8" f/8 RC Astrograph - 203 mm aperture / 1624 mm focal length

8" f/8 RC Astrograph - 203 mm aperture / 1624 mm focal length

Ritchey Crétien telescope for demanding astro imaging and observing.

Ritchey Crétien  (RC) telescopes are known for their excellent optical performance, especially for photographical applications. At the same time however RCs were always notorious for being very expensive. To change this well known US based distributor "Astro-Tech! asked GSO to produce true RC telescopes. GSO is best known for their excellent Newtonian optics which they have built for many years now.

The result of this joint venture is an instrument that is superior in most aspects to a conventional Schmidt Cassegrain, and still comes at a very affordable price.

The GSO 8" RC f/8 Astrograph was designed for full illumination of the "35 mm format" (24x36 mm). For cameras with a small sensor, particularly most CCD cameras, no field flattener is needed. For cameras with a larger sensor we recommend the use of a field flattening lens.

Since the optical layout of the RC uses no lenses or corrector plates there is no false colour at all. Also you will not need to use an IR cut filter, thus increasing the effective light-gathering power of the instrument.

How to bring a SLR camera into focus:

The GSO RC has a generous back focus allowing to attach many accessories like e.g. off-axis guiders, filter wheels etc. If you want to take photos with a DSLR camera with the focuser in a moderate position you need extension rings, we can offer you for your system.


Optical Advantages of the GSO RC Astrographs


Optical Design: True Ritchey Crétien / RC

The GSO Astrograph is a real Ritchey Crétien reflector telescope. Contrary to Maksutovs or Schmidt Cassegrains which have sperical mirrors and need image correctors, and contrary to Newtonians that need coma correctors, the RC system is a well corrected photographical instrument hat uses only mirrors (no false colour!) and does not require any corrector with many cameras.

The RC has a concave hyperbolical primary mirror and a convex hyperbolical secondary mirror.

The resulting image correction throughout the field as well as the sharpness on the optical axis are superior to typical Schmidt-Cassegrains or Newtonians.

It is for this reason that the Ritchey Crétien is very popular for astro imaging. The RC optical system (and a variant of the RC) is the most frequently used design for professional telescopes such as the 10 meter Keck telescope on Hawaii, or the VLT in Chile. Finally the RC is also available for amateurs without a "Nasa budget".

99% Dielectric High Reflectivity Coating

The primary mirror of the 8" RC is made of quarz which, similar to Zerodur, has practically zero thermic expansion for excellent optical properties during cool down.  Both primary and secondary feature a 99% high reflectivity coating which is well protected against scratches and against aging. Compared with similar instruments you get a much higher effective light gathering power.

Each optical set is tested by the manufacturer

For a constant quality level each optical set is tested and hand corrected after the automatic production with state-of-the-art machinery. Then the optical set is tested with a Zygo interferometer.


Mechanical Advantages of the GSO RC Astrographs


Fixed primary mirror cell and computer optimized baffle system

Most similar telescopes have a movable primary mirror cell. When focusing the telescope the entire mirror is moved. Unfortunately this technique envariably leads to a certain amount of image shift. Most manufacturers have controlled this problem to a fair extent, but the remaining image shift can be quite distracting for astro imaging, particularly with cameras with a small sensor size or when taking series of frames with severyl hours of total exposure time.

In the GSO RC the mirror sits in a fixed mirror cell and focus is changed with a seperate focuser. This eliminates the problem of image shift completely.

Computer optimized baffles are placed inside the tube in order to eliminate stray light.

Collimation of the secondary mirror

The primary mirror is fixed in its precisely machined mirror cell. Collimation of the telescope is done through the secondary mirror. Collimation is rather easy and can even be done by an inexperienced amateur astronomer - we do however recommend to take your time with the collimation!


Delivery contains:

• 8" f/8 RC Astrograph
• 2" Crayford Focuser with 1 1/4" Adapter
• Vixen style Dovetail Plate

Specifications: