Dual Configuration - Two Telescopes in One!
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Most Cassegrain telescopes are long focal length, slow speed instruments, typically f/8 to f/15. While this configuration is ideal for high resolution imaging, astro-imagers who desire a wide field of view and shorter exposures must resort to either a focal reducer or a second telescope. Two high-end telescopes are expensive and it is not possible to design a focal reducer capable of filling a large CCD with sharp images. |
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Peter Ceravolo's goal was to design a telescope capable of working at both short and long focal lengths and still form sharp images to the corner of a large CCD. In a reversal of the conventional approach to telescope design, Ceravolo's baseline Cassegrain optics work at a very fast f/4.9 and cover a wide field of view with the aid of the integral field corrector lenses. The high resolution configuration is achieved by swapping the f/4.9 field corrector lens group with another three-element field corrector with amplifying power, making the system f/9. |
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F/4.9 Configuration
| The Rosette Nebula imaged with the f/4.9 configuration yielding a 1.4 x 1.4 degree field of view with the Apogee U16M camera. |
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F/9 Configuration
| Install the f/9 field corrector set to achieve a 0.75 x 0.75 degree field of view with the Apogee U16M. |
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The advantages of high-resolution capability
The images of the Ring Nebula, shown below, illustrate the advantages of the Ceravolo Astrograph's dual configuration capability. The objective of the wide field configuration is straightforward: to record wide areas of sky in one shot. The aim of the high-resolution configuration is to increase the "sampling" of the image to record fine detail in smaller objects. Click on either image to view it at a larger size in a new window.
The first image of the Ring Nebula, almost lost in a sea of stars, was taken using the wide field f/4.9 mode and a modified SBIG STL-11000M CCD camera. This combination produces a medium-resolution, 1.3 arc seconds/pixel image sampling yielding a huge 1.4 x 0.94 degree field of view. The second Ring Nebula image was taken with the astrograph in the f/9 configuration and the SBIG ST-10 CCD camera. This high-resolution combination produces a 0.52 arc second/pixel image sampling yielding a 0.3 x 0.2 degree field of view.
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The wide field and high-resolution Ring Nebula images have been cropped and matched in field size to illustrate the significant increase in detail visible in the f/9, high-resolution configuration. Note that these images have not been sharpened with image processing software. Click on the image to view it at a larger size in a new window.
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