Photometers are devices that can measure starlight. They are used to determine the brightness of an object. Photometry is the business of measuring the brightness of stars and it is not necessary to have a photometer to do photometry - you can do it with a CCD camera or even a photographic camera.

Light consists of photons and photons are tiny bundles of energy that can generate minute electric currents and it this property that is used in photometers to measure the intensity of light.

There are basically two types of photometers - the PMT (Photo Multiplier Tube) and the photo diode.

The Photo Multiplier Tube

As its name implies, it is a tube that looks very similar to the ones you see in very old radio sets. (Remember those ones, usually glowing with a tiny red light with about 10 or 15 pins at the end that plugged into the electric circuit). The "multiplier" part of the name hints at the way the tube works. The end of the tube is coated with a special material and is fitted in the eyepiece of the telescope and pointed at the star. Light falls on this surface and causes electrons to be knocked off. These electrons are then attracted to a charged plate a short distance behind the front surface by a voltage difference of, say, 100v. The electrons impinge on this plate, knocking off more electrons because they have more energy because they have been accelerated by the voltage applied. These "more electrons" are then attracted to a second plate behind the first one causing more electrons to be knocked off. This process is repeated up to 10 times and so you get a "multiplier" effect. You then end up (hopefully) with enough electrons to produce a tiny electric current that can be accurately measured.

The brighter the star light you start out with, the greater the current you end up with and the higher your reading. You then feed this end current into a measuring device, like a volt meter or, as is the case with Dr Alan Cousins in Cape Town, one of the world’s most famous photometrists, onto a chart recorder, and you then have a permanent record that you can measure at will. If you have 10 "stages" in your tube and each requires a 100 volt differences you have to have a transformer giving you 1000 volts. And this voltage has to be very accurate - to better than 1 part in a million.

That is why PMT’s are not too popular with amateurs and what led to the development of the second type of photometer - the photo diode.

Photo Diodes.

These little devices also measure starlight through the effect of photons producing a tiny electric current on a special surface which is a diode ie it only transmits current in one direction and blocks electrical flow in the opposite direction. Much easier and cleaner to use as they usually require voltages of 6v - 12v, so power can be supplied by a couple of torch batteries.

Each type of photometer has its advantages and disadvantages. Astronomers would normally use filters of different colours with the photometer to determine the star’s brightness in the Blue (B),red (R), and Visual (essentially yellow - V) wavelengths. They also sometimes use ultraviolet (U) and near infrared (I) filters.

CCD photometry is becoming quite popular and relatively easy to do. If you have a CCD camera attached to your telescope, a computer attached to the CCD and some relatively simple software packages, you can measure a star’s brightness with comparative ease. Some amateurs reckon they can do variable star estimates quicker using a CCD than they can visually.

It’s astronomy for the new millenium.