High speed photometry

Many years ago (about 30) in the dark ages of astronomical instrumentation, photomultiplier tubes (PMTs) were making their farewell to astronomy and with them went almost all of the studies of high-speed phemonena. The Universe is a fast-paced place and many sources change on time scales of less than 1 second. Accretion phenomena and rotation of neutron stars are a few examples. Studies of such events and other fast changes are being resurrected thanks both to a renewed interest and to CCDs and instruments that are capable of the task.

Today, there are only a few modern instruments that can observe the Universe in under 1 second. They come in two varieties; non-CCD instruments, which use avalanche photodiodes or PMTs, and 2-D digital cameras, which use frame transfer CCDs (such as UltraCam) or shutterless readout of OTC-CDs (such as in OPTIC). UltraCam (Dhillon & Marsh, 2001) can simultaneously obtain three-color imaging photometry with integration times (actually frame transfer times, which keep a reasonable noise level) of 1 second. Subsecond readout (down to a few ms) is possible for smaller regions of interest. OPTIC (Howell et al., 2003) can perform shutterless readouts as short as 1 ms for up to four regions of interest simultaneously. New versions of OTCCDs being produced for the WIYN observatory will increase this capability to include essentially unlimited regions across the entire field of view. Some other examples of high-speed photometric applications are presented in Schwope et al. (2004) and Nather & Mukadam (2004).

High speed photometry is often used to study fast phenomena such as stellar eclipses or short period pulsations. It is also useful to obtain ultra-high

HD209458 R Band transit - - OPTIC @ WlYN

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Fig. 5.9. Transit observation of the Jupiter-like extra-solar planet orbiting the V = 7.4 magnitude star HD209458. These R band photometric data were collected with the 3.5-m WIYN telescope using OPTIC and 1 ms integrations. Only the first half of the transit was measured as the observations were halted when the source reached an airmass of 2.5. Each CCD image collected over 106 photons and the time series has been co-added here to 58-s averages. The photometric precision per point is 0.0005 magnitudes. Data provided by the author.

HD209458 R Band transit - - OPTIC @ WlYN

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precision photometry by allowing one to collect a large number of photons from (but not saturating) a bright source using rapid frame readout with later co-addition. Figure 5.9 illustrates this method as applied to the bright star HD209458, a G2V star with a Jupiter-like extra-solar planet. The planet transits the star every few days causing a ~ 1.7% drop in its light. While the transit event itself is easy to observe, even for small telescopes, much of the detailed astronomical information needed to compare with models of extra-solar planet atmospheres comes from the ingress and egress shape of the transit and from measurements of the transit depth to high precision.

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