Corrected Cassegrain Astrograph CCA-250 OTA
Introduced in 1998, the BRC-250 astrograph adopted a totally original optical design, based on Baker-Ritchey-Chrétien, perfectly designed for the imagery at that time. Following the recent evolution of CCD cameras offering larger sensors, Takahashi decided to modify its optical design. To work out this new CCA-250, successor of the BRC-250, the optician engineer Mr Mori Hitoshi opted for a Cassegrain base to lead to this new instrument perfectly adapted for the various objects of the deep sky.
CCA-250 (CCA stands for Corrected Cassegrain Astrograph) offers 3 kinds of focal lengths by the addition of optional reducers and extenders. From F/5 standard focal ratio at primary focus, it is possible to go to F/3.6 by using one of the 2 reducers or to go to F/7.5 by using one of the 2 extenders available. The choice of the model of reducer and extender will depend on the image circle needed, but also according of the size of the sensor of the camera used.
Finally Takahashi carried out a very multi-purpose astrograph offering a extremely high standard product bringing a great optical quality as well as irreproachable mechanics on top of which is added a splendid design associating aluminum and carbon.
Takahashi has chosen a modified optical design based on Cassegrain telescope, with a parabolic primary mirror, a spherical secondary mirror and a neutral field corrector. Collimation is perfectly maintained by using the whole adjustment of the spider instead of having to adjust the secondary mirror.
The main result of this formula is a real increase of the back focus (compared to that of the previous BRC-250), allowing many different possibilities and assemblies, of including those of correctors and reducers. Those will offer you an extreme flexibility to the different possible configurations, neither equipped with a camera with a large sensor or not
Thanks to the extenders and the reducers, it is possible to use instrument with 3 different focal lengthes (900mm, 1250mm or 1800mm), with 3 different circles images (40mm, 60mm or 80mm) according to the configurations (see detailed specifications). The telescope is thus not only limited to the large sensors. All the assemblies are screwed at the back of the tube to facilitate the balancing of the instrument.
The focus operation is motorized, using a translation of the secondary mirror. Three fans (coupled with temperature sensors) are installed to accelerate setting of appropriate temperature. All of the commands are centralized inside the Active focuser system and can be either manually controlled of through a compatible interface ASCOM with connector industry USB.
For the very first time, Takahashi has chosen a structure of tube associating both aluminum and carbon fiber. The result is outstanding, both from the esthetic point of view as well as regarding the efficiency of the thermic control.
The carbon fiber is used to maintain the quality of the focus during exposition time as well as to lighten the OTA. Due to the difference of dilatation rate between carbon fiber and aluminium, a compensation system is added to dovetails to keep the integrity of the tube. The photographers will thus use an instrument whose thermal behavior is perfectly known.
In spite of the presence of carbon, the tube is still massive : 23kg without accessories. It is equipped with a male Losmandy-like dovetail (with its female counterpart), as well as a top dovetail accepting TGM mounts. The CCA-250 owns a camera angle adjuster (the CAA) and of a set of extension tubes. The OTA is certified EC, in conformity with the EU law relating to the electricals appliance.
The photographer of the deep sky has, finally, a really general-purpose instrument, without compromise about optical and mechanical.