Selenium cell

Alvan Clark Polarizing photometer

Pickering visual photometer used by Stebbins in his 1904-1906 study of double stars. 

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.




Selenium cell photometer attached to the 12-inch telescope.

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)


The selenium cell photometer was connected to the galvanometer located downstairs from the telescope in the west-central transit room. 

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.

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.


Light curve of Algol from 1910.

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. The west-central transit room was converted into a galvanometer room where the signal from the selenium cell was measured. 


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.



The first photometric study using the selenium cell photometer was the moon in 1907.

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.



Reflection galvanometer by Weston.

It had taken 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.