushings to the electrometer. Deflections, due to the current produced by the photocell, were measured by the electrometer. This instrument served two functions: first accumulating the charge until the rising voltage was large enough to be measured and then the means to accurately measure the voltage. The first electrometer was by Edelmann of Munich, but it was replaced by a special electrometer built by William Gaertner of Chicago. Batteries located in the east closet provided power.

Observations began when the astronomer, using setting circles and the finderscope, located the target star. The star was centered, using the movable prism, and focused on the middle of the diaphragm. This produced an out of focus image about 1 cm in diameter on the photocell. The observer would then zero the thread in the electrometer. With one hand the grounding key was manipulated while the other hand held a stopwatch. To make a measurement, the astronomer timed how long it took the electrometer thread to move a convenient number of divisions. A typical deflection lasted around ten seconds.

In 1915 the photometer was far enough along in its development that Stebbins took it to Lick Observatory. At Lick, he used the 12-inch refractor to study Beta Lyrae for six weeks during the summer. This work provided defiant proof of distorted ellipsoidal components. Stebbins also noted the unsymmetrical eclipses that lead to the conclusion of gas streaming from one of the stars. The Lick staff duplicated the photometer for their own future use.

Stebbins and Kunz traveled to Rock Springs, Wyoming to observe the solar eclipse on 9 June 1918. They arrived a week early to find an observing site and to arrange for a small shelter to be built there. They also visited the Mt. Wilson and Yerkes expeditions stations nearby. The study the eclipse, they used two cells mounted in wooden boxes at the end of long 4-inch tubes. This was secured to an old 4-inch telescope mount. The nearby hut held the galvanometer and standard lamp to compare brightness to. Eclipse day began as a beautiful sunny day, but by first contact, clouds started to roll in. Finally, two minutes before totality, the area of the sky around the cleared and they succeed in measuring the brightness of the solar corona.

With most of the development on the photometer completed, Stebbins continued his search for eclipsing binary stars in 1916. The photometer was rebuilt in 1919 by Dr. Elmer Dershem. The program entailed checking known spectroscopic binaries for variation in light due to eclipses of the stars. His method of observation was to compare the suspect star to two or three nearby stars of the same spectral type. Analyses of the binaries orbit, derived from the spectrum, helped Stebbins determine when to study the star. Numerous measurements of 45 suspect stars over a six-year period were made. Stebbins continued this work until his departure from Illinois.

By 1919, Stebbins was recognized as of the leading astronomers in the country. At the university he was one of the most respected, and at $5,000 a year salary, one of the best paid faculty members. Besides his work on photometry, Stebbins also expanded the department to include an instructor and a research assistant. Instructors included Edward Fath (1905-1906), F. W. Reed (1907-1917), Dr. Elmer Dershem (1918-1919) and C.C. Wylie (1920-1925). Miss Iva Hamilton assisted Stebbins with routine computations. After first securing permanent funding in 1905 as a division of the Math Department, the Board of Trustees authorized the Astronomy Department on 1 August 1921 providing $1,000 for expenses and $6,800 for salaries.

Stebbins also awarded the first astronomy Masters degree in 1911 to Percy Whisler and the first Doctorate to Charles C. Wylie in 1922. Whilser assisted in many of the selenium photometer programs. He eventually went on to teach math and physics at Illinois College in Jacksonville. Wylie thesis was a study of the Cepheid variable Eta Aquilae and the eclipsing binary Sigma Aquilae. He left Illinois for the University of Iowa where he became a noted expert on meteors and meteorites.


