Optical characteristics

The Pro RC Series Ritchey Chrétien telescopes made by Officina Stellare are built around the best quality optics today available.

All optical sets are realized under precise quality requirement conditions, using the highest renewed standards in terms of quality of the materials being used and the production processes involved. Each optical set is provided with his own interferometry report stating and certifying the overall quality to the customer. In the making of the optics, we only use blanks of certified origin and perfect homogeneity.

The materials can be chosen between a wide range, varying from borosilicate to Pyrex to Sitall and other low expansion ceramic type glasses. We offer, as a standard, very high reflectivity protected aluminizings and multi layer anti reflex coatings. It is possible to make special coatings (silver, gold etc.) for specific wavelength related needs. We have an in house, pneumatically insulated, 6 meters long and 500 Kg weight optical bench using a Zygo mark IV interferometer with phase shifter coupled with a highly sophisticated analysis software for producing extremely precise measurements and tests. We verify each and every one of our instruments before delivery to the final user.

We can also produce real sky tests using our private observatory with our high capacity mount and several large format CCDs in order to match the customer's imaging train.

The RC advantages

The advantages of the original Ritchey Chrétien (RC) optical scheme are very well known since their introduction. It's the most diffused optical scheme, adopted by professionals all over the world. The Hubble space telescope itself uses the RC configuration. It is a peculiar variation of the more classical Cassegrain scheme that uses only two hyperbolic shaped mirrors.

The use of this aspherical curvature shape, very difficult to achieve, results in a very efficient optical system, particularly suited for astrophotography and professional use. The use of only two optical surfaces guarantees the minimum possible light loss and the resulting telescope is "aplanatic" which means coma free (the coma is one of the most known off ax is optical abberations that greatly degrades the quality of the images and probably the most annoying one). The resulting stellar images are particularly well corrected along the entire illuminated field and the small residual field curvature (particularly low, thanks to the smallmultiplier factor of the secondary mirror which is typically half the factor of the common Schmidt-Cassegrain telescopes) is easily corrected using custom designed corrector lenses.

Sometimes just one lens is enough, and the flatteners are only needed with the use of very large CCD sensors. Many users operate the telescopes for imaging without the use of such correctors. This is obtained with the perfect coupling between the field of view, the pixel size and the focal length in respect to the medium viewing conditions. In such a case, a very well corrected and just reflective telescope is more flexible in terms of use, allowing the user to operate in a more wide range of spectral frequency without having any chromatic aberration issues. This particular aspect of the telescope is sometimes determinant in the final decision for particular research fields.

Materials

The materials we use are mostly aluminum and carbon fiber because of their lightness, their mechanical and thermal properties and for the optimal resistance to environmental conditions. We offer to our customers the two classical configuration options for the OTA, the closed tube (aluminum or carbongraphite tube) and the most requested carbon-graphite open truss configuration.

In this last case, the combination between the near zero thermal expansion values of the optics (if produced with Sitall or other ceramics) and the carbon-graphite truss of the structure between the two mirrors, guarantees the most stable and fixed position and focus vs temperature variations thus simplifying the imaging operations.

All the mechanical parts for the the OTA are machined using CNC computer assisted machines and from a whole block of aluminum to guarantee the best possible precision. Also the carbon fiber components are produced following our strict specification and tolerances.

Each part has been computer designed and optimized for the best structural performance, rigidity and lightness. The holding systems for both mirrors, the spider, rings and all the structural components, even the smallest, are CNC machined from a single block of special aluminum (6061/7075, Ergal and Anticorodal), stainless steel, titanium and brass.

The tubes are made of true carbon-graphite fiber obtained with the use of high vacuum process, allowing us to obtain the best stability of the focus vs temperature changes still maintaining the best structural rigidity and ligtness.

Design and craftsmanship of O.T.A's

The designing and structural processes controls are entirely made with the aid of computer systems in order to maximize the system efficiency. The making of all the components and, in particular of the most critical and precision demanding ones, is done in Italy directly by us. All the preliminary, assembly, finishing and quality testing operations are made in house by us.

Great care is taken in the making of the rear holding cell of the primary mirror. The hyperbolic shaping combined with the diameter and relative weight of the mirror, imposes a very careful study and design for the holding system in order not to introduce any astigmatism,mirror flop or shift in the system. After many tests and designs we finally figured out the best solution to avoid all the above issues. The primary mirror is fixed in a multipoint cell of innovative design that holds it linearly in the center and radially on the borders in order to distribute the weights, for any given position, in an optimal way.

In the largermirror (starting from 16") we add an extra reinforcing plate in the back of the mirror and even more holding points, up to 27, optimally distributed using computer based optimization methods; this in order to give an even more increased stability factor to the whole system.

The entire rear cell for the primary mirror is figured to have the best rigidity/weight ratio and for the fastest possible thermal stabilization between the environment and mirror. The convection and thermal irradiation are greatly improved with the use of three high quality axial fans in the back of the cell.