Email Address. Sign In. Access provided by: anon Sign Out. Cold gas dynamics in Hydra-A: evidence for a rotating disc Abstract: We present multifrequency observations of the radio galaxy Hydra-A 3C located in the core of a massive, X-ray luminous galaxy cluster. These observations demonstrate that the complex line profiles found in the cold atomic and molecular gas are related to the rotating disc or ring of gas.
Rotating disc filter ODYSSEUS ®
Finally, a Hubble Space Telescope image of the galaxy shows that this gas disc contains a substantial mass of dust. Some suppliers ofter a wide selection of chopper blades with different numbers of slits, single or multiple rows, different duty cycles, outer diameters etc. During the switching, one obtains a beam with modified beam profile and possibly some effects of diffraction ; this may be relevant for some applications.
For precise control of the rotation frequency, one ideally uses a synchronous alternating current motor; the rotation frequency can then be exactly controlled via the electrical drive frequency. Alternatively, one may use some electronic feedback loop, e. A low timing jitter may also be achieved, if in addition the mechanical setup is rigid and stable, e.
ROTATING DISC SYSTEMS, BASIC PHENOMENA
High precision chopping devices may reach very high accuracy despite the mechanical operation principle. Note, however, that minimum timing jitter may be achieved only for limited rotation speeds, well below the maximum allowable speed. Some choppers work with a fixed and rather stable frequency, while others offer a variable frequency. In the latter case, the chopper wheel driver may allow the adjustment of rotation frequency with an adjustment knob or alternatively with an external DC voltage, but there can also be a digital interface for more precise control.
For convenience, there may be a digital frequency display on the front panel. The driver may also provide a reference output signal e. It may be used as a reference input for a lock-in amplifier, for example. Additional mechanical accessories may be useful, for example mounting plates and housings enclosures for blocking unwanted light. Mounts may have to be optimized such as to minimize the transfer of vibrations to the base plate.
Another operation principle of an optical chopper is based on a resonant mechanical vibration, which is excited with an electromagnet. The vibrating part contains a blade which moves into and out of the light beam. Such devices are mechanically simple. However, they can usually be operated only with a fixed frequency, since a sufficiently high oscillation amplitude is achieved only around the mechanical resonance frequency. Also, the light beam must be positioned more accurately due to the limited vibration amplitude of the blade.
There are also various kinds of beam shutters which do not necessarily operate periodically, but can be controlled with a voltage signal, which may be periodic or not. In that sense, they are more flexible. On the other hand, they are typically much more limited in terms of the maximum switching frequency, also in terms of the number of switching cycles. Therefore, they are not common for periodic dropping, particularly not at high frequencies. A typical application of choppers is to do sensitive measurements of optical powers involving lock-in amplifiers, e.
Instead of directly measuring the optical power with some photodetector , one registers an oscillating power of a chopped beam.
The lock-in amplifier can then very sensitively detect that modulation and can, if required, also determine its phase. Particularly good signal-to-noise ratios are achieved for high chopping frequencies, which however are limited with mechanical types of choppers. Influences of background light can be easily eliminated with such techniques.
With chopped laser beams, it is possible to measure certain decay times — for example, the upper-state lifetime of a laser gain medium. One simply exposes a piece of the medium to the chopped beam and registers the time-dependent power of the fluorescence.
Although switching speeds of mechanical chopper devices are limited, they can easily be sufficient for measuring decay times of the order of milliseconds or somewhat less, if a suitably focused beam is used at the blade. Choppers with a small duty cycle may be useful for experiments where the average power of the continuous beam would be too high for some system.
Rotating disc tensioner
For example, one may test a laser which would overheat for continuous pumping. Chopper discs can also be used for optical sensors, detecting rotation speeds. For example, an anti-lock braking system of a car may use it for checking whether the brake is about to completely stop a wheel.