Photometers

As the name implies, photometers measure light intensity. Modern instruments are much more sensitive than the light meter in your snap-shot camera. Indeed, some of the better photometers actually count individual photons. In astronomy, the photometer has become an invaluable professional tool which allows precise quantitative measurements to be made. Thus, the eye is no longer used for estimates of star brightness, eliminating all of the physiological problems of eyesight. This does not mean, however, that the problems such as variation of sensitivity with wavelength have been eliminated. The photometer has its own set of unique problems but they are much more amenable to analysis and calibration than the human eye.

There are two basic types of photometers, the photomultiplier tube and the solid-state photodiode. The photomultiplier, which has been around for half a century, is gradually being replaced by solidstate devices which are smaller, require less exotic high-voltage power supplies and are now more sensitive. You may elect to buy a complete photometer or build one yourself, as shown in Fig. 10.3.

While a photometer measures all of the light falling on its detector, we generally don't want all of the photons entering the telescope tube to hit the light-sensitive element. A photometer placed at the image plane of your 20-cm S–C should be adjusted so that it sees the light from only a small area, typically the light from just one star.