Helix 250 - design & construction

To maintain the optical and mechanical stability, much attention must be paid to the mechanical design. The optical elements must be exactly collimated in our small factory and then permanently locked in the right position so that there is no need to re-adjust or re-collimate.

Furthermore, the focusing system must be accurate and reproducible even during temperature changes. Only then is a astrocamera easy to use in the field. The following diagrams shows the drawing of the camera.

The camera consists of a 4 mm thick Phenol-resin tube and a number of molded and precision machined aluminum rings. CCD- and photo camera's are mounted by means of an adapter and a quick/release bayonet on the camera.

The full aperture corrector consists of two lenses and is mounted is a rigid cell. Each lens has its own cell to minimize internal stress. Three invar rods connect the corrector cell to the primary mirror cell. The thermal expansion of these invar rods is almost zero so the focal plane does not vary with temperature.

The Helix 250 is almost diffraction limited. To fully exploit the optical excellence, a small fan is built into the camera to eliminate temperature differences between the inside of the tube and the open air. Tube currents and mirror seeing can else deteriorate the image quality. The fan has a micro filter to prevent dust from entering the camera.

The primary mirror is made of Astro Sital which has an expansion coefficient of almost zero. Thermal stability is very important for an astrocamera and has been a major design issues for the Helix 250. The result is an extremely stable camera which needs no refocusing what so ever.

A micrometer screw at the back of the camera changes the position of the focus. A position accuracy better than 0,01 mm can be achieved. An auto focus stepper motor system is available which can be used with auto-focus programs. A housing protects the focusing mechanism unit.