Helix 250 - introduction

After the ASH Lurie 200, an astrocamera that has been developed for large format flat film, we now have a camera for the demanding CCD photographer.

Professional astrophotographers were enthusiastic about the ASH Lurie 200. The concept of a high speed focal ratio together with unprecedented sharpness over a large field of view is very attractive. More and more photographers asked us to make a CCD version of the ASH Lurie 200. Unfortunately this is not possible with the Lurie design since the focus lies within the camera. 

So to fulfill the demands of the CCD-photographers we had to design a completely new camera. The first task was to design an optical system that would satisfy the following requirements:

• A perfectly flat field
• Extremely high image quality so that the overall quality is limited by seeing effects and tracking errors only and not the camera
• High speed focal ratio
• A large field of view to accommodate the new generation of large format CCD-camera’s
• Neglectible vignetting and distortion
• Outstanding color correction to fully exploit modern color-filters

To find an optical solution that meets these criteria, we contacted Harrie Rutten. Mister Rutten is a well known-optical designer and co-author of “Telescope Optics”.

Extensive calculations showed that a full aperture Houghton corrector in combination with a spherical mirror and a field corrector near the focus, would meet our requirements.

The optical system of the Helix 250 has a 250 mm (10”) aperture and a focal length of 750 mm giving a geometrical focal ratio of F/3.



Although it would be possible to bring the position of the focus outside the camera through a hole in the corrector, this is not an option for two reasons. First, this would limit the use of the camera to thin CCD-camera’s. Second, the heat generated by these camera’s would deteriorate the image quality. Instead we have designed a flat diagonal to bring the focus outside the tube, just like a Newtonian reflector. 

To obtain a field free of vignetting, a heavily oversized primary mirror and large diagonal is used. The resulting obstruction is 45% linear. For a visual system this is would be too large but it is of absolutely no importance for an astrocamera. The effective focal ratio becomes F/3.4

One might wonder if a simple 250 mm F/3 Cassegrain or similar optical design would meet the requirements. The answer is no, this is not possible. On axis such a system might work but to obtain a fully illuminated field with small enough spot sizes is not possible.