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LSS principae

LSS principae. Yves Lhoumeau Lionel Ruiz LSS Team. How does it work ?. Beamer Condensor Fisheye. The master key: a fisheye. A special photographic lens Caracteristics: Very short resultant focal A standard back focal length (42-43mm) A special distorsion of image

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LSS principae

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  1. LSS principae Yves Lhoumeau Lionel Ruiz LSS Team

  2. How does it work ? • Beamer • Condensor • Fisheye

  3. The master key: a fisheye A special photographic lens Caracteristics: • Very short resultant focal • A standard back focal length (42-43mm) • A special distorsion of image • a circular picture, typically 24mm of diameter

  4. Why can't we adapt directly a fisheye to a beamer ? • Because with the original lens, the picture isn't correctly focalized, and the size of beamer's picture isn't compatible

  5. Removing and replacing the original lens of beamer: don't work either • Because the back focal length of the fisheye is too short. A standard beamer's lens has 80mm-100mm back focal length. Compare with the back focal length of the fisheye.

  6. Still not working... • Extend the back focal length of the fisheye with a divergent lens, like a barlow lens

  7. Then... What works ? • Beamer + • Condensor + • Diagonal flat mirror + • Fisheye

  8. « LSS Principae » to project an image with a fisheye...

  9. « LSS Principae » • We must build an intermediate image, with • Correct image size: typically 24 mm in diameter • Correct incident ray light, like to take a frame, but inverted...

  10. « LSS Principae » • To build an intermediate image from the beamer, we use the same process of microphotograpy, but inverted. We don't take a frame, we project a frame, by putting additionnal lenses, (photographic lenses), in front of the beamer

  11. « LSS Principae » • This intermediate image can be deflected by a flat mirror. Advantage: the beamer will work in a « made for » position with no dust and heat problems.

  12. Technics: Fisheye specs • Fullframe format (24x36mm) • Diaphragm used to limit some optical abberations in some cases (typically used at F/D8 with a standard 50mm photographic lens as condensor) • Focus unnecessary (doesn't change anything)

  13. Technics: Condensor specs • Focal length and F/D is of no importance BECAUSE: • Specifications change with: • The back focus length => change the incidence of the rays of light • The size of the rear lens of the condensor (to prevent cutting image) Diaphragm on condensor cuts the image: don't use it !

  14. Technics: Condensor specs: ideally • Back focus length as short as possible for max field of view • Diameter of rear lens minimum approximatively 24mm or larger

  15. Technics: Condensor in practice If you're lucky... find A 75 mm F/D 1.1 Heligon, which is: • 10-15mm back focus lens (vary upon models) • A big rear lens • Exist in some variants: TV, XR, etc...

  16. Technics: Condensor in practice With actual commercial lenses • A photographic objective, as Canon 50 mm F/D 1.4 USM • To reduce back focal length, add a proxy lens, +4 D to the system

  17. Distance between optical parts • A constant: back focal length of fisheye: 42 mm • Correct distance: add back focal length of fisheye and back focal length of condensor Typically between 84 mm (42+42mm) and 52 mm (42+10mm)

  18. Adjustments with Beamer • Use DLP technology to increase contrast • Zoom parameter can change the field of view and focus • Focusing the image can be made on the beamer. If you can't, change the distance between fisheye and condensor (probably too long)

  19. Field of view depend of: • Focal back lenght of Condensor • Beamer objective (zoom and focal lenght) • Beamer size of DLP (typical 0,55 or 0,65'')

  20. TIPS • Don't place the diagonal miror at intermediate focus, if you don't want to see dust…accurately • Back focal length of the condensor can be measured, using a beamer and a projected picture on a piece of cardboard (as test pattern) • You can reduce the focal back length of the photographic lens with a convergent lens in front of condensor (typicaly F=250mm = +4D) but it adds some distortions on the edge

  21. TIPS • Focusing the projected picture need a VERY precise alignement of condensor and Fisheye otherwise there will be always a part of the horizon blurred... (like telescope collimation) • You can limit horizon blur and distortion with fisheye diaphragm, but with loss of brigthness Note: unecessery with Heligon lens

  22. http://www.lss-planetariums.info

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