ASH LURIE 200 - optics

The Lurie design combines the same optical quality of a Schmidt camera with a shorter tube lenght, smaller diameter and less weight. This means that a lighter mount can be used and that the Lurie camera is easy to transport.

There are many types of camera for deep-sky photography. For small fields of view the Maksutov and Wright cameras perform very well.  Cassegrain-like designs such as the  Slevogt, Simak, Companar, Mandler, have a good performance over a large field of view but suffer from vignetting. The Schmidt camera is the winner when it comes to a large unvignetted field of view in combination with a fast focal ratio.

The technical constrains for an astrocamera are very high. The first requirement is that the image of a star in the focal plane must be as small as possible. In general telescopes that produce images smaller than 25 micron are of good quality. When using  high resolution emulsions like TP2415 a resolution of 10 micron is required to obtain the maximum result. If this criterion is  met, it means that the energy concentration is 6x higher than in conventional telescopes. That is why exposure times can be shorter to achieve the same stellar magnitude,

With a Schmidt camera it is possible to meet the necessary image quality. Even with a fast focal ratio and a almost unvignetted field of view. The biggest disadvantage of a Schmidt camera is its size and weight. The length of the tube is at least 2x the focal length.

In the march 1975 issue of ‘The Journal of The Optical Society of America’ Robert J. Lurie presented an interesting alternative in his article  “Anastigmatic Catadioptric Telescopes”. In stead of a Schmidt corrector a double corrector is placed near the focus. This reduces the length of the tube by 50%. To eliminate vignetting, the main mirror must be slightly larger than the free aperture.