Helix 250 - focusing

Focusing an astrocamera is extremely important but often very difficult. Especially for an an F/3 system where a shift of 9 micron results in a 3 micron “unsharpness". With the Helix 250 focusing is simple, accurate and easy to reproduce even after a rough ride in the back of your car. This can only be accomplished by a rigid system and a clever design as we will explain.

Maximum stability is obtained by placing the focus as close to the tube as possible using no moving parts. This also resulted in a smaller diagonal. The distance from the mounting plate to the focus is 60 mm. This is necessary for the optical quality. A spacing adapter is mounted on the CCD-camera with or without filter wheel or on the film cassette. These adapters guarantee the 60 mm distance to the focal plane.

 
1= SLR-camera
2= modified 6x6 roll film cassette
3= large format CCD camera
4= any other CCD-camera

To focus the camera, only very small displacements are needed. Just like the ASH Lurie 200,  focusing is done by turning the micrometer screw at the back of the camera. The micrometer moves the primary mirror. Focusing is completely backlash free and there is no image-shift. The position accuracy is better than 0,01 mm. Instead of a micrometer a small stepper motor can be mounted that is compliant with modern motor-focusing programs. 

As can be seen in the above diagram, there is an extremely small difference in focus position for violet and red. If the camera has been focused in green light and this position is fixed, all spot sizes will be smaller than 10 micron for colors between 397 nm and 673 nm. Although this result is already outstanding, it can even be better! Diffraction limited can be achieved by focusing for one particular wavelength. This means that diffraction limited pictures can be taken for H-alpha light! Focus differences are only a few hundreds of a mm and can easily be calibrated with the micrometer or stepper motor.