1.1. Astronomical Photometry

Astronomical photometry is the process of measuring the brightness of celestial objects. To obtain accurate brightness measurements is of fundamental importance to astronomy and astrophysics as it provides a major insight into the physical makeup of an object, from topographical variations on a nearby asteroid to the physical processes occurring deep within the core of an exploding supernova millions of light-years away.

Astronomers measure brightness on a magnitude scale that was first introduced by the Greek astronomer Hipparchus over 2,000 years ago. Since that time the scale has been refined and standardized into its current form as a logarithmic scale where a difference in intensity of 100 times is defined to be a difference of 5 magnitudes, i.e.

where m1 and m2 are the magnitudes of two objects and b1 and b2 are their brightness.

The magnitude scale has an arbitrary zero point set by convention and extends indefinitely in both directions in the sense that the more positive an object's magnitude, the fainter it is. Thus, a magnitude 2 star is 2.5 times fainter than a magnitude 1 star, while a magnitude 6 star is 100 times fainter. On this scale, the brightest stars are about magnitude 0, the faintest stars visible to the unaided eye in a dark location are about magnitude 6.5, while the faintest objects yet imaged are about magnitude 29.

In making photometric observations, an experienced observer is capable of measuring the relative brightness of two stars by eye to an accuracy of 0.1 magnitude providing both stars are in the same field of view, while photography improves this to about 0.05 magnitudes. This is inadequate for most programs and also tends to be slow. For these reasons, the use of electronic detection devices is now almost universal. Photomultipliers provide an accuracy of about 0.01 magnitudes while up to 0.001 magnitudes is claimed for charge-coupled devices (CCDs).

1.2. Project Background

For over ten years, staff at the Perth Observatory have undertaken photometric research using a DC photomultiplier photometer constructed by staff at the Lowell Observatory in Arizona, USA. However, photo-meters do have several disadvantages when used for photometry, in comparison to a CCD. They have a lower quantum efficiency, a narrower spectral response, can only be used on single objects, separate observations must be made to measure the background sky brightness, and no image is produced.

In 1988, the Perth Astronomy Research Group proposed the automation of the Lowell 61 cm telescope at the Perth Observatory for use in extra-galactic supernovae detection. Part of this automation involved outfitting the telescope with a CCD imaging system. The system is now in operation and discovered its first supernova on 28 March, 1993.

Apart from the detection of supernovae, it was proposed that photometric measurements be made of supernovae to produce light curves. In view of the relative advantages of using a CCD for photometry, it was proposed to investigate this method.

1.3. Project Aims

The aims of this project were :
(i) to investigate methods for CCD photometry;
(ii) to perform photometric measurements on stars to determine the accuracy and precision of the system;
(iii) if possible, to make photometric measurements of supernovae and produce light curves.