CCD Cameras

Which camera or camera system to choose is a matter of cost, convenience, and/or complexity. While one could go for a standard, large-format, digital imaging system, the long readout times of the CCD array make processing thousands of frames very inconvenient. In addition, many of the imaging CCDs are not equipped with accurate or reliable enough shutters. High frame rate digital CCDs (Dalsa makes this type of camera, see the list of suppliers at the end of the chapter) are another option. The disadvantage is in complexity. These cameras typically require building a custom electronic interface and writing a low-level device driver to get the image data into a location in computer memory. So, this leaves video cameras. For speckle interferometry they fall into two classes, intensified and non-intensified. Both types are useful; they simply need to have a standard video output (RS-170 or PAL). Which to choose is mostly a matter of price - a non-intensified video camera is in the range of US$250-300, and an intensified video camera is in the range of US$2000-15,000.

When choosing a video camera for use in a speckle system, be sure to get information on the physical size of the CCD detector, not just the number of pixels along each dimension (though this is handy to know as well). This video camera will be used in conjunction with a frame grabber (described below) which will rescale the output video. If this information is not available, all is not lost. Simply make sure the speckle system design has enough flexibility to optimise the Airy disk size to the output pixel size.

Any video camera will do, even a colour camcorder. It just needs to output RS-170 (NTSC if it is colour) or PAL analogue video. For sensitivity purposes, a monochrome video camera is better. For the adventurous (and moderately wealthy), a video camera with adjustable gating can increase performance on brighter objects or under faster seeing conditions. An RS-170 video camera outputs 30 frames per second, a PAL camera, 25. Normally the CCD detector exposes for the whole frame time. Through either creative "charge dumping" (reading out the CCD several times during the frame time, and only "remembering" the last readout) or an LCD shutter, the time the detector sees the sky can be less than an individual frame time while keeping the video frame rate unchanged. This process is known as gating.

The dedicated amateur with a bit of money to spend might consider an intensified video camera. Intensification adds an additional 4-5 magnitudes of sensitivity, and thereby increase the number of available objects 10- or even 100-fold. There is a penalty.

Intensifiers are electronically noisy. They use phosphors to create electrons from the original photons, amplify these electrons about a million-fold through a series of high voltage cathodes, and use phosphors again to create many photons out of the amplified electrons. An input of a photon creates an output of about a million photons. If, during the early stages of amplification, a stray thermal electron enters the series of cathodes, it gets amplified right along with photon generated electrons. A detector peering at the output of the intensifier cannot tell an object photon from a thermal electron. For binary star astrometry, using an intensifier limits the magnitude difference between the stars to about 3.6

The final issue about the camera is its size and weight. Lightweight and compact cameras are easier to use on all types of telescopes. In addition, many cameras come with the camera head (containing the detector) and the controlling electronics as separate units connected by a cable. This makes for a system that is more modular and easier to handle.

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