Astrographs

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An astrograph is a telescope designed specifically for astrophotography.  Astrographs are commonly employed when doing wide area surveys of star fields and are also used to detect comets and asteroids.  To make the astrograph effective, the focal plane is normally designed to work with a specific size photograph plate – or modern CCD chip.  This helps to deliver the largest, flattest field possible with clarity to the edge of the image.  Astrographs that have short focal ratios are most often used for wide angle responses, while longer focal ratios are used for precise measurements.  A telescope like the wide-field Astrograph should be your choice if you prefer imaging large objects such as starry vistas, extended nebulae, open star clusters, and big galaxies, or studying large, close objects like comets.

 

Ritchey-Chretien 

 

Most astrograph telescopes are a refractor-based design, but larger models can be a reflector type, such as the Ritchey-Chrétien.  The Ritchey-Chretien Telescope (RC) is similar in style to the Schmidt-Cassegrain.  It has a folded optical path but uses only mirrors.  Both the standard RC and Advanced Coma Free designs have a long focal length in a short package, making them ideal for high magnification astrophotography.


Dall-Kirkham 

 

The Dall-Kirkham Telescope is also similar to the Cassegrain, but it uses an ellipsoidal primary mirror with a spherical secondary mirror.  It also includes two lens elements ahead of the focal point to improve off-axis image quality.  Modified Cassegrain Telescopes use a small optical mirror located in front of the primary which aims the light to the side of the telescope tube and eliminates the need for a perforated primary.  The similarly designed Schmidt-Newtonian Telescopes have a flat diagonal mirror to catch the light from the secondary, and they reflect it to the side of the tube instead of exiting the rear.

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