Professor Stebbins left Illinois in September 1922 when he was offered the directorship of Washburn Observatory. Stebbins continued to improve and apply the photometer there where he enlisted the help of A. Whitford who developed a thermonic amplifier, C. M. Huffer, an Illinois math graduate who had taken a class with Stebbins at Illinois, and G. Kron at Mt. Hamilton who worked with photomultiplier tubes. Stebbins remained at the forefront of astronomical photometry until his death in 1966. For his work, Stebbins was elected to the National Academy of Science in 1922, served as Secretary (1918-1927) and President (1940-1943) of the American Astronomical Society, served on the 8 man American delegation sent to Europe in 1919 to rebuilt the war torn International Astronomical Union, received the Draper Medal from the National Academy of Science (1914), the Rumford Medal of the American Academy of Arts and Science (1915), the Bruce Medal of the Astronomical Society of the Pacific (1941), and the Gold Medal from the Royal Astronomical Society of London (1950). Stebbins is recognized the world over as the man who "furnished the continuity that lead to the permanent establishment of photoelectric photometry as one of the most important tools of modern astronomers" (Kron,Stebbins, 214).

Kunz also continued to work with photoelectric cells and their application after Stebbins left. Kunz and Stebbins remained good friends and associates. He continued to furnish Stebbins with the best cells and traveled with him on three solar expeditions in 1923, 1925 and 1932. His cells were not only used at Illinois and Washburn, but also at Lick, Yerkes and Mount Wilson Observatories and used by Stebbins to survey Mount Palomar before the establishment of the 200-inch Hale telescope. Kunz also applied his cells to other scientific fields such as biology and physiology. He worked with J. T. Tykociner who applied the cells to sound motion picture projectors. When the first sound motion picture was demonstrated in Urbana in 1922, it had a Kunz photocell at its heart. He continued to improve the cells' sensitivity and reduce the dark current making photoelectric cells invaluable to science and industry. Known as "the father of photoelectric cell" (Courier), Kunz died in his Urbana home in 1938, having served on the Illinois faculty for 29 years.

The use of photoelectric photometry grew slowly until after the Second World War when commercially made cells became available. The photometric revolution continues today. Eight decades after Stebbins first measurements, no astronomical observatory, including the Hubble Space Telescope, is complete unless it is equipped with a photoelectric photometer. The use of the photoelectric photometer is so widespread and important that even amateur astronomers can afford, build and use them from professional research.



Brown, F.C., Joel Stebbins. "A Determination of the Moon's Light with a Selenium Photometer."Astrophysical Journal. 26 (1907) 326-340.

"Jakob Kunz on U of I Faculty 29 years Dies."The Evening Courier.19 July 1938, sec A:3.

Kron, Gerald. "Joel Stebbins 1878-1966."Publications of Astronomical Society of the Pacific. 78 (1966) 62-65.

Kunz, Jacob, Joel Stebbins. "The Illinois Eclipse Expedition to Rock Springs, Wyoming."Popular Astronomy. 26 (1918) 665-676.

Kunz, Jakob, Joel Stebbins. "On the Construction of Sensitive Photo-electric Cells."Physical Review. 7 (1916) 62-65.

Schulz, W.F. "The Use of Photoelectric Cells in Stellar Photometry."Astrophysical Journal.38 (1913) 187-191.

Stebbins, Joel. "Early Photometry at Illinois."Publications of Astronomical Society of the Pacific. 69 (1957) 506-510.

Stebbins, Joel. "Jakob Kunz 1874-1938."Popular Astronomy. 47 (1939) 117-121.

Stebbins, Joel. "Electrical Photometry of Stars."Publications of Astronomical Society of the Pacific.52 (1940) 235-240.

Stebbins, Joel. "The Electric Photometry of Stars."Science. 41 (1915) 809-813.

Stebbins, Joel."Selenium Photometry of Stars."The Observatory.39 (1916) 257-263.

Whitford, A.E.. "A Half Century of Astronomy." Lick Observatory, University of California-Santa Cruz.

Wylie, C.C. "The Eclipsing Binary Sigma Aquilae, the Cepheid Variable Eta Aquilae." Urbana: University of Illinois Ph.D Thesis, 1922.

Whitford, A.E.. "American Pioneer in Photoelectric Photometry."Sky and Telescope. May 1966: 268-269.