Optical Advantages of the GSO RC Astrographs

Optical Design: True Ritchey Chrétien / RC
The GSO Astrograph is a real Ritchey-Chrétien reflector telescope. Contrary to Maksutovs or Schmidt-Cassegrains which have spherical mirrors and need image correctors, and contrary to Newtonians that need coma correctors, the RC system is a well corrected photographical instrument that 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 Chrétien 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 6" RC is made of a very homogenous BK7 blank. The primary mirror of the 8" RC is made of quartz 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. Here at TS we also test and collimate each instrument.

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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 invariably 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 several hours of total exposure time.

In the GSO RC the mirror sits in a fixed mirror cell and focus is changed with a separate 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!

Notice for visual observers:
To reach focus with a 2" diagonal, you additionally need the 2" extension TSV235. With 1.25" diagonals, you need the TSV250 extension.