Shutter time

Finally a word about efficiency observing time on large telescopes is a valuable asset, both in terms of cost and considering the ratio of observing time available to the time requested by astronomers. Marco et al. (2001) state that the observing efficiency defined as fhe ratio of science shutter time to available dark time is 10-30% for the ADONIS AO system while the corresponding ratio for other instruments is 50-80%. Some of this difference is due to the fact that most AO exposures are of short duration and the readout time is significant. In addition, AO systems use time to close the loop and optimize performance. Observations may also be necessary to characterize the PSF. 

Fig. 6. Schematic pictures of an I ion surrounded by water molecules using shutter times of 1 ps (top left) 4 ps (top right), 10 ps (lower left), and 30 ps (lower right).

A low speed shutter (S2) is used for making computer controlled dark current measurements. A high speed electromagnetic shutter (SI, 22-8411, Ealing Corp., South Natick, MA 01760) is used for controlling the time interval that the vidicon is illuminated. Exposure times are entered into the shutter control module from a multiplier and a decade switch register providing shutter times of from 30 ms to 10 s. 

If the average lifetime in one state (rex) is longer than the shutter speed, we will see two distinct peaks in the spectrum. If the average lifetime is shorter than the shutter time we will only see one averaged peak. This shutter time is formally called the coalescence time, rc, for the exchange process, or simply the NMR timescale . 

Note that on a 200-MHz instrument, Av is 30 Hz (0.15 ppm x 200 Hz/ppm) and the NMR timescale rc is 15 ms (l/(2.22 x 30)), but the shutter speed is faster as we go to higher field instruments because the chemical shift difference Av is measured in hertz, not ppm. Thus, moving to higher field shortens the shutter time rc in a way that is inversely proportional to B0. Figure 10.7 shows simulated spectra of the DMF sample at the same temperature that gives rex = 15 ms, analyzed on three different spectrometers with ywB0/27T = 60, 200, and 600 MHz. At 60 MHz (top), we have an exchange-broadened fast... 

Figure 48.2 shows the nucleation of ice which was filmed (HiSIS 2000) using 1120 frames per second (444 /rsec shutter time). Prior to the nucleation event, the bubble appeared deformed, or "dancing," and sometimes microbubble (<5 /rm) ejections were observed within one frame. As the temperature was decreased, the acoustic pressure had to be adjusted to maintain a stable bubble. In this example, the pressure was increased from... 

A typical pseudo-2D experimental setup for optical measurements is shown below in Fig. 4.2. A glass plate makes up the front of the reactor, which is fiUed with inert particles. Fluidization gas, possibly treated with a small amount of steam to prevent static charging of the particles, is fed from below through a porous plate. To prevent blurring of the particles on the images, a fast shutter time is required (in the order of milliseconds), and therefore additional Hghting is usually necessary. 

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