[simpits-tech] So I've got this huge thing in my garage now.....

Mon Aug 5 11:27:52 PDT 2013

Matt, 

First, what you have is not a collimator, it is a screen. Not picking nits, just trying to work with correct vocabulary. Moreover, given what you say about the 'optics' on the inside surface, I have a feeling that it is a screen specifically intended for rear projection, that is to say that the intent is to have the projectors on the inside and the image on the outside. The 'optics' serve to reduce the acceptance angle from the back side, improving image contrast by increasing the amount of light gathered from the direction of the projectors, and reducing the amount of light gathered from the rest of the screen. This same effect also reduces the light bleed from the rest of the screen to any point, by directing the backscatter back towards the projector. The tradeoff in this arrangement is that it won't likely make a very good front-projection screen, nor will it work all that well if back-projected in the wrong direction. All this is speculation based on high-contrast rear projection screens that I've seen, and yours may be different. 

Second, placing the projectors within the screen doesn't allow for a smaller mirror. The mirror and screen geometry are intimately joined by the requirement that the screen resides within the focal surface of the mirror. One immediate benefit of placing the projectors inside the screen is that you can project perspective images, and get the correct on-screen image with no warping required, provided the projection geometry is correct. I could write a whole page on this, but I won't bore you with the details for now. For a first-order approximation, the mirror will be just a little less than twice the width of the screen. Based on your statement of "this thing is 12ft wide", I'm showing a required mirror radius of 117", or 19.5 feet wide. Assuming your seated eyepoint is 4 feet above the floor and accounting for a few inches of supporting structure, the whole package would be about 10 feet tall. 

-Wayne 

----- Original Message -----
From: "Matt Bailey" <dabigboy at cox.net> 
To: simpits-tech at simpits.org 
Sent: Sunday, August 4, 2013 2:27:53 PM 
Subject: Re: [simpits-tech] So I've got this huge thing in my garage now..... 

Thanks guys, I may just do that. I assume you need vertical and 
horizontal FOV, height, and diameter? 

I am not sure which way I am going with this yet......there are several 
options. The original setup is almost identical to Gene's, with one key 
difference. On Gene's rig, the projectors are in front of the cockpit, 
and use direct projection onto the front of the collimator. On the big 
sims (on this one, at least), three CRT projectors (of highly 
specialized design) are mounted behind/inside this big ol' collimator, 
and shoot through it. There are two layers to this collimator: the 
inside layer is a smooth, clear piece with a bit of optics going on (I 
believe it has a moderate Fresnel effect), and the outer layer is 
frosted. This makes the projected image show up on the frosted layer, 
after passing through the inner lens. From there it works exactly like 
Gene's setup...a frame in front of the cockpit has reflective Mylar 
which is pulled with a vacuum, and reflects the image that's on the 
collimator. 

I have never really considered Gene's setup as a viable option for me, 
because of concern that I may lack the skills and tools to make a frame 
and collimator that are accurate enough to make everything look right. 
With my acquisition here, however, a significant piece of the puzzle is 
done for me. But this thing is already 12ft wide by itself.....the 
screen would be huge! I do think that having the projectors inside this 
collimator has allowed the big sim guys to get by with a smaller Mylar 
screen than otherwise possible (since no space is required in front of 
the collimator for the projector beams). 

I have more or less ruled out using the collimator with the 
projectors behind it, though I am going to test that out. Aside from my 
projectors probably not being as bright as what the big sims use, one 
of the reasons I was able to acquire this thing is because the two 
layers are starting to delaminate....the glue inside is drying up, 
which causes the screen to lose a LOT of light passing through it (or so 
I'm told..it's not noticeable as it sits right now). If I do front 
projection instead (and paint the outside of the collimator), I can 
preserve almost all the light from the projector. This setup would 
essentially be exactly what Gene is doing, just on a larger scale. 

If I don't go with a Gene-Screen, here are a couple other options: 

1: I could use the collimator itself as the screen, and mount it in 
front of my sim, with the projectors above the sim, projecting directly 
onto the collimator. Again, I would need to paint the collimator with 
some reflective paint. 

2: A second option is a variation on the above option. I could still use 
the collimator as the main screen, but I would mount the projectors in 
front of the collimator, taking advantage of the frosted finish to make 
the image project onto the screen. This is basically the reverse of how 
it was used originally. Again, the delamination and light loss that 
this collimator supposedly suffers would be a factor. It would also 
mean the entire setup would take up more room...BUT, the BIG advantage 
here is that I would have no clearance considerations between the 
projectors and the cockpit (I could put the screen as close or as far 
as I want from my cockpit). I also might get some sort of beneficial 
Fresnel effect from the inside layer. 

Like I said, this thing is 12ft wide, so I could use it as the main 
screen. I have sat down in front of it several times and looked around 
at the inside surface....I think it's big enough to fill the view of any 
standard business jet if I mount it correctly to its geometry (IOW, any 
given section of the screen would be 6ft from the pilot's viewpoint). 
Using it as the screen itself is very appealing from a simplicity 
standpoint, and from the standpoint of making the sim mobile. Right now 
my sim consists of the cockpit itself (which is on wheels, and can 
easily be moved by two people), a stand with the projector on it, and 
another stand with a flat projection screen. I could mount this thing 
on a wheeled base that rolls up to the sim, and mount three projectors 
on another base to go behind/above the sim. Options, options! 

One thing is for sure...I am about to pull my projector down from its 
lofty perch above my sim and have some fun with this big ol' 
collimator. :) 

Matt 

On Fri, 2 Aug 2013 15:41:18 +0000 (UTC) 
wledzian1 at comcast.net wrote: 

> Yep - if you ever want mirror specs to match, I can run the numbers. 
> 
> ----- Original Message ----- 
> From: "geneb" <geneb at deltasoft.com> 
> To: "Simulator Cockpit Builder's List" <simpits-tech at simpits.org> 
> Sent: Thursday, August 1, 2013 8:15:30 PM 
> Subject: Re: [simpits-tech] So I've got this huge thing in my garage 
> now..... 
> 
> On Thu, 1 Aug 2013, Matt Bailey wrote: 
> 
> > It is exactly what it looks like....this is the back-projection 
> > collimation screen from a level D commercial flight simulator. :) 
> > Now what to do with it......hey Gene, I think I have the start of a 
> > jumbo-sized version of your collimated display system. :) 
> > 
> Take very, very good care of that thing Matt. If you can figure out 
> the specs on the shape, Wayne could likely give you the numbers 
> needed to build a matching mirror cell for it. 
> 
> g. 
> 
> 

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