Photometry is the measurement of light. For some reason in astronomy it has come to mean measures with a superior detector than the old eyeball-such as  a photomultiplier tube or a CCD camera. But we are still dependent upon the visual observers, or surveys such as ASAS, to provide a skeleton on which the more specialised photometric observations can be displayed. Photometry has a wide variety of targets and many of them are illustrated at this site. It introduces a rather different approach to observing for reasons which are described below.

A typical SBIG CCD camera with a filter change system mounted on a Meade LX6 25cm telescope.  This is controlled by a CPU and a small computer. Because images can be viewed through the computer--and time exposures increase the limiting magnitude--this can work at much fainter magnitudes than PEP where centring the star in the aperture is one of the main limitations. The dynamic range and linearity of low cost CCD cameras is not as good as photomultiplier tubes.

PEP & CCD photometry are far more precise than the eye so that measures in the millimagnitude range (0.001 to 0.009 magnitudes) are possible. As well the detector range is good--pm tubes usually cover 300-600 nanometres, CCD from 450-900 nm. Taken together, this allows the use of filters with bandpasses of 100nm or so to examine the colours of stars--see the following pages for practical applications. It's a quick form of crude spectrography.

The most common filters systems used by amateurs are UBV and BVRI. JH photometry is also performed by a few. From this we derive the following colour systems: V, B-V, U-B; V, V-R, R-I, with B-V optional; J, J-H. Approximate mid-wavelengths of these filters are shown in the lower side panel.

Because of the precision attainable it is necessary to remove certain 'noise' from the measures. This takes two forms, the deviation of the telescope, filter and detector system from the standard system (sometimes called scale factors) and the effects of the Earth's atmosphere on the starlight--extinction. These are considered briefly in the next two sections.

Begin by obtaining a simple CCD camera, preferably with some type of BVRI filter system and use this with an inexpensive computer. Don't go for the nine-day-wonder projects but spend some time on bright objects, even if it means stopping down your telescope. And find someone to advise you on the results you obtain. Remember, few books on how-to are written by people who have actually done it, so get advice from an observer.

More about how to on page 9

This is explained in more detail in the section on calibration but it is as well to know the principles--that each telescope system has its own unique reponse which must be determined and corrected if measures are to be published in a true standard system. Extinction must also be removed. Taking as an example the UBV system the transformations to the standard system involve:

Epsilon      V correction        (tube, telescope, filter)
Psi             B-V correction           ditto
Mu            U-B correction           ditto
k'v            Primary extinction corrections, variable night to night k'bv          and preferably excluded. See the section--differential   k'ub           photometry and all sky photometry on page 9.
k"bv          Secondary extinction corrections--reasonably stable,
K"ub         except during dust storms, bush fires, etc.

U   Ultra-violet     340nm
B    Blue               420
V    Visual            550
R     Red               ###
I      Infra-red        ###
J                          1500
H                         1800

There are KLMN and so on in the infra-red but the atmosphere limits these. All are referred to by the letter only.

We've all seen the Sun and Moon golden at low altitude, or even red if there's any amount of dust or smoke in the air. And at sunrise and sunset the Sun is almost watchable. This illustrates the main princilpes of extinction. We are looking at objects through an atmosphere that is not completely transparent.

Extinction can be divided into two components for a practical treatment. The first is the amount by which light is dimmed in its passage, the second is the change in colour caused by the fact that light at shorter wavelengths, e.g. blue, is dimmer more than at longer wavelengths (red). By the use of filters it is possible to determine values for these extinction properties and transform any measures to the value in space.

The two components of extinction are referred to as primary--for the common reduction in brightness--and secondary--for the colour change. In measuring these, two filters are essential. Except in the special case of JH photometry, one is always V, the other either B or R. At shorter wavelengths the secondary extinction, or colour term, is important, but at wavelengths longer than visual it becomes quite small.

Extinction is always quoted in magnitudes per air mass (X). One air mass is a column of air vertically above the observer. Obviously this path length through the atmosphere increases when the target object is at lower altitudes.