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The GSO 12" F8 Ritchey-Chretien Truss Tube Astrograph is a true RC telescope and offers a flat field without additional correcting optics, reducing losses and maintaining focus across wavelengths.
Carbon Fibre Truss Tube
A lightweight serrurier truss frame is self supporting and maintains the position of the primary and secondary optics relative to each other. This keeps the system collimated. Its carbon fibre construction has low thermal expansion. Focus shift is therefore minimal.
The scope is supplied with GSO's own 3" Monorail focuser. This is a relatively basic focuser and can be substituted as an optional extra with other more specialised focusers from our focusers section.
The Ritchey-Chretien design is based upon the use of primary and secondary hyperbolic mirrors. This type of mirror allows for correction of many of the optical aberrations caused by using Spherical and Parabolic mirrors and leads to the characteristic flat field offered by an RC without the need for a separate correction lens. Traditionally a hyperbolic mirror is difficult and therefore expensive to produce. However GSO have been able to perfect a low cost method of manufacture, making the RC design accessible to all.
Both the primary and secondary mirrors are made from low expansion quartz. This reduces the cool down time required from a high ambient temperature and also further promotes focus stability. The primary mirror also features fan cooling if needed. The mirrors themselves have a highly reflective 99% dielectric coating that is anti-tarnish and anti-scratch The primary mirror is fixed to a metal mirror cell. The secondary sits in a metal housing that can be collimated.
Ritchey Chretien Advantages
Unlike other Cassegrain type designs, the RC has a fixed primary mirror. This means that image shift or flop is removed from the system during focusing or position of the telescope. No matter how good the design, any movable mirror can suffer from this problem. Apart from the issue of focus and image shift this causes, it also, by definition, affects collimation.
Schmidt Cassegrain designs require additional correctors to provide a flat field and reduce the effects of coma and other aberrations. The addition of such optics will by definition add new problems. The RC design does not require an additional correction in its basic design. It can of course be used with both flattener and reducer flatteners when even larger flat field diameters are required for use with large format 6sensors. The GSO 12" RC can be used with sensors up to 35mm format without additional correction. For larger sensors we recommend the our 0.8x RC Reducer. This provides a corrected field suitable for 24x36mm sensors and reduces the focal ratio to F6.4.
The Corrected Dall-Kirkham (CDK) design is also popular in this category of reflector. However a CDK uses a spherical mirror to lower cost which then has to be corrected with separate optics to compensate for the aberrations that spherical mirrors introduce. Although traditionally the extra optics and associated light loss from using them makes a high end CDK less expensive than an RC, the breakthrough price of GSO's hyperbolic mirrors mean this is no longer true.
|Aperture||12" / 304mm|
|Chassis||Carbon fibre truss with 2 Losmandy format mounting plates|
|Mirrors||Low Expansion Quartz with 99% dielectric coating|
|Back Focus||233mm from back plate, 127mm from supplied focuser