Boller and Chivens Microphotometer

General Information

The microphotometer was designed and engineered by Boller and Chivens, purchased by Kitt Peak National Observatory in the 1960's, and then received by McCormick Observatory from Kitt Peak in the 1972 after Kitt Peak bought one of the first PDS Microdensitometers. It is believed that only two of these very early Boller and Chivens microphotometers were ever made. They were designed specifically for the measurement of spectra.

In 1901, Max Planck showed that the amount of energy radiated by a object at any wavelength depended on the temperature of that object. Therefore, by measuring the brightness of different spectral lines (lines of different wavelengths), one could in theory, find the temperature of the object. The most difficult aspect of this technique is simply finding a star to calibrate the system. No perfect star exists for which this theory is exactly true, and so engineers designed photometers to use carefully calibrated lamps rather than real stars from which to measure. An iris or slit could then be adjusted to record the size of the star image as it compared to the calibrated lamp source. These digitized and semi-automatic iris/slit photometers were in use by several observatories, including Kitt Peak, by the early 1960's. At top speed, they could measure 300-400 stars an hour. McCormick Observatory put the microphotometer to work with a strip chart recorder, but because Kitt Peak had taken much of their added features off the microphotometer, McCormick had trouble calibrating the spectra they measured. By this time, the Grant Measuring Engine had arrived and was in full use, and so astronomers measured their spectra on the Grant engine instead of the microphotometer.

Operation

First View of the Microphotometer A photographic plate of no larger than 4 × 9 inches is placed on the stage (First View, 1). The stage is moveable in two directions by an X-motion handwheel (Second View, 2) at the right of the machine and a Y-motion handwheel (Second View, 1) at the left-front of the machine. Each of these handwheels is also equipped with a dial to measure the amount of motion. The plate is positioned on the stage under the field lamp (First View, 4) using the two handwheels and by rotating the stage using the rotary stage wheel (First View, 2) at the front of the engine. The stage can then be locked in place using the rotation locks (First View, 3) at both sides of the machine.

Second View of the Microphotometer The field lamp sends light through a slit and onto the photographic plate. This slit can be adjusted in several ways: it can be rotated and be made taller by dials (First View, 5) to the left of the field lamp and it can be made wider using a dial (First View, 6) to the right of the field lamp. The slit adjustment allows for variance in the size of the spectral lines on the photographic plate. Light procedes through the photographic plate down to the bottom half of the engine, where it hits a second stage (Third View, 1), bounces from one mirror (Third View, 2) to another (Third View, 3), and then is finally seen by eye through the microscope eyepiece (Second View, 3) at the front of the machine. Third View of the Microphotometer Additionally, there is a small slit in the second stage below the machine under which there is an RCA Type 6199 photomultiplier tube (Third View, 5) which absorbs the light and measures it. In this way, an observer can both measure the light with a precise instrument and also look at what is being measured. The width and height of the slit on the second stage can be adjusted using dials (Third View, 6 and 7) below the stage. There is also a frustration knob (Third View, 4) below the plate stage.

A control panel (Second View, 4) sits on the right side of the measuring engine. The various knobs and switches allow for varying scan speeds and field illumination. Additionally, this panel has controls for the shutter, auxiliary power, and stop motor.

At the time that this engine was produced, the RCA Type 6199 photomultiplier tube (PMT) had been newly developed. It utilized a new photocathode surface which had been first conceived in the 1950's. This surface was a partially transparent compound of CsSb (Cesium-Antimony) laid on a lime-glass window. It had a peak response at a wavelength of 4400 Å (blue light) and a higher quantum efficiency than previous PMTs, so more of the light was absorbed and could be measured. The remainder of the instrument worked by employing a focussed dynode configuration, meaning that a Fabry lens imaged the light onto the photocathode.

Boller and Chivens Microphotometer

General Information

Operation

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