JST250 (Javalambre Survey Telescope) is a 2.55m, F#3.5 alt-azimuthal telescope with a Ritchey-Chrétien-like configuration. The focal plane corresponds to a Cassegrain layout. M1 and M2 mirrors have hyperbolic aspheric surfaces. The telescope is equipped with a field corrector which is located beyond the central hole of M1. It consists of 3 lenses of fused silica with 4 aspheric surfaces and diameters in the range 500 – 600 mm.
The two mirrors and the three lenses are designed to optimize the polychromatic image quality and maintain a low distortion over the entire field of view, which has a diameter of 3 degrees (476 mm). A baffling system consisting of three conical baffles will prevent from stray light at the focal plane, resulting either from direct glimmer on the detector or unwanted reflections on M1 and M2. Overall, JST250 provides an effective collecting surface of 3.89m2 (after discounting the obscurations due to M2, spider and baffles), which translates into an effective etendue of 26.5m2 deg2.
Because of its fast optics (F#3.5), JST250 is a very compact telescope. The distance between M1 and M2 is only 2.2 m. This fact allows the dome to be compact too, having a diameter of approximately 12 m. JST250 weights about 45000 kg. The back focal distance of JST250 is 318 mm. It is able to support instruments of up to 1400 kg at 750 mm from the mounting flange. The instrument will be supported by the rotator that compensates the field rotation.
|Optical configuration||Ritchey Chrétien like, equipped with a field corrector and derotator|
|M1 diameter||2.55 m|
|FOV diameter||3 deg (476mm physical size)|
|Effective collecting area||3.89 m2|
|Focal length||9098 mm|
|Plate scale||22.67 arcsec/mm|
The first light instrumentation of JST250 will be JPCam, a panoramic camera of 14 large format CCDs which is being built by a consortium of several Spanish and Brazilian institutions within the J-PAS collaboration. The final Optical Design of JST250 has been optimized to account for the width and refraction indexes of JPCam entrance window and filters.