What is Astronomical Photoelectric Photometry
Astronomical Photoelectric Photometry or PEP is the measure of brightness of objects in the sky using electronic means. This may be the brightness of a galaxy, nebula, asteroid, planet or star.

Light has a dual nature. Sometimes it manifests itself as a wave packet with accompanying wave characteristics. Other times as an individual unit called a photon which acts like a particle. Whether light acts like a wave or a particle appears to be dependent on how it's observed. If an experiment is devised to look for wave characteristics, then light will exhibit wave-like properties. If devised to look for particle-like properties then light appears to be made of particles. In photoelectric photometry light is observed in such a way that it exhibits particle-like characteristics.

Being able to precisely measure the brightness of a star is like seeing the soul of the star. While viewing and imaging interesting objects, e.g., galaxies, nebulae and planets, with a telescope is fun, precisely measuring the photons coming from a distant star is perhaps even more exciting. In many cases you may be the only one in the world watching that star closely. With care and patience you may discover interesting and important things about the star.

Detectors
There are several devices that can be used to measure low light levels. There are many types of photomultiplier tubes available. Some are for very specialized work, e.g., hunting neutrinos. The most popular one used for photometry is the 1P21. Photomultiplier tubes have several advantages. They are very sensitive with gains of over 106 and are capable of detecting single photons. Photomultiplier tubes have large dynamic ranges in the order of 107. These characteristics coupled with a high degree of linearity make them ideal for astronomical photometry.

Many types of photodiodes are available. These are solid state devices and produce a very small current in response to light striking their sensitive surface. Unlike the photomultiplier tube these photodiodes have a gain of unity. Their output is in pico amperes. To be useful, very high gain (X 10¹²) amplifiers must be used.

A relatively new type of photodiode has been introduced. This is the avalanche photodiode or APD. It has characteristics of both the photomultiplier tube and the photodiode. The avalanche photodiode has some draw backs, however. They are expensive ($300- $400), they require high voltage (300 VDC), and they produce very high dark counts at room temperature (in the tens of thousands of counts per second).

Charge Coupled Devices (CCDs) are now being used to do astronomical photometry. This is of great interest to the professional astronomer and in time will see much use by the amateur astronomers. All of these devices can be used for photoelectric photometry. For more information on CCD Photometry see the AAVSO CCD Observing Manual at

The light from a 6th magnitude star is just barely detectable to the average unaided eye under ideal conditions. Even using a telescope the number of photons from a faint star can be very small. A very sensitive detector is needed to make accurate measurements of this feeble light. This is where the photomultiplier tubes, photodiodes and CCDs come into use.

In addition to just measuring the light levels, photoelectric photometry can also be used to obtain other information about an object. By putting filters between the detector and the light source, spectral data can be obtained. This is kind of a poor man's spectrometer, but allowing even modest amateur equipment to obtain highly accurate spectral data. The term UBV photometry is widely used and stands for Ultraviolet, Blue, and Visual filter photometry.

An example of where UBV photometry can be useful is where a large yellow star is in orbit with a small blue star (e.g., 31 Cygni). The eclipse of the blue star by the yellow star produces a shallow dip in the V band, but a pronounced dip in the B and U bands. There is little V band light from the blue star, but there is a large change in the B and U bands because most of the B and U band light came from the blue star which was occulted by the yellow star. The reverse happens during the secondary eclipse. The difference between the B and V light (B - V) is known as the color index. There is also a U - B color index. These indexes are used to classify stars . Monitoring these values over time can provide insights into what is happening with the star.

There are several other types of filter photometry (e.g., RI, RIJLMN, uvbyb). Some extend detection into the far infrared and some are very narrow band filters. They all provide unique information about the light source. UBV and BVRI are the most popular for amateur use.

This manual will discuss UBV photon counting astronomical photoelectric photometry using a photomultiplier tube.

Amateur Contribution
Amateurs can contribute real and important professional astronomical data. Obseving and imaging deep sky object or solar system objects can be fun, but such observations are unlikely to produce anything of professional interest. Photoelectric photometry can and is ideal for the amateur living in a light polluted urban area with a small to modest telescope. Living in a dark sky area, while not a requirement, will allow photoelectric photometry of fainter stars.

Bright stars need observing. Many star systems with stars brighter than 6th magnitude have interesting situations that warrant more investigation. Professional astronomers cannot get telescope time at major observatories to view bright stars. In fact, unless the apertures of teven modest sized telescopes are stopped down considerably, stars brighter than 6 magnitude or so will saturate most detectors. In addition, many stars need continued observing over days, weeks, months and even yeas. That is not normally possible for the professional. This is where amateurs with backyard observatories can make significant contributions.

As noted above, this manual will address photoelectric photometry of variable stars using a photon counting photomultiplier tube system with Visual, Blue and Ultraviolet filters. Most of the information is for work in other bands too, e.g., R and I bands. Data taken with standard filters and properly reduced can be used in combination with data from other observers producing professional quality data.