Introduction
The ASH LURIE 200 is the ultimate camera for the astrophotographer. The aperture of 200 mm and the field of view of 10 degrees make this camera very useful for deep-sky and comet photography.
The by ASH perfectioned optical and mechanical design guarantees that the starpoints are smaller than 6 micron over the whole field of 82 mm. This means that the energy concentration is at least 6x higher than conventional telescopes. That is why exposure times can be shorter to achieve the same stellar magnitude.
The small distortion of 0,0002% makes the ASH LURIE 200 the perfect instrument for astrometric measurements.
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 constraints 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 an 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”. Instead 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.
FILM & FILTERS
The field in a Lurie camera and a Schmidt camera is curved. The filmholders for the ASH LURIE 200 are designed in such a way that the film is automatically curved in the necessary radius. They can be very easily mounted in the camera and held in place by a magnet.
The following filmholders are available for the ASH LURIE 200:
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24x36 mm piece of ~ 50 mm long.
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24x36 mm on roll ( in preparation )
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56 x 56 mm (6x6) piece of sheet film 2,5” (63,5 mm)
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56 x 56 mm (6x6) on roll ( in preparation )
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Round 82 mm piece of 92 mm sheet film
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Round 90 mm piece of 100 mm sheet film.
The largest filmholder with a diameter of 90 mm covers an astonishing 10,3 degrees. Standard 2" filters can be screwed onto the smallfilm filmholders. For the larger filmholders 82 or 95 mm filters can be used.
The filmholders are high-precision components. At f/2,5 the focal distance must be accurate within 0,01 mm even with the change of temperature. In the camera there is a small hatch that can be opened to mount the filmholders. They are kept in the exact position by magnetic force. Provision has been made for a supply connector to add protection gases during exposure.
Because the filmholder attachment has been mounted on the inner corrector plate, the images are totally free of any spider effects around bright stars. A feature that is common in conventional telescopes.
The ASH LURIE 200 is temperature compensated. Three invar rods in combination with the Astro Sital mirror guarantee accurate positioning of the focus independent of temperature.
Focusing
Focusing is done by means of a built-in micrometer. This is located at the rear side of the camera and is protected by a screwcap. The camera is precision focused in the factory. The pre-adjusted micrometer reading assures that the camera is always perfectly focused.
When using filters it is quite easy to calculate the shift of focus by measuring the thickness of the filter. This shift correction is achieved by simply adjusting the micrometer.
SPECIFICATIONS
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free aperture: |
200 mm |
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focal length: |
500 mm |
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focal ratio: |
f/2,5 |
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primarymirror: |
265 mm (Astro Sital) |
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field of view: |
24x36 mm = 5° |
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circular 82 mm = 9,4° |
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circular 90 mm = 10,3° |
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definition |
< 6 micron over the whole field |
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colourcorrection: |
completely achromatic even in the far violet diffraction encircled energy curve or here for polychromatic diffraction modulation transfer function) |
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vignetting: |
circular 58 mm no vignetting(click here for relative illumination diagram) |
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circular 82 mm ~ 8 % |
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distortion: |
0,0002% (click here for grid distortion diagram) |
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max.diameter: |
~ 308 mm |
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max. length: |
~ 760 mm |
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max. weight: |
~ 18 kg |
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materials: |
special resin tube and aluminum castings |
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temperature compensation with the help of 3 invar rods. |
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focusing: |
moving the primary mirror by a micrometer (0,01 mm)(click here for RMS spot radius vs focus diagram or here for through focus spot diagram) |
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finish: |
2 component paint |
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film cassettes: |
24x36 mm piece of ~ 50 mm long, 48 mm filter. (2”) |
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24x36 mm on roll ( in preparation ) |
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56 x 56 mm (6x6) piece of sheet film 2,5” (63,5 mm), 82 mm filter. |
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56 x 56 mm (6x6) on roll ( in preparation ) |
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circ. 82 mm piece of 92 mm sheet film, 82 mm filter. |
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circ. 90 mm piece of 100 mm sheet film, 95 mm filter. |
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spider effects: |
no |
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ghost images: |
no |
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ease of use: |
big opening for access to filmholder |
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easy to operate |
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very precise focusing |