Two Star Photometers

A serious practical limitation to good photometry is a nonconstant sky. The sky is variable in both transparency and brightness. The latter may be due to atmospheric emissions (aurora, airglow) or to the reflection of city lights from clouds and haze. Therefore, it is a major advantage to be able to use a nearby comparison star to obtain differential light curves if the observations can be obtained at a faster rate than the sky undergoes variation. For this reason, two-star (more generally, "two-channel") photometers have become important for photometric work. The visual polarizing photometer used at Harvard College Observatory since the 1870s and at Princeton Observatory from about 1911 onward and the analogue electronic version, the Walraven photometer used in South Africa in the 1950s, were pioneering efforts and produced many useful light curves. Today there are many variants of these photometers which have been developed at different observatories. Most depend on having separate light path detectors, or electronics for the different channels. The one with which we are most familiar does not. It is the Rapid Alternate Detection System (RADS), used at the University of Calgary's Rothney Astrophysical Observatory since about 1981 (Milone et al. 1982). This system (Figs. 2.1 and 2.2) employs a single pulse-counting detector and a swiveled secondary mirror which is driven by the dial-in settings of a function generator. The amplitude of the throw, the

Fig. 2.1 RADS instrument. Shown is the controller for the RADS. It consists of a function generator which controls the successive positions of the secondary mirror within a cycle involving successive settings on the program star, sky near the comparison star, the comparison star, and the sky near the program star

Fig. 2.1 RADS instrument. Shown is the controller for the RADS. It consists of a function generator which controls the successive positions of the secondary mirror within a cycle involving successive settings on the program star, sky near the comparison star, the comparison star, and the sky near the program star duty cycle, and the delay time for mirror settling can be entered separately for each of four positions. The delay time depends on the aperture, because a smaller aperture requires more stability for the image as the mirror ringing dies down. The sum of

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