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CCD Astronomy Magazine Article Page 3

The Mega-Pixel Approach: The 2048x2048 CCD

The McDonald 30-inch Prime Focus camera (f/2.98) uses a thick, front-side illuminated Loral-Fairchild 2048x2048 CCD with a Metachrome II coating. This device has an active imaging area of 47x47 mm spread over 4 million pixels. A typical 1-meter telescope has an f/3.0 -- f/4.0 primary mirror. Hence, placing a 2048x2048 CCD at the Prime Focus would result in an angular field of view of approximately 1 square degree. The 30-inch telescope at the McDonald Observatory has such a setup which results in a total field size of 46 x 46 square arcminutes. The CCD data can be flatfielded well over the entire field size. Figure 1 (683 Kbytes) shows an image, taken in the Blue Band, of the Pegasus cluster of galaxies. The red arrows point to some galaxies in the field. These galaxies all have relatively high contrast with respect to the background night sky and hence are easily detected. The inset shows an example of an extremely diffuse galaxy (known as Low Surface Brightness Galaxies ) that can be discovered with this kind of data. Note how noisy the sky appears in this contrast setting as the galaxy itself is only 2% above the sky background. These kinds of galaxies, previously unknown, represent a new population of objects that has recently been discovered by members of our team (see May 1994 issue of Discover magazine) using wide field CCD imaging techniques.

Typically, prime focus imaging requires the fabrication of an expensive corrector plate so that good image quality is maintained over the entire field. This was done for the McDonald 30-inch. However, most 1-m class telescopes do not have a budget for such a corrector and hence continue to operate at the Cassegrain focus, thereby reducing the field size by typically a factor of 3. Nonetheless, field sizes of ~ 20x20 arcminutes can be achieved with a 2048x2048 detector at Cass. This is still relatively wide field imaging by traditional professional astronomical standards. Figure 2 (319 Kbytes) shows a blue band image of the Hercules cluster of galaxies obtained using a 2048x2048 Textronix CCD detector on the back of the Kitt Peak National Observatory 0.9-m telescope. The improved image scale offered at CASS is helpful in measuring the details of galaxies which have angular sizes that are less than one arcminute, as in the case here.

The Newtonian Focus of the Burrell-Schmidt telescope at Kitt Peak National Observatory is capable of producing a field size of slightly larger than one square degree. Figure 3 (685 kbytes) shows a deep V-band CCD image taken with this system. The extended objects in center of the field is a dwarf spheroidal companion to M31 (named And I). This exposure shows that it can be resolved into individual stars to some extent. The low galactic latitude of this field also means there are lots of foreground Milky Way stars present. Special care and data processing is required for data taking with this telescope as the optics of the Schmidt are known for aliasing the light from bright sources that are off the field onto the detector. This produces a series of ghost images which can be identified and removed, to some extent, the data processing stage. Thus, with some work, a very old telescope can be made into a state-of-the art wide field digital camera.

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