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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Hi guys, I hoped to tap your brains for something optical but not really telescope related - astronomy related though.
I am thinking of using a projector with a curved screen or even better a dome. I know that there are problems with this, for instance the picture will be skewed but there is software that can fix this by mapping points on any non flat screen.
What this software wont do is fix the focus. Some areas of the screen will be focused and some will not. In a curved screen you would normally focus in the center but as you get farther around the sides they get out of focus, especially if the sides are 3 feet forward for the center.
I was wondering what sort of effort would be involved in designing a lens that would sit in front of the projector to fix the focus issue and if possible what it might cost roughtly , $500, $1000, $2000, $10000? I have no idea
The best would be something that would let you project on a dome with an in focus image. This would be something like a quarter sphere about 5-6 feet diameter.
If anyone has any ideas - even if you can say strainght away it's going to cost a bomb then I would like to know!
Thanks,
Jason
Edited by jasonharris (10/28/09 06:49 PM)
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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It's not hard to design a lens with a spherical image surface - I've done very wide-angle flight simulator projector lenses for domes with very good, uniform image quality and brightness over 120 deg. dome coverage. But those were $30K lenses in 1982 - OUCH!
It's always cheaper to see if you can use off-the-shelf optics, with 35mm/DSLR wide-angle lenses or 80-100 degree FOV eyepieces, etc. for the final projection. But the problem remains in relaying the previous lens's exit pupil to the projection lens entrance pupil. If that doesn't happen, you will rapidly learn a lot about vignetting! Also, the image space focal ratio is very high for wide angle projection, possibly above f/100, requiring a BOATLOAD of light going into the lens to get a usable screen brightness (on the order of 20-50 foot-lamberts for comfortable viewing with a typical 0.7-0.8 reflectivity Lambertian screen). High brightness light sources can sometimes damage cemented projection lens elements, depending on the optical gain.
But this is not to say you can't do it, and maybe even affordably. Put up a drawing of what you want here, and me, Cuzzin Ed, Dave Groski, Dick Parker, Vlad and others can take a crack at it. This is also an Optics forum as well as ATM, so it doesn't absolutely have to only focus on astronomical systems. I think a community optics project would be fun, and interesting and educational to a lot of people.
Mike
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1251
Loc: Ottawa, Ontario, Canada
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I'm not so sure the focus will be too adversely affected. let me give you a couple of examples of projects I dabbled in:
I used to carry a 16 foot diameter Starlab portable planetarium around Eastern Ontario's schools. In addition to my own home-made pinhole star projector (stars down to 5.0m), I showed 35mm slide images using a 25mm f/2.8 lens designed for projection onto a flat screen. But on the curved dome, the relatively short distance of about 8 feet caused no visible de-focus. And the angularly large image was impressive for the audience, too.
My other device was a home-made all sky projector, Which projected slides taken by my home-made all-sky camera. The same lens was used for both instruments, namely a Minolta 16mm f/2.8 fisheye. That lens imaged about 200 degrees (10 deg. below the horizon), and when used in projection produced a perfectly focused image from the zenith on down. I should mention also that I had to use a 1,000W lamp in order to achieve reasonable brightness, and indeed it was enough to clearly see color in the faint nebulae that were recorded.
The aperture-to-projection distance ratio has some bearing on criticality of focus. If sufficiently small, the depth of field is probably good enough to not have to worry about focus differential at all. That was the case for my projectors used inside domes, where the aperture was no larger than 9mm, and the projection distance was 8 feet (2.4m).
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Thanks Mike, I have attached a couple of pictures with the types of screens, I just need to decide which is preferable..
To be honest, I think the sphere is preferable but I also think that it is the one that would pose the biggest problem to get an image that looks good. I think that I might be pushing past what is possible with a single projector to get that 120 degrees. A quick estimate with my hands shows that I can see ~120deg field when looking forward - of course I cant make out a lot of whats on the periphery.
The other option is the curved screen which is much easier to make and I can project on to the front of the screen saving a lot of space. With the dome the only option is to project from the rear when someone is stiing inside of it..
It's a catch 22 really.. I guess the dome would be more immersive but I have a feeling the the curved screen has a better chance of pulling it off.
The width of the curved screen would be something around 78" from the left edge to the right with a radius of 51" so it does have a bit of a curve to it. You would end up sitting at about the center of the screens arc.
The sides would be about 24" in front of the center of the screen. Given that other people I know would like to do something similar I think the curved screen is the best option as many more people are likely to be able to build such a screen over a dome.
A pity in a way because it would be really cool to sit inside it with a planetarium program running..
Being able to fill your periphery really helps with the immersion but as you cannot make out a lot of that detail at the very edges of the screen I guess it's not as important to get that perfect when cost is a consideration, after all you do spend most of your time looking forward. Unless this is the only area where you get something for nothing, didn't think so though hehe  As to the height, it would make sense to follow most projects which are HD so thats 16:9 and the height would be about 43" not allowing for the curve as I am not sure if you should or not..
So this is where I am with the basic idea, lots have people have wanted to do it at a DIY level but no one I know has had guys like yourself to give advise on how to take it to the next level.
The only other option I have seen is to get 3x projectors and blend the edges, this way the focus doesnt get too bad for each.. Thats a lot of $$ and on going bulb costs. For all I know a lens might make that look like a good option though!
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jasonharris
sage
Reged: 09/16/06
Posts: 248
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and the dome, I know a whole half dome cannot be used but whatever part can be..
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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Hi Glenn, I fired up the projector and had a quick look at a few things.
As the projector is set up now, for watching movies it is about 12.1 feet from the screen which is about 7.2 feet wide. With the center in focus I took a piece of paper to the edge and moved it forward 24". At this point it was focused enough that you wouldnt notice if you had pictures or video etc on it.
The warping was evident though, I didn't have a big enough sheet to play with the keystone to see if this would fix it but I figure that would just make the top and bottom unused which is not ideal..
Now, when I change the distance to the screen on my projector things get a little bit more of a problem and this is what most people are finding. Most of the guys using this set up on their computer don't have the typical layout that we have for watching movies, I.e. it's in a smaller room so we are trying to throw a large image from a small distance. When I try to do this then the focus can go out very quickly as I move away from the focal plane and the 24" forward is a problem as the image is not viewable at all.
In fact for the space I was hoping to use this set up I have just found that my projector will not project 82" wide from 82" back. Some will but mine wont  I may have to use it in a different place which means a screen that can be moved when I am not using it..
So those are the issues I am seeing, if not focussing then there is a real warp problem. I tried putting a 17mm camera lens in front of the PJ, expecting warping but I couldn't tell because so little light was coming through.. It was only an f/4 zoom so it wasn't the largest aperture I could try.
Thanks,
Jason
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