CCD Photometry

Very, very brief overview

The technique of differential photometry was used for the data collected during my PhD studies. In this, one star in the field is assumed to be constant, and the brightness of another star (or stars) in the field is measured relative to the brightness of this so-called camparison, or C, star. Usually, the other star is variable, and is denoted by V. It is also considered extremely helpful to measure at least one other star- a check star, K- as a means of checking the constancy of C. If several K stars are measured, these may be used to estimate the expected error in the differential magnitudes, C-V (or, alternatively, V-C), of the variable star.

If C is indeed constant, the values of C-V will vary as V itself changes brightness, and (apart from measurement errors), only because V is variable. Also, the values of C-K will vary about a straight, flat line, and the scatter in these values will be an indication of the measurement errors of the photometry. If C is not constant, there will be some change, over time, of the C-K values. Of course, in such a case, K may itself be variable (and C not), so it would be wise to measure two or more K stars in any case.

There are certain provisos in the above statements, however. The first is that the colours of the C, V and K stars should be similar. If not, differential atmospheric extinction will cause rising or falling trends in the C-V or C-K values over time. Perhaps not over a short observing run with the field near the meridian, but certianly when the field is closer to the horizon, or when data from different months (and therefore likely at different air masses) are compared.

Secondly, that the check star(s) (if only one or two) should be of similar magnitude to the V star, in order for the statistics of the C-K data series to be able to be used as an estimate of the statistics of the expected errors in the C-V data series. If this is not so, then the C-K stats will either over- or under-estimate the errors in the C-V series.

On with the show...

Some of the results obtained at the Woomera Optical Observatory will be presented. Apart from cataclysmic variables (CV's), other variable objects were examined- active galaxies for the most part. Only the choicest subjects will be presented here for your delectation, and with, admittedly, only minor discussion of the results.

In brief summary, the objects which were observed at Woomera are as follows:

Cataclysmic variables : EUVE J1429-38.0, EUVE J2115-58.6, AX J2315-59.2, AE Aquarii, AH Mensae, TX Columbae, HP Librae

EUVE No Optical Identifications : EUVE J1200-36.5, EUVE J1707-76.3

Black-hole binary GRO J1655-40

Active galaxies: PKS 0208-512, PKS 0548-322, PKS 2005-489, PKS 2155-304, PKS 2255-282

Woomera Optical Observatory

EUVE J1429-38.0

euve j2115-58.6

ax j2315-59.2

GRO J1655-40

Extinction measurements

Error Estimation