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> the Lhoumeau optical system, a clever way of adapting a fisheye lens to a projector
albert
post Jun 14 2009, 10:09 AM
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Last week in France, along with the Cuisery planetarium I had a chance to see M. Yves Lhoumeau, the inventor of a new system of adapting a fisheye lens to a video projector. This has the great advantage of keeping the video projector intact so it can be used for other things or replaced with a higher resolution system as they become available.
Mr. Lhoumeau uses the Russian PELENG 8 mm fisheye which is of good quality and can be found on Ebay or some other places for around 300 Dollars.
He makes use of a 50 to 75 mm photographic lens turned around to focus the image of the projector onto a small front silvered mirror. From there it goes up 90 degrees into the fisheye lens. this way the problem of the extremely short back focus distance of the fisheye is worked around. The image is brought out of the projector by simply putting the projectors lens face to face with the 50 mm lens.
I had no chance to take any pictures during my visit so I took these pics from the various web sites where M . Lhoumeau has published them.
Here you can see a small cart containing the projector and the entire computer system.

Attached File  lhoumeau_beamer.jpg ( 60.88k ) Number of downloads: 14
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albert
post Jun 14 2009, 10:13 AM
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The optical block consisting of the first lens in this case a 75 mm Heligon Rodenstock Lens, got at an auction. This goes into the mirror block which might well be a 90 degree mirror from a telescope. Then comes the Peleng fisheye. The video projector is tilted because it projects upwards and the electronic trapeze correction affects the image. the projector I saw was an OPTOMA 1920x1080 full hd beamer. Mr Lhoumeau has a small experimental 3 m cloth dome set up in his garage. He uses Linux as his operating system and a greatly modified version of Stellarium.
Attached File  lhoumeau_optical_system.jpg ( 18.7k ) Number of downloads: 15
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albert
post Jun 14 2009, 10:16 AM
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Attached File  aulaprojector.jpg ( 27.92k ) Number of downloads: 19

The Lhoumeau system will be commercialized by auladelcosmos, a Spanish company, complete with computer and Stellarium for less than 10 000 Euros. Besides the astronomical part, it can also serve as a projector for properly warped full dome movies or panoramic images.
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albert
post Jun 14 2009, 10:23 AM
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Here is another picture of the optical block.

The images on the 3 m test dome were sharp all the way around. Being so close to the dome we could see the grid structure of the pixels which might be not as obvious in a larger dome. The brightness was very good, the projector gives out 2500 lms.
Video images, we saw several trailers for current fulldome movies, were impressive. We stood in the dome and had to grab for something to hold onto when the images moved!
The tuned version of Stellarium had several interesting scripts developed by M. Lhoumeau and Lionel Ruiz. It could be remote controlled with a wireless remote control, I think from a game console.
the whole thing is extremely interesting since it is the first time we have access to fulldome projection without using a mirror. And the system is not out of the reach of the average do it yourselfer.

Attached File  Lhoumeau_optical_block.jpg ( 44.18k ) Number of downloads: 12
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Ken Miller
post Jun 15 2009, 01:49 PM
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Every time I see these video projection techniques I start twitching all over because I would really like to try a video option in the small planetarium that we have in the Children's Natural History Museum. The problem is that front surface mirrors are expensive, as are fisheye lenses and digital projectors. I would like to see something that really works well in a 10 ft dome before I invest a lot of money in something that may give disappointing results. I have seen this particular lens setup before, and I'm really tempted to experiment with something like it. I would like to see the comments of others on this particular approach.
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Ken Miller
post Jun 16 2009, 09:41 AM
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QUOTE(albert @ Jun 14 2009, 09:09 AM) *
Mr. Lhoumeau uses the Russian PELENG 8 mm fisheye which is of good quality and can be found on Ebay or some other places for around 300 Dollars.
He makes use of a 50 to 75 mm photographic lens turned around to focus the image of the projector onto a small front silvered mirror. From there it goes up 90 degrees into the fisheye lens. this way the problem of the extremely short back focus distance of the fisheye is worked around. The image is brought out of the projector by simply putting the projectors lens face to face with the 50 mm lens.

The Peleng lens comes with one of several camera mounts. Is there one type of mount that is better than the others for this application (for instance a screw type mount as opposed to bayonet)? Is the 50 to 75 mm lens a variable focal length, or just a fixed lens that falls in that range? I expect that you refer to one that is adjustable.
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Ken Miller
post Jun 18 2009, 11:09 AM
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QUOTE(Ken Miller @ Jun 16 2009, 08:41 AM) *
The Peleng lens comes with one of several camera mounts. Is there one type of mount that is better than the others for this application (for instance a screw type mount as opposed to bayonet)? Is the 50 to 75 mm lens a variable focal length, or just a fixed lens that falls in that range? I expect that you refer to one that is adjustable.

Chirp, chirp, chirp (sound of crickets). Is anybody here (ECHO, ECHo, ECho, Echo, echo)? I'm assuming there aren't any easy answers to my questions. I'll see what I can figure out. The video enthusiasts all seem to be over at the Yahoo Small Planetarium group. This group is pretty much focused on mechanical projectors. (I hope I don't sound sarcastic, that is really not my style).

Ken
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albert
post Jun 18 2009, 12:11 PM
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Hi Ken,
I'm sorry I can't answer your questions. I think you might ask M. Lhoumeau directly, he is known here as ZEMT.
I just wanted to attract a little more attention to this process, since it seems to me the only one I could do with my home workshop.
Albert
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Ken Miller
post Jun 18 2009, 02:18 PM
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QUOTE(albert @ Jun 18 2009, 11:11 AM) *
Hi Ken,
I'm sorry I can't answer your questions. I think you might ask M. Lhoumeau directly, he is known here as ZEMT.
I just wanted to attract a little more attention to this process, since it seems to me the only one I could do with my home workshop.
Albert

About two years ago on this forum, ZEMT was involved in a discussion of this system with Ron Walker and Oscar Kim.

See: http://www.observatorycentral.com/index.ph...ost&p=13670

I have not seen any reports of success or failure from anyone using this system, but I would like to experiment with it. I just have to find the time to do it. Right now, as always, I am totally swamped. I keep waiting to get laid off from my job, but it never seems to happen. If I cut back my hours, or get laid off, I won't have the money to play with this stuff.
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