A 16" Ritchey Chretien Reflecting Telescope

The primary use of our telescope will be deep space imaging.  From this goal a Ritchey-Chrétien (hyperbolic primary and hyperbolic secondary mirror) or a Cassegrain (parabolic primary and hyperbolic secondary) design was chosen for the optical tube assembly (OTA).  A Dobsonian mount was chosen because of easy of manufacturing & assembly, as well as its capability to properly support large optics.  A Nasmyth focus will be utilized since a Dobsonian mount does not allow for a traditional Cassegrain focus.  A diagonal will allow switching between CCD and eyepiece focus.   Additionally, we chose a sixteen-inch diameter primary for the telescope because of the large light collecting ability and appropriate degree of design and manufacturing difficulty.   We are adding a Poncet platform to provide sidereal tracking .  A focuser with three inches of travel will allow for two inch eyepieces ranging in size from a 31 Nagler to a 4 Plossl.  

The most up to date CAD rendering of the telescope.  The Dobsonian base has been optimized for stiffness, and weight.  

A early CAD rendering of the telescope.

A 3D rendering of the ray trace.
A dimensioned ray trace.

When choosing the optical design we had the choice between a classical Cassegrain and a Ritchey-Chrétien design due to our desired application of deep space imaging and predicted cost constraints.  The Ritchey-Chrétien design was chosen because it corrects for coma while a classical Cassegrain design becomes dominated by coma off-axis.  The Ritchey-Chrétien becomes astigmatism limited off-axis. The Nasmyth focus was added for the convenience of the observer and based on the mechanical design limitations on access to the back of the primary mirror. The spacing between the primary and the secondary was chosen as a compromise between the optical and mechanical performance. The spacing from the tertiary mirror to the sensor was chosen with the idea that a corrector can be added in later to compensate for the astigmatism.  The dimensions of the design can be seen in figure below. Currently, the design is corrected for half a degree full field of view (see spot diagrams, Figure 2 below).  However, we choose a CCD sensor commonly used in deep sky imaging, Kodak KAF 0900, that has a 0.9° field of view.  With the addition of a future corrector lens system it is possible to have correction up to a 0.9° full field of view.  Also, a Barlow lens can be used when necessary to change the operating f/# such that the well-corrected field of view (without additional corrector lenses) will fill the entire detector.  Baffles will be added for stray light removal and will be integrated in the mechanical design of the tube assembly.   

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